Magnetic Effect of a Current and Electromagnets
A current produces a magnetic field. That one discovery links electricity and magnetism, and it underlies electromagnets, motors, generators and transformers.
1. The field around a straight wire
A current-carrying wire is surrounded by a magnetic field in the form of CONCENTRIC CIRCLES centred on the wire.
Properties:
- The circles are closer together near the wire, where the field is strongest
- The field gets weaker with distance
- Reversing the current reverses the field direction
- Increasing the current strengthens the field
The field lines are circles around the wire, not straight lines along it. Not understanding the concentric circle pattern was a recorded error — the field wraps around the wire.
The right-hand grip rule
Point the RIGHT thumb in the direction of the CONVENTIONAL CURRENT (positive to negative). Your curled fingers show the direction of the magnetic field.
Showing direction on a diagram:
- A dot (⊙) means current coming out of the page
- A cross (⊗) means current going into the page
A plotting compass placed near the wire points along the MAGNETIC FIELD, at a tangent to the circle — not along the current. This exact error was recorded. The compass aligns with the field, which is perpendicular to the wire, not parallel to it.
2. The field of a solenoid
A solenoid is a coil of wire. Its magnetic field is just like a bar magnet’s — with a north pole at one end and a south at the other.
Inside the solenoid the field is strong and uniform; outside it looks exactly like a bar magnet’s field.
Finding the poles: use the right-hand grip rule for a solenoid — curl the right hand’s fingers in the direction of the current around the coil, and the thumb points to the NORTH pole.
Reversing the current swaps the poles.
3. Electromagnets
An electromagnet is a solenoid with a soft iron core.
To increase the strength of an electromagnet:
- Increase the current
- Increase the number of turns on the coil
- Add or use a soft iron core
- Wind the turns closer together
Write “increase the number of turns”, not “add more coils” — the same precise wording that matters throughout electromagnetism.
Why soft iron?
Soft iron magnetises strongly when the current flows and LOSES its magnetism as soon as the current stops. That is exactly what makes an electromagnet switchable.
Steel would be the wrong choice — it retains magnetism, so the electromagnet could not be turned off. Misidentifying the material of a magnet was recorded: soft iron for electromagnets, hard steel for permanent magnets.
Advantages over a permanent magnet: it can be switched on and off, and its strength can be varied by changing the current.
4. Uses of electromagnets
Scrapyard crane — switched on to lift steel, off to release it.
The relay — a small current in one circuit switches on a much larger current in another:
- A small current flows in the coil, creating a magnetic field
- The electromagnet attracts an iron armature
- The armature pivots and closes the contacts of the second circuit
- When the small current stops, a spring returns the armature and the second circuit opens
This lets a safe, low-current switch control a high-current circuit — such as a car starter motor.
The circuit breaker — an excessive current makes the electromagnet strong enough to pull open a switch, breaking the circuit.
Electric bell — the electromagnet attracts an armature, which strikes a gong and simultaneously breaks its own circuit; the armature springs back, remaking the circuit, and the cycle repeats rapidly.
Loudspeaker — a varying current in a coil in a magnetic field makes the cone vibrate.
5. Comparing the rules
Three different rules appear in electromagnetism, and mixing them up is a reliable way to lose marks:
| Rule | Used for |
|---|---|
| Right-hand GRIP rule | field direction around a wire or solenoid |
| Fleming’s LEFT-hand rule | force on a current-carrying wire — motors |
| Fleming’s RIGHT-hand rule | induced current direction — generators |
Grip rule for fields; left hand for motors; right hand for generators.
6. Mistakes that cost marks
Drawing the field as straight lines along the wire instead of concentric circles.
Saying a compass points along the current.
Using the left hand for the grip rule.
Using electron flow instead of conventional current.
Saying “more coils” instead of “more turns”.
Using steel for an electromagnet core.
Not explaining why soft iron is chosen.
Confusing the grip rule with Fleming’s rules.
Getting the solenoid’s poles the wrong way round.
Frequently asked questions
What shape is the magnetic field around a wire? Concentric circles centred on the wire.
How do I find the field direction around a wire? The right-hand grip rule — thumb along the conventional current, fingers curl in the field direction.
Which way does a compass point near a wire? Along the magnetic field, tangential to the circles — not along the current.
What does a solenoid’s field look like? Like a bar magnet’s, with a north and a south pole.
How do I find the north pole of a solenoid? Curl the right hand in the direction of the current; the thumb points to the north pole.
What is an electromagnet? A solenoid with a soft iron core.
How can I make it stronger? More current, more turns, a soft iron core, turns wound closer.
Why soft iron and not steel? Soft iron loses its magnetism when the current stops, so the magnet can be switched off.
How does a relay work? A small current energises an electromagnet that closes the contacts of a high-current circuit.
What’s the difference between the grip rule and Fleming’s rules? The grip rule gives the field around a current; Fleming’s left hand gives the force in a motor; right hand gives the induced current in a generator.
Quick revision checklist
- I know the field around a wire is concentric circles
- I can use the right-hand grip rule
- I know dot means out of the page, cross means into it
- I know a compass points along the field, not the current
- I know a solenoid’s field resembles a bar magnet’s
- I can find a solenoid’s north pole
- I know reversing the current reverses the field
- I know what an electromagnet is
- I can list four ways to strengthen it, in the right words
- I know why the core is soft iron
- I can describe a relay, circuit breaker and electric bell
- I can tell the grip rule from Fleming’s two rules
These notes cover the magnetic effect of a current and electromagnets in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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