Chapters 27 and 28
Note: When studying the following material, make sure to completely redraw the figures on your notebook and write the formulas as you proceed. This will help you learn better. Make sure to always draw horizontal fraction bars.
Electric Current:
The current, I through a wire is defined as the change in charge, dq per change in time, dt at any crosssectional area of that wire. We may generally write:
The SI unit for I is of course Coul./sec called Ampere or Amp or simply A.
Example 1: A 5.00Amp current flows through a wire. Calculate the amount of electric charges that cross any section of that wire in 20.0 minutes.
Solution: For a constant current, instead of the differential form I = dq /dt, we may write: I = Δq /Δt . Crossmultiplication solves for Δq. We get: Δq =IΔt or,
Δq = (5.00Amps)(20.0*60sec.) = 6000 Coul.
Besides Coulombs, another unit for the amount of electricity (q) is Amperehour (Ah). It is easy to see that:
1 Ah = 3600 Coul. The reason is : 1Ah = 1 (Coul./s)(3600 s) = 3600 Coul.
Example 2: A car battery has been under charge for 40.0 hours at an average current of 0.80 Amps. How much electricity in (Ah) is stored in it? How much is that amount in (Coul.)?
Solution: I =Δq /Δt ; Δq = IΔt ; Δq = (0.80 A)( 40.0 h) = 32.0 Ah
Δq = (32.0)( 3600) Coul. = 115,000 Coul.
Ohm's Law:
For any linear electric device, the ratio of voltageV across the device to the current I through the device is a constant. The constant is called the resistance R of the device. Mathematically,
where in SI, the unit of R is volts/amp called "Ohms" with the symbol Ω , pronounced "Omega."
Example 3: Calculate the resistance R of a light bulb that allows a current of 2.5 Amps to flow through it when connected to a 12volt battery.
Solution: The light bulb resists toward the passage of the current through it. We use a zigzag line to show the resistance of an electric device. We also use two parallel lines one shorter and one longer as a symbol for a battery. The longer line means the positive pole. The following figure is selfexplanatory:
Solution: (Write with horiz. fraction bars). R = V / I = (12 V) / ( 2.5 A) R = 4.8 Ohms, or R = 4.8 Ω When key K is turned on, the circuit is closed and the current, I flows making the light bulb go on.

Fig. 1
Test Yourself 1:
1) The electric current, I, is defined as (a) the # of gallons of water crossing a section of a pipe per second (b) the # of Coulombs of electric charge crossing a section of a wire per second or per unit of time (c) neither a nor b.
2) The unit for electric current is (a) ampere (b) Coulombs per second (c) both a & b. click here
3) If through any crosssection of a wire, 480 Coulombs of electric charge pass per minute, the currents is (a) 8.0 Amps (b) 480 Amps (c) 480 mA.
4) If through any crosssection of a wire, 7200 μCoul. of electric charge pass every hour, the current is (a) 2.0A (b) 2.0μA (c)2.0mA. click here
5) If through any crosssection of a wire, 108 μCoul. of electric charge pass in 1/2 hour, the current is (a) 6.0mA (b) 6.0μA (c) 60nA.
6) 2Amps my be written as (a) 2,000mA (b) 2000,000μA (c) both a & b. click here
7) 350mA may be written as (a) 350,000A (b) 0.35A (c) 3.5A.
8) 3600Coul. is (a) 1.0 Ah (b) 10.Ah (c) 0.10Ah.
9) If a current of 5.0A passes through an electric device for 6.0hours, the amount of electric charge, Q flown through that device is (a) 30.Ah (b) 108,000Coul. (c) both a & b. click here
10) If a battery goes under charge for 24h at an average current of 1.5A, the amount of electric charge, Q accumulated in the battery is (a) 20Ah (b) 36Ah (c) neither a nor b.
11) Ohm's Law states that (a) the voltagetocurrent ratio for a resistor remains constant (b) the voltagetocurrent ratio for a resistor is not a constant (c) the coulombstocurrent ratio is constant. click here
12) Ohm's law may be written as (a) V = RI (b) I = V/R (c) both a & b.
13) The voltage across a resistor is 10.0volts and the current through it 2.50A. Its resistance is (a) 0.250Ω (b) 4.00Ω (c) 25.0Ω.
14) The current through a resistor is 2.30A and the voltage across it 9.20V. Its resistance is (a) 4.00Ω (b) 0.400Ω (c) 21.2Ω. click here
15) The current through a 2.0Ωresistor is 3.0A. The voltage across it is (a) 6.0V (b) 0.67V (c) 1.5V.
16) The voltage across a 4.5Ωresistor is 27V. The current through it is (a) 120A (b) 0.17A (c) 6.0A.
17) If V is known, and either R or I is to be calculated, then (a) to find R, V must be divided by I (b) to find I, V must be divided by R (c) both a & b. click here
18) If R and I are known, to find V, (a) R and I must be multiplied because V = RI (b) R and I must be divided by each other somehow because V = RI (c) both a & b.
19) In Fig. 1, if V_{battery} = 12.0V, the voltage across the resistor, R (a) is 12.0V because there is only one resistor in the circuit and all the battery voltage drop across that single resistor (b) would be 12.0V anyway even if other resistors R_{1}, R_{2}, R_{3}, ... were present in series with R (c) neither a nor b. click here
20) Although the ammeter in Fig. 1 has some small resistance itself ; however, in calculations, it is neglected and therefore the voltage across R is (a) 12.0V (b) slightly more than 12.0V (c) neither a nor b.
Problem: A 12V battery is connected to a 2.4Ω car light bulb.
21) The current through the bulb is (a) 29A (b) 5.0A (c) 0.20A. click here
22) If the resistance in the circuit is doubled (by adding another 2.4Ω resistor in series with the first one), the current will be (a) halved to 2.5A (b) doubled to 10.0A (c) does not change.
23) Based on the previous question, for a fixed voltage, if the resistance is doubled, the current (a) doubles (b) remains unchanged (c) halves. click here
24) Read the problem's statement again. If the 2.4Ω resistor is replace by a 0.80Ω one (the resistance cut to 1/3), the current (a) nine folds (b) does not change (c) triples.
Problem: Suppose there is a source that can supply a fixed current of 3.0A to a user regardless of the resistance of the user. Answer the following questions: click here
25) If this 3.0A current is passing through an 8.0Ωresistor, the voltage across it is (a) 2.67V (b) 0.375V (c) 24V.
26) If the 8.0Ωresistance is doubled to 16Ω, the voltage across it (a) doubles to 48.0Ω (b) halves to 12V (c) does not change. click here
27) From the previous question, one may conclude that for a fixed current, if the resistance doubles, then the voltage (a) halves (b) doubles (c) triples.
