Magnetism
Senior 2 · Physics
Magnetism
Lesson 3 of 4
20 min
A mechanic in Gikondo magnetises the tip of his screwdriver so that tiny screws cling to it instead of falling into the engine. A compass needle has to be magnetised before it can work. And sometimes magnetism is a nuisance: a magnetised watch runs wrongly, and old magnets grow weak when stored carelessly. This lesson shows how to make a magnet, and how to destroy one on purpose.
You learned that an unmagnetised piece of steel contains domains pointing in random directions. Magnetisation is any process that lines them up. There are three main methods.
Single-touch stroking. Lay a steel bar on the bench. Stroke it from one end to the other with one pole of a magnet, then lift the magnet high above the bar and bring it back to the starting end. Repeat 20 to 30 times, always in the same direction.
The rule: the end where each stroke finishes becomes the pole OPPOSITE to the stroking pole. The moving pole drags the domains round, and unlike poles attract.
Worked example. Keza strokes a steel needle from its eye to its point with the N pole of a magnet. The strokes finish at the point, so the point becomes a S pole and the eye becomes a N pole.
Double-touch (divided-touch) stroking. Use two magnets with opposite poles. Place them together at the centre of the bar and stroke outwards to the two ends at the same time, lift, return to the centre and repeat. Each end becomes the pole opposite to the stroking pole that finishes there. This gives a stronger, more even magnet than single touch.
Wind many turns of insulated copper wire around a tube to make a solenoid (a long coil). Place the steel bar inside, connect the solenoid to a direct current (d.c.) supply for a few seconds, then switch off and remove the bar. The strong, steady field inside the solenoid lines up the domains, so this method makes the strongest magnets.
Which end becomes N? Use the right-hand grip rule: grip the solenoid with your right hand so that your fingers curl round in the direction of the current. Your thumb then points to the N pole. Reverse the current and the poles swap.
Hold a steel or iron bar pointing north–south (tilted at the angle the Earth's field makes with the ground) and hammer it several times. The blows shake the domains loose and the Earth's weak field nudges them into line. This is why iron railings, steel window frames and old tools often turn out to be weakly magnetised.
Demagnetisation destroys a magnet's magnetism by putting its domains back into random directions.
A bar magnet slowly weakens on its own, because the poles at its ends tend to turn the domains nearby out of line. Store bar magnets in pairs, side by side with opposite poles together, and with pieces of soft iron called keepers across the ends. The field then runs round a closed loop of iron and the domains stay lined up. Keep magnets away from heat, and do not drop them.
Tip
Stroking rule: "The end where the stroke finishes gets the opposite pole."
Three ways to kill a magnet: HEAT it, HAMMER it (lying east–west), or pull it out of an a.c. coil. D.c. makes magnets; a.c. destroys them.
A steel bar XY is stroked from X to Y with the S pole of a magnet (single touch). Which pole is formed at Y?
Solution: The strokes finish at Y, so Y becomes the opposite of the stroking pole: a N pole. X becomes a S pole.
Give two reasons why the electrical method is preferred for making magnets.
Solution: It produces the strongest magnets (the field inside a solenoid is strong and uniform), it is quick, and the polarity can be chosen by choosing the direction of the current.
A student tries to demagnetise a magnet by putting it in a coil connected to a battery. Explain why this fails, and what she should do instead.
Solution: A battery gives direct current, which makes a steady field that lines the domains up even more: she is magnetising it. She should use an alternating current and slowly withdraw the magnet while the current still flows.
Why should a magnet be hammered while it lies east–west, not north–south, if you want to demagnetise it?
Solution: Lying north–south, the Earth's field would line the loosened domains up again and could even magnetise the bar. Lying east–west, the Earth's field does not favour either end, so the domains settle randomly.
Why are bar magnets stored in pairs with keepers?
Solution: With opposite poles side by side and soft-iron keepers across the ends, the field lines form a closed loop through iron. This stops the poles from turning nearby domains out of line, so the magnets do not weaken.
Remember
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