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Some PHY6 questions.

You have an apparatus (which I can't show because I only have the question on paper) which consists of a 400 turn coil, with a steel rod of clampstand in the middle of it and on this rod there's an aluminium ring. Here is the question.

When there's an alternating current in the 400-turn coil the aluminium ring rises to a few centimetres above the coil. Changes in the size of the alternating current make the ring rise to different heights.

Explain why, when there is a varying current in the coil, there's an induced current in the aluminium ring. Suggest why the ring then experiences an upward force.

The aluminium ring becomes hot if the alternating current is left on for a few minutes. In order to try and measure its temperature it is removed from the steel rod and then dropped into a small plastic cup containing cold water.

State what measurements you would take and what physical properties of water and aluminium you would need to look up in order to calculate the initial temperature of the hot aluminium ring.

Explain whether experimental errors would make you value for the initial temperature of the aluminium ring too big or too small.

Thanks.

Reply 1

Mehh

Mohit_C

Explain why, when there is a varying current in the coil, there's an induced current in the aluminium ring. Suggest why the ring then experiences an upward force.

The current in the first coil generates a magnetic field. This causes the ring (to be known as the second coil from hence forth) to have a voltage in it. This drives a current. The second coil thus generates a second magnetic field, which interacts with the first by pushing against it.

Mohit_C

State what measurements you would take and what physical properties of water and aluminium you would need to look up in order to calculate the initial temperature of the hot aluminium ring.

You would need the specific heat capacities of water and aluminium. You would also need to know the mass of the water and the ring.
This is because
heat = specific heat capacity x mass x temperature

From this formula you can work out the change in heat and hence temperature...

Reply 2

Mohit_C

I understand the first part but can you expand a bit on the second. How exactly can I find the initial temperature?

Reply 3

phys981

You know how much water you have and what the temperature change of the water is. So you calculate how much energy the water gained.

This must be the same as the energy lost by the aluminium ring. Since you know its final temp (same as water) you can work out its initial temp.

I think the bit about errors probably refers to the fact that some of the energy is lost to the surroundings so the initial temperature was probably higher than you calculate.

Reply 4

Mohit_C

Ah right. That makes sense. I just thought that as they didn't mention we have a thermometer, how were we supposed to find the final temperature. Thanks.