Part 9

What Did You Learn?

(Underline correct answer)

1. A Junior Hard Service Cord is known as an (SO-Type) (SJO-Type) cord.

2. You disconnect a cord by (jerking it from the socket) (grasping plug and pulling it out).

3. Brass sockets are unsafe because (they break too easily) (the exposed metal can cause short circuits).

4. Rubber-covered cord is safer for emergency cords than fabric because (it will stretch) (it will insulate and protect the wires inside).

5. In a trouble light (any kind of bulb will do) (a rough service bulb is best).

Ideas for Demonstrations and Exhibits

1. Show how to make your trouble light and a method of storing it.

2. Show a safe trouble light, and an unsafe trouble light with danger points marked.

3. Show cutaway pieces of different types of cord.

For More Information

Ask your power supplier, county highway engineer, police official or leader to tell you about various types of portable emergency lights and their uses.

LESSON NO. B-5

Credit Points 5

WHAT MAKES MOTORS RUN

What makes an electric motor run? Can you make an electric motor that will run? Certainly you can, and by doing so you'll learn why it runs. It won't be mysterious any more and you'll be ahead of all the millions of people who use motors every day and never know why or how the motor converts electrical energy into useful power.

Motors Are Magnets

You know how one end of a compass needle always points to North. No matter how you turn the compass, the same end of the needle always swings to the North. The earth itself and that small compass are both magnets (Figure 1). Each has a North pole and a South pole. Around the poles of each there are magnetic fields, invisible lines of force that attract and repel.

The N poles repel each other and so do the S poles. The N and S poles attract each other. In other words, opposite poles attract; poles that are alike repel each other.

Lay 2 bar magnets on a table side-by-side. If both N poles are at one end, they'll repel each other and almost flip around until there's a N pole lying next to a S pole (Figure 2).

Now suppose we place one of the bar magnets on the table. The other, we'll fix on a pivot so it can spin around. This one we'll move so its N pole almost touches the fixed magnet's N pole. As soon as we release it, the movable magnet will spin around so its S pole will be near the N pole of the stationary magnet. That's an electric motor--almost.

It's not quite a motor because the rotating magnet will just move as far as it has to in order to get the opposite poles together. You might be able to cause the movable bar magnet to make turn after turn. You could do this by turning the fixed magnet quickly end for end. This wouldn't be very practical as a motor.

We Can Improve It

If we could change the pole on one end of the rotating magnet just as soon as it reaches the attracting pole, it could make a complete circle. In doing that, the pole at the near end of the rotating magnet would be repelled by the stationary magnet and pushed away. As soon as the opposite end of the rotating magnet would come into the magnetic field, it would be drawn to the stationary magnet. In order to keep the "motor" running, we would have to constantly change the poles at each end on every half revolution.

We Need An Electromagnet

We can't reverse the poles on simple bar magnets, but we can on electromagnets. We can make one by wrapping a wire several times around an iron core to form a coil. This magnet will also have a N and a S pole when connected to electrical current. The big difference is that the poles can be changed instantly by reversing the current in the wire.

Switching Poles Automatically