PHYSICS MAIN ELECTRICITY REVISION QUESTIONS

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MAIN ELECTRICITY

  1. What current will a 500W resistor connected to a source of 240V draw?
  2. Name a device used to change light energy directly into electrical energy.
  3. When a current of 2.0A flows in a resistor for 10 minutes, 15000 Joules of electrical energy is dissipated. Determine the voltage across the resistor.
  4. An electric bulb rated 40W is operating on 240v mains. Determine the resistance of its filament.
  5. An electric heater rated 240V, 3000V is to be connected to a 240V mains supply, through a 10A fuse. Determine whether the fuse is suitable or not.
  6. A 60W bulb is used continuously for 36 hours. Determine the energy consumed, giving your answer in kilowatt hour (Kwh)
  7. How many 100W electric irons could be safely be connected to a 240V mains circuit fitted with a 13A fuse?
  8. Find the maximum number of 75W bulbs that can be connected to a 13A fuse on a mains supply of 240V.
  9. Determine the cost of using an electrical iron box rated 1500W, for a total of 30 hours given that the cost of electricity per Kwh is Kshs. 8.
  10. State Ohm’s law.
  11. Electrical energy costs Kshs. 1 per Kwh unit. Find the cost of using an electric heater of power 1.5 KW for a day.
  12. The figure below represents part of the main circuit.

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  1. i) Explain why it is not advisable to fix a fuse on neutral line.
  2. ii) Explain why there are fuses of different rating in the distribution box.
  3. Calculate the power of a devise which has a p.d of 250V applied across it when a current of 0.5A passes through it.
  4. An electric iron box is rated 2500W and uses a voltage of 240V. Given that electricity costs Kshs. 1.10 per Kwh, what is the cost of using it for 6 hours?
  5. Four 40 w bulbs and seven 100W are switched on for 3 hours a day for domestic monthly bill given the cost of electricity is sh. 3.50 per unit (Take 1 month = 30 days)
  6. a) Give one advantage of transmitting mains electricity as a.c and not d.c.
  7. b) The figure below shows part of a wiring circuit for a house.

 

Identify two faults in the wiring.

  1. c) The figure shows a connection to the pin plug.

 

  1. i) State the colour of the leads labelled A, B and C.

A ……………………………

B ……………………………

C ………………………………

  1. ii) Why is the earth pin normally longer than the two pins?

iii) What is the function of the fuse?

  1. d) The power company supplied electrical energy and charges the consumption to ordinary domestic wiring as follows;

– A monthly fixed charge of Ksh. 75

– Ksh. 1.55 per unit for the first 50 units consumed.

– Ksh. 6.65 per unit for the next 51- 300 units

1 unit = 1kilowatt- hour (kwh)

A consumer uses 1.98 x 105 kilojoules of electrical energy in a given month. Determine the total month bill.

  1. The figure below shows a connection to the three pin plug.
  2. i) Identify the mistakes in this wiring.
  3. ii) State two reasons why the earth pin is normally longer than the other two pins.
  4. (a) Figure 10 shows a section of a house wiring syste

(i).       Name:

The circuit labelled P

The terminals labelled X and Y

X……………………………………

Y……………………………………

(ii)      State the purpose of R in the circuit

(iii)     Give the reason why R is connected to Y but not to X

(iv)     Why is the earthing necessary in such a circuit?

(b)      Determine the cost of using an electric iron rated 1500W, for a total of 30 hours given that the cost of electricity per kWh is Ksh. 8

  1. (a)A transformer has 800 turns in the primary windings. The alternating e.m.f connected to the primary is 240V and the current flowing is 0.2A. Find the power in the secondary coil if the transformer is 90% efficient
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(b) The figure below shows a domestic wiring system.

 

 

 

(i) Point out two faults in the circuit.

(ii)     What is the fuse made of and why is it necessary in the circuit?

(c) State 2 ways of minimizing power loss in transmission lines.

  1. A transformer has 800 turn in the primary and 40 turns in the secondary winding. The alternating e.m.f connected to the primary is 240v and the current is 0.5A.

