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F3.6 · Explain how motors and generators operate

Learn to explain how motors and generators operate through clear examples and targeted practice.

Ontario Grade 11 Physics

Electricity and Magnetism

Electrical energy and motion are linked by magnetic forces and electromagnetic induction

Motors and generators connect electricity and motion. A motor uses an electric current to produce motion. A generator uses motion to produce an electric current. Both rely on the interaction between magnets and electric currents. Before comparing them, remember that electric current is the movement of electric charge, and a magnet produces a magnetic field in the space around it. A magnetic field is a region where a magnetic force can act. In the diagrams and examples, the device is the system being described. A stated direction, such as clockwise rotation or current toward the right, is a chosen reference direction. Direction matters because force and current have directions, while energy and speed are scalars: they have size but no direction.

What you will learn

1. Prerequisite bridge: fields, currents, and directions

A bar magnet has a north pole and a south pole. Outside the magnet, magnetic field lines point from north to south. The lines are a drawing tool that shows the field's direction. They do not show the path of a moving object.
A wire carrying current also produces a magnetic field around itself. When the wire is placed in another magnetic field, the fields interact and the wire can experience a force. A force is a push or pull. The wire may move if it is free to move.
Current direction is defined as the direction positive charge would move. In a metal wire, electrons move the other way, but circuit diagrams use conventional current. In this lesson, arrows show conventional current.
A direction rule helps connect current, magnetic field, and force. For a motor, use the left-hand rule: point the first finger in the magnetic field direction, the second finger in the conventional current direction, and the thumb shows the force direction on the wire. Keep the three directions at right angles. This is a way to organize directions, not a separate force-producing process.

2. The motor: electrical energy produces motion

A simple motor has a coil of wire between the poles of a magnet. The coil is connected to a power source. A coil is wire wound into one or more loops. When current flows, opposite sides of the coil carry current in opposite directions.
Each side of the coil experiences a magnetic force. The forces point in opposite directions on opposite sides. Because the forces act on different sides of the coil, they turn it. The coil's turning motion is called rotation. The motor changes electrical energy from the source into mechanical energy, which is energy of motion.
A basic direct-current motor uses a split-ring commutator. This is a pair of conducting segments that connects the rotating coil to the power source through brushes. Brushes are stationary contacts that let current enter the rotating part. Each half-turn, the commutator reverses the current in the coil. That reversal also reverses the forces on the two sides, so the turning continues in the same rotational direction.
Without this reversal, the forces would switch the rotation's direction after half a turn. The coil could rock back and forth instead of continuing to rotate. Real motors may use different designs, but the key idea remains: current in a magnetic field produces force and motion.

3. The generator: motion produces electrical energy

A generator has a coil and a magnetic field. An external source of motion turns the coil, or turns a magnet near the coil. As the coil moves through the magnetic field, the magnetic conditions through the coil change. This change can produce a voltage across the coil. Voltage is the electrical energy transferred per unit of charge, and it can drive current in a connected circuit.
This process is electromagnetic induction: producing a voltage by changing the magnetic conditions through a coil. If the circuit is complete, the voltage can produce a current. If the coil stops moving relative to the magnetic field, the changing conditions stop, so this process no longer produces a voltage in the simple generator model.
The generator changes mechanical energy into electrical energy. The energy does not appear from nowhere: the motion source must do work to keep turning the coil against effects that oppose its motion. A generator and a motor share important parts and principles, but the direction of energy transfer is reversed.
In a simple generator, rotating the coil makes the induced current reverse direction every half-turn. This gives alternating current (AC), a current that repeatedly changes direction. A commutator can instead make the output current flow in one direction in a simple direct-current generator. The output type depends on the design.
mechanical energy→electrical energy\text{mechanical energy} \rightarrow \text{electrical energy}

4. Compare the devices and track direction

A useful way to tell the devices apart is to ask what energy goes in and what comes out. A motor takes in electrical energy and produces motion. A generator takes in motion and produces electrical energy. The devices are not simply identical objects with different names: their energy transfer and their operating cause are described in opposite directions.
Use labelled arrows to keep the direction ideas clear. For a motor, magnetic field direction and current direction determine force direction. For a generator, coil motion relative to the magnetic field produces an induced voltage; the circuit and coil arrangement determine the current direction. Reversing a relevant direction can reverse the force or induced current.
The sketches below use symbols rather than a scale drawing. A cross can represent a direction into the page, and a dot can represent a direction out of the page. These are useful only when their meaning is stated. Here, the motor coil's two sides carry current in opposite directions, so their forces can turn the coil.

