How does a current make a magnetic field, and how does the motor effect turn a coil?
The magnetic field around a current-carrying wire and a solenoid, electromagnets and what affects their strength, the motor effect, Fleming's left-hand rule, and the electric motor.
A focused answer to OCR Gateway GCSE Physics A topic P4 on electromagnetism, covering the magnetic field around a current-carrying wire and a solenoid, electromagnets and their strength, the motor effect, Fleming's left-hand rule, and how an electric motor works.
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What this topic is asking
OCR wants you to describe the magnetic field around a current-carrying wire and a solenoid, explain electromagnets and what affects their strength, and explain the motor effect, Fleming's left-hand rule and the electric motor. This is topic P4.2 of the OCR Gateway Physics A (J249) specification.
The field around a wire and a solenoid
So the simple straight wire gives a circular field, while coiling the wire into a solenoid concentrates the field and gives it clear poles, which is far more useful.
Electromagnets
Soft iron is used for the core because it magnetises strongly when the current is on but loses its magnetism when the current is off, so the electromagnet can be controlled. Electromagnets are used in scrapyard cranes, electric bells, relays and loudspeakers.
The motor effect
The direction of the force is found with Fleming's left-hand rule: hold the thumb, first finger and second finger of the left hand at right angles, point the First finger along the Field (north to south), the seCond finger along the Current (conventional, positive to negative), and the thuMb points in the direction of the Motion (force). Reversing either the current or the field reverses the force.
The electric motor
In an electric motor, a coil carrying a current sits in a magnetic field. By the motor effect, the two sides of the coil feel forces in opposite directions (because the current flows opposite ways on each side), which makes the coil turn. A device called a split-ring commutator swaps the current direction in the coil every half turn, so the coil keeps rotating the same way instead of stopping. This is how electrical energy is turned into rotation in motors, from fans to electric cars.
Try this
Q1. State two ways to increase the strength of an electromagnet. [2 marks]
- Cue. Increase the current, increase the number of turns, or add an iron core (any two).
Q2. State which hand and which finger represents the magnetic field in Fleming's rule for the motor effect. [1 mark]
- Cue. The left hand; the first finger represents the field.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDescribe the magnetic field produced by a current flowing through a solenoid (a coil of wire), and state two ways to increase the strength of an electromagnet made from such a coil.Show worked answer →
A P4 question worth four marks. A solenoid produces a magnetic field like that of a bar magnet: the field outside has a north and a south pole, while inside the coil the field is strong and uniform (field lines parallel and evenly spaced) (2 marks for the bar-magnet-like field and the strong uniform field inside). To increase the strength of the electromagnet you could: increase the current, increase the number of turns on the coil, or add an iron core (1 mark each, two needed). Markers reward the bar-magnet field with a uniform interior and any two valid ways to strengthen it. A common error is to describe a single straight wire's circular field instead of the solenoid's.
OCR 20214 marksA current-carrying wire is placed at right angles to a magnetic field and experiences a force. State the name of this effect, describe how to use Fleming's left-hand rule to find the direction of the force, and state two ways to reverse the direction of the force.Show worked answer →
A P4 question worth four marks. This is the motor effect (1 mark). To use Fleming's left-hand rule, point the first finger in the direction of the magnetic field (north to south), the second finger in the direction of the conventional current, and the thumb then points in the direction of the force (motion) (2 marks for the correct assignment of field, current and force to the fingers and thumb). To reverse the force you can reverse the current or reverse the magnetic field (1 mark). Markers reward naming the motor effect, the correct Fleming's left-hand rule assignment, and reversing either the current or the field.
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