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CHAPTER 20 SWITCHING ON
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QUESTIONS
20.1 Direct and alternating voltages and currents
1.
What is the peak current of a circuit if it has an RMS current of 8 A?
2.
What DC voltage would produce the same heating effects as a peak voltage of 300 V
AC?
3.
The peak current moving through a resistor is equal to 10 A AC. What is the effective
value of the current?
20.5 Power
4.
Calculate the electric potential energy dissipated in 1.00 h by a 2.00 kW electric heater.
5.
An electric jug has a rating of 1.00 × 103 W. If it requires 4.00 × 105 J of heat energy to
heat the water in the jug to boiling point, calculate the time it will take.
6.
A light globe has a power of 1.00 × 102 W and the voltage drop across it is 2.40 × 102 V.
Calculate the current passing through the light globe.
7.
The current through a power supply is 2.0 A. The voltage rise across the power supply is
1.2 × 101 V. If the current passes for 1.0 min, calculate the electric potential energy
generated by the power supply.
8.
A current of 6.0 A passes through a resistor for 1.2 × 101 s and dissipates 3.20 × 102 J of
electric potential energy. Calculate the voltage drop across the resistor.
Review questions
Understanding
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QUEENSLAND PHYSICS
1.
Explain why a fuse or a circuit breaker is connected into each circuit of a household
electrical system.
2.
Explain why a fuse or circuit breaker is connected into the active wire rather than into the
neutral wire.
3.
Explain the advantage of a circuit breaker over a fuse.
4.
Explain the function of the earth wire in a household electrical system.
5.
Explain the function of double insulation.
6.
Discuss the use of a residual current device.
7.
Tom claims that the effect of an electric shock depends on the voltage. Bill claims that
the effect depends on the current. Explain the confusion.
8.
Explain why a more serious shock is experienced if the skin is wet than if it is dry.
9.
A student finds that a set of Christmas tree lights do not come on when the power is
turned on. What is the most likely cause of this?
10. Two globes have filaments that are equal in length, yet one gives out twice the electrical
power of the other. How is this possible?
Application
11. Calculate the power generated by a battery that produces 6.00 × 102 J of electric potential
energy in 2.00 × 101 s.
12. Calculate the heat energy produced in 1.00 minute by a heater generating 5.00 × 102 W of
power.
13. Calculate the time it takes a 4.00 × 101 W lamp to dissipate 8.00 × 102 J of electrical
energy.
14. Calculate the electrical energy (in kWh) used by a 2.0 kW heater in 3.0 hours.
15. The following table refers to the energy, E, produced by a device that generates power, P,
for time t. Calculate the missing values and complete the table.
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QUEENSLAND PHYSICS
16. The following table refers to the power, P, generated when a current, I, passes through a
potential difference (voltage), V. Calculate the missing values and complete the table.
17. Refer to the electricity account in figure 20.16 (page 497).
(a)
In what units is the energy measured?
(b)
What is the normal cost per unit of energy?
(c)
What was the average amount of energy used by this household per day?
18. Eight identical lights are connected in parallel to a 240 V power source by two leads that
have a total resistance of 2 Ω. If 560 mA flows through each bulb, determine:
(a)
the resistance of each light
(b)
the percentage of the total power that is lost in the leads as heat.
19. A 75 W, 240 V bulb is connected in parallel with a 60 W, 240 V bulb. What is their
combined effective resistance?
Challenges
20. The amp-hour rating of a battery is equal to the current that is draining the battery
multiplied by the number of hours it would take to drain the battery completely at that
current. A person accidentally leaves their car headlights on overnight. If each of the two
front lights uses 40 W and the two rear lights use 60 W each, how long will it take the
12 V battery to go flat if it is rated at 45 amp hours? Assume that the battery was fully
charged to start with and that the lights are all connected in parallel.
21. Design the wiring system for a flat consisting of a living room, kitchen, bedroom and
bathroom. Draw a plan of the unit showing the wiring. Use the table below to determine
the rating of the fuse for each circuit (8 A, 20 A, 32 A). You will place the fuse box in the
kitchen. Guidelines and hints for your design:

Work out how many outlets for lights and how many outlets for power points you
will need. The stove and the hot water heater will each have a separate circuit.

How many light switches will you need?

How many lighting circuits will you need?

How many three-point power outlets will you need?

How many power circuits will you need?
22. A coil of wire dissipates energy at a rate of 5.0 W when a potential difference of 200 V is
applied across it. A second coil made of the same wire dissipates 15 W when the potential
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QUEENSLAND PHYSICS
difference is applied to it. What is the ratio of the length of wire in the second coil to that
in the first coil?
23. A solid square-faced rectangular prism made of a conducting material has edges of
lengths L, L and 10L. A constant potential difference is applied across parallel sides
separated by 10L and then L. Find the ratio of:
(a)
the resistance
(b)
the current
(c)
the power dissipated.
Notes:
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