Edexcel IGCSE Physics 6(c) Electromagnetism — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1), Science Double Award (4SD0) and Science Single Award (4SS0), linear and modular.
Which parts do I need?
Single Award and Double Award candidates can skip every section tagged Physics only · 2P. On this page that is the construction of electromagnets, drawing the field patterns of a wire, a flat coil and a solenoid, and the force on a moving charged particle. Everyone needs the field around a current, the motor effect, the left-hand rule, the factors that change the force, and how motors and loudspeakers work.
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 6.8 | 7.8 | know that any conductor carrying a current has a magnetic field around it | Single · Double · Physics |
| 6.9P | 7.9P (4XPH1 only) | describe how an electromagnet is built | Physics only · 2P |
| 6.10P | 7.10P (4XPH1 only) | draw the field around a current in a straight wire, a flat circular coil and a solenoid | Physics only · 2P |
| 6.11P | 7.11P (4XPH1 only) | know that a charged particle moving across a magnetic field feels a force, but not if it moves along the field | Physics only · 2P |
| 6.12 | 7.12 | explain the force on a current-carrying wire in a magnetic field, and its use in d.c. motors and loudspeakers | Single · Double · Physics |
| 6.13 | 7.13 | predict the direction of that force with the left-hand rule | Single · Double · Physics |
| 6.14 | 7.14 | describe how the size and direction of the force depend on the current and the field | Single · Double · Physics |
Modular candidates: electromagnetism is Topic 7 in the modular specifications — statements 7.8–7.14 of 4XPH1 Unit 2 (4WPH2); 7.8 and 7.12–7.14 of 4XSD1 Physics Unit 6 (4WSD6).
In one minute
- A current in a wire produces a magnetic field around the wire.
- A current-carrying wire in a magnetic field feels a force (the motor effect) because its own field interacts with the magnet's field.
- Fleming's left-hand rule: First finger = Field (N to S), seCond finger = Current (+ to −), thuMb = Motion (force).
- A bigger current or a stronger field gives a bigger force. Reversing the current or the field reverses the force. A wire along the field feels no force.
- A d.c. motor uses the force on a coil; the split-ring commutator reverses the current every half turn so the coil keeps turning. A loudspeaker uses an alternating current so the coil and cone vibrate.
- Wires, flat coils and solenoids have characteristic field patterns; the right-hand grip rule gives the direction. Physics only · 2P
Does an electric current produce a magnetic field? Single · Double · Physics
- A plotting compass placed next to a wire carrying a current is deflected; when the current is switched off, the needle swings back to point north.
- Reversing the current reverses the direction of the field, so the compass needle points the opposite way.
- A larger current produces a stronger field. The field is strongest close to the wire.
- Unlike a permanent magnet, this field can be switched on and off and its strength controlled — the idea behind electromagnets, motors and loudspeakers.
What are the field patterns for a wire, a flat coil and a solenoid? Physics only · 2P
Physics only (4PH1 Paper 2P). Not required for Single or Double Award.
In diagrams a wire is often drawn end-on. ● (a dot, the tip of an arrow coming towards you) means current out of the page; × (the tail feathers of an arrow going away) means current into the page.
A long straight wire
- The lines are concentric circles centred on the wire, in a plane at right angles to it.
- The spacing increases with distance from the wire — the field is weaker further away.
- Current out of the page (●): field anticlockwise. Current into the page (×): field clockwise.
A flat circular coil
- Close to each side of the coil the lines are near-circles around the wire, in opposite senses on the two sides.
- Through the centre of the coil the fields from the two sides add, so the field there is stronger and the central line is straight, along the axis.
A solenoid (a long coil)
- Inside the solenoid the field is strong and almost uniform: straight, parallel, equally spaced lines along the axis.
- Outside it the field is weak and has the same shape as the field of a bar magnet. The end where the lines come out is a north pole.
- End rule: look at one end of the solenoid. If the current flows anticlockwise it is a N pole; if clockwise, an S pole. (Or grip the solenoid with your right hand, fingers along the current in the turns: your thumb points to the N end.)
A solenoid lies along a bench. Seen from end A, the current in the turns flows anticlockwise. (a) State which pole forms at end A. (b) A plotting compass is placed inside the solenoid, near its middle. State the direction in which its N end points. (c) The current is reversed. Describe what happens to the field.
(a) Anticlockwise when viewed from an end means that end is a north pole.
(b) Inside the solenoid the field runs from the S end to the N end, so the compass N end points towards end A.
(c) The field has the same shape and strength but every field line reverses direction: end A becomes a south pole.
Common error in (b): saying the needle points away from A because “field lines go from N to S”. That is true only outside the solenoid; inside, the lines go from S to N.
How is an electromagnet constructed? Physics only · 2P
Physics only (4PH1 Paper 2P). Not required for Single or Double Award.
- A coil (solenoid) of many turns of insulated copper wire. The insulation stops the current short-circuiting between turns, so it flows around every turn.
