Big question: how can an electric current make a magnet that we can switch on and off?
Make a prediction
A soft iron nail has 40 turns of wire round it. A current of 1 A lets it hold a few paper clips.
We make the current 2 A. The number of clips it holds will…
Switch on, count and test
Count the clips the end holds. Use a compass to find the N pole. Then reverse the current.
Ammeter—
Clips held—
Right end is—
Recorded readings
0 readings
| No. | N turns | I A | Core | Clips | Right end | N × I |
|---|
Leave N × I (ampere-turns) empty and work it out. Plot clips against N × I.
Why the coil is a magnet
A current makes a magnetic field. More turns (N), more current (I) and a soft iron core make it stronger.
Right-hand grip rule: curl your right-hand fingers the way the current flows round the coil. Your thumb points to the N pole. Reverse the current and the poles swap.
Worked solution: magnetising force H (7 steps)
Record readings to see your graph of clips against N × I.
Check yourself
Why is the core of an electromagnet made of soft iron?
Electromagnet challenge
Three tasks. Set up the electromagnet, then press Check my set-up.
Score: 0 / 3
Big question: how does a motor change electrical energy into movement, and how much is wasted?
Make a prediction
A DC motor runs from 12 V. We tighten the brake, so the motor has to work harder.
The current taken from the supply will…
Load the motor and measure
Switch on. Change the brake force. Read the voltmeter, ammeter, tachometer and spring balance.
Voltmeter—
Ammeter—
Tachometer—
Brake force—
Recorded readings
0 readings
| No. | V | I A | F N | N rev/min | P in W | P out W | η % |
|---|
Brake drum radius r = 0.025 m. Leave the last three columns empty and work them out.
Power in, power out
P in = V × I. Torque T = F × r. P out = 2π × N × T ÷ 60. Efficiency η = P out ÷ P in × 100%.
Fleming's left-hand rule: First finger = Field (N to S), seCond finger = Current, thuMb = Motion (force). The two sides of the coil are pushed opposite ways, so it turns. The commutator reverses the current every half turn, so it keeps turning the same way.
Worked solution: efficiency (7 steps)
Check yourself
What is the job of the split-ring commutator?
Calculate the efficiency
Read the meters, work out η, type it, then check. Three rounds.
Score: 0 / 3