Chapter 21
Electric Current and Direct-Current Circuits
Answers to Even-numbered Conceptual Questions
2.
No. An electric current is produced when a net charge moves.
If your body is electrically
neutral, no current is produced when you walk.
4.
Yes. There is a net charge on the comb, and by moving it from one place to another you
have created an electric current.
6.
Car headlights are wired in parallel, as we can tell by the fact that some cars have only one
working headlight.
8.
The equivalent resistance decreases because there are now more paths through which the
electric current can flow.
10.
Material A satisfies Ohm’s law because the relationship between current and voltage is
linear.
12.
Yes. Just connect two of these resistors in parallel and you will have an equivalent
resistance of
R
/2.
14.
Assuming the lights are connected to the same potential difference,
V
, the resistance of the
lights can be compared using
P
=
V
2
/
R
. Since light A has twice the power of light B, it
must also have half the resistance of light B.
16.
Resistors connected in parallel have the same potential difference across their terminals.
18.
When the switch is closed, light 2 is shorted out and the equivalent resistance of the circuit
drops from 3
R
to 2
R
.
It follows that the current in the circuit increases.
20.
Originally, light 3 is dark; after the switch is closed, light 3 is illuminated. The intensity
of lights 1 and 2 are unaffected by closing the switch; after all, they still have the same
potential difference and the same resistance.
Therefore, it is clear from
P
=
V
2
/
R
that
they will continue to dissipate the same amount of power.
22.
The total power dissipated in the circuit increases when the switch is closed. Before the
switch was closed light 1 dissipated the power
P
=
ε
2
/
R
, as did light 2.
Now all three
lights dissipate the power
P
=
2
/
R
.
24.
Resistivity is an intrinsic property of a particular substance.
In this sense it is similar to
density, which has a particular value for each particular substance.
Resistance, however,
is a property associated with a given resistor. For example, the resistance of a given wire
can be large because its resistivity is large, or because it is long.
Similarly, the weight of a
ball can be large because its density is large, or because it has a large radius.
26.
If two heaters, each with resistance
R
, are connected in series, the equivalent resistance is
now 2
R
.
The heaters are connected to the same potential, however, and therefore they
draw half the original current. From
P
=
I
2
R
we can see that each heater now dissipates
one quarter the power of the single heater, for a total power consumption of one half the
original value.
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