Problem: Suppose there is a source that can supply a fixed voltage of 24V to a user regardless of the resistance of the user. Answer the following questions: click here
28) When the resistance connected to this source is 1.2Ω, the current is (a) 20A (b) 0.02A (c) 26.4A.
29) If the resistance is changed to 12Ω ( increased by a factor of 10), the current (a) also increases by a factor of 10 to 200A (b) decreases by a factor of 10 to 2.0A (c) remains unchanged.
30) From the above, one may conclude that for a fixed voltage, if the resistance increases by a certain factor, the current then (a) decreases by that same factor (b) increases by that same factor (c) remains unchanged. click here
Resistance of a Wire:
Running electric current through a wire is similar to running water in a pipeline. The longer the pipe, the smaller its crosssectional area, and the rougher it is inside, the more difficult is and the more forceful must the pushing of water through the pipe be. An electric wire acts in a similar manner. The greater its length, L, the smaller its crosssectional area, A, and the greater its resistivity, ρ, the higher its electric resistance, R, is and it takes a greater voltage to push a higher current through it. The dependence of R on L, A, and ρ is given in the following formula:
where ρ, pronounced "rho", depends on the material. For copper, ρ_{cu} = 1.70x10^{8}Ωm at 20ºC. Table 20.1 gives the values of ρ for a few selected materials:
Table 20.1
ρ ( Ωm )  α ( ºC ) ^{1}  ρ ( Ωm )  α ( ºC ) ^{1} 
Conductors Aluminum 2.82 x 10^{8} Copper 1.70 x 10^{8} Iron 10 x 10^{8} Nickel 7.8 x 10^{8} Platinum 10 x 10^{8} Tungsten 5.6 x 10^{8} 
4.29 x 10^{3} 6.80 x 10^{3} 6.51 x 10^{3} 6.0 x 10^{3} 3.93 x 10^{3} 4.5 x 10^{3} 
Semiconductors Carbon 3.6 x 10^{5} Germanium 4.6 x 10^{1} Silicon 2.5 x 10^{ 2} Insulators Glass 10^{12} Rubber 10^{15} Wood 10^{10} 
5.0
x 10^{4} 5.0 x 10^{2} 7.0 x 10^{2}

Example 4: Calculate the resistance of a copper power line that is 425km long and 1.27cm thick (1/2 inch.).
Solution: The diameter of the circular wire is D =1.27cm. R = 0.635cm = 6.35x10^{3}m.
R = ( ρL) /A ; R = (1.7x10^{8} Ωm)( 425000m) / [π (6.35x10^{3})^{2} m^{2}] = 57.0 Ω
Example 5: Calculate the resistance of a thin copper wire 0.100mm thick and 50.0m long.
Solution: The diameter of the circular wire is D =0.100mm. R = 0.0500mm = 5.00x10^{5}m.
R = ( ρL) /A ; R = (1.7x10^{8} Ωm)(50.0m) / [π (5.00x10^{5})^{2} m^{2}] = 108 Ω
Power dissipation in a resistor :
Any electric device that pulls some current I from a source and has some voltage V across it, consumes some electric energy per unit of time. This means that each electric device has a power, P.
It is easy to show that (electric power) is equal to (Voltage) times (Amperage), Simply, P = V I.
The SI unit for power is [P] = [ V ][ I ] = ( Joule / Coul )(Coul / sec ) = (Joule / sec).
Joule / sec is called watt as was learned in Physics I. This means that 1watt = ( 1volt )( 1amp ).
Example 6: A lightbulb that works with 120 Volts pulls a current of 0.83 Amps from a wall electric outlet. Find its power.
Solution: P = V I = (120 V )( 0.83 A ) = 100 watts.
Example 7: A 60.0watt lightbulb is connected to a wall outlet, V = 120 volts. Calculate (a) the current it pulls and (b) its resistance.
Solution: (a) P = VI ; therefore, I = P / V ; I = (60.0 watts) / (120 volts) = 0.50 Amps.
(b) V = RI ; R = V / I. ; R = (120 volts) / (0.5 Amps) = 240 Ω.
Another Version of P = V I
Since V = R I, We may write P = ( R I )( I ) or P = R I^{2}.
Example 8: A current of 5.0Amps is flowing through an electric heater with a resistance of 40.0 Ω. Find (a) its electric power in watts, (b) its daily ( 24 hours ) energy consumption in ( Joules ), and (c) the daily heat energy in calories it gives to the room. Note: 1 cal = 4.186 J
Solution: (a) : P = RI^{2} = (40.0 Ω )( 5.0 A )^{2} = 1000 watts ( 3 sig. fig.)
(b) P = (Work / time) ; therefore, Energy = ( P )( time) , Here Energy = (1000. watts )(24)(3600s).
Energy = 86,400,000 Joules (c) Energy = (86,400,000J) / (4.186 J/cal) = 2.1x10^{7} calories.
A commercial Unit for Energy (kwh):
It is easy to show that kilowatthour (kwh) is a unit of energy. Since P = W/t, work or energy (W) can be solved in terms of power (P) and time (t) by crossmultiplying the formula P = W/t. This crossmultiplication yields:
W = Pt ; expressing P in kilowatts and t in hours, gives us the energy or work, in kwh.
Example 9: A water heater works at 240V and pulls 12.5A. Find (a) its power in watts and kilowatts. If this heater is on 12h a day during a 30day month, find (b) its energy consumption in kwh. If electric energy sells for 9.00 cents per kwh, calculate (c) the cost per month of this heater.
Solution: (a) P = VI ; P = ( 240.V )( 12.5A ) = 3000 watts ; P = 3.00 kw.
(b) W = Pt ; W = ( 3.00kw)( 360h) = 1080 kwh.
(c) Cost = ( 1080 kwh )( $ 0.0900 / kwh ) = $ 97.2.
Test Yourself 2:
1) The resistance, R of a wire depends (a) directly on its length, L (b) directly on its resistivity, ρ (c) directly on its crosssectional area (d) directly on the reciprocal of its crosssectional area (1/A) (e) a, b, & d. click here
2) The resistivity, ρ of a resistor depend on (a) the material of the resistor (b) the length of it (c) the crosssectional area of it (d) a, b, and c.
3) Comparing two wires of the same thickness and the same material, (a) the longer wire has a greater resistance (b) the longer wire has a smaller resistance (c) both have the same resistance regardless of their length. click here
4) Comparing two wires of the same length and the same material, (a) the thicker wire has a greater resistance (b) the thicker wire has a smaller resistance (c) both have the same resistance regardless of thickness.
