UniversityPhysicsVolume2-Ch10. Old Dominion University PHYS 232N

University Physics Volume II
Unit 2: Electricity and Magnetism
Chapter 10: Direct-Current Circuits
Conceptual Questions
1. What effect will the internal resistance of a rechargeable battery have on the energy being used
to recharge the battery?
Solution
Some of the energy being used to recharge the battery will be dissipated as heat by the internal
resistance.
2. A battery with an internal resistance of r and an emf of 10.00 V is connected to a load resistor
R r  . As the battery ages, the internal resistance triples. How much is the current through the
load resistor reduced?
Solution
The current is cut in half.
3. Show that the power dissipated by the load resistor is maximum when the resistance of the
load resistor is equal to the internal resistance of the battery.
Solution
P I 2R e
r + R
æçè
ö÷ø
2
Re2R(r + R)- 2 , dP
dR e2 éëê(r + R)- 2 - 2R(r + R)- 3ùûú 0,
(r + R) - 2R
(r + R)3
éêêë
ùúúû
0, r R
4. A voltage occurs across an open switch. What is the power dissipated by the open switch?
Solution
No power is dissipated by the switch because zero current flows through the circuit when the
switch is open.
5. The severity of a shock depends on the magnitude of the current through your body. Would
you prefer to be in series or in parallel with a resistance, such as the heating element of a toaster,
if you were shocked by it? Explain.
Solution
It would probably be better to be in series because the current will be less than if it were in
parallel.
6. Suppose you are doing a physics lab that asks you to put a resistor into a circuit, but all the
resistors supplied have a larger resistance than the requested value. How would you connect the
available resistances to attempt to get the smaller value asked for?
Solution
connect two or more in parallel
7. Some light bulbs have three power settings (not including zero), obtained from multiple
filaments that are individually switched and wired in parallel. What is the minimum number of
filaments needed for three power settings?
Solution
two filaments, a low resistance and a high resistance, connected in parallel
8. Can all of the currents going into the junction shown below be positive? Explain.
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