physics... · web viewwhy is earthing necessary in such a circuit (1mk) (b)an electric cooker has...

NAIROBI SCHOOL

APRIL HOLIDAY FORM 4 PHYSICS ASSIGNMENT

MAINS ELECTRICITY

1. A power line from a sub-station to a town a few km away has a resistance of 0.1Ω/km. determine the rate of energy loss in transmission of power over 50

km at a current of 60 A. (2mks)

2. An electric heater takes a current of 12.5 A from a 240 power supply i Calculate the power rating (2mks) ii Calculate its resistance (2mks) iii Calculate the cost of running the heater for 3 hours at ksh. 6.70 per

unit. (3mks) iv If all the energy used in running the heater for 3 hours was used to

heat water from 200C to 1000C, determine the mass of water heated. (Shc of water = 4200 Jkg-1k-1 (3mks)

3. Outline the main energy changes that occur during the production and transmission of the mains electricity from a hydroelectric power station to

the consumer. (4mks)

4. Find the maximum number of 75W bulbs that can be connected to a 13A fuse on a main supply of 240V

5. The figure shows the electric wiring of an electric heater A, B, C are the main wires.

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(a)Identify A, B, and C (3mks) (b) Give a reason why earthling is useful is electrical appliance

(1mk)

6. An 8kW heater is used for a total of 1.5 housing a day. Calculate the cost of using this shower for a month of 30 days if the cost of an electrical unit is

Ksh 7.40 (3mks)

7. Cables of resistance 3Ω supply 3kW of power. Calculate the power loss in the cable if the power is transmitted at 300V.

(3mks)

8. Explain the following observation i Bulbs in a house circuit are always connected in parallel

(2mks) ii Fuses are usually connected to the live wires

(2mks) iii There are different fuse ratings for mains ring and for lights in a

house circuit. (2mks) iv It is cheaper to transmit mains electricity at a high voltage.

(2mks) v The earth pin of a plug is longer than the neutral and live pins

(2mks)

9. (a) Define the kilowatt hour (1mk) (b) State 2 causes of power loss in transmission of mains electricity.

(2mks)

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B

A

( c) State how the power losses can be minimized. (2mks)

10. A man uses six, 60W bulbs for four hours daily and a 50W T.V for five hours daily. Calculate his monthly bill if electricity costs Ksh 6.80 per unit

and on top there is a fixed charge of Ksh 75.( 1month = 30days) (3mks)

11. A 3 pin mains plug is fitted to the lead for a 1kW electric kettle to be used on a 240V a.c supply. What value of the fuse can be used safely in the

plug? (3mks)

12. The diagram shows a three pin plug which fits into an electric socket and is connected to 3 wires A, B, and C

Name the colours of the connecting leads A, B and C. (3mks)

13. The figure shows part of the wiring system of a house

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B

C

A

(a)(i) Name the circuit labeled P. (1mk)

(ii) Identify the terminals labeled X and Y (2mks)

(iii) State the purpose of component R in the circuit (1mk)

(iv) Give a reason why component R is connected to Y and not to X (1mk)

(v) Why is earthing necessary in such a circuit (1mk)

(b)An electric cooker has an oven rated 3kW, a grill rated at 2kW and two burners each rated at 500W.The cooker operates from 240V mains. What is the cost of operating all the pars for 30min if electricity costs ksh 7.50 per unit? (3mks)

14. Outline the main energy changes that occur during the production and transmission of mains electricity from a hydroelectric power station to the

consumer. (3mks)

ELECTROMAGNETIC INDUCTION

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1. State the condition under which an emf is induced in aconductor. (2mks)

2. The figure shows awire XY at right angles to amagnetic field.XY is part of acircuit containing agalvanometer.

when xy is moved,the current flows in the direction shown.State the direction in which xy is moved. (1mk

3. (a) State one property of soft iron that makes it suitable for use as atransformer core. (1mk)

(b)Atransformer with 1200 turns in the primary circuit and 120 turns in the secondary circuit has its primary circuit connected to a400V a.c suply.It is found that when aheater is connected to the secondary circuit, it produces heat at the rate of 600w. Assuming 100% efficiency determine

(i) voltage in the secondary circuit. (2mks)

(ii) current in the primary circuit. ( 2mks)

(iii) the current in the secondary circuit. ( 1mk)

4. The figure shows an electric generator.The points X and Y are cnnected to a CRO.

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Y

X

G S N

Sketch on the axes provided the graph of the voltage output as seen on the CRO, given that when t = 0 the coil is at the position shown in the figure. (2mks)

5. (a) State Fleming’s right hand rule. (1mk)

6. (i) State Lenz’s law. (1mk) (ii) Explain how Lenz’s law illustrates the principle of conservation of energy. (2mks) (iii).A copper plate swinging between the poles of a magnet is strongly damped unless it is slotted in the right direction .Explain (2mks)

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7. An ideal transformer has 900turns on the primary and 180 turns on the secondary. Resistor of 500Ω is connected across the secondary and then an alternating Pd of 50V is applied to the primary. Calculate

(i) The Pd across the secondary (3mks

(ii) The current in the resistor. (2mks)

(iii) The power output (2mks

(iv) The current flowing in the primary (2mks)

8. The diagram below shows a bar magnet being withdrawn from the coil

State the polarity at X and Y (2mks)

State one way in which the deflection of the galvanometer could be increased (1mk)

9. The figure below shows a diagram of a moving coil microphone

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tBar magne

Galvanometer

x

Y

connected to a galvanometer

i. Label the parts marked A,B and C (2mks)

ii. Explain how the loudspeaker represented by the diagram above works (3mks)

10. (a) State Faraday’s law of electromagnetic induction (1mk) (b)State 3 factors that affect the size of induced emf in a conductor (3mks)

(c)The figure below shows Faraday’s iron ring

i. What is observed when the switch is closed

(1mk) ii. Why is soft iron ring most preferred to straight iron bar

(2mks) iii. Sketch a graph to show the variation of secondary emf during

the make and break (2mks)

11. The figure below shows an induction coil

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(a) Give a reason for the following:

i. winding the secondary coils on top of the primary coils (1mk)

ii. The primary coil is made of thick wire while the secondary is made of thin wire (1mk)

iii. the presence of a capacitor (1mk)

iv. Contacts are made from platinum (2mks)

(b)Explain how the induction coil works (3mks

(c) State one application of the induction coil (1mk

(d).State two differences between an induction coil and step-up transformer. (2mk)

12. The figure shows a coil being rotated in a clockwise direction in a magnetic field. The set up produces an alternating current. Initially the coil lies in the plane of the field as shown in the diagram a diagram.

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i. Name the parts labeled A and B (2mks) ii. State two factors that would affect the

value of the voltage output. (2mks

iii. On the axis provided in the diagram below, sketch the output as seen on the screen of a CRO when the armature is rotated starting at the position shown

(b)The same set up can be used to produce a varying direct current (d.c)

(i) What changes needs to be made on the set up.

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B

A

T (s)

V

Output

(1mk)

(ii) On the same axes sketch graphs of emf against time for I varying d.c emf (1mk

13. The diagram shows a dynamo that may be used to light the head lamp of a bicycle

a. State the function of the part labeled P .( 1mk)

b. Suggest with reasons a suitable material for P (1mk)

c. Describe how the dynamo works ( 4mks)

d. Using this dynamo how can the brightness of the head lamp be increased.( 1mk)

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physics... · web viewwhy is earthing necessary in such a circuit (1mk) (b)an electric cooker has...

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