Electromagnetism: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 20 · Read time: ~9 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Magnetic effect of a current
A current in a wire produces a magnetic field. Around a straight wire, the field lines are concentric circles, closer together near the wire. Use the right-hand grip rule for the direction.
A solenoid produces a field like a bar magnet's, strong and uniform inside the coil.
Increasing the current makes the field stronger. Reversing the current reverses the field direction.
An electromagnet is a solenoid with a soft iron core. It can be switched on and off, which is how a circuit breaker works: too large a current makes the electromagnet strong enough to pull a switch open and cut the circuit.
Trap: Reversing the current reverses the poles of an electromagnet. Reapply the grip rule with the new current direction instead of assuming the poles stay the same.
Force on a current-carrying conductor
A current-carrying wire placed across a magnetic field experiences a force. So does a beam of charged particles moving across a field.
Fleming's left-hand rule gives the direction when field and current are at right angles: First finger = Field (N to S), seCond finger = Current (conventional), thuMb = Motion (force).
Reversing either the current or the field reverses the force. Reversing both leaves it unchanged.
Trick: For a beam of electrons, the conventional current points the **opposite** way to the electrons' motion. Point your second finger against the beam before applying the rule.
The d.c. motor
A current-carrying coil in a magnetic field has forces in opposite directions on its two sides, producing a turning effect.
The turning effect increases with more turns on the coil and a larger current (and a stronger magnet).
A split-ring commutator reverses the current in the coil every half turn, so the coil keeps turning in the same direction.
Winding the coil on a soft iron cylinder concentrates the magnetic field, making the motor stronger.
Trap: The commutator does not reverse the direction of rotation. It reverses the current in the coil so that the rotation keeps going the same way.
Worked Example
A horizontal wire carries a current from left to right. It lies between the poles of a magnet, with the field pointing into the page.
(a) Use Fleming's left-hand rule to find the direction of the force on the wire. [2]
First finger (field): into the page. Second finger (current): left to right.
Thumb (force): upwards.
(b) State two changes that would each reverse the direction of the force. [2]
Reverse the direction of the current.
Reverse the direction of the magnetic field, by swapping the poles.
(c) The wire is replaced by a beam of electrons moving from left to right. State the direction of the force on the beam. [1]
Conventional current is now right to left, so the force is downwards.
Why this question is a good test of the topic: Fleming's rule marks are usually lost by using the wrong hand or mixing up the fingers. Part (c) adds the electron beam twist, where conventional current runs opposite to the electrons.
Frequently Asked Questions
What is the right-hand grip rule used for?
Finding the direction of the magnetic field around a straight wire or solenoid. Thumb points along the current, fingers curl in the direction of the field.
What is Fleming's left-hand rule used for?
Finding the direction of the force on a current-carrying conductor, or a beam of charged particles, in a magnetic field.
How can the turning effect on a motor coil be increased?
Increase the number of turns on the coil, increase the current, or use a stronger magnet.
What does the split-ring commutator do?
It reverses the current in the coil every half turn, so the coil keeps rotating in the same direction.
How does a circuit breaker use electromagnetism?
A large current makes an electromagnet strong enough to pull a switch open, cutting off the current.
Fleming's left-hand rule still confusing?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.