Determine

  1. The secondary e.m.f
  2. The power in the secondary if the transformer is 95% efficient.
  3. c) Explain how energy losses in a transformer are reduced by having
  4. A soft-iron core
  5. A laminated core. (
  6. A transformer with 2000 turns in the primary circuit and 150 turns in the secondary circuit has its primary circuit connected to a 800V a.c source. It is found that when a heater is connected to the secondary circuit, it produced heat at the rate of 1000W. Assuming 100% efficiency, determine the;
  7. Voltage in the secondary circuit.
  8. Current in the primary circuit.
  • Current in the secondary circuit.
  1. State the type of transformer represented above.
  2. State the reason why long distance power transmission is done at a very high voltage and using thick cables.
  3. Calculate the cost of using the following appliances in one month (30 days ) if the company rate is ksh. 9.50 per unit.

I        A 2000W water heater for 2 hours per day.

II      A 75W bulb for 10 hour per day.

III     A 1500W electric iron for 1 hour per day.

  1. Figure 6 below shows a part of housing wiring system

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. Name (i) The  circuit labeled p
  2. The terminal labeled X and Y
  • State the purpose of R in the circuit
  1. Give a reason why R is connected to Y but not to x
  2. Why is earthing necessary in such a circuit.

Determine the cost of using an electric iron rated at 1.5Kw for a total of 40 hours if he cost of  electricity is Ksh 8 per unit.

  1. (a) Long distance transmission of power is done at a very high voltage.
  • Figure 10 below shows a section of a house wiring system.

 

           

 

 

 

 

 

 

 

 

 

 

  • Name:

I           The circuit labelled F.

II          The terminals A and B.

  • Give a reason why R is connected to B and not A.
  • What is R?
  • Why is earthing necessary in such a circuit?
  • 54kg of water in a metallic cylinder of heat capacity 9000JKˉ¹ is heated from 10°C to 80°C using an immersion heater rated 1800W, 240V. Assuming that no heat is lost to the surrounding and the immersion heater works at its correct voltage.
  • The current flowing through the water.
  • The cost of heating the same mass of water from 10°C to 80°C, every day for 30 days, if electricity cost sh.6.70 per unit.
  1. (i) State the reason why long distance power transmission is done at a very high voltage and using thick cables.

(ii)  Calculate the cost of using the following appliances in one month (30 days) if the company rate is Ksh. 9.50 per unit.

  1. A 2000W water heater for 2 hours per day.
  2. A 75W bulb for 10 hours per day.
  • A 1500W electric iron for 1 hour per day.

Find the total monthly bill for the above household if in addition to the energy consumed, the power company charges each consumer a standing charge of Ksh. 200 and fuel cost at 70 cents per unit.

  1. The following appliances are operated in a home for the time indicated everyday.

Applicance                              Time

Three 40W bulbs                    3hrs

One 2000W television            5 hrs

Four 60W bulbs                      30 min

One 500W freezer                  10 hrs

Calculate:

  1. a) The total power used by the applicance everyday.
  2. b) The total electrical power consumed in 30 days in Kwh.
  3. c) If the cost per unit is ksh. 6.20, calculate the cost of using the applicances in 30 days.
  4. (a) Appliances which draw current from a ring’s main circuit have a third cable connected to the earth.  Give a reason why?
    • In a lighting circuit the wires used are relatively thinner than those of a cooker circuit. Give an explanation for this.
    • A transformer with 6000 turns in the primary circuit and 300 turns in the secondary circuit has its primary circuit connected to a 400V a.c. source. A heater connected to the secondary circuit produces heat at the rate of 600W.  Assuming that the transformer is 100% efficient determine:-
      • The voltage in the secondary circuit.
      • The current in the primary circuit.
      • The current in the secondary circuit.