5) Two wires A and B have the same length and the same material. The diameter of wire A is 1/3 of that of wire B. We may write (a) R_{A} = (1/3) R_{B} (b) R_{A} = 3 R_{B} (c) R_{A} = 9 R_{B}. click here
6) Two wires C and D of same length and same material, wire C has a diameter 1/6 that of wire D. We may write (a) R_{C} = 36R_{D} (b) R_{C} = 6R_{D} (c) R_{C} = (1/6)R_{D}.
7) The resistance of a copper power line 255.0 miles long (1mile = 1609m) and 2.00cm thick is (a) 45.3Ω (b) 22.2Ω (c) 11.1Ω. click here
8) The resistance of a copper wire 41.0m (135 ft) long and 0.200mm thick is (a) 45.3Ω (b) 22.2Ω (c) 11.1Ω.
9) The reason why the resistances of the two wires in the previous two questions are equal is that (a) the second wire is much shorter but also much thinner (b) the first wire is much thicker but also much longer (c) both a & b. click here
10) If a wire is passed through a narrow hole to where its diameter is reduced by a factor of 2, its length becomes (a) 2 times longer (b) 4 times longer (c) 6 times longer.
11) The reasoning for the previous question is that (a) when the diameter or radius is cut by half, the crosssectional area is reduced by a factor of 4 (b) A = π r^{2} (c) the volume of the wire is kept constant in the process. If the crosssectional area is reduced by a factor of 4, the length has to increase by a factor of 4 to keep the volume constant (d) a, b, & c.
12) If a wire is passed through a narrow hole to where its diameter is reduced by a factor of 2, its resistance, R becomes (a) 16 times greater (b) 4 times greater (c) 8 times greater. click here
13) If a wire is passed through a narrow hole to where its crosssectional area, A is reduced by a factor of 3, its resistance, R becomes (a) 3 times greater (b) 9 times greater (c) 6 times greater.
14) If a wire is passed through a narrow hole to where its length, L is increased by a factor of 3, its resistance, R becomes (a) 3 times greater (b) 6 times greater (c) 9 times greater. click here
15) The resistivity, ρ of conductors is of the order of (a) 10^{8} Ωm (b) 10^{5 }Ωm (c) 10^{+12} Ωm.
16) The resistivity, ρ of semiconductor, germanium is of the order of (a) 10^{8} Ωm (b) 10^{ 1 }Ωm (c) 10^{+12} Ωm.
17) The resistivity, ρ of insulator, glass is of the order of (a) 10^{8} Ωm (b) 10^{ 1 }Ωm (c) 10^{+12} Ωm. click here
18) Power is defined as (a) the work done per unit of time (b) the energy given off per unit of time (c) the energy consumed per unit of time (d) a, b, & c.
19) The electric power, P may be written as (a) P = VI (b) P = RI*I (c) P = RI^{2} (d) a, b, & c.
20) If the voltage across a resistor is 15 volts and the current through it is 2.0A, the power dissipation in it is (a) 7.5 watts (b) 30. watts (c) 60. watts. click here
21) The voltage across a vacuum cleaner is 120 volts and the current through it is 4.0A, the power dissipation in it is (a) 480 watts (b) 90. watts (c) 560watts.
22) The voltage across a vacuum cleaner is 120 volts and the current through it is 4.0A, the electric resistance it shows while in use is (a) 480 ohms (b) 160 ohms (c) 30. ohms. click here
23) A vacuum cleaner with an inuse resistance of 30.Ω allows a 4.0A current to flow through it. Its power is (a) 30(4)^{2}watts (b) 480 watts (c) both a & b.
24) An alternate version of the power formula, P = VI, is (a) P = RI^{2} (b) P = RI (c) both a & b. click here
25) The voltage across an electric iron is 120V and the inuse electric resistance of it is 15Ω. The electric current it pulls from the socket and the power it dissipates as heat are (a) 10A and 1000watts (b) 8.0A and 960watts (c) 15A and 1800watts.
26) A light bulb rated as 75 watts is designed for 120V. The current it pulls from the socket and its inuse resistance are (a) 0.625A and 192Ω (b)1.25A and 384Ω (c) 1.60A and 75Ω. click here
27) P = VI may be written as (a) P = V (q / t) (b) P = Vq / t (c) P = energy / t (d) P = energy per unit of time (e) a, b, c, and d. click here
28) Since power = energy / time, solving for energy, we get: (a) energy = (power)( time) (b) U_{e} = P t (c) both a & b.
29) The power of a light bulb is 150watts. The electric energy, U_{e}, it consumes in 30.0s is (a) 5.0 J (b) 4500J (c) 0.20J.
30) An electric motor is rated as 2.50hp. Its power in watts is (a) 1890 watts (b) 1380watts (c) 790 watts.
31) If a 2.50hp motor is on for 2.00 minutes, the energy (in Joules) it consumes is (a) 3960J (b) 990J (c) 227000J.
32) The power in kw (kilowatt) of a 2300watt electric motor is (a) 2,300,000kw (b)2.3kw (c) neither a nor b.
33) The power in kw of a 150watt lightbulb is (a)0.15kw (b)150,000kw (c) neither a nor b. click here
34) The power in kw of a 4.00hp electric pump is (a)4.00kw (b)6.00kw (c)2.98kw. click here
35) The electric energy, U_{e}, a 4.00hp pump consumes in 2.0h, expressed in kwh, is (a)5.96kwh (b)12.0kwh (c)18kwh.
36) A 100w light bulb has been on for 25h. The consumed energy in kwh is (a)2.5kwh (b)2500kwh (c)0.25kwh.
37) The energy a 40.0w bulb consumes in 30.0 days in Joules and kwh is (a)1200J, 288kwh (b)1.04x10^{8} J, 28.8kwh (c) neither a nor b. click here
38) For a 6.5¢ / kwh enrgy price, the cost of leaving a 75.0watt lightbulb on for 10.0 days is (a) $1.77 (b) $1.50 (c) $1.17.
Now let us consider cases where there are more devices, resistors in a circuit. The resistors could be in series or in parallel.
