F5 Physics Second Term Test (16-17) Marking Scheme

1. (a) Magnetic field (or B) must be at right angles to velocity (or v) 1M

(b) (i) Into paper 1M

(ii) Because the magnetic force and the electric force have equal magnitude and act in opposite

directions and they balance (or cancel) each other (the resultant force is zero).

(If just mention magnetic force and electric force cancel, 1M)

(iii) QvB = QE (1M for magnetic force = electric force)

(1M for E = V/d, 1A)

It is because the upward magnetic force increases but the electric force is unchanged. (1M)

(Or magnetic force now greater than electric force)

2. (a) The bulb flashes once at the moment the switch is closed (or flashes momentarily). (1A)

(b) VpIp x 0.9 = Pout (1M)

Ip === 0.0606A (1A)

(c) 1) The a.c. continually magnetises and demagnetises the core. This requires extra energy and this energy is wasted as heat. The energy loss can be reduced by using soft iron to make the core because soft iron is a material which can be easily magnetised and demagnetised. (1M for magnetise and demagnetize, 1M for soft iron)

2) The core is made of soft iron and is in a changing magnetic field, eddy currents are induced in it, and as a result, the core heats up. The energy losses due to eddy currents can be reduced by using a laminated core. (1M for eddy currents, 1M for laminated core)

3. (a) (i) Into paper (1A)

(ii) B =

(1M +1A)

(iii) Magnetic flux = BA = 2.5 x 10-5 x p (0.02)2 = 3.14 x 10-8 Wb (1M for flux = BA, 1A)

(b) (i) Clockwise direction (1A)

(ii) e == (1M for +1A)

(iii) The magnitude of induced emf depends on d because e = and =A.

The induced emf does not depend on resistance.

(1M for dependence on d, 1M for e dependence on flux change, 1M for flux change depends on d. 1M for independence on resistance.)

(a) X should be connected to B.
Y should be connected to C.
Z should be connected to A. / 2A for all correct
1A for 2 correct
(b) S1 is placed in wire X to ensure that no part of the hand-dryer is still at high electric potential ( or ‘live’) when the switch is off. / 2M
(c) (i) By V = IR, / 1M
current through the heating element === 4.4 A / 1A
(ii) Power of the heating element =
=
= 968 W / 1M
1A
(iii) Total power of the hand-dryer = 968 + 200 = 1168 W
Cost = 1.168 ´ 5 ´ 0.9
= $5.26 / 1M
1A

5. (a) There is an upward magnetic force on side PQ. (1A)

There is no magnetic force on side QR. (1A)

(b) X is a commutator. (1A)

It reverses the direction of current flowing through the coil. (1A)

Thus the coil will continue to rotate in the same direction. (1A)

Number of revolutions made by the coil in one minute = 60/0.2 = 300 (1A)

(ii) At t = 0.05 s, 0.15 s and 0.25s (1A for all correct)

(iii) (1) The peak value of the voltage will be halved. (1A)

(2) The shape of the graph will be inverted. (1A)

(d) Slip-rings and brushes are not needed because they are easily damaged by sparks / the generators are more efficient / durable. (1A for no slip-rings or brushes, 1A for less damage/more efficient/more durable)

1-5 A B B A A 6-10 A B A C D 11-15 D C B D A 16-20 D B D D D

MC explanations

1. F = qE = qV/d = 4 x 10-12 x 400 x 106 / 1000 = 1.6 x 10-6 N

2. E = I (R+r) = (8/16) (16+2) = 9V

3. When K is closed, bulb M becomes shorted, so total resistance decreases.

4. Vrms = 2V0 / => doubled.

5. Statement 3, he will not receive electric shock because of the presence of earth wire which carries most of the leakage current to the ground.

6. Use P = V2 /R to calculate resistance of each bulb. The brightest bulb corresponds to highest resistance.

7. B = independent of radius of solenoid.

8. Change in flux linkage = BA (cos 0o – cos90o) = 3 x 10-3 x p (0.05/2)2 = 5.89 ´ 10–6 Wb

If I à 2I, F will be 4 times. r à 2r, F à F/2. All together, Fà 2F.

10. F = BIl sin q = 0.2 x 4 x 0.08 x sin 60o = 0.055 N

11. The iron becomes an electromagnet twice for each cycle.

12. BQv = mv2 /r => r = mv/BQ

Statement 2, P has a smaller radius than Q, so vP is smaller.

T = 2π/w = 2πm/BQ independent of radius.

13. 42 x 1 + (-2)2 x 1 = <I2> x 2 => Irms =

14. XY can be regarded as a cell with Y being higher potential.

15. By Lenz’s law, the bar magnet rotates in the same direction as the disc to reduce relative motion.

16. e =

17. <P> = Vrms2 / R =

18. Magnetic field into paper is increasing in loop B, so the induced current produced in loop B is in anticlockwise direction. As a result, the two adjacent currents are in opposite directions and so the force is repulsive.

19. For output, I = P/V = 10000/500 = 20A

Power input = 202 x 0.2 + 10000 = 10080W

Voltage required = P/I = 10080/20 = 504V

20. Otput power = 22 x 2 = 44W

220 x 0.25 x efficiency = 44

Efficiency = 80%