AP Physics – Electric Current – 2 Ans
1. A hot plate has an internal resistance of 22.0 . It operates on 120 V household AC electricity. (a) How much
current did it draw? (b) How much power did it develop? (c) If it operated for 15 minutes, how much heat did it
develop? (d) If a kWh costs 4.5 cents, how much did it cost to run the thing?
(b) P  IV  5.5 A 120 V  
660 W
(a) I  V  120 V 
5.5 A
R
22.0 
(c) P 
W
t
W  Pt
 660
 60 s 
J
15 min  

 
s
 1 min 
590 000 J
 1 h  1 kW  4.5 cents 
660 W 15 min  
0.74 cents


 
 60 min  1000 W  1 kWh 
2. Examine this useless circuit. What is: (a) the total resistance, ( b) the total
current, (c) the voltage drop for the 75.0  resistor?
(d)
(a) R  R1  R2  R3
 100.0   50.0   75.0  
100.0 W
50.0 W
225.0 
75.0 W
1.20 A
(b) I 
1.20 A
(c) V  IR
 1.20 A  75.0   
A
90.0 V
3. What happens to the light intensity of a set of identical lamps in series when you add an additional lamp? How
come? The Total Resistance increases, thus the current going through each resistor will decrease. Since
P=IV, the power of each individual resistor will now be decreased and thus be dimmer.
4. What happens to the light intensity of a set of identical lamps in parallel when you add an additional lamp? How
come?The Total Resistance of the circuit will decrease which will cause a total current increase, but since
the potential difference across ALL resistors in parallel to each other is the same, the current through every
resistor will remain unchanged. So as long as the power source can supply the new larger amount of total
current with dropping is voltage (something almost impossible for a battery) the bulbs will remain at the
same brightness.
R 1 120 W
5. (a) Draw a circuit which has a 120  resistor in series with three
resistors that are in parallel with each other; a 25 , 35 , and 45 
R4
R2
R3
resistor. The voltage source is a 9.0 V battery. (b) What is the current
9.0 V
45 W
35
W
W
25
for this circuit?
R  R1  R234
1
1
1
1



R234 R2 R3 R4
V
9.00 V
I


131 
R

1
1
1


25  35  45 
R234  11.0 
R  120   11   131 
0.069 A
6. What is: (a) the current that goes through the 45.0  resistor and (b) what is the total current through the circuit?
V
12.0 V


0.267 A
R
45.0 
1 1
1
1
1
1
1
1
1
(b)
 
 




R R1 R2 R3 R4
25  15  45  35 
V
12.0 V
1.89 A
I


6.35 
R
(a)
I3 
25.0 W
R  6.35 
15.0 W
12.0 V
45.0 W
35.0 W
7. What is (a) the voltage drop across the 75  resistor, (b) the total current supplied by the battery?
1
1
1
1
1




R23 R2 R3
75  55 
R  25   31.7   35   91.7 
V
12.0 V
I 
 0.131 A
91.7 
R
(a)
R  R1  R23  R4
R2
R1
R23  31.7 
25 W
12.0 V
75 W
R3
R4
35 W
55 W
V23  IR23
 0.131 A  31.7   
(b) I 
0.131 A
4.2 V
8. What is (a) the total resistance of the circuit, (b) the total current in the circuit, (c) the power developed in the
circuit?
1
1
1


R23 R2 R3
(a) R  R1   R23
85.0 W
1
1
1


R23  74.78 
R23 112  225 
R  85   74.78 
112 W
0.250 A A
225 W
159.78 
(b)
V23  I 2 R2
I3 
 .250 A(112)  28V
V23
R3
I3 
I 23  I 2  I 3
28V
225
I3  .124 A
I 23  .250 A  .124 A  .374 A
V  IR  0.374 A 159.78    59.76V
(c)
 0.374 A  59.76 V  
P  IV
22.35 W
9. What is (a) the total resistance of the circuit, (b) the total current in the circuit, (c) the power developed
by R3, the 115  resistor, and (d) the electrical potential provided by the battery?
1
1
1
1
(a) R  R123  R4
 

85.0 W
R123 R1 R2 R3
1
1
1
1
R1



R123  20.4 
R4
R123 85  35  115 
R  20.4   25.0  
25.0 W
45.4 
(b)
I123 
V123
R123
V
(c) I  3
3
P  IV
(d)

6.0 V
20.4 

0.294 A
6.00V
I =
 .0522 A
3 115
R
3
P3  I 3V123
 0.0522 A  6.00 V  
V  IR  0.294 A  45.4   
.313 W
13.35 V
35.0 W 115 W
R2
R3
V
6.00 V