Resistors in Series:
Devices ( resistors ) in a portion of a circuit are said to be in series if they experience the same current. The following diagram shows three resistors in series with a battery. We can show that if R1, R_{2}, and R_{3} are in series, there is a single resistor R_{t} (called the total or the equivalent resistance) that can replace them all such that
R_{t} = R_{1} + R_{2} + R_{3}
In a series
circuit as shown:
1) The current, I , is the same in all resistors, but the voltagedrop across each resistor is different. 2) Positive current leaves the (+) pole of a battery. Going clockwise, along with the current shown, from the () pole to the (+) pole of the battery, the voltage (V) goes up. Then as the current flows through each of R_{1}, R_{2}, and R_{3}, the voltage drops proportionally. The total voltage drop must be equal to the voltage jump produced by the battery. We can write: V_{b} = V_{1} + V_{2} + V_{3}. Also, V_{1}= I R_{1} and V_{2}= I R_{2} and V_{3} = I R_{3} ; Consequently , V_{b} = I R_{1} + I R_{2} + I R_{3}. But V_{b} is used up for the total resistance R_{t}, and according to Ohm's law V_{b} = R_{t} I , or,

R_{t} I = I R_{1} + I R_{2} + I R_{3}. simplifying : R_{t} = R_{1} + R_{2} + R_{3}

Fig. 2
Example 10: In the above figure, let V = 20.0 volts, R_{1} = 25.0Ω, R_{2} = 50.0Ω, and R_{3} = 25.0Ω. Calculate R_{t}, I, V_{1}, V_{2}, and V_{3}.
Solution: R_{t} = R_{1} + R_{2} + R_{3} = 100.0Ω. Using Ohm's Law: I = (V / R) = (20.0 volts / 100.0Ω ) = 0.200 Amps.
Now: V_{1} = I R_{1} = (0.200Amps)(25.0Ω) = 5.00 volts
V_{2 } = I R_{2} = (0.200Amps)(50.0Ω) = 10.0 volts
V_{3} = I R_{3} = (0.200Amps)(25.0Ω) = 5.00 volts
Resistors in Parallel:
Resistors that are between the same two nodes experience the same voltage and form parallel connection. At one node the arriving current divides amongst the resistors and at the other node the currents join and form the main undivided current. The greater a resistor, the smaller the current its branch pulls. Parallel resistors divide the main current amongst themselves proportional to 1/R . The voltage for all resistors between two nodes is the same. The equivalent resistance, R_{t} or R_{eq} may be calculated from the formula:
In the figure shown, since there is no resistance between points d and a ( Connecting wires have negligible resistance) the voltages of points d and a are equal. So are the voltages of points b and c. This means that if we use a voltmeter to measure V_{dc} and V_{ab}, we get the same readings. V_{dc} = V_{ab }= V_{bat} . Since V_{ab} is known, V_{bat} , we may calculate the currents in each branch by using the Ohm's law.
I_{1} = V_{ab }/ R_{1}, similarly I_{2} = V_{ab }/ R_{2} and I_{3} = V_{ab }/ R_{3}.
The voltage V_{ab} = V_{dc} is the same for all resistors and is equal to the battery voltage (V_{bat}); therefore, I _{1} = V_{bat} / R_{1} I _{2} = V_{bat} / R_{2} I_{ 3} = V_{bat} / R_{3} Of course, the 3 resistors can be replaced by one resistor, R_{eq} or R_{t} such that I = V_{bat} / R_{t} Since I = I _{1} + I _{2} + I _{3 } ; therefore, (1 / R _{t }) = (1 / R_{1}) + ( 1 / R_{2}) + ( 1 / R_{3}) 

Fig. 3
Example 11: In the following figure, find the equivalent resistance in each case.
Fig. 4
Solution: In Fig. 1, the two resistors are in series and they simply add up to a greater resistance (15Ω ).
In Fig. 2, the two resistors are in parallel and their reciprocals add up to yield the reciprocal of the total that is smaller than the smaller resistor (6Ω ).
In Fig. 3, the two parallel resistors yield 8Ω. The 8Ω is then in series with the 9Ω . They add up to 17Ω.
In Fig. 4, each parallel section must be calculated first. The first set yields 18Ω, and the second set yields 8Ω. The 18Ω and the 8Ω are then in series and add up to 26Ω.
A Shortcut for Two Parallel Resistors:
There is a shortcut formula that calculates the equivalent of two parallel resistors quickly. The formula is:
Example 12: In the previous example, verify the parallel sets by using the above formula.
Solution: Should be done by students.
Example 13: In the above figure, let R_{1} = 30.0Ω, R_{2} = 6.00Ω, and R_{3} = 3.33Ω. Also let V_{bat} = 4.00 volts. Find R_{t}, I, I_{1}, I_{2}, and I_{3}.
Solution: R_{t} = 1 / (1/R_{1} +1/R_{2} +1/R_{3} ) = 2.00Ω. Ohm's Law: I = V_{bat} / R_{t} ; I = 4 volts / 2Ω = 2.00Amp.
V_{ab} = V_{dc} ;therefore, I _{1} = V_{ab }/ R_{1} = 4 / 30 = 0.133A
I _{2} = V_{ab }/ R_{2} = 4 / 6 = 0.667A
I _{3} = V_{ab }/ R_{3} = 4 / 3.33 = 1.200A, Obviously : I_{1} + I_{2} + I_{3} = 2.00A
Mixed Series and Parallel Circuits:
There are many cases in which a parallel portion is in series with other portions. The other portions may also be comprised of series or parallel portions. A typical case is shown in the following example:
Example 14: In the figure shown, find the current through, voltage across, and power dissipation in each resistor:
Let R_{1} = 12 Ω, R_{2} = 60. Ω, R_{3} = 5.0 Ω, and V = 45 volts.
Solution:
R_{ab} = 1 / ( 1/ R_{1} + 1 / R_{2} ) = 10. Ω R_{ac} = R_{ab} + R_{bc} = 10 + 5 = 15. Ω I = V_{ac} / R_{ac} = (45 volts) / (15 ohms) = 3.0 amps V_{bc} = ( I )( R_{bc} ) = 15 volts V_{ab} = V_{ac}  V_{bc} = 45  15 = 30. volts I _{1} = V_{ab} / R_{1} = 2.5 amps I _{2} = V_{ab} / R_{2} = 0.50 amps P_{1} = (V_{1})(I_{1}) = 30x2.5 = 75 watts P_{2} = 30x0.5 = 15 watts ; P_{3} = 15x3.0 = 45 watts


Fig. 5
Example 15: In the figure shown, find the equivalent resistance between points a and b.
Fig. 6
Solution: In the Problem, if a current flows from a to m, it has to divide at m, partially toward n, and partially downward. The part that arrives at n must also divide, partially through the 5Ωresistor and partially through the 60Ωresistor. The part that flows through the 5Ωresistor must also flow through the 15 resistor as well, and therefore, the 5Ω and 15Ωresistors are in series. The first step is to simply add the 5Ω and 15Ωresistors to get an equivalent of 20Ω resistance as shown in Fig.1. In Fig. 1, Since n is a dividing point, the 20Ω and 60Ω resistors are in parallel. They can be replaced by their equivalent resistance of (20*60)/(20+60) = 15Ω as shown in Fig. 2.
