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Electricity and Magnetism Exercises

Complete the following exercises.

1. Why are good conductors of heat also good conductors of electricity?

They contain free electrons which can move freely hence conducting both heat and charge.

2. What is the difference between a good conductor and a good insulator?

A good conductor allows electric current to flow freely through it while a good insulator prevents or resists any electric current from flowing through it.

3. Which of the following are conductors? (Select all that apply)
4. Wood
5. Copper
6. Gold
7. Plastic
8. Distilled water

B, C

4. Which of the following are insulators? (Select all that apply)
5. Wood
6. Copper
7. Gold
8. Plastic
9. Distilled water

A,D,E

5. A nail contains trillions of electrons. Given that electrons repel from each other, why do they not then fly out of the nail?

Most nails are made of iron which means it contains 26 positively charged protons and 26 negatively charged which attract each other hence the force of attraction prevents the electrons from flying out of the nail.

6. How do the number of electrons flowing into a light bulb compare to the number of electrons flowing out of it?

The number of the electrons flowing into the light bulb is equal to the number of electrons flowing out of it.

7. A light bulb connected to a 9-V battery has a current of 1.5 A flowing.
   1. Find the resistance of the light bulb (in ohms)

R = V/A

9-V/1.5A

R = 6 ohms

1. Find the power drawn by the light bulb (in W)

W = V x A

9 x 1.5 = 13.5 W

8. Consider three identical light bulbs connected as shown in the circuit below.

1. Which light bulb(s) will be brightest? Explain.

Because the wires are directly connected to the positive and negative charges.

1. What will happen to B and C if light bulb A is removed? Explain.

They will both increase in brightness. Because the number of the bulbs sharing the voltage will be reduced.

1. What will happen to A and B if light bulb C is removed? Explain.

Bulb A will not change and will continue to light while bulb B will go off. Bulb A will still be connected to both the positive and the negative charges while bulb B will be lacking the positive charge.

9. Rank the following from smallest current to largest current.
   1. A 1200W microwave connected to 110V outlet.
   2. A 1500W water heater connected to 220V outlet.
   3. A 100W light bulb connected to 110V outlet.
   4. A 2000W oven connected to a 220V outlet.
   5. A 40W Light bulb connected to a 12V battery.

C, E, B, D, A.

10. Your friend says an appliance uses energy. How would you correct his statement?

Appliances converts the energy from one form to another.

11. Describe the differences and similarities between an electromotor and a generator.

An electromotor converts electrical energy into mechanical energy while a generator converts mechanical energy into electrical energy. Both the electromotor and the generator work on the magnetic effects of an electric current and both have the armatures to rotate the loop.

12. Your friend with great excitement tells you about his newest idea to solve the energy crisis: He wants to use an electromotor to drive a generator and then use part of the electric power generated to power the electromotor while using the rest to power his home.  What would you tell him? It is not advisable because the generator will always produce less power than it is needed to run the motor and this will lead to power shortage and the machine will eventually stop turning.
13. Why is a paper clip attracted to a magnet, while is pencil is not?

A pencil is made of wood which is an insulator and magnetic forces only affect metal hence attracts the paper clip.

14. A loop of wire embedded in the road at an intersection can be used to trigger a traffic signal. This is an application of magnetic induction.

The loop of wire operates by sensing a change in frequency to the electromagnetic field over the coil of wire such that when a vehicle pulls up, it senses the vehicle and the light changes.

The remaining questions are multiple-choice questions:

15. Two like charges:
    1. Attract each other
    2. Repel each other
    3. Must be neutrons
    4. Neutralize each other
    5. Repel each other

16. When a plastic rod is rubbed with fur, the rod will become negatively charged. Which of the following best explains how this happens?
    1. Electrons are transferred from the plastic rod to the fur.
    2. Electrons are transferred from the fur to the plastic rod.
    3. Negative charges are created on the plastic rod.
    4. Positive charges are removed from the plastic rod.
    5. Electrons are transferred from the fur to the plastic rod.

17. What is characteristic of a good insulator?

• Electrons are usually not moving at all.
• Electrons are free to move around.
• Electrons are semi-free to move around.
• Electrons are tightly bound to the nuclei.
• Electrons are tightly bound to the nuclei

18. In a solid conductor such as a metal, what is the cause of an electric current?

• A flow of positive and negative charges.
• A flow of positive charges only.
• A flow of negative charges only.
• A flow of protons.
• A flow of neutrons.

1. A flow of negative charges only

19. How does the resistance of a 100W light bulb compare to the resistance of a 60W light bulb?

• The 100W light bulb has a higher resistance than the 60W light bulb.
• The 100W light bulb has a lower resistance than the 60W light bulb.
• The 100W light bulb has the same resistance as the 60W light bulb.
• Not enough information to tell.
• The 100W light bulb has a lower resistance than the 60W light bulb
• Consider the five circuits in the diagram below.

20. In which of the circuits will the light bulb light up?

B.

21. What causes an electric field?
    1. A moving change only.
    2. A stationary charge only.
    3. Any charge, moving or stationary.
    4. A stationary magnet.
    5. A stationary charge only.

22. What causes a magnetic field?
    1. A moving change only.
    2. A stationary charge only.
    3. Any charge, moving or stationary.
    4. A moving change only.

23. What does a step-up transformer do?
    1. It steps up the energy.
    2. It steps up the power.
    3. It steps up the voltage.
    4. It steps up the current.

1. It steps up the voltage.

24. Which of the following is true for a generator?
    1. It transforms thermal energy to electrical energy.
    2. It transforms mechanical energy into electrical energy.
    3. It transforms electrical energy into mechanical energy.
    4. It transforms solar energy into electrical energy.

1. It transforms mechanical energy into electrical energy.

25. Which of the following is true for an electromotor?
    1. It transforms thermal energy to electrical energy.
    2. It transforms mechanical energy into electrical energy.
    3. It transforms electrical energy into mechanical energy.
    4. It transforms electrical energy into potential energy.
    5. It transforms electrical energy into mechanical energy.