Fig. 44.1 shows an aluminium ring placed loosely over the top of a vertical solenoid. The solenoid is connected to a d.c. power supply through a switch. Whe...
Question 1 Report
Fig. 44.1 shows an aluminium ring placed loosely over the top of a vertical solenoid. The solenoid is connected to a d.c. power supply through a switch. When the switch is closed rapidly, the ring jumps upward off the solenoid. A student repeats the experiment with different ring materials and observes that a copper ring also jumps but a plastic ring does not move. The solenoid has 600 turns and the power supply provides 12 V. When the switch is closed, the current rises rapidly from zero to its maximum value. The aluminium ring has a mass of 5.0 g. The solenoid is clamped vertically to a heavy stand.
(a) Explain why the aluminium ring jumps upward when the switch is closed. Use Lenz's law in your explanation. [4]
(b) Explain why the plastic ring does not move. [1]
(c) State the effect on the height the ring jumps if the power supply voltage is increased. [1]
(d) State what happens to the ring if the switch is held closed for a long time and then opened suddenly. [2]
Answer Details
(a) This phenomenon is explained in four steps using electromagnetic induction and Lenz's law:
- When the switch is closed, the current in the solenoid rises rapidly from zero to its maximum value, creating a rapidly increasing magnetic field through and around the solenoid
- The changing magnetic field passes through the aluminium ring, which is a conductor. By Faraday's law, this changing flux induces an e.m.f. and hence a current in the ring
- By Lenz's law, the induced current must flow in a direction that opposes the change causing it. The change is an increasing upward magnetic field (assuming north pole at top), so the ring's induced current creates its own magnetic field that opposes the increase
- The ring's magnetic field is directed opposite to the solenoid's field, making the ring behave like a magnet with the same pole facing the solenoid's top. Like poles repel, so the ring experiences an upward repulsive force that launches it off the solenoid
(b) Plastic is an electrical insulator. No current can be induced in an insulator because it has no free electrons to carry a current. Without an induced current, there is no magnetic field produced by the ring, so there is no magnetic force and the ring does not move.
(c) If the power supply voltage is increased, the current in the solenoid rises to a larger value and does so more rapidly. This produces a greater rate of change of magnetic field, which induces a larger current in the ring and a stronger repulsive force. The ring therefore jumps higher.
(d) When the switch is opened suddenly after being held closed for a long time:
- The current in the solenoid drops rapidly to zero, causing the magnetic field to decrease rapidly
- This decreasing field again induces a current in the ring (by Faraday's law), but now Lenz's law dictates that the ring's field opposes the decrease, so the ring is attracted downward towards the solenoid rather than repelled upward
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