Energy transfer and main operating idea

DeviceEnergy inEnergy outMain operating idea
MotorElectricalMechanicalCurrent in a magnetic field experiences force
GeneratorMechanicalElectricalMotion in a magnetic field induces voltage

Worked example

Predicting force in a motor wire

A straight wire is part of a motor coil. The magnetic field points from left to right, and conventional current in this wire points into the page. In which direction is the force on this wire?
  1. Identify the system and directions
    The system is the wire segment. Take right as the field direction and into the page as the current direction. The unknown is the force direction.
  2. Apply the motor direction rule
    Use the left-hand rule with the first finger pointing right and the second finger pointing into the page. The thumb points downward, so the force on the wire is downward.
Answer: The force on this wire is downward.
Check: The answer is a direction, as required for a force vector. In a coil, the other side has the opposite current direction and its force points oppositely, allowing the pair of forces to turn the coil.

Worked example

Explaining why a motor keeps turning

A simple direct-current motor coil turns clockwise. What must the commutator do as the coil passes through each half-turn, and why?
  1. Track the rotating coil
    The system is the coil and its commutator. The chosen positive rotation direction is clockwise. Without a current change, the coil's sides would move into positions where the magnetic forces would oppose continued clockwise turning.
  2. Connect current to force
    The commutator reverses the current in the coil each half-turn. This reverses the forces on the coil sides at the right time, so the turning continues clockwise.
Answer: The commutator reverses the coil current every half-turn so the coil continues rotating clockwise.
Check: The explanation links the current reversal to the force reversal and then to the maintained rotation direction.

Worked example

Explaining generator output

A coil is connected to a closed circuit and is rotated in a magnetic field. The coil completes another half-turn. What happens to the direction of the induced current in a simple alternating-current generator?
  1. Identify the system and motion
    The system is the rotating coil and its connected circuit. The coil is moving relative to the magnetic field, so the magnetic conditions through it change and induce a voltage.
  2. Use the generator model
    In a simple alternating-current generator, the induced current reverses direction after each half-turn. The coil's continued rotation therefore produces current that alternates direction.
Answer: The induced current reverses direction after the half-turn.
Check: The circuit is stated to be closed, so induced current can flow. If the coil were not moving relative to the field, this simple model would not produce an induced voltage.

Common mistakes and how to avoid them

Saying that a motor produces electricity and a generator produces motion.
Correction: State the energy transfer: a motor changes electrical energy into motion, while a generator changes motion into electrical energy.
Treating magnetic field direction, current direction, and force direction as interchangeable.
Correction: They are separate directions. Use the motor direction rule to connect field and current directions to force direction.
Saying a generator creates current just because a coil is near a magnet.
Correction: In the simple model, the coil and magnetic field must move relative to each other so the magnetic conditions through the coil change. A closed circuit is needed for current to flow.
Thinking the motor commutator merely connects the power source to the coil.
Correction: It also reverses the current in the coil every half-turn in a simple direct-current motor, helping the coil keep turning in the same direction.

Lesson summary

Check your understanding

Question 1

Which statement best describes a motor?
  1. It changes electrical energy into motion using magnetic forces on a current-carrying coil.
  2. It changes motion into electrical energy by keeping a coil still in an unchanging magnetic field.
  3. It produces motion because current has no direction.
  4. It changes electrical energy into motion by reversing the magnetic field every half-turn in every motor.
Show answer and explanation
It changes electrical energy into motion using magnetic forces on a current-carrying coil.
A motor uses the force on current-carrying wire in a magnetic field to produce motion. The other statements either reverse the energy transfer or make claims that do not describe the simple motor model.

Question 2

A simple generator's coil is rotating in a magnetic field, and its circuit is closed. What does the changing magnetic condition through the coil produce?
  1. A voltage that can drive current in the circuit
  2. A force that always makes the coil stop instantly
  3. A constant current even when the coil stops moving
  4. A magnetic field with no electrical effect
Show answer and explanation
A voltage that can drive current in the circuit
Changing magnetic conditions through the rotating coil induce voltage. With a closed circuit, that voltage can drive current.

Question 3

What is the purpose of the commutator in a simple direct-current motor?
  1. To reverse the current in the coil each half-turn so rotation can continue in the same direction
  2. To stop current from entering the coil
  3. To change mechanical energy into electrical energy
  4. To make both sides of the coil carry current in the same direction at all times
Show answer and explanation
To reverse the current in the coil each half-turn so rotation can continue in the same direction
The commutator reverses coil current each half-turn. The resulting force changes help keep the coil turning in the same rotational direction.

Key terms

Magnetic field
A region where a magnetic force can act; field lines show its direction.
Conventional current
The direction positive charge would move in a circuit.
Coil
Wire wound into one or more loops.
Commutator
A device that connects a rotating coil to a circuit and, in a simple motor, reverses coil current each half-turn.
Electromagnetic induction
Producing voltage by changing the magnetic conditions through a coil.
Alternating current
Electric current that repeatedly changes direction.
Vector
A quantity with both magnitude and direction.
Scalar
A quantity with magnitude but no direction.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Physics (SPH3U), expectation F3.6. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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