Electric motors

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Lesson notes

Construction of an Electric Motor

An electric motor consists of a **coil of wire** (armature) placed inside a **magnetic field** produced by two permanent magnets.
A **battery** supplies current to the coil.
The **brushes** (made of carbon) rub against the split ring commutator, allowing the coil to rotate.

How the Motor Works

When current flows through a wire, it creates its own **magnetic field**.
This magnetic field interacts with the permanent magnetic field, producing a **force** on the coil.
On the left side of the coil, the interaction causes an **upwards force**; on the right side, a **downwards force** (because current direction changes).
These forces cause the coil to **rotate**.

Role of the Split Ring Commutator

The **split ring commutator** reverses the direction of current every **half turn**.
This reversal ensures the motor continues to rotate in the **same direction**.
Without it, the forces would reverse after 180° and the coil would stop or reverse.

Increasing Motor Speed

**Increase the current** flowing through the coil.
**Increase the number of turns** in the coil.
**Increase the strength** of the magnetic field.

Reversing Motor Direction

**Reverse the direction of the current** (e.g., swap battery terminals).
**Reverse the direction of the magnetic field** (e.g., swap the permanent magnets).

Key Facts

Magnetic field lines point from **north to south**.
The more turns in the coil, the **greater the force** on the coil.
If current decreases, the motor spins **slower**.

Simple circuit showing battery, switch, coil (bulb symbol), and ammeter.

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Practice questions

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1.Which of the following statements is true?
Easy
- AWhen a current flows through a wire, it creates a magnetic field.
- BMagnetic field lines point away from a south pole towards a north pole.
- CThe split ring commutator reverses the direction of the magnetic field every half turn.
- DIncreasing the number of turns in the coil decreases the force on the coil.
2.What are the components of an electric motor?
Easy
- ACoil of wire, permanent magnets, split ring commutator, brushes, battery
- BCoil of wire, electromagnet, slip rings, brushes, battery
- CPermanent magnets, split ring commutator, resistor, capacitor
- DCoil of wire, permanent magnets, diode, brushes, battery
3.Why is there a force on the coil in an electric motor?
Medium
- AThe magnetic field of the current-carrying coil interacts with the permanent magnetic field, producing a force.
- BThe coil is attracted to the north pole of the permanent magnet.
- CThe current in the coil creates an electric field that pushes the coil.
- DThe brushes push against the commutator, causing the coil to move.
4.What is the role of the split ring commutator in an electric motor?
Medium
- AIt reverses the direction of the current every half turn so the motor continues to rotate in the same direction.
- BIt reverses the direction of the magnetic field every half turn.
- CIt increases the current supplied to the coil.
- DIt connects the coil to the battery continuously.
5.Which of the following is NOT a way to make an electric motor rotate faster?
Medium
- ADecrease the number of turns in the coil.
- BIncrease the current flowing through the coil.
- CIncrease the number of turns in the coil.
- DIncrease the strength of the magnetic field.
6.Which two changes will cause an electric motor to rotate in the opposite direction?
Easy
- AReverse the direction of the current or reverse the direction of the magnetic field.
- BIncrease the current or increase the magnetic field strength.
- CReverse the current and reverse the magnetic field at the same time.
- DDecrease the current or decrease the magnetic field strength.
7.What are the brushes in an electric motor made of?
Easy
- ACarbon
- BCopper
- CIron
- DPlastic
8.The current supplied to an electric motor is switched off. Why does the motor stop spinning?
Medium
- ANo current flows through the coil, so no magnetic field is produced and no force acts on the coil.
- BThe magnetic field disappears when the current is off.
- CThe commutator stops reversing the current.
- DThe brushes no longer make contact with the commutator.

