Magnets and magnetic fields

A permanent magnet produces its own lasting field. An induced magnet becomes magnetic in a field and may lose magnetism when removed.

Near the poles.

Like poles exert repulsive forces on each other.

Like poles repel (1); unlike poles attract (1).

A permanent magnet produces its own field at all times (1); an induced magnet is only magnetic while it is in a magnetic field and loses most of its magnetism when removed (1).

The field consists of concentric circles around the wire (1), and its direction can be found with the right-hand grip rule and reverses if the current reverses (1).

Increase the current (1); increase the number of turns on the coil (1); add or use an iron core (1).

An electromagnet can be switched on to $pic$k up the car and switched off to release it, which a permanent magnet cannot do (1). Its strength can also be controlled by changing the current (1). A solenoid produces a strong, uniform field inside the coil with external field lines like those of a bar magnet (1), and adding an iron core greatly increases the field strength (1).

The Earth has a magnetic field that acts like a giant bar magnet, with a magnetic south pole near the geographic north pole (1); a compass needle is a small permanent magnet that is free to rotate; its north pole is attracted towards Earth's magnetic south pole (geographic north), so the needle aligns along the Earth's field lines and points north (1)

When the bar magnet is brought close, the iron nail becomes an induced magnet; a south pole is induced in the end of the nail nearest the bar magnet's north pole (unlike poles attract), so the nail is attracted (1); iron is a magnetically soft material, meaning it magnetises easily but also loses its magnetism quickly when the external field is removed (1); when the bar magnet is taken away the nail is no longer in a magnetic field, so it demagnetises and falls (1)

Place the bar magnet on a large $pie$ce of paper and mark its outline; put the plotting compass close to the north pole and mark a dot at the tip of the needle (1); move the compass so its tail is at the previous dot and mark a new dot at the tip; repeat until you reach the south pole or edge of the paper; join the dots to form a smooth field line and draw an arrow showing the direction from north to south (1); lines spread out from the north pole and converge into the south pole; the lines are closest together at the poles (strongest field) and further apart in the middle (weaker field); field lines never cross each other (1)

A small current in the control circuit flows through a coil wound on a soft iron core, creating a magnetic field and magnetising the core (1); the magnetised core attracts a soft iron armature (a hinged metal strip), pulling it down and closing a separate set of contacts in the output circuit — this allows a large current to flow through the output circuit (1); when the control current is switched off the core demagnetises, the spring pulls the armature back up, and the output circuit opens (1); relays are useful when a low-voltage control circuit (such as a computer output or sensor) needs to switch a high-voltage or high-current circuit safely, keeping the two circuits electrically separate — for example, starting a car engine from the ignition switch (1)