Magnetic Effects of Electric Current • Topic 2 of 3

Force on a Conductor & Electric Motor

Force on a current-carrying conductor. Oersted showed that a current creates a magnetic field; the reverse is also true. When a current-carrying conductor is placed in a magnetic field, the field exerts a force on it. This is because the external field and the field of the current interact. The conductor is pushed sideways and may even jump out of the field region.

What the force depends on. The force is largest when the current and the field are at right angles (90°) to each other. It is zero when the conductor is placed parallel to the field. The force also increases if the current increases or if the magnetic field is made stronger.

Directions of the three quantities. Three things are always mutually perpendicular here: the current (I), the magnetic field (B) and the resulting force/motion (F). To find the direction of the force we use Fleming's Left-Hand Rule.

Fleming's Left-Hand Rule. Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular. Then:

  • Forefinger points in the direction of the magnetic field (F for Field).
  • Middle finger (centre finger) points in the direction of the current (C for Current).
  • Thumb then points in the direction of the force / motion (Thrust) on the conductor.

This rule is used wherever a current-carrying conductor experiences a force — most importantly in the electric motor.

Electric motor. An electric motor is a device that converts electrical energy into mechanical (rotational) energy. It is the heart of fans, mixers, washing machines and electric cars.

Construction. It has a rectangular coil ABCD of insulated wire mounted on an axle and placed between the poles of a strong magnet. The ends of the coil are connected to a split ring (commutator), whose two halves touch carbon brushes connected to a battery.

Working. Current enters the coil and flows in opposite directions in arms AB and CD. By Fleming's left-hand rule the two arms feel forces in opposite directions, so the coil rotates. After a half turn the split ring reverses the current in the coil. This keeps the force on each arm acting in the direction needed to continue the rotation, so the coil spins continuously in the same sense.

Role of the commutator. The split-ring commutator reverses the direction of current through the coil after every half rotation, which is essential for continuous rotation in one direction. Commercial motors use an electromagnet, many turns of coil and a soft-iron core (armature) to greatly increase the power.

Electric motor: rectangular coil between N and S poles with force directionsNSADBCForce upForce downSplit-ring commutator + brushes
Solved Examples
1
Worked Example
When is the force on a current-carrying conductor in a magnetic field maximum and when is it zero?
Solution
  1. Step 1: The force depends on the angle between the current and the field.
  2. Step 2: It is maximum when the conductor is perpendicular (90°) to the field.
  3. Step 3: It is zero when the conductor is parallel to the field.

Answer: Maximum at 90° to the field; zero when parallel to the field.

2
Worked Example
State Fleming's Left-Hand Rule and say what each finger represents.
Solution
  1. Step 1: Hold the thumb, forefinger and middle finger of the left hand mutually perpendicular.
  2. Step 2: Forefinger shows the magnetic field.
  3. Step 3: Middle finger shows the current.
  4. Step 4: Thumb then shows the direction of the force (motion).

Answer: Forefinger = field, middle finger = current, thumb = force/motion.

3
Worked Example
A horizontal wire carries current from west to east in a magnetic field directed vertically downward. Find the direction of the force on the wire.
Solution
  1. Step 1: Point the left forefinger downward (field).
  2. Step 2: Point the middle finger toward the east (current).
  3. Step 3: The thumb then points toward the north.

Answer: The force on the wire is directed towards the north.

4
Worked Example
What energy conversion takes place in an electric motor?
Solution
  1. Step 1: Electrical energy is supplied to the coil by the battery.
  2. Step 2: The force on the current-carrying coil makes it rotate.
  3. Step 3: Rotation is mechanical energy.

Answer: An electric motor converts electrical energy into mechanical (rotational) energy.

5
Worked Example
Why is a split-ring commutator used in an electric motor?
Solution
  1. Step 1: After every half rotation the arms of the coil change sides between the poles.
  2. Step 2: To keep the coil turning the same way, the current direction in the coil must be reversed at that instant.
  3. Step 3: The split ring reverses the current through the coil every half turn.

Answer: The commutator reverses the current after each half turn so the coil keeps rotating continuously in one direction.

6
Worked Example
In a motor the two arms of the coil carry current in opposite directions. Why does the coil rotate instead of moving as a whole?
Solution
  1. Step 1: Both arms are in the same magnetic field but carry current in opposite directions.
  2. Step 2: By Fleming's left-hand rule the forces on the two arms are equal but opposite in direction.
  3. Step 3: One arm is pushed up while the other is pushed down, forming a couple (turning effect).

Answer: The opposite forces on the two arms form a couple that makes the coil rotate.

Key Points

  • A current-carrying conductor in a magnetic field experiences a force.
  • The force is maximum when current is perpendicular to the field and zero when parallel.
  • Fleming's Left-Hand Rule: forefinger = field, middle finger = current, thumb = force.
  • An electric motor converts electrical energy into mechanical energy.
  • A split-ring commutator reverses the current every half turn for continuous one-way rotation.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Fleming's Left-Hand Rule is used to find the direction of:
Explanation: It gives the force/motion when current and field are known.
Q2.In Fleming's Left-Hand Rule the middle finger represents:
Explanation: Forefinger = field, middle finger = current, thumb = force.
Q3.An electric motor converts:
Explanation: It is the opposite of a generator.
Q4.The function of the split-ring commutator in a motor is to:
Explanation: Reversing the current keeps the coil rotating in the same direction.
Practice more MCQs → 📝 Assignment · 30 marks