 

  1. State why alternating current (a.c.) is used for transmitting electricity over long distances.
  2. State the reason why electrical power is transmitted over long distances at very high voltages.
  3. A house has one 100W bulb. Two 60W bulbs and one 30W bulb. Determine the cost of having all the bulbs switched on for 70 hours, given that the cost of electricity is 40cents per kilowatt hour.
  4. The figure below illustrates a system for transmitting electricity around a country, from the PowerStation to the consumers
  • At different points in the systems, the voltage is 220v, 11000v or 132v. At the three places marked, write the appropriate values of the voltage.
  • State one advantage of using high voltages for transmission of electricity
  1. Figure below shows the arrangement of elements in a circuit of an electric cooker. The two elements P and Q are each of resistance 40 ohms. One side of the mains is connected to terminals A. The other side of the mains is connected to a selector with three positions.
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  1. i) In the first position of the switch, the mains are connected to terminal which terminals?
  2. ii) Determine the magnitude of the current through the element

iii) In the second position of the switch the mains are connected to terminal B and to terminal A. Calculate the total resistance in the circuit.

  1. iv) Determine the magnitude of current in the second connection.
  2. v) In the third connection of the switch, terminal B is connected to terminal A and then to one side of the mains while the other side of the mains is connected to terminal C.

(i) . Determine the total resistance in the circuit

(ii) . Determine the magnitude of the power consumed by this connection

  1. a) Give a reason why a fluorescent tube is preferred to a filament bulb for domestic lighting

(b)       What property does a fuse wire have that makes it suitable for controlling excessive current in circuit?

  1. c) State two factors which affect heating in electric circuits.
  2. d) An electric heater takes a current of 12.5A from 240V power supply.
  3. i) Calculate
  4. a) Its power rating.
  5. b) Its resistance
  6. e) State Faraday’s law of Electromagnetic induction
  7. (a) Electrical energy is distributed as alternating current (a.c) at high voltage.

Explain why:

(i)        A very high voltage is necessary.

(ii)       Thick aluminium transmission lines are recommended to carry the current.

(b)       The figure shows the electric wiring of an electric cooker X, Y and Z are main wires.

 

 

 

 

 

 

 

 

 

 

 

Identify Y and X giving reasons.

(c)       A student has a large number of 240V, 60W coloured bulbs he wishes to use for decorations so that the bulbs operate normally.

(i)        How many bulbs can be connected to a 240V supply through a 5A fuse?

(ii)       If electric energy cost Kshs. 3.00 per unit, what will be the cost of running the above circuit for 5 hours a night for 20 nights?

  1. A hydro-electric power station produces 500KW at a voltage of 10KV. The voltage is then stepped up to 150KV and the power is transmitted through cables of resistance 200Ώ to a step down transformer in a sub-station. Assuming that both transformers are 100% efficient calculate;

(i) The current produced by the generator

(ii) The current that flows through the transmission cables

(iii) The voltage drop across the transmission cables

(iv) The power loss during transmission

(v) The power that reaches the sub-station

  1. a)         State one danger of high voltage transmission
  2. b) In domestic ring there are various circuits which may be used. Name three
  3. c) Figure 5 shows part of a house wiring system

 

  1. i) I using X and Y identify the live and the neutral cables respectively
  2. ii) Insert in the circuit, SL, SS and SC for switches of the lamps, sockets and

cooker respectively

iii)       Insert the earth connection in the circuit diagram

  1. iv) Explain why the lamps are connected in parallel
  2. An electric iron rated 240V, 750w is to be connected to a 240v mains supply through a 3A fuse. Determine whether the fuse is suitable or not.
  3. (a) The earth pin of the three-pin plug is longer to open the blind for neutral and live pins. State another reason for its being longer.

(b) An electrician installed electric wiring in a house and connected the bulbs and the switches as shown in figure below.

 

(i)        State what happens when switch:

I           S1 only is closed

II          S2 only is closed

III        S3 only is closed

IV        All the switches are closed.

(ii)      Redraw a diagram showing the best position the bulbs should be installed.

I:          Explain why you consider this your arrangement to be the best.

(c)      A 75W lamp and a 1.5kW water heater are connected in turn to a 240V supply.

