🌊PAPER 04-QNS

 

O-STUDY LIBRARY

TOPIC QUESTIONS:ELECTROMAGNETISM

LEVEL:FORM FOUR

PHYSICS 1

SECTION A 16 MARKS

Answer ALL questions in this section.

1. For each of the items (i) – (x), choose the correct answer from among the given alternatives.

1. The main function of a step-down transformer is to

1. Decrease current.

2. Decrease voltage.

3. Change a.c. to d.c.

4. Change d.c. to a.c.

5. Decrease the resistance of a circuit.

2. The direction of induced current is obtained by

1. Lenz’s law.

2. Fleming’s left hand rule.

3. Clock rule .

4. Right hand thumb rule.

5. Fleming’s right hand rule.

3. An electric current flows into the page. What is the direction of the magnetic field?

1. To the bottom of the page.

2. To the top of the page.

3. Clockwise.

4. Counter-clockwise.

5. To the right.

4. A transformer can’t function normally with a d.c because a d.c. A. Has an extremely high heating effect.

2. Reduces the efficiency of the transformer.

3. Cannot produce a changing magnetic field.

4. Its an ideal.

5. Cannot provide voltages required for power transmission.

4. The Lenz’s law is applied to describe:

   1. Magnitude of the back emf in a circuit.

   2. Magnitude of induced current in a circuit.

   3. Direction of induced emf in a circuit.

   4. Direction of applied emf in circuit.

   5. Direction of the applied emf within the circuit.

4. The figure below shows a transmission line from power station to a consumer several kilometers away.

Which one of the following is the correct type of transformers at P and Q?

	P	Q
A	Step-up	Step-up
B	Step-down	Step-down
C	Step-up	Step-down
D	Step-down	Step-up
E	Generator	Motor

4. Which one of the following is the most economical means of transmitting electricity over a long distance?

1. At a high voltage and a low current.

2. At a high voltage and a high current.

3. At a low voltage and a low current.

4. At a low voltage and a low current.

E At current and low emf.

4. Consider the diagram below

An electron beam is incident into the page at right angles to the magnetic field formed between two magnets R and S as shown in the above diagram. The beam will be deflected.

1. Down wards.

2. Towards magnet R.

3. Towards magnet S.

4. Upwards.

5. Horizontally.

4. Which of the following factors affect the magnitude of force on a current carrying conductor in a magnetic field?

(i) Direction of current. (ii) Amount of current

(iii) Direction of the magnetic field (iv) Strength of the magnetic field. A. (i) and (ii) only.

2. (ii) and (iii) only.

3. (i) and (iii) only.

4. (ii) and (iv) only.

5. (i) only.

4. The induced current in a generator

   1. Is a maximum when the coil is vertical.

   2. Is a minimum when the coil is horizontal.

   3. Changes direction when the coil is horizontal.

   4. Increases when the speed of rotation increases.

   5. Decreases when the coil is horizontal.

4. A transformer has twice as many turns in the secondary as in the primary coil. The a.c input to the primary is 4V. Find the output voltage.

   1. 2 V B. 8 V C. 4 V D. 16 V E. 32

2. Match the item in LIST A with responses LIST B by writing the letter of the correct response beside the item number in the answer booklet provided.

	LIST A	LIST B
(i)	Are used to provides a constant magnetic field
(ii)	As it rotates, the magnetic field through it changes, which induces an e.m.f.
(iii)	External circuit.
(iv)	They turn with the coil. They are made of a good conductor such as copper.
(v)	They keep electrical contact with the slip rings but do not move. They link the slips rings to the external circuit.

SECTION B (54 Marks)

Answer all questions from this section

2. (a) An engineer determines that the power input of a transformer is twice the power output.What would be the efficiency of the given transformer?

(4 marks)

2. The figure below shows an electromagnet made by a student in a laboratory

State two changes that the student can make so that a heavier iron block could be lifted by the magnet. (5 marks)

2. (a) Husna wants to determine the direction of a force, experienced by a straight current carrying conductor placed in a magnetic field which is perpendicular to it. Which rule will she apply to determine it? And how? (4 marks)

2. A moving coil galvanometer gives a full scale deflection of 4mA and has a resistance of 5Ω.How can such instrument be converted into an ammeter giving a ful-scale deflection of 10A? (5 marks)

2. (a) The current in the secondary coil of a transformer is 1A and that in the primary is 0.5A. Explain the working principle of a transformer. (4 marks)

2. Explain, with the help of a suitable example, how we can show that Lenz’s law is a consequence of the principle of conservation of energy. (5 marks)

2. (a) Eddy currents is undesirable.What is the statement mean? (3 marks)

2. The figure below shows a coil and a magnet being removed from the coil

1. State the polarity at end B (3 marks)

2. Indicate the direction of flow of current on the coil. (3 marks)

2. (a) Iron core of a transformer made laminated (thin sheets) instead of being in one solid piece.What would be the reason behind. (4 marks)

   2. A bicycle wheel is mounted vertically on a metal axle in a horizontal magnetic field from the Figure below. Sliding connections are made to the metal edge of the wheel and to the metal axle.

1. Explain why an e.m.f. is induced when the wheel rotates. (2 marks)

2. State and explain two ways in which this e.m.f. can be increased. (3 marks)

2. (a) Explain how can you convert galvanometer into voltmeter. (4 marks)

2. What will be the multiplier required to covert a galvanometer of a coil resistance 10Ω and current 15mA to make it read 3V full scale-deflection? (5 marks)

SECTION C (30 marks)

Answer only two (2) questions.

2. (a) Basing on electromagnetic induction, when the current is circulating (in a closed loop) within a metal block pouses a problem like heating effect.Identify the current and explain how can you minimize the problem. Three (3) points. (3 marks)

   2. Consider the diagram below then answer the questions below

The secondary coil is removed and a small coil connected to a low voltage lamp is placed as shown. Explain the following observations:

1. The lamp lights, (3 marks)

2. If the coil is moved upwards, the lamp gets dimmer, (3 marks)

3. If a soft iron rod is now placed through the coil, the lamp brightens

again, (3 marks)

4. The lamp will not light if a d.c. supply is used instead of an a.c. one.

(3 marks)

2. (a) A student thinks that electrical current passes through the core in a transformer to the secondary coil. Describe how you might demonstrate that this is not true and explain how an electrical current is actually induced in the secondary coil. Use Faraday’s law in your explanation. (5 marks)

2. Calculate the number of turns on the secondary of a step- down transformer which would enable a 12 V lamp to be used with a 230V a.c. mains power, if

there are 460 turns on the primary. (5 marks)

2. Alternatively the 6V, 24W lamp can be operated normally from a 240V d.c. supply using a suitable fixed resistor, R, as in the diagram.

What is the resistance of the lamp? (5 marks)

2. (a) The diagram shows a circuit used in a charger for a mobile phone.

1. Name the parts labelled F, G and H. (3 marks)

2. Describe the function of G in this circuit. (4 marks)

(b)The diagram below shows the circuit for an electric bell.

1. Explain in your own words why the bell will not work if the electromagnet is replaced with a permanent magnet. (4 marks)

2. Explain why the core of the electromagnet used in an electric bell must not

be made from steel. (4 marks)

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