1. a light coil P is suspended to a spring balance by an inextensible string.
Another coil Q is connected to a battery and a rheostat. If the resistance of the
rheostat is suddenly reduced significantly, what happens to the reading of the
spring balance at that instant ?
A. Unchanged
B. Increased
C. Decreased
D. Cannot be determined as the polarity of the battery is not given.
2. Given the power dissipated in a fixed cable is P when a current I flows through it. If I is reduced by 25%, what is he percentage change of P ?
A. Reduced by 94%
B. Reduced by 75%
C. Reduced by 44%
D. Reduced by 25%
3. In the circuit , the rating of the light bulb is " 20 W, 5 V " . The efficiency of the
transformer is 60% . If the bulb works at its rated value , find the current in the
primary coil .
A. 0.1 A
B. 0.15 A
C. 0.2 A
D. 0.25 A
物理 MC choice
2013-01-17 12:03 am
回答 (2)
2013-01-17 12:43 am
✔ 最佳答案
1. I suppose there is a diagram associated with this question. Without the diagram, I assume coil Q is placed beneath coil P.Once the current in coil Q is suddenly decreased, the magnetic flux passing through coil P because of the current in coil Q decrases. By Lenz's Law, induced emf and thus induced current must flow so as trying to mainatain the magnetic flux. As such, the opposite polarity will be produced at the lower end of coil P to that at the upper end of coil Q. Attractive force is thus produced, causing the balance reading to increase.
The answer is option B.
2. New current I' = (1-0.25)I = 0.75I
Initial power P = (I^2).R where R is the resistance of the cable
New power P' = (I'^2).R = (0.75I)^2.R = 0.5625(I^2.R) = 0.5625P
Percentage change of power = [(P' - P)/P] x 100%
= (0.5625 - 1) x 100% = -43.75% (the -ve sign indcates a power reduction)
The answer is option C
3. Again, I suppose there is a diagram given, showing the turn-ratio of the transformer.
Power delivered at primary coil = 20 x 100/60 w = 33.33 w
Let the turn-ratio = N (N = turns in primary/turns in secondary)
Then voltage at primary coil = 5N
Current in primary = 33.33/5N
2013-01-17 7:30 am
天同 ,
thankyou for your help !!!!
thankyou !!!!
thankyou for your help !!!!
thankyou !!!!
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