Cranks, magnets and lodestones

Cranks, the fixed pulley, lodestones, magnets and electromagnets, and circuit components. Read one part at a time and tap “I know this”. Then test yourself in the quiz.

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Cranks and pulleys

The crank makes the drum easier to turn.

Mechanical advantage tells you how many times a mechanism multiplies the effort you put in. A single fixed pulley at the top of a flagpole mainly helps by changing the direction of the force. A crank is an adapted class 2 lever: the axle is the fulcrum, the load is close to the axle, and you push on the handle far from the axle, so a small effort turns a big load. A pulley is a type of wheel and axle: a wheel with a groove for the rope, turning on an axle.

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Magnets

Two north poles repel; north and south attract. The force is strongest at the poles (the ends).
A fridge magnet is a permanent magnet. Steel keeps its magnetism well. An iron nail touching a magnet becomes a temporary magnet.
Magnetise a steel needle: stroke it one way with a magnet.
Store bar magnets N next to S, with iron keepers. Don’t drop or hit them, and keep them away from bank cards.
A compass needle points north because the Earth acts like a giant magnet.

Rule

The only sure test for a magnet: it repels a magnet’s pole. Attracting only proves it is magnetic material.

An electromagnet’s iron core makes the field much stronger; it can be switched on and off.

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Remember this: your summary

A crank is the handle on a wheel: sewing machine, bicycle pedals, hand drill, well winch.
A lodestone (magnetite) is a naturally magnetic rock; early sailors used it as a compass.
A horseshoe magnet is strong because both poles are close together.
Magnets do not attract all metals: only iron, nickel and cobalt (and steel).
Circuit: cell = energy source; switch = control; bulb, buzzer, motor = load.

Write these points in your book. Read them again the night before the test.

Memory trick

Like poles fight, unlike poles hug: N-N repel, N-S attract.

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