More questions

9.The battery powering an electric motor has been used for a long time and the current supplied by the battery is starting to decrease. What happens to the electric motor?
Hard
- AThe motor rotates more slowly because the force on the coil is smaller.
- BThe motor stops rotating because no current flows.
- CThe motor rotates faster because the lower current reduces resistance.
- DThe motor rotates in the opposite direction because the current direction reverses.
10.In an electric motor, the coil of wire is also called the:
Easy
- AArmature
- BCommutator
- CBrushes
- DMagnet
11.What would happen if the split ring commutator were replaced by a continuous ring (slip ring)?
Medium
- AThe coil would rotate back and forth instead of continuously in one direction.
- BThe motor would rotate faster.
- CThe motor would not rotate at all.
- DThe direction of rotation would reverse every half turn.
12.Which of the following correctly describes the direction of forces on the coil in an electric motor?
Medium
- AOn the left side, force is upwards; on the right side, force is downwards.
- BOn both sides, force is upwards.
- COn both sides, force is downwards.
- DOn the left side, force is downwards; on the right side, force is upwards.
13.An electric motor is connected to a battery. The coil has 50 turns. To make the motor rotate faster, which single change would be most effective?
Medium
- AReplace the battery with one of higher voltage to increase current.
- BReplace the coil with one having 25 turns.
- CReplace the permanent magnets with weaker ones.
- DReverse the connections to the battery.
14.A student has an electric motor that rotates clockwise. To make it rotate anticlockwise, which two changes could be made?
Medium
- AReverse the current direction and reverse the magnetic field direction.
- BReverse the current direction only.
- CIncrease the number of turns in the coil.
- DSwap the positions of the north and south poles.
15.In an electric motor, the split ring commutator reverses the current every half turn. What would happen if the commutator were replaced by a continuous ring (slip ring)?
Hard
- AThe motor would rotate in the same direction continuously.
- BThe motor would rotate in the opposite direction after each half turn.
- CThe motor would not rotate at all.
- DThe motor would rotate faster.
16.Which of the following statements about electric motors is correct?
Easy
- AThe brushes are made of copper.
- BThe split ring commutator ensures the motor rotates in one direction.
- CIncreasing the number of turns in the coil decreases the force on the coil.
- DThe magnetic field produced by the permanent magnets is called the armature.
17.A student builds an electric motor with a coil of 20 turns. The motor rotates at 60 rpm. If the student replaces the coil with one having 40 turns, what is the most likely effect on the rotation speed?
Medium
- AIt decreases to 30 rpm.
- BIt increases to 120 rpm.
- CIt increases but not necessarily to 120 rpm.
- DIt remains at 60 rpm.
18.An electric motor is connected to a variable power supply. The graph shows how the rotation speed (in rpm) changes with current (in A). What can be concluded from the graph?
Hard
- ARotation speed is inversely proportional to current.
- BRotation speed is directly proportional to current.
- CRotation speed increases with current but not proportionally.
- DRotation speed is independent of current.
19.A student measures the rotation speed of an electric motor with different numbers of turns in the coil. The results are shown in the table. What is the missing value?
Medium
- A100 rpm
- B150 rpm
- C200 rpm
- D250 rpm
20.In an electric motor, the force on the left side of the coil is upward. What is the direction of the force on the right side of the coil?
Hard
- AUpward
- BDownward
- CLeftward
- DRightward
21.What material are the brushes in an electric motor made of?
Easy
- ACopper
- BCarbon
- CIron
- DPlastic
22.A student designs an electric motor with a coil that has 100 turns and a current of 0.5 A. The motor rotates at 120 rpm. To double the rotation speed, which change would be most effective?
Hard
- AIncrease the current to 1.0 A while keeping turns at 100.
- BIncrease the number of turns to 200 while keeping current at 0.5 A.
- CReplace the permanent magnets with stronger ones.
- DAny of the above could double the speed.
23.A student builds an electric motor and measures its rotation speed. She then replaces the two permanent magnets with stronger ones and measures the rotation speed again. What is the most likely effect on the rotation speed?
Medium
- AIt increases because the magnetic field is stronger.
- BIt decreases because the stronger magnets create more resistance.
- CIt stays the same because the number of turns in the coil has not changed.
- DIt becomes zero because the motor cannot work with stronger magnets.
24.A student wants to reverse the direction of rotation of an electric motor. Which two changes could she make?
Medium
- AReverse the current direction and reverse the magnetic field direction.
- BIncrease the current and increase the number of turns.
- CSwap the brushes and reverse the magnetic field.
- DReverse the current direction only and increase the strength of the magnets.