- A core of soft iron inside the coil. Iron is magnetised by the coil's field and makes the field much stronger. Because iron is magnetically soft, it loses its magnetism when the current is switched off.
- A d.c. supply and a switch to turn the current, and therefore the magnetism, on and off.
- To make an electromagnet stronger: increase the current, increase the number of turns, or add (or use a larger) iron core.
- A steel core would not do — steel is magnetically hard and would stay magnetised after the current was switched off.
Why is there a force on a current-carrying wire in a magnetic field? Single · Double · Physics
Place a wire carrying a current between the poles of a magnet and it is pushed sideways. This is the motor effect.
- The current produces its own magnetic field around the wire.
- This field interacts with the field of the permanent magnet. On one side of the wire the two fields are in the same direction and add (strong field); on the other side they are in opposite directions and partly cancel (weak field).
- The wire is pushed from the stronger field towards the weaker field. By Newton's third law the magnet feels an equal and opposite force.
Single and Double Award: you do not have to draw the wire's own field, but you must explain the force as an interaction between the field of the current and the field of the magnet.
How do you use Fleming's left-hand rule? Single · Double · Physics
- First finger → Field, from N to S.
- seCond finger → Current, conventional current from + to −.
- thuMb → Motion, the direction of the force on the wire.
- The rule applies when the current is at right angles to the field. Use your left hand — the right hand gives the opposite answer.
(a) A wire runs into the page between a N pole on the left and a S pole on the right. The current is into the page. Determine the direction of the force on the wire.
(b) A horizontal wire lies across the page with the current flowing to the right. A magnet's N pole is above the wire and its S pole is below. Determine the direction of the force.
(a) First finger to the right (N to S), second finger into the page: thumb points down the page.
(b) First finger down the page (N above, S below), second finger to the right: thumb points into the page.
Check (a) with the field-interaction picture above: it is the same arrangement, and the wire is pushed towards the weak-field side, downwards.
What changes the size and direction of the force? Single · Double · Physics
| Change | Effect on the force |
|---|---|
| increase the current | larger force |
| use a stronger magnetic field (stronger magnet, poles closer) | larger force |
| reverse the current | force reverses direction (same size) |
| reverse the field (swap the poles) | force reverses direction (same size) |
| reverse both the current and the field | force in the original direction |
| turn the wire so the current is parallel to the field | no force |
The force is greatest when the current is at right angles to the field and zero when the current runs along the field lines.
A short copper rod rests on two horizontal metal rails between the poles of a U-shaped magnet. When a current flows through the rod, it rolls along the rails towards the open end of the magnet. Describe three separate changes and their effect on the motion of the rod.
- Increase the current (raise the supply voltage) → larger force, so the rod accelerates more in the same direction.
- Reverse the connections to the supply → current reverses, so the rod rolls the other way, into the magnet.
- Turn the magnet over (swap N and S) → field reverses, so the rod also rolls the other way. Doing both changes together would make it roll towards the open end again.
How does a simple d.c. motor work? Single · Double · Physics
- A rectangular coil of wire on an axle sits in the magnetic field between two poles.
- Current flows in opposite directions along the two sides of the coil that are at right angles to the field.
- By the left-hand rule, one side is pushed up and the other down. The two forces form a turning effect, so the coil rotates.
- The split-ring commutator turns with the coil; carbon brushes press on it to carry the current in and out.
- Each half turn, when the coil is vertical, the gaps in the commutator pass the brushes and the current in the coil reverses. The side that was being pushed up now lies on the other side and is pushed down, so the coil keeps turning in the same direction. The coil's momentum carries it past the vertical position.
- Faster motor: larger current, more turns on the coil, stronger magnet.
- Reverse the rotation: reverse the current (swap the supply connections) or reverse the field (swap the magnets).
A student builds a motor but connects the ends of the coil to two complete metal rings (slip rings) instead of a split-ring commutator, with a d.c. supply. Explain why the coil does not turn continuously, and what it does instead.
- With complete rings the current in each side of the coil always flows the same way.
- So the force on each side is always in the same direction (for example, the left side is always pushed down).
- After the coil passes the vertical, these forces turn it back the way it came.
- The coil therefore rocks to and fro and settles in the vertical position. A split ring reverses the current each half turn so the forces keep turning the coil the same way.
How does a loudspeaker work? Single · Double · Physics
- A coil of wire is attached to a paper (or plastic) cone and sits in the field of a permanent magnet.
- An alternating current from the amplifier flows in the coil.
- The current is at right angles to the magnetic field, so there is a force on the coil (the motor effect).
- The current keeps changing direction, so the force keeps reversing: the coil and cone are pushed out and pulled in.
- The cone vibrates at the frequency of the a.c. and pushes the air, producing a sound wave of the same frequency.
- A larger current gives a larger force, a larger amplitude of vibration and a louder sound.
- A higher frequency of a.c. gives a higher pitch.
Is there a force on a charged particle moving in a magnetic field? Physics only · 2P
Physics only (4PH1 Paper 2P). Not required for Single or Double Award.
A current is a flow of charge, so a single charged particle moving through a magnetic field also feels a force.
