The magnetic effect of a current and the motor effect
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Electricity makes magnetism
- A current has a field
- The direction
Read the transcript of the teaching part
Last time
Before we start: last time, in Induction, generators, transformers and the oscilloscope, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Electricity makes magnetism
In 1820, a Danish teacher noticed that a compass needle twitched whenever a wire nearby carried a current. That tiny twitch showed that electricity and magnetism are connected. It led to electromagnets, electric motors, loudspeakers and everything in a modern power station.
A current has a field
A wire carrying a current has a magnetic field around it. The field lines are concentric circles, centred on the wire. The field is strongest close to the wire and gets weaker with distance, so the circles get further apart. Reverse the current, and the field direction reverses.
The direction
Imagine gripping the wire with your right hand, with your thumb pointing in the direction of the conventional current. Your fingers curl in the direction of the magnetic field lines. In the diagram, a current coming out of the page gives field lines running anticlockwise.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Force on a current-carrying conductor · Magnetic effect of a current · Force on a current-carrying conductor.
The whole chapter: Electromagnetic Effects, Electromagnetic Effects.
Chapter 4.5: Electromagnetic effects
- The magnetic effect of a current and the motor effect 5 min
- The d.c. motor, relays and loudspeakers 5 min
- Electromagnetic induction and the a.c. generator 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.