Electromagnets and motors · Grade 7
An electric current produces a magnetic field. Coil the wire around an iron core and you get an electromagnet you can switch on and off. Put a current-carrying wire in a magnetic field and it feels a force – the motor effect, which drives motors, loudspeakers and hard-drive heads.
Key facts
- A straight wire’s field is circles around it; a coil (solenoid) makes a field like a bar magnet.
- An electromagnet is stronger with more current, more turns, and a soft-iron core.
- F = BIL (force on a wire at right angles to the field; B in tesla). At angle θ: F = BIL sin θ.
- Fleming’s left-hand rule: First finger = Field, seCond finger = Current, thuMb = Motion (force).
- No force acts when the wire is parallel to the field.
- A charge moving across a field feels F = Bqv; a solenoid’s field is B = μ₀NI ÷ L.
Worked examples
Example 1
A wire with 5 cm of its length in a magnetic field is held over a balance. When 3.0 A flows, the reading changes by 0.92 g. Find B. (g = 9.8 N/kg)
Answer: about 0.060 T