Magnetic Effect of a Current, the Motor Effect and the d.c. Motor
PhysicsΒ· 4.5.3, 4.5.4, 4.5.5Β· 25 min read
1. 1. Magnetic Field from Current-Carrying Conductors (Core)β β ββββ± 6 min
When electric current flows through a conductor, a magnetic field is produced around it. This is called the magnetic effect of a current, the basis for electromagnets, motors and generators.
Magnetic Effect of a Current
The production of a magnetic field around any conductor carrying an electric current.
For a straight current-carrying wire, magnetic field lines form concentric circles perpendicular to the wire, centered on the wire. The direction of these field lines is found using the right-hand grip rule: grip the wire with your right hand, point your thumb in the direction of conventional current (positive to negative), and your curled fingers point in the direction of the magnetic field lines.
A straight vertical wire carries conventional current upwards. What is the direction of the magnetic field directly to the right of the wire, when viewed from above?
- 1
Grip the wire with your right hand, thumb pointing upwards to match the conventional current direction.
- 2
Your curled fingers move anticlockwise around the wire when viewed from above.
- 3
At the point directly to the right of the wire, the field is a tangent to this anticlockwise circle, so it points towards the top of the page. The field circles the wire in the plane of the page (perpendicular to the wire), so it is never directed into or out of the page here.
A solenoid is a long coil of insulated wire. Its magnetic field is identical to a bar magnet: strong and uniform inside the coil, with a north and south pole at the ends. Use the right-hand grip rule for solenoids: grip the solenoid with your right hand, curl your fingers in the direction of conventional current around the coils, and your thumb points to the north pole end of the solenoid.
You look at one end of a solenoid, and conventional current flows clockwise around that end. Is this end a north or south pole?
Reveal answer
South pole βIf your fingers curl clockwise to match the current direction, your thumb points away from you, so the end you are viewing is the south pole.
2. 2. The Motor Effect (Core)β β β βββ± 6 min
When a current-carrying conductor is placed in an external magnetic field, the field of the current interacts with the external field to produce a force on the conductor. This is called the motor effect, the principle behind all electric motors.
Motor Effect
The force exerted on a current-carrying conductor when it is placed in a magnetic field that is not parallel to the direction of the current.
A simple experiment to demonstrate this uses a stiff copper wire, or a strip of foil, resting between the poles of a magnet and connected to a d.c. supply. When the current is switched on, the wire is pushed out of the magnetic field. This shows that a current-carrying conductor in a magnetic field experiences a force.
Reversing the direction of the current, or reversing the direction of the magnetic field, reverses the direction of the force. Reversing both at the same time leaves the direction of the force unchanged.
In an experiment, a current-carrying wire between the poles of a magnet is pushed downwards when the current is switched on. State the direction of the force if (a) only the current is reversed, (b) both the current direction and the magnetic field direction are reversed.
- 1
(a) Reversing one quantity reverses the force, so the wire is now pushed upwards.
- 2
(b) Reversing both the current and the field reverses the force twice, so the two changes cancel and the force is still downwards.
Exam tip:
For Core you only need to show that a force acts and describe how reversing the current or the field reverses it. Finding the exact direction with Fleming's left-hand rule is Extended (Supplement) content.
3. 3. The Motor Effect: Force Direction and Size (Extended Only)β β β β βExtended onlyβ± 6 min
For Extended tier you need the relative directions of force, magnetic field and current, and the qualitative factors that change the size of the force. The force is maximum when the current and magnetic field are perpendicular to each other, and zero when they are parallel.
Fleming's Left-Hand Rule
Stretch the thumb, first finger and second finger of your left hand so they are all at right angles to each other. The first finger points in the direction of the magnetic field (north to south), the second finger points in the direction of conventional current, and the thumb points in the direction of the force (motion) on the conductor.
A horizontal wire carries conventional current from left to right, placed in a magnetic field pointing vertically upwards. Use Fleming's left-hand rule to find the direction of the force on the wire.
- 1
Hold your left hand so your first finger points upwards, matching the magnetic field direction.
- 2
Turn your hand so your second finger points to the right, matching the conventional current direction.
- 3
Your thumb points out of the plane of the page, which is the direction of the force on the wire.
The size of the force increases if the current in the conductor increases, or if the strength of the external magnetic field increases. These are qualitative relationships only.
Exam tip:
Always use your LEFT hand for the motor-effect force direction, and your RIGHT hand for the magnetic field produced by a current. Mixing up the hands is one of the most common marks lost on this topic.
4. 4. The d.c. Motor (Core + Extended)β β β βββ± 7 min
A d.c. motor converts electrical energy from a direct current supply into kinetic (movement) energy, using the motor effect. Core tier requires you to name its components and describe its basic operation: it consists of an insulated copper coil mounted on an axle, placed between the poles of a permanent magnet, connected to a d.c. power supply via carbon brushes and a split-ring commutator.
When current flows through the coil, each side of the coil experiences a force in the opposite direction due to the motor effect, causing the coil to rotate around the axle.