In Fig. 2, the 25Ω and 15Ωresistors are in series because whatever current flows through the 25Ω must also flow through the 15Ωresistor. They add up to 40Ω as shown in Fig. 3. In Fig. 3 , the 40Ω and the 120Ω resistors are in parallel because the current that arrive at m from a must divide. They can be replaced by their equivalent resistance of (120*40)/(120+40) = 30Ω as shown in Fig. 4. In Fig. 4, the 50Ω and the 30Ωresistors are in series because the flow through the 50Ωresistor must also flow through the 30Ωresistor. They add up to a final value of 80Ω.
Test Yourself 3: click here
1) Resistors in series experience (a) the same current (b) the same voltage (c) the same power dissipation.
2) Resistors in parallel experience (a) the same current (b) the same voltage (c) the same power dissipation.
3) Resistors in mixed series and parallel experience (a) the same current (b) the same voltage (c) neither a nor b.
4) Total resistance for a group of resistors in series is (a) greater than each individual resistance (b) equal to the sum of the individual resistances (c) both a & b. click here
5) Total resistance for a number of resistors in parallel is (a) greater than the greatest resistance (b) smaller than the smallest resistance (c) equal to the sum of the individual resistances.
6) The R_{total} or R_{eq} for a 120.Ω, an 80.Ω, and a 48Ωresistor connected in series is (a) 96Ω (b) 248Ω (c) 280Ω.
7) The R_{total} or R_{eq} for a 120.Ω, an 80.Ω, and a 48Ωresistor connected in parallel is (a) 96Ω (b) 248Ω (c) 24Ω.
8) R_{1} = 120.Ω and R_{2} = 80.Ω are in parallel and the result in series with R_{3} = 48Ω. Draw a figure and place the parallel set between points A and B, and the third resistor, R_{3}, between points B and C. The overall resistance between points A and C is (a) 96Ω (b) 248Ω (c) 24Ω. click here
9) Between points E and F, a 6.00Ω and a 30.0Ωresistor are in parallel, and between points F and G, a 12.0Ω and a 60.0Ωresistor are also in parallel. The set between E and F is in series with the set in between F and G. Draw a diagram for the problem. The overall resistance between E and G is (a) 3.33Ω (b) 108Ω (c) 15.0Ω.
10) In Example 15, redraw the figure. Double the even numbers and triple the odd number in the main problem. The new equivalent resistance between points a and b is (a) 173Ω (b) 273Ω (c) 178Ω. click here
11) In Example 10, let V = 60volts, R_{1} = 50Ω, R_{2} = 150Ω, and R_{3} = 100Ω. Redraw the figure. Without looking at the solution, recalculate new values. The values of R_{t}, I, V_{1}, V_{2}, and V_{3} all to 3 sig. fig. are (a) 600Ω, 0.4A, 20V, 60V, and 40V (b) 300Ω, 0.2A, 10V, 30V, and 20V (c) 200Ω, 0.1A, 25V, 65V, and 45V.
Problem: Let a 24volt battery be connected to following three resistors in parallel : R_{1} = 8Ω , R_{2} = 12Ω, and R_{3} = 6Ω. Draw a diagram for the problem. Answer the following questions: click here
12) We may say that the voltage across (a) R_{1 }is 24volts (b) R_{2 }is 24volts (c) R_{3 }is 24volts (d) a, b, & c.
13) The currents through the individual resistors are respectively (a)3A, 2A, & 4A (b)3A, 3A, & 3A (d) neither a nor b.
14) The current that leaves the battery is (a) 24A (b) 12A (c) 9A. click here
15) Using the formula for equivalent resistance of parallel resistors, the equivalent resistance is (a)2.67Ω (b)26Ω (c) 6Ω.
16) The current that leaves the battery is controlled by the equivalent resistance that the battery faces. Based on the equivalent resistance, the current that is pulled from the battery is (a)9A (b)12A (c)24A. click here
Problem: Let a 36volt battery be connected to the following three resistors : R_{1} = 24Ω, R_{2} = 48Ω, and R_{3} =14Ω. Let R_{1} and R_{2 }be in parallel between points a and b, and place R_{3 }between points b and c. In your calculations to follow, name the resistance between a and b as R_{ab}, and the resistance between b and c, as R_{bc}. It is very important to draw a diagram for the problem. Answer the following questions: click here
17) R_{ab}, the equivalent resistance for the two parallel resistors R_{1} and R_{2 }is (a)72Ω (b)24Ω (c)16Ω.
18) R_{ac}, the total resistance the battery is facing is (a)86Ω (b)30.Ω (c)10.4Ω.
19) If the total resistance that the 36V battery is facing is 30Ω, the current that the battery can push out is (a)1.2A (b)2.4A (c)3.6A. click here
20) The current through R_{ac}portion is (a) 2.57A (b) 1.2A (c) equal to the current that flows out of the battery (d) b & c.
21) The current through R_{ab}portion is (a)2.25A (b)1.2A (c)equal to the current that flows out of the battery (d) equal to the current that flows through the R_{ac}portion (e) b, c, & d. click here
22) To find the voltage (Potential difference) between points a and b, named "V_{ab}", (a) R_{ab } must be multiplied by the1.2A (b) we think that V_{ab} is just equal to the 36V that the battery supplies (c) neither a nor b.
23) To find the voltage between b and c, or V_{bc}, (a)R_{bc } must be multiplied by the 1.2A (b) we think that V_{bc} is also equal to the 36V that the battery supplies (c) neither a nor b. click here
24) The current that flows through R_{1} is (a) more than 1.2A (b)less than 1.2A (c)equal to 1.2A.
25) The current in any portion of the circuit in this problem (a) can become more than 1.2A (b) cannot be more than 1.2A (c) must be set equal to 1.2A, even through R_{1} alone or R_{2} alone. click here
26) Since V_{ab} is known as well as R_{1}, the current I_{1} through R_{1} is (a) 19.2V / 24Ω = 0.80A (b) 36V / 24Ω = 1.5A (c) just 1.2A.
27) Since V_{ab} is known as well as R_{2}, the current I_{2} through R_{2} is (a) 19.2V / 48Ω = 0.40A (b) 36V / 24Ω = 1.5A (c) just 1.2A.
28) The sum of the currents in R_{1 }and R_{2 }must be (a) 2.4A (b) 3.6A (c) 1.2A. click here
29) The sum of the voltage drops across the abportion and the bcportion must be (a) 72V (b) 36V (c) 0.0.
