Cost of Electricity
1. Find the cost of operating an electric toaster for 3.0 h if it draws 5.0 A from a 120 V outlet. Electric energy
costs 5.5 ¢/kW·h.
2. What is the cost to a storekeeper of leaving a 40 W light burning near his safe over the weekend, for 60
h, if electricity costs 4.5 ¢/kW·h?
3. The blower motor on an oil furnace, rated at 250 W, comes on, for an average of 5.0 min at a time, a total
of 48 times a day. What is the monthly (30 d) cost of operating the motor, if electricity costs 4.2 ¢/kW·h?
4. The following appliances were operated for a thirty day month, in a 120 V circuit: a coffee percolator of
resistance 15 Ω for 0.50 h/d, a 250 W electric drill for 2.0 h/d, and a toaster that draws 5.0 A for 15 min/d.
Calculate the electric bill for the month at an average cost of 6.0 ¢/kW·h.
Electric Circuits Review
1. Describe the current and potential difference characteristics of a series circuit and of a parallel circuit.
2. Draw a schematic diagram of a circuit consisting of a power source and one resistor connected in series
with a combination consisting of two resistors connected in parallel with each other. Include a fuse, a
switch, a voltmeter, and an ammeter. The fuse should protect the whole circuit, the switch should
interrupt the whole circuit, the voltmeter should measure the potential difference across the series
resistor, and the ammeter should measure the current through one of the parallel resistors.
3. Describe the effect on the rest of the circuit when one of three lamps in a circuit burns out, if the three
lamps are connected (a) in series, and (b) in parallel. Explain your answers.
4. A battery is constructed by connecting several 1.5 V dry cells together. Draw the circuit symbol of such a
battery, if it consists of (a) three cells connected in series, and (b) four cells connected in parallel. What is
the potential difference of the battery in each case?
5. Research and draw a schematic diagram for a circuit that may be used to determine the resistance of an
unknown resistor. Provide an explanation of how it works.
6. Describe what is meant by resistance and list four factors that affect the resistance of a conductor.
Describe the effect of each factor on the resistance of a conductor.
7. When a car battery is "low", it cannot deliver sufficient current to the starting motor to cause it to turn
over. A healthier battery from another vehicle is often used to "boost" or recharge the weak battery.
Should this second battery be connected in series or parallel with the first one? Why? What terminals
from each should be connected together?
8. What is the effect on the total resistance in a circuit, when an extra resistor is added (a) in series, and (b)
in parallel?
9. State three equations for calculating the power dissipated in a resistor. Show that when proper units are
used in these equations, the unit that results for power is watts.
10. (a) What quantity is measured in kilowatt hours?
(b) What values of electric potential are found in the home, and where are these different values used?
11. Draw a graph of potential difference versus current for two different resistances, and indicate which has
the greater resistance, and why.
Problems
12. A voltmeter connected across the ends of a heating coil indicates a potential difference of 60 V when an
ammeter shows a current through the coil of 3.0 A. What is the resistance of the coil?
13. A flashlight bulb has a resistance of 7.5 Ω and is connected to a dry cell with a potential difference of
3.0 V. What is the current through the bulb?
14. What is the potential difference across a motor with a resistance of 40 Ω if the motor draws a current of
6.0 A?
15. A string of eight Christmas tree lights connected in series to a 120 V source draws a current of 0.75 A.
Find:
(a) the total resistance of the string of lights
(b) the resistance of each light
(c) the potential difference across each light
16. (a) What is the resistance of a toaster that draws a current of 6.0 A from a 120 V source?
(b) What resistance would have to be added in series with the same toaster to reduce its current to
4.0 A?
17. Calculate the total resistance in each of these cases. (a) 10 Ω, 30 Ω, and 50 Ω in series
(b) 6 Ω, 5 Ω, and 30 Ω in parallel
(c) 9 Ω in series with a combination consisting of 4 Ω and 12 Ω connected in parallel with each other.
18. How many 160 Ω resistors must be connected in parallel to draw a current of 6.0 A from a 120 V
source?
19. The potential difference across a heating coil is 6.0 V when the current through it is 3.0 A. What
resistance must be added in series with the coil to reduce the current through it to 2.0 A?
20. A portable radio is designed to operate at a potential difference of 6.0 V and a current of 250 mA, but the
only source available has a potential of 10.0 V. What resistance must be added in series with the radio
to make it operate properly?
21. Examine these circuits and find the values indicated.
(a) Find: R2, R3 and V3
(b) Find: V0, R1 Rtotal
(c) Find: R3, I1, I2, I4, I5, I6
(d) Find: I1, R1, and R2
(c) Find: I0, I1, I2, I3, I4, V1, V2, V3, and V4
22. Calculate the electrical energy dissipated in 1.5 min when there is a current of 4.0 A through a potential
difference of 60 V.
23. Calculate the power dissipated by each of the following loads.
(a) a clothes dryer drawing 125 A from a 240 V source.
(b) a kettle that draws 12.0 A and has a resistance of 8.3 Ω.
(c) a 240 Ω heating pad plugged into a 120 V source.
24. (a) What maximum power can be used on a 240 V circuit with a 15 A fuse?
(b) How much more current can safely be drawn from a 120 V outlet fused at 20 A if an 800 W toaster
and an 1180 W kettle are already operating in the circuit?
25. A 1.0 kW toaster, designed to operate at 120 V, is mistakenly connected to a source of 240 V.
(a) What current is the toaster designed to draw?
(b) What current will it draw when connected to 240 V?
(c) What power will it use on 240 V, and what will be the likely result?
26. A refrigerator compressor draws 2.5 A from a 120 V source and operates for an average of 15 min out of
each hour. Calculate the annual cost of operating the refrigerator if the average cost of electrical energy
is 4.2 ¢/kW·h.
Answers!
Cost Of Electricity
1. 10¢
2. 11¢
3. $1.26
4. $2.03
Electric Circuits
12. 20 _
13. 0.40A
2
14. 2.4 X 10 V
2
15. (a) 1.6 x 10 _ (b) 20 _ (c) 15 V
16. (a) 20 _ (b) 10 _
17. (a) 90 Ω (b) 2.5 Ω (c) 12 Ω
18. 8
19. 1.0 Ω
20. 16 Ω
21. (a) 10 Ω, 12 Ω, 4.8 V
(b) 36 V, 18 Ω, 6 Ω
(c) 4 Ω, 1 A, 5 A, 3.3 A, 1.6 A, 1.1 A
(d) 3.0 A, 2 Ω, 6 Ω
(e) 1.5 A, 1.5 A, 0.30 A, 0.90 A, 0.30 A, 4.2 V, 1.8 V, 1.8
V, 1.8 V
4
22. 2.2 x 10 J
3
3
23. (a) 3.00 x 10 W (b) 1.2 x 10 W (c) 60.0 W
3
24. (a) 3.6 x 10 W (b) 3.5 A
3
25. (a) 8.3 A (b) 17 A (c) 4.1 X 10 W
26. $27.59