Extended Only: The split-ring commutator is a copper ring split into two insulated halves, attached to the axle. As the coil rotates past the vertical position, the commutator swaps its connections to the two carbon brushes, reversing the direction of current in the coil every half turn. This reverses the direction of the forces on each side of the coil, so the coil continues to rotate in the same direction instead of stopping at the vertical position.
Explain why a d.c. motor stops rotating if the split-ring commutator is replaced with a solid continuous metal ring (Extended).
- 1
A solid continuous ring does not reverse the current in the coil when it passes the vertical position.
- 2
The forces on the two sides of the coil will reverse direction once the coil is past vertical, pulling it back towards the vertical position instead of continuing rotation.
- 3
The coil will oscillate around the vertical position and eventually stop due to friction.
5. 5. Applications: Relays and Loudspeakers (Core)β β ββββ± 6 min
The magnetic effect of a current is used in many everyday devices. Two that you must be able to describe for Core are the relay and the loudspeaker.
Relay
A switch that uses a small current in one circuit to switch on a separate circuit that carries a much larger current.
In a relay, a small current flows through a coil wound on a soft-iron core, turning it into an electromagnet. The electromagnet attracts a soft-iron armature, which pivots and closes (or opens) a pair of contacts in a second, separate circuit. Because a small current controls a much larger one, relays are used in car starter motors and to let low-power electronic circuits switch mains-powered appliances safely.
In a loudspeaker, a coil is attached to a paper or plastic cone and sits in the field of a permanent magnet. A varying (alternating) current from the amplifier passes through the coil, so by the motor effect the coil experiences a force that changes size and direction in step with the signal. The coil and cone vibrate back and forth, pushing on the air to produce sound waves. A larger current gives a larger force and a louder sound, and a higher signal frequency gives a higher-pitched sound.
Why is a relay useful when a low-power circuit needs to switch on a high-power device?
Reveal answer
A small current in the relay coil switches a separate circuit that carries a much larger current, so the two circuits stay isolated. βThe relay coil only needs a small current to make the electromagnet close the contacts of the high-current circuit, keeping the low-power control circuit safe and separate.
6. Common Pitfalls
Wrong move:
Using the right hand for Fleming's motor effect rule
Why:
The right hand is used for magnetic field direction from current, while the left hand is used exclusively for motor effect force direction
Correct move:
Always use your LEFT hand for motor effect force direction questions, and right hand for magnetic field direction questions
Wrong move:
Stating magnetic field lines around a straight wire are parallel to the wire
Why:
Magnetic field lines around a straight current-carrying wire are concentric circles perpendicular to the wire
Correct move:
Visualize field lines as circles around the wire, with direction given by the right-hand grip rule
Wrong move:
Claiming the split-ring commutator reverses the direction of the supply current
Why:
The commutator only reverses the direction of current in the coil, not the supply current
Correct move:
Explain that the commutator swaps connections to the coil every half turn, reversing coil current while the supply current direction stays constant
Wrong move:
Assuming force is produced when current is parallel to the magnetic field
Why:
The interaction between the two magnetic fields only produces a force when they are perpendicular to each other
Correct move:
State that force is maximum when current and field are perpendicular, and zero when they are parallel
Wrong move:
Identifying the wrong pole of a solenoid by reversing the right-hand grip direction
Why:
Fingers must curl exactly in the direction of conventional current flow around the solenoid coils
Correct move:
Curl your fingers to match the direction of current around the solenoid, your thumb points to the north pole end of the solenoid
7. Quick Reference Cheatsheet
Concept | Core Requirement | Extended Requirement |
|---|---|---|
Magnetic effect of current | Describe fields around straight wire and solenoid; use right-hand grip rule; describe uses in relays and loudspeakers | Same as Core |
Motor effect | Show a force acts on a current-carrying conductor; reversing the current or the field reverses the force | Use Fleming's left-hand rule for force direction; describe qualitatively how the current and field strength affect force size |
d.c. Motor | Name components; describe basic rotation operation | Explain split-ring commutator function; describe how changing current/field affects motor speed |
8. Frequently Asked
Which hand rules do I need, and for which tier?
Core tier requires the right-hand grip rule for the direction of the magnetic field around a current-carrying wire or solenoid. Fleming's left-hand rule for the direction of the motor-effect force is Extended-tier (Supplement) content.
Do I have to calculate force size for the motor effect?
No, numerical calculations using are not required for 0625. Extended tier only needs to describe qualitatively how the force changes when the current or the magnetic field strength changes.
Going deeper
What's Next
Now that you have mastered the magnetic effect of current and d.c. motor operation, you are ready to move on to electromagnetic induction, the reverse process that converts kinetic energy to electrical energy, which is the foundation of generators and transformers. These topics are also high-weighted in the CIE IGCSE Physics 0625 exam, and build directly on the hand rules and magnetic field principles you learned here. Make sure you practice applying Fleming's rules to different orientation questions, as these are common in both structured and multiple-choice exam papers.