Magnetic effect of a current
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Magnetic Field Around a Straight Wire
When current flows through a straight wire, a magnetic field is produced around it.
Oersted's Discovery
In 1819, Hans Oersted accidentally discovered the magnetic effect of an electric current. Holding a wire over a compass needle pointing N–S, he found the needle moved when the current was switched on, and moved in the opposite direction when the current was reversed.
A current-carrying wire deflects a nearby compass needle, showing that it produces a magnetic field
The magnetic field around a straight wire forms concentric circles centred on the wire
Right-Hand Screw Rule (for a straight wire)
If a right-handed screw moves forwards in the direction of the current (conventional), the direction it rotates gives the direction of the magnetic field.
- Point thumb in direction of current
- Fingers curl in direction of magnetic field
Properties of the Field
- Field lines are concentric circles around the wire
- Field is stronger closer to the wire (lines closer together)
- Field strength increases with larger current
- Reversing current reverses field direction
Magnetic Field Around a Solenoid
A solenoid is a coil of wire. When current flows through it, the field pattern resembles that of a bar magnet.
Inside a solenoid the magnetic field is strong and uniform; outside it resembles the field of a bar magnet
Right-Hand Grip Rule (for a solenoid)
- Curl fingers in direction of current
- Thumb points to N pole of the solenoid
Factors Affecting Solenoid Field Strength
- Increasing current → stronger field
- Increasing number of turns → stronger field
- Adding a soft iron core → much stronger field (electromagnet)
- Reversing current → reverses polarity (N and S swap)
Experiment: Investigating Magnetic Fields
Method 1: Plotting Compasses
- Place plotting compass near current-carrying wire/solenoid
- Mark position of compass needle
- Move compass and repeat to trace field pattern
- Add arrows showing N pole direction
Method 2: Iron Filings
- Place card around wire/solenoid
- Sprinkle iron filings on card
- Tap gently – filings align with field
- Note: cannot show direction, only pattern
Common Mistakes
- ❌ Mixing up the straight-wire rule and the solenoid rule
- ✓ the right-hand screw rule (straight wire): thumb = current, fingers curl = field. The right-hand grip rule (solenoid): fingers curl = current, thumb = N pole — the roles of thumb and fingers swap between the two
Applications: Relays
A relay is an electromagnetic switch that allows a small current to control a larger current in a separate circuit.
A small control current energises the electromagnet, which closes the contacts to switch on the larger main current
How a Relay Works
- Small current flows through electromagnet coil
- Electromagnet becomes magnetised
- Iron armature is attracted to electromagnet
- Armature movement closes contacts in main circuit
- Large current can now flow in main circuit
Pull-on and Drop-off Current
The current needed to operate (energise) a relay is called the pull-on current. The drop-off current is the smaller current in the coil at which the relay just stops working.
Worked Example: Relay Pull-on Current
A relay has a coil resistance of 185 Ω and an operating p.d. of 12 V. Find the pull-on current.
- Step 1: write down the equation. I = (V/R)
- Step 2: substitute the values. I = (12/185)
- Step 3: calculate the answer. I = 0.065 A = 65 mA
Uses of Relays
- Car starter motor: Small current from ignition switch controls large starter current
- Industrial machines: Safe control of high-power equipment
- Home automation: Low-voltage switches controlling mains appliances
Applications: Loudspeakers
A loudspeaker converts electrical signals into sound using the magnetic effect of current.
An alternating current in the coil makes it move back and forth in the magnetic field, vibrating the cone to produce sound
How a Loudspeaker Works
- A.c. signal from amplifier passes through voice coil
- Coil experiences force in permanent magnet's field
- Direction of force alternates with a.c. signal
- Coil (and attached cone) vibrates back and forth
- Cone vibrations create sound waves in air
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