Lesson focus: Electromagnets convert controlled current into forces in relayA switch worked by an electromagnet, so a small current can turn on a bigger one., bells, loudspeakerA device that turns electrical signals into sound using a magnet and a coil., motors and lifting systems.
Trace input, field, force and output through electromagnetic devices and evaluate design choices.
Build on earlier learning
ElectromagnetsThe previous lesson established how to control an electromagnet. This lesson turns controlled fields into useful forces and motion.
You will be successful when you can…
- Explain a relay sequence.
- Describe the motor effect.
- Explain loudspeaker vibration.
- Evaluate safety and control advantages.
Words that carry the science
- relay
- An electrically operated switch that lets one circuit control another.
- motor effect
- A force on a current-carrying conductor in a magnetic field.
- armature
- A moving magnetic part attracted by an electromagnet.
- loudspeaker
- A device using a varying current and magnetic force to produce vibrations and sound.
- electric motor
- A device that uses magnetic forces on currents to produce rotation.
- electromagnetic induction
- Production of a potential difference when magnetic flux linkage changes; distinct from the motor effect.
A relay uses a small control current
01A relay uses a small control current
When current flows in the relayA switch worked by an electromagnet, so a small current can turn on a bigger one. coil, its soft-iron core magnetises and attracts an armatureThe moving metal part that an electromagnet pulls, such as the part that closes a switch in a relay.. The armature moves contacts that open or close a separate circuit.
The control and load circuits can use different currents and voltages. Electrical isolation and suitable ratings allow a low-power sensor circuit to control a larger load safely.
02A current in a magnetic field experiences force
A current-carrying conductor placed across a magnetic field experiences the motor effectWhen a wire carrying a current in a magnetic field feels a force and moves.. Reversing either the current or the magnetic field reverses the force; reversing both leaves the force direction unchanged.
In a motor, forces on opposite sides of a coil produce a turning effect. A split-ring commutator reverses coil current every half-turn so the turning effect continues in the same rotational sense.
03Devices match field control to an output
In a loudspeakerA device that turns electrical signals into sound using a magnet and a coil., an alternating current in a voice coil creates a changing force in a permanent-magnet field. The coil and cone vibrate, transferring energy to sound waves.
Scrapyard cranes use switchable electromagnets; doorbells use repeated make-and-break motion; MRI scanners use very strong controlled magnetic fields but require specialised superconducting systems and strict projectile safety. Device explanations should trace input → field → force → output.
Strength of an electromagnet
A current in a coil of wire makes a magnetic field. More turns, a bigger current or an iron core make the electromagnet stronger.
Controls
Open the switch and the clips drop: an electromagnet can be turned off.
Trace a relay action
A temperature sensor supplies a small current when a greenhouse overheats.
- Sensor current flows through the relay coil.
- The coil and soft-iron core produce a magnetic field.
- The field attracts the armature and closes the fan-circuit contacts.
- The separate higher-current fan circuit runs until the control condition changes.
How to make the reasoning visible: The relay couples circuits mechanically through magnetic force while keeping their electrical paths separate.
Match devices to electromagnetic sequences
Complete each explanation using input, field, force and output.
| Device | Electrical input | Magnetic action | Useful output |
|---|---|---|---|
| relay | small direct current | core attracts armature | switches separate circuit |
| loudspeaker | varying current | changing force on voice coil | cone vibration and sound |
| motor | current in rotating coil | opposite forces form a turning effect | rotation |
| scrapyard crane | large controlled current | core strongly magnetises | lifts and releases steel |
| doorbell | current switched repeatedly | armature attracts and releases | hammer strikes bell |
Why does a loudspeaker need a varying current?
Why is soft iron useful in a scrapyard magnet?
Check your understanding
Answer from memory, then check against the suggested answer.
What does a relay do?
What is the motor effect?
What happens if current direction reverses?
A loudspeaker cone moves in only one direction and stays there. What input fault is plausible?
Why does reversing both current and magnetic field leave motor force direction unchanged?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A loudspeaker converts an electrical signal into sound.
Explain why the coil in a loudspeaker moves.
(3)Explain how this produces sound.
(3)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., write the causal chain from changing current to changing force, cone motion and sound.