IB MYP 4-5 Physics- Electric motors - Study Notes - New Syllabus

IB MYP 4-5 Physics-Electric motors – Study Notes

Electric Motors

An electric motor is a device that converts electrical energy into mechanical energy (rotation or motion).
It works on the principle that a current-carrying conductor placed in a magnetic field experiences a force.

Principle (Motor Effect)

A conductor carrying current in a magnetic field experiences a force given by:

\(F = B \, I \, L \, \sin\theta\)

- \(F\) = force on conductor
- \(B\) = magnetic field strength
- \(I\) = current in conductor
- \(L\) = length of conductor in field
- \(\theta\) = angle between conductor and field

This force is due to the interaction between the magnetic field of the conductor and the external magnetic field.

Construction of a Simple DC Motor

Armature (coil): A rectangular coil of wire through which current flows.
Magnet: Provides a uniform magnetic field (between north and south poles).
Commutator (split-ring): Reverses the current every half turn to keep the coil rotating in the same direction.
Brushes: Provide electrical contact between the power supply and the rotating coil.

Working of a DC Motor

When current flows through the coil → forces act on the two sides of the coil (one up, one down).
This creates a turning effect (torque) on the coil, causing it to rotate.
The commutator reverses current every half turn → ensures continuous rotation in the same direction.

Factors Affecting the Strength of Motor

Applications of Electric Motors

Example:

A wire of length 0.2 m carrying 5 A of current is placed perpendicular to a 0.3 T magnetic field. Find the force acting on the wire.

▶️ Answer/Explanation

Step 1: Use the formula \(F = BIL\sin\theta\).

Step 2: \(B = 0.3 \,T\), \(I = 5 \,A\), \(L = 0.2 \,m\), \(\theta = 90^\circ\).

Step 3: \(F = 0.3 \times 5 \times 0.2 \times \sin 90^\circ = 0.3 \, N\).

Final Answer: \(\boxed{0.3 \, N}\)

Example

Explain why a split-ring commutator is necessary in a DC motor.

▶️ Answer/Explanation

Step 1: Without a commutator, the current direction in the coil would remain constant.

Step 2: After half a turn, the forces would reverse → the coil would oscillate instead of rotating continuously.

Final Answer: The commutator reverses current every half turn, ensuring continuous rotation in one direction.

Example:

List two ways to increase the turning effect in an electric motor.

▶️ Answer/Explanation

Step 1: Increase the strength of magnetic field (\(B\)).

Step 2: Increase the current (\(I\)) supplied to the coil.

Step 3: Increase the number of turns in the coil or its area.

Final Answer: Use a stronger magnet and higher current (or larger/more turns coil) to increase torque.