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# Help on Physics Question (GCSE) watch

You may find the following formula and information useful when answering some parts of this question.
Change in gravitational potential energy = weight _ change in vertical height
(joule, J) (newton, N) (metre, m)
On Earth, 1 litre of water weighs 10 N.

A hydroelectric power station uses two lakes, A and B.
10.1 When 6 litres of water fall from lake A to lake B, 1500 J are transferred.
How far has the water fallen?
A 2.5m
B 25.0m
C 250.0m
D 2500.0m

10.2 If the input power to the turbine is 1.5kW, how long has it taken for the water to fall?
A 0.1 s
B 1.0 s
C 10.0 s
D 100.0 s

10.3 Another hydroelectric power station allows water to fall 20 m to the turbine.
Only 20% of the gravitational potential energy lost is transferred into electricity.
How many litres of water must fall in order to generate 4 kWh of electricity?
A 6000
B 72 000
C 360 000
D 3 600 000
2. anyone?
3. B
B
A
4. Keep thinking about definitions of power and the equation they give you.
Write it out and plug in the information you are given.
Then try to work out what they want.

and watch out for unit changes kW--> 1000W, hr--> 60secs, that sort of thing.

A hydroelectric power station uses two lakes, A and B.
10.1 When 6 litres of water fall from lake A to lake B, 1500 J are transferred.
How far has the water fallen?
A 2.5m
B 25.0m
C 250.0m
D 2500.0m
6 litres--> weighs 60N
using the equation ^GPE=weight^h (^ as delta/change in)
1500 = 60*h
h = 1500/60 = 25.0m (B)

10.2 If the input power to the turbine is 1.5kW, how long has it taken for the water to fall?
A 0.1 s
B 1.0 s
C 10.0 s
D 100.0 s
Power = energy/time
since 1.5kW = 1500 W
1500 = 1500/t
t = 1500/1500 = 1.0s (B)

10.3 Another hydroelectric power station allows water to fall 20 m to the turbine.
Only 20% of the gravitational potential energy lost is transferred into electricity.
How many litres of water must fall in order to generate 4 kWh of electricity?
A 6000
B 72 000
C 360 000
D 3 600 000
Energy = Power*time
E = 4kW*60s = 4000*60 = 240000J

only 20% efficient, so work done by water is
240000/20% = 1200000J

Since ^GPE=weight^h
1200000 = (weight of water)*20
weight of water = 1200000/20 = 60000N

Since "On Earth, 1 litre of water weighs 10 N."
weight of water = 60000N = 10*n (n being number of litres)
n = 60000/10 = 6000litres

-----------------------
Is that ok?

selkie
5. (Original post by selkie222)

Is that ok?

selkie
that's excellent!

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Updated: June 28, 2004
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