QUEENSLAND PHYSICS

CHAPTER 20 SWITCHING ON
Name:

QUESTIONS20.1 Direct and alternating voltages and currents

1.  What is the peak current of a circuit if it has an RMS current of 8A?

2.  What DC voltage would produce the same heating effects as a peak voltage of 300V AC?

3.  The peak current moving through a resistor is equal to 10AAC. What is the effective value of the current?

20.5 Power

4.  Calculate the electric potential energy dissipated in 1.00h by a 2.00kW electric heater.

5.  An electric jug has a rating of 1.00×103W. If it requires 4.00×105J 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 1.00×102W and the voltage drop across it is 2.40×102V. Calculate the current passing through the light globe.

7.  The current through a power supply is 2.0A. The voltage rise across the power supply is 1.2×101V. If the current passes for 1.0min, calculate the electric potential energy generated by the power supply.

8.  A current of 6.0 A passes through a resistor for 1.2×101s and dissipates 3.20×102J of electric potential energy. Calculate the voltage drop across the resistor.

Review questions

Understanding

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 6.00×102J of electric potential energy in 2.00×101s.

12.  Calculate the heat energy produced in 1.00minute by a heater generating 5.00×102W of power.

13.  Calculate the time it takes a 4.00×101W lamp to dissipate 8.00×102J of electrical energy.

14.  Calculate the electrical energy (in kWh) used by a 2.0kW heater in 3.0hours.

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.

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 figure20.16 (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 240V power source by two leads that have a total resistance of 2Ω. If 560mA 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 75W, 240V bulb is connected in parallel with a 60W, 240V 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 40W and the two rear lights use 60W each, how long will it take the 12V battery to go flat if it is rated at 45amp 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 (8A, 20A, 32A). 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 5.0W when a potential difference of 200V is applied across it. A second coil made of the same wire dissipates 15W when the potential 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:

© John Wiley & Sons Australia, Ltd 4