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物理 MC choice
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
4. The rated values of both bulbs are " 12 W , 6 V " . The two bulbs are operating at their rated values. If the reading of the ammeter is 125 mA , fing the efficiency of the transformer .
A. 40 %
B. 60 %
C. 80 %
D. 100 %
5. PQ and RS are two parallel metal rails with separation L .
A metal rod XY resting on the rails moves with velocity v perpendicular to its length across a uniform magnetic field of flux density B pointing into the paper as shown. If the rod makes an angle θ with the rails , what is the potential difference across Q and S ?
(Assume that the rod is always in contact with the rails.)
6. A rigid rectangular conducting loop PQRS is connected to a cell as shown. It is held at rest horizontally within a uniform magnetic field which is parallel to the plane of the loop and perpendicular to its side RS.
Which of the following statements is / are correct ?
(1) The side RS of the loop experiences an upward magnetic force.
(2) The loop experiences a torque due to the magnetic field.
(3) The magnetic flux linking the loop due to the uniform magnetic field is zero .
A. (1) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
2 個解答
- 天同Lv 78 年前最愛解答
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
