Electric motors · Grade 10
In Grade 10 you get fluent with: motors: electrical to kinetic energy; the motor effect and the direction of the force; the useful energy a motor gives a load.
A wire carrying a current in a magnetic field feels a force: the motor effect. In an electric motor, the forces on the two sides of a coil push in opposite directions and turn it, and a split-ring commutator keeps it turning the same way. Loudspeakers use the same effect to make a cone vibrate.
Key facts
- The motor effect: a wire carrying a current across a magnetic field feels a force. Fleming’s left-hand rule gives its direction.
- Motor: electrical → kinetic energy. Generator: kinetic → electrical energy.
Worked examples
Example 1
What energy transfer happens in an electric motor?
Answer: electrical → kinetic (plus some heat)
Example 2
How can you make a d.c. motor turn in the opposite direction?
Answer: reverse the current (or swap the magnet’s poles)
Example 3
How does a loudspeaker make sound?
Answer: An alternating current in a coil in a magnetic field makes the coil and cone vibrate
Example 4
A loudspeaker is fed a 440 Hz alternating current. How often does its cone vibrate?
Answer: 440 times a second
Example 5
A small electric motor lifts a 1 kg load through 4 m. How much useful energy does it transfer to the load? (g = 9.8 N/kg)
Answer: 39.2 J