Course discussions

Questions

Comments

Do you enjoy the discussion intensive way of teaching in this course? — 乐永康 2015/07/12 23:44

You are encouraged to raise questions, write comments here on this page. Above is just an example. — 乐永康 2015/07/12 23:44

Questions

Do you think that the current homework load is appropriate? — 乐永康 2014/07/10 23:46
Agree. — Shuo Wang 2014/07/11 22:49

In our course demo with the falling magnet in an aluminum pipe. One way to derive the terminal velocity of the falling magnet is to start from the energy conservation law. In this approach, we assume that the decrease of the potential energy is all converted into thermal energy when the terminal velocity is reached. From this point of view, I have two questions:
1) How much potential energy is converted into thermal energy in one falling? How large will be the raise in the temperature of the aluminum pipe? How many times of falling is necessary for about one degree temperature raise?
2) To raise the temperature of the aluminum pipe as much in one falling, how should be the setup? Especially, should we choose a heavier, stronger magnet? — 乐永康 2014/07/13 21:39
In reply to the first question, I made some approximate calculations with some assumptions.
First, let's assume the pipe is approximately 1 meter long and the ball weighs 50 grams (Very approximate). The heat capcity of aluminum is 0.88 kJ/(kg*K), the mass of the pipe is 300 grams, and the final velocity of the ball is around 0.2 m/s.
Next, the total mechanical energy comsumption is about 0.49J (≈0.5J), equal to the increase of heat energy. That means there is a temperature increment of 1.9*10^-3K.
Finally, we can find out that after about 528 falls, the pipe will rise 1°C in temperature. — 刘知平 2014/07/14 19:15
Liu Zhiping, thank you very much. Anyone has other comments? — 乐永康 2014/07/15 21:58