30) The power dissipation in R_{1}, R_{2 }, and R_{3 } are: (a)15.36w, 7.68w, & 20.16w (b)24w, 48w, & 14w (c)20w, 40w, & 11.7w. click here
The Effect of Temperature on Resistance:
Resistivity ( ρ) increases with increase in temperature (T). This change is linear and given by
ρ_{T} = ρ_{20 }[ 1 + α ( T  20) ]
where ρ_{T }is the resistivity at temperature T, and ρ_{20 }is the resistivity at 20º Celsius. The symbol ( α ) denotes the temperature coefficient of resistivity. Values of (α) measured for different materials at room temperature (20ºC) are given in Table 20.1.
For a wire, temperature increases causes thermal expansion. Length increase causes resistance to increase while crosssection increase causes resistance to decrease. Assuming these two effects to somehow offset each other, for tiny wires we may approximately write: R _{T} = R_{20 }[ 1 + α ( T  20) ]. When alternating currents pass through the elements of light bulbs, there are other effects that can affect the overall resistance, we neglect those here for simplicity.
Example 16: The resistance of a 60watt lightbulb is measured 18.2Ω when cold (unlit). When connected to 120V and is lit, calculate (a) the current it pulls as well as its resistance when hot. (b)What temperature does it have when hot? The elements of light bulbs are made of tungsten for which [α =4.5x10^{3}(ºC)^{1}].
Solution: P = VI or, I = P / V = 60watts / 120V = 0.50 Amps. (when hot)
R = V / I = 120V / 0.50A = 240Ω ( Resistance when hot!).
Using the approximate formula R _{T} = R_{20 }[ 1 + a ( T  20) ], we get
240 = 18.2 [ 1 + 4.5x10^{3 }( T  20.) ] from which T = 2728 rounded to 2700 ºC. The melting point of tungsten is 3400 ºC.
Multiloop Circuits:
Sometimes the electric elements are connected in such a way that it is not possible to label them as being in series or parallel. This happens when there are loops or closed circuits in the system. For example, in the figure shown below, we can not say that R_{1}, R_{4}, and R_{3} are connected in parallel or in series. We also see that there are two loops: Loop ABCDA and Loop ABFEA. Between A and B, there are 3 branches. There will be 3 different currents in these 3 branches. Elements R_{1}, R_{2}, and battery V_{1} experience same current. This current is labeled I_{1}. Element R_{4}, and battery V_{2} experience a different current. This current is labeled I_{2}. The elements in the last branch (AGB), R_{3} and V_{3}, experience another current that is named I_{3}. The purpose is to calculate I_{1}, I_{2}, and I_{3}. To do this, we use the socalled "Kirchhoff's Rules."

KLR in Loop ABCDA: V_{2} + I_{3}R_{3} + V_{1} + I_{1}R_{2} + I _{1}R_{1} = 0 (1) KLR in Loop ABFEA: V_{2} + I_{3}R_{3}  I_{2}R_{4} + V_{3 } = 0 (2) KJR at Node A:  I_{1} + I_{3} + I_{2} = 0 (3) 
Fig. 7
Kirchhoff's Loop Rule (KLR):
This rule states that the sum of voltage jumps and drops in a closed loop is equal to zero. In each branch, we don't know the direction of the current to begin with. We simply assume a direction for it. If our assumption is correct, the answer we find at the end of our calculations will be positive. If our guess is wrong, the calculations will yield a negative value for that current. The negative sign indicates that we chose a wrong direction. To write down the jumps and drops of voltage in a closed loop, we need to start from a point of the loop, go around it, complete it, and come back to that point. In two ways, a loop can be traveled around. 1) By going with the assumed direction of current, and 2) by going against the assumed direction of current.
In adding voltages across the elements of any closed loop to set the total equal to zero, For Resistors: If we follow the assumed direction for the current, it is like swimming with the flow of a river. Swimming with the current means going toward points of lower and lower potential. On this basis, traveling through a resistor with the assumed current direction means a drop in voltage and we give a negative sign to the product (RI) in the equation. (RI) is the voltage drop across R. Now, if we travel opposite to the assumed direction of current, it is like swimming against the flow of a river or simply swimming toward points of higher potential. On this basis, traveling through a resistor against the assumed current direction means a jump in voltage and we give a positive sign to to the product (RI) in the equation. (+RI) is the voltage jump across R. 
For Batteries: It doesn't matter if we are traveling along a loop with the assumed current or against it, going from () to (+) is always a voltage jump (+V_{bat}) and going from (+) to () is always a voltage drop (V_{bat}). 
A KLR is written for each loop of the above figure. 
Kirchhoff's Junction Rule ( KJR ):
The sum of currents going toward a junction is equal to the sum of currents leaving that junction. In other words, the algebraic sum of currents to and away from a junction is zero. A junction is the point of connection of 3 or more wires. A KJR is written at Junction A in the above figure.
Example 17: In the figure shown, the directions for currents I_{1}, I_{2}, and I_{3} are already chosen. Find their values and determine if the assumed directions are correct?
Fig. 8
Solution: KLR in Loop ABCDA: + 40 I_{3}  5 + 6 + 30 I_{1} + 20 I_{1} = 0
KLR in Loop ABFEA: + 40 I_{3}  5  9 + 20 I_{2} + 50 I_{2} = 0
KJR at Node B : + I_{1} + I_{2}  I_{3} = 0
Making the above equations ready for calculator, we get:
+ 50 I_{1} + 0 I_{2} + 40 I_{3} = 1 0 I_{1} + 70 I_{2} + 40 I_{3} = 14 + 1 I_{1} + 1 I_{2}  1 I_{3} = 0

Results of calculation:
I_{1} =  0.0807 A I_{2} = 0.1566 A I_{3} = 0.0759 A 
In the above figure, the direction of I_{1} must be reversed. 
RC Circuits:
A circuit containing a resistor and a capacitor forms the socalled "RC circuit." A typical RC circuit in which the capacitor is to be charged requires a battery as shown below. The resistance R is used to control the charging speed and depending on the capacity C, the charging time varies. The product RC is called the time constant ( τ )of the circuit. τ is pronounced " tau ."
τ = RC has unit of time, that is of course, seconds in SI. Note: I_{max} , I_{o} , and I(0) mean the same thing. 
