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Form 4 Physics electromagnetic induction topical questions and answers
State how eddy currents are reduced in a transformer.
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1.
(a) (i) State the laws of electromagnetic induction. (ii) Describe an experiment to demonstrate Faraday’s law. (b) (i) A researcher studying the behavior of set-up transformer made the following observations: “More joules per coulomb and fewer coulombs per second at the output than at the input terminals” Explain why the observation does not imply a violation of the principle of conservation of
2.
What causes electromagnetic damping in a moving coil galvanometer?
3.
A transformer in a welding machine supplies 6 volts from a 240 V mains supply, if the current used in the welding is 30A. Determine the current in the mains.
4.
State how eddy currents are reduced in a transformer.
5.
The primary coils of a transformer have 2000 turns and carries a current of 3A. If the secondary coil is designed to carry a current of 30A, calculate the maximum number of turns in the secondary coil.
6.
An armature composed of turns of insulated copper wire wound on laminated soft-iron core is rotated in a magnetic field to generate an e.m.f. State two factors other than the speed of rotation that affect the magnitude of the e.m.f generated.
7.
An armature composed of turns of insulated copper wire wound on laminated soft-iron core is rotated in a magnetic field to generate an e.m.f. State the reason why softiron is laminated.
8.
(i) State one property of soft iron that makes it suitable for use as a transformer core. (ii) Figure 6 represents a step-down transformer with 500 turns in the primary and 50 turns in the secondary. The turns are wound uniformly on the core. The length of PQ and QR are indicated. Determine the p.d across PQ.
9.
Figure 15 shows a wire XY placed in a magnetic field. State the direction in which the wire must be moved for the current to move in the direction shown.
10.
Two identical copper coils P and Q are placed close to each other as shown in figure 17. Coil P is connected to a D.C, power supply and coil Q is connected to a galvanometer, G. State and explain what would be observed on the galvanometer immediately the switch S is closed.
11.
Two identical copper coils P and Q are placed close to each other as shown in figure 17. Coil P is connected to a D.C, power supply and coil Q is connected to a galvanometer, G. State with reason the difference that would be noted in the observation made in question 10 if the number of turns in coils Q were doubled.
12.
(a) Figure 6 shows a simple generator. The coils are rotated in the anticlockwise direction as shown. (i) Indicate using an arrow on the figure, the direction of the induced current as the coil passes the position shown. (ii) State two ways of increasing the magnitude of the induced current in this type of generator. (iii) On the axis provided, sketch the graph of the induced e.m.f with time.
13.
In the set up in figure 19, the magnet is moved towards the coil and stopped when inside the coil. It is observed that the galvanometer deflects to one side and then goes back to zero. Explain this observation.
14.
(a) State Lenz’s Law of electromagnetic induction. (b) Figure 13 shows a simple microphone in which sound waves from the person talking cause the cardboard diaphragm to vibrate. (i) Explain how a varying current is induced in the coil when the diaphragm vibrates. (ii) State two ways in which the induced current in (i) above can be increased. (c) A transformer with 1200 turns in the primary circuit
15.
(a) Figure 12 shows two circuits close to each other. When the switch is closed, the galvanometer shows a reading in the opposite direction and then returns to zero. Explain these observations. (b) Explain how energy losses in a transformer are reduced by having: (i) A soft-iron core. (ii) A laminated core. (c) An ideal transformer has 2000 turns in the primary circuit and 200 turns in the
16.
Figure 5 shows a magnet being moved towards a stationary solenoid. It is observed that a current flows through the circuit in a direction Q to P. Explain: (i) How the current is produced. (ii) Why the current flows from Q to P.
17.
Explain why electric power is transmitted over long distances at high voltages.
18.
Figure 6 shows a horizontal conductor in a magnetic field parallel to the plane of the paper. State the direction in which the wire may be moved so that the induced current is in the direction shown by the arrow.
19.
(a) State what is meant by the term “electromagnetic induction”. (b) Figure 9, shows a simple electric generator. (i) Name the parts labeled P and Q. (ii) Sketch on the axis provided, a graph to show how the magnitude of the potential difference across R, changes with the time, t. (iii) State two ways in which the potential difference produced by such a generator can be increased.
20.
(a) Figure 15, shows two coils A and B placed close to each other. A is connected to a steady D.C supply and a switch, B is connected to a sensitive galvanometer. (i) The switch is now closed. State the observations made on the galvanometer. (ii) Explain what would be observed if the switch is then open. (b) The primary coil of a transformer has 1000 turns and secondary coil has 200 turns.
21.
Figure 4 shows a simple transformer connected to a 12V a.c source and an a.c voltmeter. By counting the number of turns in each coil, determine the reading on the voltmeter.
22.
(a) One of the causes of energy loss in a transformer is heating in the coils when current flows. State: (i) The reason why the current causes heating. (ii) How the heating can be minimized. (b) The input voltage of a transformer is 240V and its output is 12v. When an 80W bulb is connected across the secondary coil, the current in the primary coil is 0.36A. Determine: (i) The ratio #N_p/N_s# of
23.
(a) Figure 8, shows a conductor AB connected to a galvanometer and placed between two permanent magnets. (i) Conductor AB is moved perpendicular to the magnetic field. State and explain the observation made on the galvanometer. (ii) State the effect of moving the conductor faster. (b) A transformer is used to step down 240V to 12V for use in an electric appliance operating at 0.5A. if the primary
24.
A transformer has 600 turns in the primary coil and 9000 turns in the secondary coil. If the transformer is 100% efficient and the current in the secondary coil is 0.15A, determine the current in the primary coil.
25.
Figure 12 shows two identical copper coils X and Y placed close to each other. Coil X is connected to a DC power supply while coil Y is connected to a galvanometer. (i) Explain what is observed on the galvanometer when the switch is closed. (ii) State what is observed on the galvanometer when the switch is opened. (iii) State what would be observed if the number of turns in coil Y is doubled.
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