Phy 數問

Question

1.When a guitar wire is plucked harder, will quality and pitch change? why?

2.Which of the following can control rods control? Why? (<<冇錯題目)
a. Number of neutrons.
b. Temperature in a nuclear reactor.
c. Energy released in a fission reaction.

3. A kettle of power 1200W takes 15 mins to boil 3 kg of water. If electrical energy cost $0.9 per kWh, how much does it cost to boil 1-kg water by this kettle in term of cents per litre ?

Answer 1

1. No. Only the loudness (i.e. wave amplitude) will change. There will not be any change in quality (waveform) and pitch (wave frequency).

2. The answer is (a) no. of netrons.
Control rod is made of neutron absorbing material. It is used to limit the no. of netrons so as to control the rate of fission reaction in the reactor.

3. Energy supplied by the kettle = 1200 x 15 x 60 J
therefore, energy needed to boil 1 kg of water = 1200x15x60/3 J = 360 000 J
= 360 000/3600000 kWh = 0.1 kWh
(since 1 kWh = 3 600 000 J)

Cost = $0.1 x 0.9 = $0.09 = 9 cents
since 1 kg of water has a volume of 1 litre
thus, cost per litre = 9 cents