25.A student sets up an electric motor with a coil of 50 turns and a current of 0.4 A. The motor rotates at 100 rpm. She then increases the current to 0.8 A and the number of turns to 100. Assuming all other factors remain constant, what is the new rotation speed?
Hard
- A400 rpm
- B200 rpm
- C100 rpm
- D50 rpm
26.Which of the following statements about magnetic field lines is correct?
Easy
- AThey point away from a north pole towards a south pole.
- BThey point away from a south pole towards a north pole.
- CThey point from south to north inside a magnet.
- DThey are circles around the magnet.
27.A student investigates how the rotation speed of an electric motor varies with the number of turns in the coil. The results are shown in the table. What is the missing value? (Table: Turns: 10 → speed 20 rpm; 20 → 40 rpm; 30 → ?; 40 → 80 rpm)
Hard
- A60 rpm
- B50 rpm
- C70 rpm
- D30 rpm
28.An electric motor is connected to a variable power supply. The graph shows how rotation speed (in rpm) changes with current (in A). What can be concluded from the graph? (Graph: straight line through origin, positive slope)
Medium
- ARotation speed is directly proportional to current.
- BRotation speed is inversely proportional to current.
- CRotation speed is independent of current.
- DRotation speed increases but not proportionally with current.
29.A student builds an electric motor with a coil of 20 turns. The motor rotates at 60 rpm. If the student replaces the coil with one having 40 turns, what is the new rotation speed assuming the current and magnetic field are unchanged?
Hard
- A120 rpm
- B60 rpm
- C30 rpm
- D80 rpm
30.What are the main components of an electric motor?
Easy
- ACoil of wire, permanent magnets, split ring commutator, brushes, battery
- BCoil of wire, electromagnets, slip rings, brushes, battery
- CCoil of wire, permanent magnets, slip rings, carbon rods, battery
- DCoil of wire, temporary magnets, split ring commutator, brushes, power supply
31.Why does the coil in an electric motor experience a force?
Medium
- ABecause the current in the coil creates a magnetic field that interacts with the permanent magnetic field.
- BBecause the coil is made of a magnetic material.
- CBecause the brushes push the coil.
- DBecause the split ring commutator attracts the coil.
32.A student builds an electric motor with a coil of 20 turns. The motor rotates at 60 rpm. If the student replaces the coil with one having 40 turns, what is the expected rotation speed? (Assume the current and magnetic field strength remain the same.)
Hard
- A120 rpm
- B60 rpm
- C30 rpm
- D180 rpm
33.A student builds an electric motor with a coil of 50 turns and a current of 0.4 A. The motor rotates at 100 rpm. She then increases the current to 0.8 A and the number of turns to 100. Assuming the magnetic field strength remains constant, what is the new rotation speed?
Medium
- A200 rpm
- B400 rpm
- C100 rpm
- D800 rpm
34.In an electric motor, the force on the left side of the coil is upward. The direction of the current in the left side is from top to bottom. What is the direction of the magnetic field produced by the permanent magnets?
Hard
- ALeft to right
- BRight to left
- CUpwards
- DDownwards
35.A student measures the rotation speed of an electric motor with different numbers of turns in the coil, keeping current and magnetic field constant. The results are shown in the table. What is the missing value?
Medium
- A120 rpm
- B160 rpm
- C180 rpm
- D200 rpm
36.A student builds an electric motor with a coil of 20 turns. The motor rotates at 60 rpm. If the student replaces the coil with one having 40 turns, what is the new rotation speed assuming the same current and magnetic field?
Hard
- A30 rpm
- B60 rpm
- C120 rpm
- D240 rpm
37.A student sets up an electric motor with a coil of 50 turns and a current of 0.4 A. The motor rotates at 100 rpm. She then increases the current to 0.8 A and the number of turns to 100. What is the new rotation speed?
Medium
- A200 rpm
- B400 rpm
- C100 rpm
- D800 rpm
38.A student designs an electric motor with a coil that has 100 turns and a current of 0.5 A. The motor rotates at 120 rpm. To double the rotation speed, which change could be made?
Medium
- AIncrease the current to 1.0 A and keep turns at 100.
- BIncrease the turns to 200 and keep current at 0.5 A.
- CDecrease the current to 0.25 A and increase turns to 200.
- DBoth A and B are correct.
39.An electric motor is connected to a variable power supply. The graph shows how rotation speed (in rpm) changes with current (in A). What can be concluded from the graph?
Medium
- ARotation speed is directly proportional to current.
- BRotation speed is inversely proportional to current.
- CRotation speed is independent of current.
- DRotation speed increases but not linearly with current.
40.A student designs an electric motor with a coil that has 100 turns and a current of 0.5 A. The motor rotates at 120 rpm. To double the rotation speed, which single change would be most effective?
Hard
- AIncrease the current to 1.0 A.
- BIncrease the number of turns to 200.
- CIncrease the magnetic field strength by using stronger magnets.
- DIncrease both current and turns by 50%.

