Magnetism & electromagnetism flashcards

Key points

• Every magnet has a north pole and a south pole.
• Like poles repel; unlike poles attract.
• The magnetic materials are iron, steel, cobalt and nickel. A magnet attracts them even if they are not magnets.
• Copper, aluminium, gold and brass are metals, but they are not magnetic.

Key points

• A permanent magnet produces its own magnetic field all the time, like a steel bar magnet.
• An induced magnet becomes a magnet only while it is in a magnetic field.
• It loses most or all of its magnetism quickly when it is taken out of the field.
• Induced magnetism always attracts: a paperclip near a magnet can pick up another paperclip.

Key points

• A magnetic field is the region around a magnet where a force acts on another magnet or a magnetic material.
• Field lines run from the north pole to the south pole outside the magnet.
• The lines never cross. They are closest together at the poles, where the field is strongest.
• Further from the magnet, the lines spread out and the field is weaker.

Key points

• A compass needle is a small magnet that lines up along the field.
• Mark where the needle points, move the compass to that mark, and repeat. Join the dots to trace a field line.
• Iron filings sprinkled round a magnet show the pattern quickly, but not the direction.
• With no magnet near, the needle lines up with the Earth's magnetic field: evidence that the Earth's core is magnetic.

Key points

• A current in a wire makes a magnetic field around it, in circles round the wire.
• The field is strongest close to the wire, and a bigger current makes a stronger field.
• Reversing the current reverses the direction of the field.
• Right-hand grip rule: thumb along the current, and the fingers curl the way the field goes.

Key points

• A solenoid is a coil of wire. The fields of its turns add up, so the field inside is strong and uniform.
• Outside, the field has the same shape as a bar magnet's.
• An iron core makes it an electromagnet: the iron becomes an induced magnet and adds to the field.
• Make it stronger with a bigger current, more turns on the coil, or an iron core.

Key points

• The current makes its own field round the wire, which interacts with the magnet's field: the motor effect.
• The force is biggest when the wire is at right angles to the field, and zero when it is parallel.
• Reversing the current or the field reverses the force.
• Fleming's left-hand rule: first finger Field (N to S), second finger Current (+ to −), thumb the Motion.

Key points

• B is the magnetic flux density, the strength of the field, measured in tesla (T).
• It works for a wire at right angles to the field.
• A bigger current, a stronger field or a longer wire in the field all mean a bigger force.

Formula

20 cm of wire, 4 A, in a 0.5 T field

Key points

• The current flows opposite ways along the two sides of the coil, so one side is pushed up and the other down.
• Those two forces make a turning effect, and the coil rotates.
• A split-ring commutator reverses the current every half turn, so the coil keeps turning the same way.
• It turns faster with a bigger current, a stronger magnet or more turns on the coil.

Key points

• An electromagnet can be switched on and off, and its strength changed with the current.
• A crane's electromagnet is switched on to lift scrap iron and steel.
• Switching off the current makes it lose its magnetism, so the load drops where it is wanted.
• A permanent magnet cannot let go of what it holds.

Key points

• A small current flows through the coil of an electromagnet.
• The electromagnet pulls an iron switch closed in a second circuit, which carries the large current.
• When the small current stops, a spring pulls the switch open again.

A car's starter motor

Key points

• A coil fixed to a cone sits in the field of a permanent magnet.
• The signal is an alternating current, so the force on the coil keeps reversing.
• The cone vibrates back and forth, pushing on the air to make the pressure waves of sound.
• The signal's frequency sets how often the cone vibrates, which sets the pitch.