The current in the circuit that is also the
current through the capacitor decreases as an exponential function of time and is given by:
I_{C} = I_{max} e ^{(t / }RC) where I_{max} = V_{b}_{/}R The capacitor voltage, V_{C}, increases exponentially to its limiting value, V_{bat}, according to the following equation: V_{C} = V_{max}[1  e ^{(t / RC)}] where V_{max} = V_{bat.} 
Fig. 9
As soon as key K is closed the battery with voltage V_{b} faces an empty capacitor C and a resistor R. At the very beginning, the capacitor, being empty, does not impose any resistance toward filling up. At t = 0, the only element showing resistance is R, and that determines the amount of the initial current ( I_{o}). I_{o}_{ }can also be shown as I_{max} . Maximum current occurs at the very beginning, t = 0. As the capacitor fills up, its develops resistance, and therefore the battery faces an increasing resistance in excess of R. This causes the current decrease as the capacitor fills up. When the capacitor is fully charged, its voltage equals the battery voltage. At this stage the current through the circuit approaches zero and so does the voltage across the resistor. At any given instant, V_{bat.} = V_{R} + V_{C}. As V_{C} increases, the current ( I or I_{C} ) goes down causing V_{R} to decrease and approach 0.
Example 18: In the figure shown, find (a) I_{o} or I_{max } , and (b) the values of I at t = 1s, 5s, 10s, and 100s; in other words, find I(1), I(5), I(10), and I(100). Assume 3 significant figures on all numbers.
Solution:
Fig. 10 
RC = (100Ω)(0.05 Farads) = 5.00
sec.
I _{max} = V_{bat.} / R = 20 volts /100Ω = 0.2 A I ( t ) = I_{max} e ^{( t / RC)} I ( 1 ) = 0.2 exp (1 / 5) = 0.164 A I ( 5 ) = 0.2 exp (5 / 5) = 0.0736 A I ( 10 ) = 0.2 exp (10 / 5) = 0.0270A I ( 100 ) = 0.2 exp (100 / 5) = 0 A 
Example 19: In the previous example, calculate V_{R} and V_{C} at the given instants.
Solution:
t = 0  V_{R} = RI = (100 Ω)(0.200 A) = 20 volts  V_{C} = V_{b}  V_{R} = 20  20 = 0 volt 
t = 1  V_{R} = RI = (100 Ω)(0.164 A) = 16.4 volts  V_{C} = V_{b}  V_{R} = 20  16.4 = 3.6 volt 
t = 5  V_{R} = RI = (100 Ω)(0.0736 A) = 7.36 volts  V_{C} = V_{b}  V_{R} = 20  7.36 = 12.64 volt 
t = 10  V_{R} = RI = (100 Ω)(0.0270 A) = 2.70 volts  V_{C} = V_{b}  V_{R} = 20  2.70 = 17.30 volt 
t = 100  V_{R} = RI = (100 Ω)(0 A) = 0 volts  V_{C} = V_{b}  V_{R} = 20  0 = 20.0 volt 
Example 20: In the previous example, find the values of V_{c} directly by using the equation for V_{c}, that is V_{c} = V_{bat}[ 1  e ^{ (t /}^{ RC}^{)} ] and compare the results with the values of V_{C } in Example 19.
Test Yourself 4:
1) The resistance of a resistor (a) increases with temperature increase (b) decreases with temperature increase (c) is not a function of temperature. click here
2) Knowing that ρ_{20} is the resistivity of a material at 20ºC, the temperature coefficient of electric resistivity, α is (a) the change in resistivity, Δρ (b) the change in resistivity per unit resistivity, Δρ /ρ_{20} (c) the change in resistivity per unit resistivity per unit change in temperature, Δρ /(ρ_{20}ΔT). click here
3) If we write α as α = Δρ /(ρ_{20}ΔT) that you may want to write it with a horizontal fraction bar, it is easy to find out that the unit of α is (a) ºC^{1} (b) Ω / ºC (c) [Ω ºC]^{1}.
4) One of the elements used in the filament of light bulbs is tungsten. The resistivity of tungsten (From Table 20.1) at 20ºC is ρ_{20} = 5.6x10^{8} Ωm and its temperature coefficient of resistivity is α = 4.5x10^{3}[ºC]^{1}. The resistivity of tungsten at 2720ºC is (a) ρ_{2720 ºC} = 7.3x10^{7} Ωm (b) ρ_{2720 ºC} = 5.9x10^{7} / ºC (c) neither a nor b. click here
Problem: A student uses an ohmmeter to measure the resistance of a light bulb when cold and not in use. Its cold resistance is R_{20 ºC} = 11Ω. If the element is made of tungsten, and we neglect the changes in its dimensions due to temperature increase, find its resistance at 2700ºC. We may approximately use R_{T} = R_{20}[1 + α (T  20) ] instead of ρ_{T} = ρ_{20}[1 + α (T  20) ]. Answer the following questions:
5) The resistance at 2700ºC of the element is (a)129Ω (b)144Ω (c)192Ω. click here
6) Since light bulbs are designed to operate at 120V in the United States, the current this light bulb draws from a wall socket is (a)1.2A (b)0.833A (c)17000A.
7) The power of the light bulb may be calculated by (a) P = VI (b) P = RI^{2} (c) both a & b. click here
8) The power of the light bulb is (a)150watts (b)75watts (c)100watts.
9) If you pick up a 100watt GE light bulb and measure its resistance cold, you expect to measure a resistance of (a)144Ω (b)about 11Ω (c) about 72Ω. click here
10) What current does a 60watt light bulb draw from a wall socket when in use? (a) 0.5A (b) 2A (c) 1A.
11) What is the resistance of a 60watt light bulb when in use (Assume a filament temperature of 2700ºC.) (a) 240Ω (b)120Ω (c)180Ω.
12) What is the resistance of a 60watt light bulb when cold and not in use (assume 20ºC)? (a) 9Ω (b) 5Ω (c)18.4Ω.
13) According to Kirchhoff's Loop Rule (KLR), the sum of (a) battery voltages must be zero (b)voltages across all elements must be zero (c) voltage jumps and drops across the elements in a closed loop is zero. click here
14) A junction is a point in an electric circuit to which (a) only two wires are connected (b) 3 or more wires are connected (c) only a single wire is connected.
15) According to Kirchhoff's Junction Rule (KJR), the sum of (a) currents going to and away from a junction is zero (b) the sum of voltages at a junction is zero (c) neither a nor b. click here
Problem: Draw a square loop, and starting from the lower left corner, move in the clockwise direction, and label it abcd. In the ab side draw a resistor and label it 10Ω. In the bc side place a 3V battery with its pos. pole on the left. In the cd side place an 18V battery with its neg. pole up and follow it by a 20Ω resistor. In the da side place a 15Ω resistor. Answer the following questions: click here
We want to write a KLR for loop abcda. We do not know the actual current direction. We may assume it clockwise or counterclockwise. Let's assume it clockwise. If we start from point b, for example, and go clockwise, we are going with the flow or current, I. click here
16) From b to c, we have to go through the 3V battery from its pos. pole to its neg. pole. This is associated with a (a) voltage drop and we write 3 volts (b) voltage increase and we write +3 volts (c) neither increase nor decrease.
