2 physics question

Roland

2009-02-21 11:15 pm

1. An electric kettle uses 2.4 KW and runs for 100s.
a. If the water it heats gains 228 kj in this time, how efficient is the kettle?
b. Where has the wasted energy gone?
c. The kettle is amde from polished stainless steel. Would it be more or less efficient of it was made from dark blue platic?

2. A gas fire has a power input of 6.6 KW. It provides 4 KW of room heating.
a. How efficint is the gas fire?
b. What energy transfers take place?
c. In what form is the wasted energy?
d. Where does the wasted energy go?
e. How does the efficiency of the gas fire compare with an electric fire?
f. In answering question e, what another energy transfers need to be considered?

回答 (1)

Prof. Physics

2009-02-22 2:19 am

✔ 最佳答案

1.a. Energy input

= Power input X time

= 2400 X 100

= 240 kJ

Energy output = 228 kJ

Efficiency of the kettle

= Energy output / Energy input X 100%

= 228 / 240 X 100%

= 95%

b. The wasted energy has gone to heat up the air in the surrounding, or heat up the container.

c. It would be less efficient. It is because plastic is a poor conductor of heat, in which the energy supplied by the heater will be difficult to be transferred to heating the water.

2.a. Efficiency = Power output / Power input X 100%

= 4 / 6.6 X 100%

= 60.6%

b. The chemical energy is transferred to the internal energy of the air in the room.

c. The wasted energy is in form of light.

d. The wasted energy has gone to the light energy of the fire.

e. The gas fire has a much lower efficiency that the electric fire.

f. Light energy.

參考: Physics king

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