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Physics: Post your doubts here!

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amal sharkawi said:
A wind turbine has blades that sweep an area of 2000 m
2. It converts the power available in the

wind to electrical power with an efficiency of 50%.
What is the electrical power generated if the wind speed is 10 m s
–1? (The density of air is

1.3 kg m
–3.)

A
130 kW B 650 kW C 1300 kW D 2600 kW
can anyone explain this question
Click to expand...
mass flow rwte intercepted by wind turbine blades is 2000*10*1.3=26000 kg/sec
energy=1/2* 26000 * 100 Joules/sec or watts.
due to 50% efficiency,power produced is half of that. Divide by 1000 o express in KW
Yahoo answer.
 
A

amal sharkawi

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The diagram shows a wheel of circumference 0.30 m. A rope is fastened at one end to a force
meter. The rope passes over the wheel and supports a freely hanging load of 100 N. The wheel is
driven by an electric motor at a constant rate of 50 revolutions per second.
When the wheel is turning at this rate, the force meter reads 20 N.
load 100N
force
meter
25

20
15
10
5
0
50rev s
–1

wheel of
circumference 0.30m
N
What is the output power of the motor?
A
0.3 kW B 1.2 kW C 1.8 kW D 3.8 kW
any one can explain this question
 
A

amal sharkawi

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A light wave of amplitude
A is incident normally on a surface of area S. The power per unit area

reaching the surface is
P.

The amplitude of the light wave is increased to 2
A. The light is then focussed on to a smaller

area
. 3

1
S

What is the power per unit area on this smaller area?
A
36P

B
18P

C
12P

D
6P

can any one explain this
 
Suchal Riaz

Suchal Riaz

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amal sharkawi said:
The diagram shows a wheel of circumference 0.30 m. A rope is fastened at one end to a force
meter. The rope passes over the wheel and supports a freely hanging load of 100 N. The wheel is
driven by an electric motor at a constant rate of 50 revolutions per second.
When the wheel is turning at this rate, the force meter reads 20 N.
load 100N
force
meter
25

20
15
10
5
0
50rev s
–1

wheel of
circumference 0.30m
N
What is the output power of the motor?
A
0.3 kW B 1.2 kW C 1.8 kW D 3.8 kW
any one can explain this question
Click to expand...
It wpuld be better if you would just tell me the question number and year of paper.
 
Suchal Riaz

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if you have casio fx 991ES or other good calculator here is how to do this:
press mode press 4
now write a decimal number, lets say 125 and press =
now press shift+log
it will show the binary as 000000001111101
all you need it 11111101
 
talha-_-khan

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Need this answers with a little explanation please
001.jpg

 
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chocolate_girl said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w04_qp_1.pdf
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w04_er.pdf

helpp with exp in mcq
2
7
20
31
ty in advance
Suchal Riaz
Click to expand...
2) A and D must not be the answer, Its obvious.
the answer is C. because unit current is not like unit charge.
current is how many electrons in a second i.e. e/second, which has Amps as its unit.
Charge is Coulomb with a unit C. so both of them are NOT the same.
p.d.= work done/unit charge NOT work done/current SO Answer is not B

7) 20) Suchal Riaz ZaqZainab

31) Before we select an answer, let's review what each answer means.

A. The mathematical expression of this statement is F = E*q. Voltage (potential difference) is not a force, and the E here is an electric field not a difference in potential, so this answer cannot be correct.

B. Energy = V*q = d*E*q is the mathematical expression referred to here. Since I (current) = dq/dt, q = I*t (current is the derivative of charge with respect to time, thus total charge is current times total time passed). This means Energy = V*q (voltage times charge) = V*I*t (voltage times average current times time elapsed), which can be rearranged to say V = E/(I*t). This is not the ratio of energy to current, rather the ratio of energy to charge passed, so this answer cannot be correct.

C. Power = V*I. Thus V = P/I. This is a correct assessment of potential difference. Saying that potential difference in a wire is equal to the ratio of power to current is correct, since V = P/I, so this answer is acceptible.

D. P = V*I = V*dq/dt, which means V = P/(dq/dt). You can go one step further and integrate the top and bottom of this equation and say V = int(dE/dt)/int(dq/dt) = E/q. In neither case is the potential difference the ratio of power to charge moved, it is either the ratio of power to charge moved per second (current), or it is the ratio of the energy dissipated to the charge moved. So this answer is unacceptible.

Thus the answer is C.
 
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