Magnetism and Electromagnetism

Electricity and magnetism are two views of one phenomenon, and the subtest asks you to know which way the conversion runs in a given device.

Magnets and fields

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Every magnet has two poles, north and south.

Like poles repel; unlike poles attract. North pushes north away and pulls south toward it.

Field lines run out of the north pole and into the south pole outside the magnet, and they show the direction a compass needle would point.

You cannot have a single pole. Cutting a magnet in half does not give you a north piece and a south piece; it gives two complete magnets, each with a north and a south. That is a question, and it is what makes magnetism different from electric charge, where a lone positive or negative is ordinary.

Magnetic materials

Iron, nickel and cobalt are the magnetic metals, along with alloys containing them such as steel.

Most metals are not magnetic. Aluminum, copper, gold and silver are not, which is worth noting because "metal" and "magnetic" are easily conflated. A question asking which of four metals a magnet attracts is usually testing exactly that.

Earth as a magnet

Earth has a magnetic field, generated by the motion of molten iron in its outer core. That connects this topic back to Earth's structure.

A compass needle aligns with Earth's field, and its north-seeking end points toward the geographic north.

Which means Earth's magnetic pole near the geographic north is, magnetically speaking, a south pole - because unlike poles attract, and the needle's north end is drawn to it. That is a genuine curiosity rather than a trick, and it is occasionally asked.

Magnetic north and geographic north are not the same place, and the difference is why navigation requires a correction.

Electromagnetism

A current through a wire produces a magnetic field around it. That is the discovery the whole field rests on.

Coil the wire and the fields add, making a stronger, more concentrated field. Put an iron core inside the coil and it is stronger still.

That is an electromagnet, and its defining property is what the figure's caption says: it can be switched off. Current on, magnetism on; current off, magnetism gone.

Three ways to strengthen an electromagnet:

1. More current.
2. More turns of wire in the coil.
3. An iron core.

Those three are the answer to a question asked almost verbatim.

Reversing the current reverses the poles, which is useful and is also asked.

Uses: scrapyard lifting magnets, doorbells, relays, speakers, and the magnetic locks on doors - anything where the magnetism needs to be controllable.

Motors and generators

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A motor converts electrical energy into mechanical energy. Current through a loop of wire in a magnetic field experiences a force, and the loop turns.

A generator converts mechanical energy into electrical energy. Turning a loop of wire in a magnetic field induces a current.

They are the same device run in opposite directions, which is the most useful thing to know here. A question describing current going in and a shaft turning is a motor; a shaft being turned and current coming out is a generator.

Electromagnetic induction is the name for the second effect, and the rule is called Faraday's law: a changing magnetic field induces a current in a conductor. The field has to be changing - a stationary magnet next to a stationary wire induces nothing.

A transformer uses induction to change AC voltage. A changing current in one coil induces a current in a second coil, and the ratio of turns sets the voltage ratio. More turns on the output side steps voltage up; fewer steps it down.

Transformers work on AC only, because AC is already changing and DC is not. A transformer fed steady DC produces nothing after the initial switch-on, and that is why the grid is AC.

What you can skip

Across all 2,104 General Science questions in our bank:

• Units of magnetic field - teslas and gauss - never appear on General Science. (Electronics Information asks once that flux density is measured in teslas.)
• Magnetic domains never appear by name.
• Hand rules for field direction come up once. Knowing that reversing the current reverses the poles answers the direction questions.

Where people lose points

Thinking a single magnetic pole can be isolated.

Assuming all metals are magnetic. Only iron, nickel, cobalt and their alloys.

Swapping motor and generator. Current in gives rotation; rotation gives current.

Saying a stationary magnet induces a current. The field must change.

Saying transformers work on DC.

Forgetting that an electromagnet can be switched off, which is the whole point of having one.

Work one in under a minute

An electromagnet is not lifting enough. Name three changes that would make it stronger.

Increase the current, add more turns to the coil, and use an iron core if it does not already have one.

All three increase the field, and the question is asked in this form often enough that the list is worth holding as a list.

Where this leads

This is where General Science hands off to Electronics Information, which asks about transformers, motors and generators in more detail.