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Chapters 18&19 Sample Quiz
1. Wetting human skin lowers the electrical resistance of the body to 1000 . If a student licks their fingers then holds a 1.50 volt calculator battery between thumb (+) and forefinger (–), what current (in mA) flows between them?
a) 1500. b) 667. c) 150. d) 1.50 e) none of these
2. An aluminum power line cable 2.54 cm in diameter and 100 km in length has what resistance (in ) at 20°C?
a) 5.23 b) 5230 c) 5.23 x 106 d) 5.23 x 109 e) none of these
3. If an aluminum wire has a resistance of 10 at 20°C, find its resistance (in ) at 50°C
a) 8.7 b) 11.3 c) 87 d) 113 e) none of these
4. A laser printer operating at 120 V draws 3.60A for 5.00s while printing a page. At 10¢ per kWh, how much will the electricity cost to print 600 one page report cards?
a) less than $0.01 b) $0.036 c) $36.00 d) none of these
5. A 720 W toaster operating in the US draws how many amps of current?
a) 6 b) 3 c) 1/3 d) 1/6 e) none of these
6. In Europe, the effective or rms voltage supplied by the AC power grid is 240 V at a frequency of 50 Hz. Calculate the peak voltage (in volts).
a) 120 b) 170 c) 340 d) 480 e) none of these
7. Which draws more current, a 100W light bulb or a 75W bulb? Which has the higher resistance?
a) 100W, 100W b) 100W, 75W c) 75W, 100W d) 75W, 75W e) none of these
8. Why do light bulbs almost always burn out as they are turned on and not after they have been on for some time?
a) at low temperatures, R is higher so I is higher causing more I2R heating and burn out.
b) at high temperatures, R is lower so I is higher causing less I2R heating and burn out.
c) at low temperatures, R is higher so I is lower causing more I2R heating and burn out.
d) at low temperatures, R is lower so I is higher causing more I2R heating and burn out.
9. A person leaves their car lights on after parking their older model car. If the headlights use 40W each (for a total of 80W), are wired in parallel so 12V from the battery appears across each light, and if the freshly charged battery is rated at 90 Ah, how long (in hours) will the battery last?
a) 2.4 b) 8.7 c) 13.5 d) 39 e) none of these
The following circuit diagram applies to problems 10-13
12v
10. Find the equivalent resistance (in k) of this circuit.
a) 2 b) 3 c) 4 d) 9 e) none of these
11. Find the current I (in mA) that flows through the battery.
a) 12 b) 3 c) 4 d) 6 e) none of these
12. Find the current I1 (in mA) that flows through R1.
a) 12 b) 3 c) 4 d) 6 e) none of these
13. Find the voltage drop (in V) across resistor R3.
a) 12 b) 8 c) 4 d) 1 e) none of these
The following circuit diagram applies to problems 14-15
C1 C2
9.00V
14. Find the equivalent capacitance (in F) of this circuit.
a) 125 b) 75 c) 50 d) 20 e) none of these
15. Find the charge (in C) on the + plate of capacitor C2.
a) 225 b) 635 c) 180 d) 540 e) none of these
The following circuit diagram applies to problems 16-17
R
16. Find the time constant (in s) of this circuit.
a) 47 b) 4.7 c) 2.13 d) 0.47 e) none of these
17. If the capacitor has potential difference between its plate of 9.00V at t = 0 when the switch is closed, find the charge (in C) on the capacitor at t = 2 (two time constants).
a) 3.31 b) 12.3 c) 4.50 d)57.3 e) none of these
18. Mr. Watson loves toast. Given an unlimited number of outlets, how many toasters (720W at 120V) can he plug into a single US kitchen circuit protected by a 15A fuse and use simultaneously?
a) 2 b) 3 c) 5 d) 15 e) none of these
19. Mr. Watson is celebrating National Toast Week by wrapping a string of 100 tiny light bulbs around his kitchen. One bulb falls out and the whole string goes dark. Were the lights in the string connected in series or in parallel?
a) series b) parallel c) none of these
20. Dry human skin has a resistance of 105 . If we measure a brown black yellow gold resistor while holding it and the multi-meter leads between our fingers what will be the meter resistance (in )?
Hint: we will be in parallel with the resistor.
a) 200,000 b) 100,000 c) 50,000 d) 0.00002 e) none of these
SOME USEFUL FORMULAS
I =
V = I R
R = /A
R = R0 [1 + (T - T0)]
rAl = 2.65 x 10-8 Wm ; at T = 20° C
Al = 0.00429/C° ; at T = 20° C
P = IV = I2R = V2/R = energy / time
Req = R1 + R2 + ... resistors in a series; current same, voltage adds
... resistors in parallel; current adds, voltage same
While charging, = RC is time for current to decrease from its maximum (given by I = V / R) to 37% of that value:
and
______Q = (V)C
Black(0), Brown(1), Red(2), Orange(3), Yellow(4), Green(5), Blue(6), Violet(7), Gray(8), White(9) is the resistor color code.
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