Question 1
What is the role of a transformer at a power station? (1 mark)
A. To reduce heating in the transmission lines by stepping up the current
B. To reduce heating in the transmission lines by stepping up the voltage
C. To increase heating in the transmission lines by stepping up the current
D. To increase heating in the transmission lines by stepping up the voltage
Question 2
The total flux in the core of an electrical machine is 40 mWb and its flux density is 0.5 T.
What is the cross-sectional area of the core? (1 mark)
A. 0.01 m2
B. 0.08 m2
C. 12.5 m2
D. 80 m2
Question 3
The diagram shows an ideal transformer.
When the switch is closed, the pointer on the galvanometer deflects.
How could the size of this deflection be increased? (1 mark)
A. Decrease the number of primary coils.
B. Decrease the number of secondary coils.
C. Replace the iron core with a copper core.
D. Place a resistor in series with the galvanometer.
Question 4.
An ‘electron gun’ like that used by JJ Thomson is shown.
Electrons leave the cathode and are accelerated towards the anode.
(a) Show that the acceleration of the electrons as they just leave the cathode is 4 × 1016 ms-2. (2 marks)
(b) Calculate the velocity of an electron as it reaches the anode. (2 marks)
Question 5.
A current of 5.0 A flows in a wire that is placed in a magnetic field of 0.50 T. The wire is 0.70 m long and is at an angle of 60° to the field.
Calculate the force on the wire. (2 marks)
Question 6
The diagram shows two rings A and B, connected to a balancing arm which swings freely on a pivot. Ring A has a split in it as shown
When a bar magnet is pushed into one of the rings, the whole balancing arm begins to rotate on the pivot. When the magnet is pulled out, the balancing arm begins to rotate in the opposite direction. When the magnet is pushed in and out of the other ring, the apparatus does not move at all.
Account for these observations using Lenz’s Law and conservation of energy. (5 marks)
Question 7
A metal loop, WXYZ is connected to a battery and placed in a uniform magnetic field. A current flows through the loop in the direction shown.
The loop is then allowed to rotate by 90° about the axis PQ.
Compare the forces acting on WX and XY before and after this rotation. (3 marks)
Question 8.
A proton and an alpha particle are fired into a uniform magnetic field with the same speed from opposite sides as shown. Their trajectories are initially perpendicular to the field.
Explain ONE similarity and ONE difference in their trajectories as they move in the magnetic field.
Question 9
A charged particle, q1, is fired midway between oppositely charged plates X and Y, as shown in Figure 1. The voltage between the plates is V volts. The particle strikes plate Y at point P, a horizontal distance s from the edge of the plate. Ignore the effect of gravity.
Plate Y is then moved to the position shown in Figure 2, with the voltage between the plates remaining the same.
An identical particle, q2, is fired into the electric field at the same velocity, entering the field at the same distance from plate X as q1.
(a) Compare the work done on q1 and q2. (3 marks)
(b) Compare the horizontal distances travelled by q1 and q2 in the electric field. (3 marks)
Question 10
Two identical solenoids are mounted on carts as shown. Each solenoid is connected to a galvanometer, and the solenoid on cart 1 is also connected to an open switch and a battery. The total mass of cart 1 is twice that of cart 2.
Explain what would be observed when the switch on cart 1 is closed. In your answer, refer to the current in each galvanometer and the initial movement of the carts. (7 marks)
KIS Academics
Answers
Question 1. B
Question 2. B
Question 3. B
Question 4. a)
Question 4. b)
Question 5.
F = lIBsin(theta) = 0.7 × 5.0 × 0.5 × sin 60° = 1.5 N out of the page
Question 6.
When the magnet is pushed into ring B the ring is repelled, but is attracted when the magnet is pulled back out. This is due to the fact that the moving magnet induces a current in the ring. Lenz’s Law states that the induced current is in the direction such that the magnetic field produced by this current opposes the original change caused by the moving magnet. This means that pushing a magnet into the ring creates a ‘like’ pole, repelling the magnet, and pulling the ring out creates an ‘opposite’ pole, attracting the magnet. This is an application of the law of conservation of energy, as, if the current were in the other direction, the field produced would cause a movement that increases the change in flux even more, thereby producing even more current, and violating conservation of energy. When the magnet is pushed into ring A, no repulsive or attractive force is observed because the gap in the ring prevents a current from being induced, so no magnetic field is created as a result.
Question 7.
The magnitude and direction of the force on WX remains the same when it is rotated. Initially, XY experiences no force, whereas after rotation it experiences a force to the right.
Question 8.
Similarity – both particles will experience a force perpendicular to both their velocity and the magnetic field lines. This will result in both particles experiencing circular motion (in a clockwise direction). Difference – since the radius is proportional to the mass and inversely proportional to the charge of the particle, the radius of the alpha particle trajectory is greater.
Question 9. a)
Question 9. b)
Question 10.
When the switch is closed, a constant direct current will be observed on G1. This current will create a magnetic field in the solenoid with a south pole facing cart 2. As this field is generated, the solenoid on cart 2 experiences a momentary change in magnetic flux, inducing an emf and a current in the solenoid. By Lenz’s law, the direction of this current opposes the original change in flux, forming a south pole facing cart 1, and causing a momentary deflection of G2 in the opposite direction to the current in G1. The equal and opposite magnetic repulsion of the south poles causes the carts to move away from each other. To conserve momentum, cart 1 will have half the initial velocity of cart 2 since its mass is double that of cart 2.
Manoj Arachige
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