Revision notes

## Electric Motors An electric motor consists of a **coil of wire (armature)** placed between two **permanent magnets** that produce a magnetic field. A battery supplies current to the coil. ### How it works - When current flows through a wire, it creates its own magnetic field. - This field interacts with the permanent magnetic field, producing a force on the wire. - On the left side of the coil, the force is **upwards**; on the right side, it is **downwards** (because current direction changes around the coil). These forces cause the coil to **rotate**. - Using a coil (many turns) increases the magnetic field strength and the force. ### Split ring commutator - Without it, after a half-turn (180°), the forces would reverse and the motor would stop or reverse. - The **split ring commutator** reverses the direction of current every half-turn, so the motor keeps rotating in the same direction. - **Brushes** (made of **carbon**) rub against the commutator, allowing it to rotate while maintaining electrical contact. ### Making the motor spin faster 1. Increase the **current** through the coil. 2. Increase the **number of turns** in the coil. 3. Increase the **strength of the magnetic field**. ### Reversing direction - Reverse the direction of the **current**. - Reverse the direction of the **magnetic field** (swap the magnets). ### Key formula/definition - "When a current flows through a wire, it creates a magnetic field." - "The more turns in the coil the greater the strength of the overall magnetic field produced and the greater the force on the coil."

Armature: The coil of wire in an electric motor that rotates.
Split ring commutator: A device that reverses the direction of current every half turn to keep the motor rotating in the same direction.
Brushes: Carbon components that rub against the split ring commutator to allow rotation while maintaining electrical contact.
Magnetic field: A region around a magnet or current-carrying wire where magnetic forces are experienced.
Current (I): The flow of electric charge through a conductor.

Flashcards

What are the main components of an electric motor?

A coil of wire (armature), two permanent magnets, a split ring commutator, and carbon brushes.

What produces the magnetic field in an electric motor?

Two permanent magnets produce the magnetic field.

What happens when a current flows through a wire?

It creates a magnetic field around the wire.

Why is a coil of wire used instead of a single straight wire?

More turns in the coil increase the strength of the overall magnetic field and the force on the coil.

What causes the force on the coil in an electric motor?

The interaction between the magnetic field created by the current in the coil and the permanent magnetic field produces a force.

Why does the coil rotate in an electric motor?

The force on the left side is upward and on the right side is downward (due to current direction change), creating a turning effect.

What is the role of the split ring commutator?

It reverses the direction of current every half turn to keep the motor rotating in the same direction.

What are the brushes made of in an electric motor?

Carbon.

What do the brushes do in an electric motor?

They rub against the split ring commutator and allow it to rotate while maintaining electrical contact.

State three ways to make an electric motor rotate faster.

Increase the current, increase the number of turns in the coil, or increase the strength of the magnetic field.

State two ways to reverse the direction of rotation of an electric motor.

Reverse the direction of the current or reverse the direction of the magnetic field.

Why does the motor stop spinning when the current is switched off?

No current means no magnetic field from the coil, so no force is produced to keep it rotating.

What happens to an electric motor if the battery current decreases?

The motor rotates slower because the force on the coil is reduced.

True or false: Magnetic field lines point away from a south pole towards a north pole.

False. They point away from a north pole towards a south pole.

True or false: When a current flows through a wire, it creates a magnetic field.

True.

What is the armature in an electric motor?

The coil of wire that rotates.

Why does the direction of force on the coil change if it rotates 180° without a commutator?

Because the current direction relative to the magnetic field reverses, reversing the force direction.

What would happen if the split ring commutator failed?

The motor would stop rotating after half a turn because the forces would reverse.

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