Electric and magnetic fields Notes

How Can Electricity Create Magnetism?

1. Physicists often prefer a small number of universal laws rather than many separate rules for specific situations.
2. A major success of this approach was James Clerk Maxwell's theory of electromagnetism, which showed that electricity and magnetism are deeply linked.

The Magnetic Effect of an Electric Current

1. When an electric current flows through a wire, it creates a magnetic field around the wire.
2. This magnetic field exists only while the current is flowing.
3. If the current stops, the magnetic field disappears immediately.
4. This shows that electricity and magnetism are closely linked.

Shape of the Magnetic Field Around a Straight Wire

1. The magnetic field around a straight current-carrying wire forms concentric circles.
2. These circles are centred on the wire.
3. The magnetic field gets weaker as the distance from the wire increases.
4. There are no north or south poles around a straight wire.

The Right-Hand Grip Rule Gives The Field Direction

1. To find the direction of the magnetic field around a straight current-carrying wire:
   1. Point your right thumb in the direction of the conventional current $I$.
   2. Your curled fingers show the direction of the magnetic field $\mathbf{B}$.
2. If the current is towards you (out of the page), the field circles anticlockwise.
3. If the current is away from you (into the page), the field circles clockwise.

Solenoids (Coils of Wire) Make Stronger, Bar-Magnet-Like Fields

1. A single straight wire produces a circular field, but we can shape and strengthen magnetic fields by using coils.
2. A solenoid is a long coil of wire.
3. When current flows through a solenoid:
   1. A strong magnetic field is produced inside the coil.
   2. The field has a north and south end
   3. The field inside is uniform, meaning it has the same strength and direction.
4. Outside the solenoid, the magnetic field is weaker and more spread out.
5. The overall field pattern is similar to that of a bar magnet.

Electromagnets And How To Increase Their Strength

1. A practical way to build one is to wind insulated wire around an iron nail or rod.
2. You can increase the strength of an electromagnet by:
   1. Increasing the current through the coil
   2. Increasing the number of turns of wire
   3. Adding an iron core
3. The iron core greatly increases the field because its magnetic domains become more aligned when the coil's field is present.
4. This can make the magnet about a hundred times stronger than without the core.

Why Electromagnets Are Useful

1. Electromagnets are used when controlled magnetism is needed.
2. Common uses include:
   1. Lifting heavy metal objects
   2. Electric bells
   3. Relays and switches
3. Electromagnets are useful because:
   1. Their strength can be adjusted.
   2. They can be turned off instantly.

Linking Back To Unification: From Magnets To Music

1. Electromagnetism is not just an abstract unification of ideas.
2. It supports technologies for personal and cultural expression.
3. Microphones and loudspeakers rely on interactions between currents and magnetic fields to convert between electrical signals and sound.
4. In a typical loudspeaker, a current in a coil interacts with a magnetic field, producing forces that make the cone vibrate.
5. Those vibrations create pressure waves in the air that we perceive as sound.