Currents and Magnetic Fields Revision Notes | GCSE AQA Higher Physics

1. The Hidden Force Around Wires

When you plug in your phone charger, something invisible happens around the wire. Current flows through the cable, and this moving charge creates a magnetic field in the space around it. You can't see this field, but it's there — and it's powerful enough to deflect compass needles and attract iron filings.

This discovery changed everything. Before 1820, scientists thought electricity and magnetism were completely separate forces. Then Hans Christian Oersted noticed his compass needle twitching near a current-carrying wire during a lecture. That moment revealed the deep connection between electricity and magnetism.

Understanding this magnetic effect isn't just academic curiosity. Every electric motor in your home — from washing machines to computer fans — relies on the magnetic fields created by currents. The same principle powers MRI scanners, maglev trains, and the generators that produce our electricity.

2. Demonstrating the Magnetic Effect

The magnetic effect of current can be demonstrated using surprisingly simple equipment. You need a straight wire, a battery, some iron filings or small compass needles, and a piece of card. When you pass current through the wire and sprinkle iron filings on the card around it, the filings arrange themselves in circular patterns.

The iron filings reveal the invisible magnetic field lines. Each filing acts like a tiny compass needle, aligning itself with the magnetic field direction. The result is a clear pattern of concentric circles centred on the wire. This visual proof shows that current creates magnetism.

A more sensitive demonstration uses plotting compasses placed at different positions around the wire. When you switch on the current, every compass needle swings to point along the circular field lines. Switch off the current, and the needles return to pointing north. This proves the magnetic field appears and disappears with the current.

3. Key Definitions

4. Magnetic Field Patterns

The magnetic field around a straight current-carrying wire forms concentric circles. These circles get further apart as you move away from the wire, showing that the field strength decreases with distance. The field lines have no beginning or end — they form complete loops around the wire.

To determine which way the field lines point, use the right-hand rule. Point your right thumb along the wire in the direction of conventional current flow (positive to negative). Your fingers naturally curl in the direction of the magnetic field lines. This rule works for any straight conductor.

When you coil the wire into a solenoid, something remarkable happens. The circular field lines from each turn of wire combine and strengthen each other. Inside the coil, the field lines run parallel from one end to the other, creating a strong, uniform field. Outside the coil, the field pattern looks exactly like a bar magnet.

The solenoid has a definite north pole (where field lines emerge) and south pole (where field lines enter). You can identify the poles using the right-hand rule: curl your fingers in the direction of current flow around the coil, and your thumb points toward the north pole.

5. Factors Affecting Field Strength

The strength of the magnetic field around a current-carrying wire depends on two key factors: the size of the current and your distance from the wire. Double the current, and you double the field strength at any given point. This makes sense — more moving charges create a stronger magnetic effect.

Distance matters too, but in a specific way. As you move away from the wire, the field strength decreases. The field lines spread out over larger circles, so the field becomes weaker. This is why the iron filings cluster more densely near the wire and become more sparse further away.

For a solenoid, additional factors come into play. More turns of wire mean more field lines adding together, creating a stronger field. The field strength also depends on what's inside the coil — an iron core can increase the field strength by hundreds of times compared to an air core.

6. Practical Applications

Electromagnets are everywhere in modern technology. The solenoid in your car's starter motor uses the magnetic field from current to pull a metal plunger, engaging the starter gear. Loudspeakers contain a coil that moves back and forth in a magnetic field, converting electrical signals into sound waves.

MRI scanners use powerful electromagnets to create magnetic fields thousands of times stronger than Earth's magnetic field. These fields align hydrogen atoms in your body, allowing doctors to create detailed images of internal structures. The same principle helps maglev trains float above their tracks, eliminating friction for incredibly smooth, fast travel.

Even your doorbell relies on electromagnetic principles. When you press the button, current flows through a solenoid, creating a magnetic field that pulls a striker against the bell. Release the button, the current stops, the magnetic field disappears, and a spring pulls the striker back to its original position.

7. Exam Technique and Common Mistakes

When drawing magnetic field patterns, examiners look for specific details. Field lines must be smooth curves with arrowheads showing direction. For a straight wire, draw concentric circles with the wire at the centre. For a solenoid, draw field lines emerging from the north pole and entering the south pole, with parallel lines inside the coil.

Many students confuse the right-hand rule directions. Remember: thumb follows conventional current (positive to negative), fingers curl in the field direction. Don't use your left hand — that's for the motor effect, which we'll study later. Practice the rule with different wire orientations until it becomes automatic.

In calculation questions, watch out for unit traps. Current might be given in milliamps (mA) when you need amps (A) for your formula. Distance could be in centimetres when you need metres. Always check units before substituting values, and include units in your final answer.

Worked example: Worked Method: Currents and Magnetic Fields

Use the relevant formula, substitute values with units, calculate, then state the final answer clearly.

1. Write down the correct physics formula. — Write down the correct physics formula.
2. Substitute known values with units. — Substitute known values with units.
3. Rearrange if needed and calculate carefully. — Rearrange if needed and calculate carefully.
4. Give the final answer with correct unit and sensible precision. — Give the final answer with correct unit and sensible precision.

Answer: Final answer should include unit and appropriate significant figures.