The magnetic effect of a current and the motor
Introduced| English | Meaning |
|---|---|
| solenoid/ˈsəʊlənɔɪd/ | A coil of wire that produces a magnetic field when an electric current flows through it. |
| d.c. motor/ˌdiː ˈsiː ˈməʊtə/ | A current-carrying coil in a magnetic field that spins continuously using a split-ring commutator. |
| motor effect/ˈməʊtə ɪˈfekt/ | A wire carrying current in a field feels a force (left-hand rule). |
| left-hand rule/left hænd ruːl/ | left-hand rule |
| commutator/ˈkɒmjuːteɪtə/ | commutator |
Electricity makes magnetism
- A wire carrying a current creates a magnetic field around itself.
- Put that wire in another magnet's field and it feels a force — the idea behind every electric motor.
- This is where electricity and magnetism meet.
The magnetic effect of a current
- A current in a straight wire makes circular field lines around it; reverse the current and the field reverses.
- A solenoid 螺线管 (a long coil) makes a field just like a bar magnet, with a N and a S pole.
- Make it stronger by: more current, more turns, or a soft-iron core.
- Used in relays and loudspeakers.
A d.c. motor 直流电动机
The force that spins a motor
A current in a magnetic field is pushed at right angles to both — reverse the current or flip the magnet and it pushes the other way. That is what turns a motor.
The magnetic field of a solenoid (a long coil carrying current) looks like the field of:
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A solenoid produces a bar-magnet-like field, with a north pole at one end and a south at the other.
Which change makes an electromagnet stronger?
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More current, more turns, and a soft-iron core all make the field stronger.
The motor effect 电动机效应
- A current-carrying wire in a magnetic field feels a force — the motor effect.
- Reverse the current, or reverse the field, and the force reverses.
- A bigger current or stronger field gives a bigger force; the force is greatest when the wire is at right angles to the field.
- Find the direction with the left-hand rule 左手定则: thumb = force, first finger = field (N→S), second finger = current.
In the motor effect, reversing the current direction:
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Reversing either the current or the field reverses the force. Reversing both leaves it unchanged.
In Fleming's left-hand rule, the thumb points in the direction of the:
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Thumb = force, first finger = field, second finger = current — all at right angles.
The force on a current-carrying wire is greatest when the wire is at right angles to the magnetic field.
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The force is largest when the wire is perpendicular to the field, and zero when the wire lies along the field.
Match each idea to what it describes.
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Each item links the term to its correct meaning.
The d.c. motor
- A current-carrying coil in a field feels a turning effect: the forces on its two sides act in opposite directions.
- A split-ring commutator 换向器 swaps the current direction every half-turn, so the coil keeps spinning the same way.
- The turning effect is bigger with more turns, a bigger current, or a stronger field.
The motor effect on a current-carrying wire
What is the job of the split-ring commutator in a d.c. motor?
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Without the commutator the coil would stop after half a turn. Swapping the current each half-turn keeps it rotating the same way.
You've got it
- a current makes a magnetic field: circles round a wire, bar-magnet field for a solenoid
- stronger electromagnet: more current, more turns, soft-iron core
- motor effect: a wire carrying current in a field feels a force (left-hand rule)
- a d.c. motor spins; the split-ring commutator reverses the current each half-turn