Chapter 12 · Question 7

Describe an activity to demonstrate that a force acts on a current-carrying conductor placed in a magnetic field. On what factors does the direction of this force depend, and when is the magnitude of the force maximum?

Answer Revealed

Direct Answer:

Activity 12.7: Suspend a small aluminium rod AB (about 5 cm) horizontally from a stand using two connecting wires. Place a strong horse-shoe magnet such that the rod lies between its poles — the north pole vertically below and south pole vertically above the rod, so the magnetic field is directed upwards. Connect the rod in series with a battery, a key, and a rheostat. When current flows from B to A, the rod is displaced towards the left. Reversing the current to flow from A to B causes the rod to displace towards the right. This shows that a force acts on a current-carrying conductor in a magnetic field. The direction of the force depends on (i) the direction of current and (ii) the direction of the magnetic field. The magnitude of the force is maximum when the direction of current is at right angles (

90^\circ

) to the direction of the magnetic field.

Simple Explanation

Hang a small aluminium rod between the poles of a horseshoe magnet using two connecting wires, like a tiny swing. Connect it to a battery through a key. When you press the key, current flows, and the rod swings to one side — a force is pushing it. Reverse the battery, and the rod swings the other way. Flip the magnet upside down, and the direction reverses again. The force is strongest when the current flows perpendicular to the magnetic field — point the current and the field at right angles, and you get the biggest push. This is the principle behind every electric motor.

Exam-Ready Structure

The force experienced by a current-carrying conductor in a magnetic field is demonstrated through Activity 12.7 and forms the principle behind devices like the electric motor. 1. Experimental setup (Activity 12.7): (a) Take a small aluminium rod AB (about 5 cm). Using two connecting wires, suspend it horizontally from a stand (Fig. 12.12). (b) Place a strong horse-shoe magnet such that the rod lies between the two poles. For a vertically upward magnetic field, place the north pole of the magnet below and the south pole above the aluminium rod. (c) Connect the aluminium rod in series with a battery, a key, and a rheostat. 2. Observations: (a) When current passes through the aluminium rod from end B to end A, the rod is displaced towards the left. (b) When the direction of current is reversed (from A to B), the rod is displaced towards the right. (c) When the magnet poles are interchanged (field direction reversed), the direction of rod displacement also reverses. 3. Direction dependence: The direction of the force on the conductor depends on (i) the direction of the electric current, and (ii) the direction of the magnetic field. 4. Maximum force condition: The displacement of the rod is largest (i.e., the magnitude of the force is highest) when the direction of current is at right angles (

90^\circ

) to the direction of the magnetic field. In this condition, the force is perpendicular to both the current and the magnetic field. 5. Significance: Devices that use current-carrying conductors and magnetic fields include electric motors, electric generators, loudspeakers, microphones, and measuring instruments.

Key Points

Activity 12.7: suspended aluminium rod in a horse-shoe magnet's field gets displaced when current flows
Rod displacement direction reverses when current direction or magnetic field direction is reversed
Direction of force depends on direction of current and direction of magnetic field
Force is maximum when current direction is at right angles ( $90^\circ$ ) to magnetic field direction
Applications: electric motor, electric generator, loudspeakers, microphones

Describe an activity to demonstrate that a force acts on a current-carrying conductor placed in a magnetic field. On what factors does the direction of this force depend, and when is the magnitude of the force maximum?