CHAPTER
13
Review
SUGGESTED ANSWERS
WHAT DO YOU REMEMBER?
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1. (a)
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(b)
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(c)
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This circuit is series.
2. (a)
(b)
This circuit is parallel.
3.
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V
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A
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High Resistance
Voltage (V)
4. (a)
Current (A)
(b)
Voltage (V)
Low Resistance
Current (A)
5.
Resistance is the ability of a material to oppose (resist) the flow of electric current through it.
6.
Both fuses and circuit breakers function like switches. They interrupt the flow of current in a circuit when
the current through the circuit exceeds a certain value. Without fuses or circuit breakers, the risk of fire
would be significant. Fuses are one-use devices and must be replaced after they have blown, whereas a
circuit breaker can be reset and used multiple times.
7.
Too much current through a conductor eventually causes the conductor to heat up. If the circuit is not
shut down, a fire may occur.
8.
The terminals of an ohmmeter should be placed across the terminals of a load with the circuit turned off.
Switching the position of the terminals of the ohmmeter does not matter.
9.
The circuit in diagram (a) would have a greater total resistance. From my lab results in section 13.11, a
series circuit always has a greater total resistance than a parallel circuit.
WHAT DO YOU UNDERSTAND?
10.
Electrons flow from the negative terminal of an energy source through the load and then to the positive
terminal of the energy source. The potential difference across the two terminals of the energy source causes
the electrons to continue flowing throughout the circuit because electrical potential energy from the energy
source gets transferred to the electrons. The electrons then convert the potential energy into kinetic energy
as they flow through the circuit.
11.
An electron at the positive terminal of the battery has an electric potential energy that is greater than an
electron at the negative terminal. The difference between these two potential energy values is 1.5 V.
12.
Touching a live circuit can be dangerous because it can deliver electrical energy that is capable of
disrupting the nervous system and the muscles it controls, as well as damaging living tissue.
13.
A load always has some internal resistance. When the load converts electric potential energy into thermal
energy, the load is a resistor.
14.
A good extension cord should have low resistance to keep it from becoming a load and heating up. To
accomplish this, it should be thick, made from a good conductor such as copper, and be well insulated.
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15.
Aluminum is inexpensive and a good conductor. By twisting smaller wires together, the line can be made
thick and kept flexible while keeping costs down.
16.
The loads will have different amounts of current going through them depending on their resistance. Only
loads of identical resistance will have identical currents going through them.
17.
The resistance of dry skin is much higher than the resistance of wet skin. If a potential difference is put
across the dry skin, only a small current will be generated as compared to much larger current on wet skin.
The reason has to do with Ohm’s law, I 5 V/R.
18.
The purpose of a fuse is to break the flow of current if the current exceeds a certain value. This ensures that
the conducting wires do not become too hot. Replacing the fuse with a conductor defeats this purpose and
increases the risk of fire.
19.
Some outlets in the room may be wired in series if the loads on those outlets do not have low resistance
values. But the room should be wired in parallel relative to the rest of the house. This ensures that all the
appliances in the room will be supplied with the appropriate voltage and current that they need to operate
correctly.
20.
This setup will not trip a circuit breaker as long is the total current drawn through the circuit is below
the rating of the circuit breaker. If all the appliances have relatively high resistances and low loads this
condition will be met.
21. (a) Table 1 Data Table
Location
Current (A)
Voltage (V)
Total resistance (V)
Total
3.0 (at source)
6.0 (across source)
2.0
R1
1.0
6.0
4.0
R2
1.0
6.0
6.0
R3
1.0
6.0
8.0
The larger bold figures do not make sense. The circuit is a parallel circuit where the voltage drop across
each resistance is the same as across the source, as shown in the table. Also the currents appear to be
correct because they add up to the total current at the source. This would mean that a resistance of 2 Ω
would be incorrect (R 5 V/I). If they were all 6 Ω, everything would work out.
(b) Table 2 Data Table
Location
Current (A)
Voltage (V)
Total resistance (V)
Total
3.0 (at source)
6.0 (across source)
5.0
R1
3.0
2.0
5.0
R2
3.0
2.0
5.0
R3
3.0
2.0
5.0
Almost nothing in this table makes any sense. It is a parallel circuit so the voltage at the source should
match the voltages across each resistor, but they do not in the table. Also, the total resistance should
be significantly less than the resistance of any one loop. Finally, the currents through all the resistors
should add up to the current at the source, but they do not.
SOLVE A PROBLEM
22.
The first thing to try would be to connect a voltmeter in parallel across the batteries to see if they were any
good. I could check to see if any current was getting from the batteries to the headphones. To do this, I
would connect an ammeter in series, with one lead touching the centre part of the jack on the MP3 player
and another touching the tip of the jack on the headphones. If there is zero current, then it would mean
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that no current is getting to the headphones. In both cases, if the voltmeter read zero that would identify
the problem.
23.
To calculate the resistance, use R 5 V/I. In this case, R 5 120 V/5 A 5 24 V.
24.
To calculate the current, use I 5 V/R. In this case, I 5 12 V/25 V equaling 0.48 A.
25.
To calculate the voltage drop, use V 5 IR. In this case, V 5 0.035 A 3 120 V 5 4.2 V.
26.
Resistance is the slope on the graph. Using the points (0, 0) and (0.6, 23) on the graph it appears the
resistance is about
23 2 0
or 38.3 V.
0.6 2 0
CREATE AND EVALUATE
27.
To operate independently, the motors should be connected in parallel. The indicator lights should be
connected in series with the motors so that they would go out if the motors fail.
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REFLECT ON YOUR LEARNING
28.
Sample answer: The hard part about understanding electrical potential difference is that electricity, unlike
water, is not something you can see. Making analogies to things that we have everyday experience with can
be helpful in understanding electric potential difference.
WEB CONNECTIONS
29.
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Muscle movement is a result of an electric current being sent from the brain, along nerves, to muscles.
Electrical currents are sent to the brain through nerves, and this enables us to sense the world around us.
A cochlear implant can be used to replace the part of the inner ear responsible for sending electrical signals
to the brain to produce hearing. This device can restore hearing to some people who had been deaf.
Unit E: The Characteristics of Electricity
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