Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Magnetic Swing

Introduction

Oersted discovered that a current carrying conductor experiences a force when placed in a magnetic field.

In this demonstration we make a magnetic swing which moves by this force. We can learn more about the direction of this force and the factors which affects its magnitude.

Apparatus

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Two metal bars (or two cycle spokes), a wooden base (or a cardboard base), copper wires, battery, two ring magnets, crocodile clips.

Procedure

  1. Fix the metal bars in the wooden base at some distance from one another.
  2. Make small loops with some copper wire at the upper end of the two bars
  3. Take a single copper wire, give it the shape of swing and hang it from the two metal bars from the loops made above. Make sure that the copper wire swings freely from the loops.
  4. Connect the two metal bars to a battery with the help of crocodile clips and copper wires. The circuit is now complete.
  5. Place a ring magnet below the swing.
  6. Switch on the power source. See the deflection of the swing.
  7. Flip over the ring magnet. See that the deflection is now in the opposite direction.
  8. Interchange the terminals of the battery. The deflection again gets reversed.
  9. Take out the crocodile clip from one of the metal bars to disconnect the circuit. Now touch it again with the metal bar. See the copper wire swing to and fro.
  10. Increase the voltage in the battery. Repeat the above process and see that wire swings with a greater amplitude.
  11. Now put two ring magnets instead of one below the swing. Again repeat the process and see the increase in amplitude of the swing.

Discussion

When the power is switched on, a current \(i\) passes through the copper wire. In the magnetic field \(\vec{B}\) of the ring magnet, current carrying wire experiences a force \(\vec{F}\) given by

$$\vec{F}=i(\vec{l}\times \vec{B}),$$

where \(l\) is the length of the conductor. The direction of this force is given by the right hand thumb rule. When the direction of magnetic field reverses on flipping over the magnet, the direction of the force also reverses. Similarly when the current direction is reversed, the force also reverses direction.

Increasing the voltage of the battery increased the current which in turn increased the magnitude of the force and the amplitude. When instead of one magnet we put two magnets the magnetic field strength increases causing the force to increase again.

An interesting observation: If the magnet is given a to and fro motion near the current carrying copper wire it starts swinging. This can be explained on the basis of Lorentz force experienced by the wire.

Now the magnet is removed and only the fingers make the to and fro motion. The wire again starts swinging!

In another variation the current source is removed from the copper wire and the magnet is given a to and fro motion in the vicinity of the copper wire. Interestingly the copper wire again starts swinging!

Is it phenomenon of resonance which we are seeing or something else?

Puzzle: This set-up can be used as an interesting puzzle. Ask the students to find poles (north/south) of ring magnet by using this set-up. This will eliminate the confusion about Fleming's right hand rule/ left hand rule/ screw rule etc.