17) From c to d, we have to go through the 18V battery from its neg. pole to its pos. pole. This is associated with a (a) voltage drop and we write 18 volts (b) voltage jump and we write +18 volts (c) neither increase nor decrease.
18) After the 18V battery, there comes the 20Ω resistor. Since we are following the assumed direction for the current, I, we are moving toward lower potentials. The voltage difference across this 20Ω resistor must be taken to be negative and we write (a) 20I (b) +20I (c) +20. click here
19) From d to a , going with the flow is again associated with another (a) voltage increase across the 15Ω resistor and we write +15I (b) voltage decrease across the 15Ω resistor and we write 15I (c) voltage decrease across the 15Ω resistor and we write 15.
20) Going from a to b to complete the loop, we run into the 10Ω resistor only. Again, going with the assumed direction for I, there is a (a) voltage increase across the 10Ω resistor and we write +10I (b) voltage decrease across the 10Ω resistor and we write 10I (c) voltage decrease across the 10Ω resistor and we write 10. click here
21) The completed KLR to Loop abcda is (a) 3+1820I15I10I (b) 3+1820I15I10I.= 0 (c) neither a nor b.
22) Solving for I, the current in the loop is (a) 3.0A (b) 6.0A (c) 0.33A click here
23) Since the current turned out positive, our assumed clockwise direction is (a) correct (b) wrong (c) partially correct.
Now solve the problem assuming a counterclockwise direction for the unknown current I. Redraw the diagram with the new assumed direction for the current. However, start from b again and go clockwise again, that is opposite to the assumed direction for the current, I. click here
24) From b to c, we have to go through the 3V battery from its pos. pole to its neg. pole. This is associated with a (a) voltage drop and we write 3 volts (b) voltage increase and we write +3 volts (c) neither increase nor decrease.
25) From c to d, we have to go through the 18V battery from its neg. pole to its pos. pole. This is associated with a (a) voltage drop and we write 18 volts (b) voltage increase and we write +18 volts (c) neither increase nor decrease.
26) After the 18V battery, there comes the 20Ω resistor. Since we are going against the assumed direction for the current, I, we are moving toward higher potentials. The voltage difference across this 20Ω resistor must be taken to be positive and we write (a) 20I (b) +20I (c) +20. click here
27) From d to a , going against the flow is again associated with another (a) voltage increase across the 15Ω resistor and we write +15I (b) voltage decrease across the 15Ω resistor and we write 15I (c) voltage increase across the 15Ω resistor and we write +15.
28) Going from a to b to complete the loop, we run into the 10Ω resistor only. Again, going against the assumed direction for I, there is a (a) voltage increase across the 10Ω resistor and we write +10I (b) voltage decrease across the 10Ω resistor and we write 10I (c) voltage increase across the 10Ω resistor and we write +10. click here
29) The completed KLR to Loop abcda is (a) 3+18+20I+15I+10I (b) 3+18+20I+15I+10I = 0 (c) neither a nor b.
30) Solving for I, the current in the loop is (a) 3.0A (b) 6.0A (c) 0.33A
31) Since the current turned out negative, our assumed direction is (a) correct (b) wrong and must be reversed to clockwise (c) partially correct. click here
Problem: Redraw Example 17, but double the battery voltages and triple the resistances. Also, reverse the assumed directions for I_{1} and I_{3}. Answer the following questions:
32) KLR for ABCDA is (a) 120 10 +12 90I_{1}  60I_{1} = 0 (b) 120I_{3} 10 +12 90I_{1}  60I_{1} = 0 (c) a & b.
33) KLR for ABFEA is (a) 120I_{3} 10 +18 90I_{1}  60I_{1} = 0 (b) 120I_{3} 10 18 +60I_{2} +150I_{2} = 0 (c) a & b.
34) KJR at Node B is (a) I_{1}  I_{2} + I_{3} (b) I_{1}  I_{2} + I_{3} = 0 (c)  I_{1} + I_{2} + I_{3} = 0. click here
35) Answer for I_{1}, I_{2}, &I_{3} are: (a) 0.05382A, 0.10442A, 0.0506A (b) 53.82mA, 104.42mA, 50.60mA (c) a&b.
Problem: Consider a RC circuit as shown in Fig. 9. Suppose that at t = 0 the capacitor is empty. As soon as key, K is turned on, the capacitor starts charging. Charges flow from the battery to the plates of the capacitor. As the capacitor accumulates charges on its plates, it builds up greater and greater voltages; however, the limiting voltage is the voltage of the battery that feeds it. Answer the following questions:
36) At t = 0, the voltage across the capacitor, V_{C} is (a) 0, because there is no current in the circuit (b) 0, because no charge is accumulated on its plates yet (c) both a & b. click here
37) At t = 0, since V_{C} = 0, all the battery voltage drop across (a) the resistance R (b) the capacitor, C (c) neither a nor b.
38) The initial current, I_{o}, is I_{o} = V_{bat} _{/ R} because (a) the capacitor is initially full and does not accept any current; therefore, all the current flows toward the resistor (b) the capacitor is empty and creates no resistance in the circuit, and the only resistance the battery faces is R (c) neither a nor b. click here
39) As the capacitor fills up it develops resistance toward accepting more charge and the total resistance in the circuit keeps increasing beyond R. We may think that when the capacitor is nearly fully charged, the current in the circuit (a) approaches zero (b) becomes very high (c) neither a nor b.
40) As the current approaches zero, so does the (a) voltage across the capacitor (b) the voltage across the resistor (c) neither a nor b. click here
Problem: The derivation of RC Circuit Formula
In the figure shown, 41) Write the voltage balance equation for V_{bat}, V_{R} and V_{C} that is good for any instant. See (*) below the figure. 42) In your answer for (41), Replace V_{R} by its equivalent in terms of I. Also replace V_{C} by its equivalent in terms of C and Q_{C}. What formula do you know that relates Q_{C} to C and V_{C}? How does your new equation read now? 43) Do you know a differential form for the current I in terms of Q? Use it to write your result in (42) completely in terms if Q or indeed Q(t). This gives you the differential equation from which you can solve for Q(t). 44) One way of solving the differential equation you obtained in (43) is to multiply both sides of it by e^(t /RC). Do so and show that one side of your equation is exactly the time derivative of Qe^(t /RC). Continue and show that the final result after integration can be written as V_{C} = V_{bat}[ 1  e^(t/RC)]. 
(*) Note that Q = Q(t), I = I(t) , V_{C} = V_{C}(t) , and V_{R} = V_{R}(t). 