- There is a force on a charged particle moving in a magnetic field as long as its motion is not parallel to the field. A particle moving along the field lines feels no force; a stationary particle feels no force.
- Use the left-hand rule with the second finger along the direction of conventional current: the direction of motion for a positive particle, and the opposite direction for a negative particle such as an electron.
- The force is at right angles to the motion, so the particle's path curves. Positive and negative particles curve opposite ways.
An electron moves to the right, across a magnetic field that points into the page. (a) Determine the direction of the force on the electron as it enters the field. (b) The electron is now fired so that it travels into the page, parallel to the field. State and explain what happens to its path.
(a) An electron moving right is a conventional current to the left. First finger into the page, second finger to the left: thumb points down the page. (A proton would be pushed up.)
(b) Moving parallel to the field there is no magnetic force, so the electron continues in a straight line at constant speed.
Exam tips for 6(c)
“Why is there a force?” is not answered by quoting the left-hand rule. Say that the current produces a magnetic field, and that this field interacts with the field of the magnet, producing a force.
For the motor, the mark is for “reverses the current in the coil every half turn”, followed by “so the coil keeps turning in the same direction”. “Stops the wires tangling” or “lets the coil spin” does not earn it.
For a beam of electrons, turn the second finger the opposite way to the electrons' motion before you apply the left-hand rule. Many candidates lose the mark by using the electron direction directly. Physics only · 2P
For a wire, draw at least three circles with increasing spacing and an arrow on each. For a solenoid, draw parallel lines inside and a bar-magnet shape outside, with arrows that form closed loops. Physics only · 2P
A complete loudspeaker answer links the chain: alternating current → force on the coil that changes direction → the cone vibrates → the air vibrates → sound. Missing out “changes direction” loses the key mark.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| The force on the wire is towards one of the poles. | The force is at right angles to both the field and the current. | Use the left-hand rule; the force is never along the field lines. |
| Reversing both the current and the field reverses the force. | Each reversal flips the force once; two flips cancel. | Reversing both leaves the force in its original direction. |
| The commutator changes a.c. to d.c. in a motor. | The supply to a d.c. motor is already d.c. | It reverses the current in the coil every half turn. |
| Outside and inside a solenoid the lines go from N to S. (Physics only) | Field lines are closed loops. | Outside: N to S. Inside: S to N. |
| Any charged particle in a magnetic field feels a force. (Physics only) | The force needs motion across the field. | No force on a stationary particle or one moving parallel to the field. |
Key facts
| Fact | Route |
|---|---|
| A current in a conductor produces a magnetic field around it; reversing the current reverses the field. | All |
| Straight wire: concentric circles, spacing increasing outwards; right-hand grip rule for direction. | Physics only · 2P |
| Solenoid: uniform field inside, bar-magnet pattern outside; anticlockwise current seen from an end = N pole. | Physics only · 2P |
| Electromagnet: many turns of insulated wire on a soft iron core; stronger with more current, more turns, iron core. | Physics only · 2P |
| Motor effect: the field of the current interacts with the magnet's field, giving a force. | All |
| Left-hand rule: First finger Field, seCond finger Current, thuMb Motion. | All |
| Bigger current or stronger field → bigger force; reversing current or field reverses the force; parallel → no force. | All |
| d.c. motor: forces on opposite sides of the coil turn it; split-ring commutator reverses the current every half turn. | All |
| Loudspeaker: a.c. in the coil → force that reverses → cone vibrates → sound. | All |
| A charged particle moving across (not along) a field feels a force; + and − particles curve opposite ways. | Physics only · 2P |
Check your understanding
1. A compass is placed next to a vertical wire. Describe what happens to the needle when a current is switched on, and then reversed. All
With the current on, the needle is deflected (it lines up with the magnetic field of the current). When the current is reversed, the field reverses and the needle points the opposite way.
2. State the quantity represented by each of the three digits in Fleming's left-hand rule. All
First finger: magnetic field (N to S). Second finger: current (conventional, + to −). Thumb: force / motion.
3. State three ways of making a d.c. motor turn faster. All
Increase the current; use more turns on the coil; use a stronger magnet.
4. A wire carrying a current is turned until it lies along the field lines between two poles. State the force on it. All
Zero — there is no force when the current is parallel to the field.
5. Explain why the cone of a loudspeaker vibrates. All
- The coil carries a current in the magnet's field, so there is a force on it (motor effect).
- The current is alternating, so the force keeps reversing direction.
- The coil, and the cone attached to it, are pushed in and out — they vibrate.
6. A beam of electrons travels up the page through a magnetic field that points out of the page. Determine the direction of the force on the electrons. Physics only · 2P
Conventional current is down the page. First finger out of the page, second finger down: thumb points to the left.
7. Describe how to construct an electromagnet that can be switched off to release the objects it is holding. Physics only · 2P
Wind many turns of insulated wire around a soft iron core; connect the coil to a d.c. supply through a switch. Soft iron loses its magnetism when the current is switched off, so the objects are released.
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