Magnetic fields are places where magnets ‘feel’ a force.
• Magnetic field lines show the pattern of the magnetic field.
• Magnetic field lines point from North poles to South poles
Field pattern of a bar magnet
Field pattern of a straight wire
Field pattern of a plane coils
Field pattern of a solenoid
The motor effect
When a conducting wire moves inside a magnetic field OR a magnetic field changes
around a conducting wire, an electric current is induced inside the wire – this is called
electromagnetic induction. The size of the induced current depends upon the rate of
cutting of magnetic flux (field) lines.
• Simple a.c. electric generators work as a result of electromagnetic induction. The
output of the generator (current or voltage) is proportional to the speed of rotation
and the number of turns – and increases with the strength of the magnetic field.
• The direction of the induced current in a generator depends upon the direction of the
magnetic field and the direction of rotation of the coil. The direction of the induced
current can be determined by using the fingers on your right hand (sometimes called
Fleming’s Right Hand Rule).
The motor effect part 2
The size of the force can be increased by:
- Increasing the magnetic field strength;
- Increasing the length of the wire in the field;
- Increasing the current in the wire.
To work out the direction of force experienced we use Fleming's Left Hand Rule.
- Your first finger points in the direction of the magnetic field (North to South).
- Your second finger points in the direction of conventional current (positive to negative).
- Your thumb points in the direction of the thrust or force on the conductor.
If a wire is moved to cut accross lines of flux, then a current is induced in the wire. The current can be increased by:
- using a stronger magnetic field
- moving the wire faster
Simple A.C Generator
As the coil rotates, it cuts lines of force and so a current is induced. As the coil rotates, the current is conducted in and out by way of slip-rings and carbon brushes.
It can be increased by
- Using a coil with more turns
- Using a stronger magnet, or using a powerful electromagnet to make the field stronger, or winding the coil round a soft-iron core so that the field is stronger
- rotating the coil faster
- using a coil with a larer area
Transformers can change alternating current (a.c.) allowing it to be transmitted
around the country at low current (but high voltage) – this reduces the energy lost in
the National Grid. Transformers work by using electromagnetic induction; a changing magnetic field in
the primary coil induces an alternating current in the secondary coil. The output voltage of a transformer depends upon the number of turns on the coils.
For an ideal transformer (that is 100% efficient).
• Step-up transformers change low-voltage/high-current to high voltage/low-current
and step-down transformers do the reverse.
• Commercial transformers can be up to 98% efficient
- The primary coil is connected to an AC supply.
- An alternating current passes through a primary coil wrapped around a soft iron core.
- The changing current produces a changing magnetic field.
- This induces an alternating voltage in the secondary coil.
- This induces an alternating current (AC) in the circuit connected to the secondary coil.