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(i)        Calculate the current in each appliance.

(ii)      Explain the difference you would expect to find between the wiring used to connect the bulb and the water heater to the mains supply.

  1. a) State one source of mains electricity
  2. b) How are the high voltages required during power transmission achieved?
  3. c) i) Explain why three pin plugs are preferred for two pin plugs in the use with iron box
  4. ii) Explain the letters L, N and E usually marked on the three pin plug
  5. (a) Long distance transmission of power is done at a very high voltage.
  6. b) Figure 10 below shows a section of a house wiring system.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  • Name:

I     The circuit labelled F.

II    The terminals A and B.

(ii) Give a reason why R is connected to B and not A.

(iii)      What is R?

(iv)      Why is earthing necessary in such a circuit?

  • 54kg of water in a metallic cylinder of heat capacity 9000JKˉ¹ is heated from 10°C to 80°C using an immersion heater rated 1800W, 240V. Assuming that no heat is lost to the surrounding and the immersion heater works at its correct voltage.

(i)  The current flowing through the water.

(ii) The cost of heating the same mass of water from 10°C to 80°C, every day for 30 days, if electricity cost sh.6.70 per unit.

  1. Define power as used in electric circuits.
  2. a) A hoist motor powered by a 240V mains supply requires a current of 30A to lift a load of mass 3 tonnes at the rate of 5m per minute.

Calculate:

  1. i) The power input
  2. ii) The power output

iii)       Overall efficiency E

  1. The following appliances are operated in a home for the time indicated everyday.

Appliance                               Time

Three 40W bulbs                    3hrs

One 2000W television                        5 hrs

Four 60W bulbs                      30 min

One 500W freezer                  10 hrs

Calculate:

  1. a) The total power used by the appliance everyday.
  2. b) The total electrical power consumed in 30 days in Kwh.
  3. If the cost per unit is ksh. 6.20 calculate the cost of using the appliances in 30 days.   The voltmeter in figure 7(a) and the ammeter in figure 7(b) below read 2.1v and 0.35A respectively.

 

Fig. 7(a)
Fig 7(b)

 

 

 

Determine the reading in the voltmeter and ammeter in figure 7 (c ) and 7 (d) respectively. (The lamps are identical)

 

 

Fig. 7(d)
Fig. 7(c)

 

 

 

  1. Give any two causes of energy loss in a transformer
  2. a) A part from the amount of current drawn by an electric kettle, state the other factors which affects its heating effects.

(b) If the current passing through the kettle is doubled, the power dissipated increases by 3000W.

Determine:

  1. i) The power dissipated before doubling the current.
  2. ii) The current flowing if the internal resistance of the kettle element is 10W
  3. (a) What is the purpose of a fuse?

(b) The diagram in figure 9 below shows a ring – main circuit used by an electrician in a certain house.

 

  • Identify two faults in the installation.
  • Explain why the 3 – pin plug fuse has a longer earth pin than the live and neutral pins.
  • Identify the type of transformer T used in the diagram in Fig. 9

(c) A cooker rated 2.0kW was operated for 40minutes each for 30days.

If the cost of each kilo – watt – hour unit is Shs. 15.50, calculate the cost of electricity used.

  1. (a) A house has five rooms each with 240V, 60W bulbs. If the bulbs are switched on from 7:00pm to 10:30 pm
    • Calculate the power consumed in the month of April in Kilowatt-hours
    • Find the cost per month for lighting these rooms at Ksh6.70 per unit.
      • A student designed a transformer to provide power to an electric bell marked 24W, 6V from a mains supply 240V. He wound coils, 50 turns and N turns on an iron ring core. When he connected the coil of 50 turns to the bell and N turns coil to the a.c source, he found out that the transformer was only 80% efficient. Find;

(i) The value of N

(ii) The current in the primary coil.

  • The figure below shows a connection to the three- pin plug.
    • Name the cables A, B and C and state their colours.
    • Why is the fuse connected to cable C.?
    • State one reason why in domestic wiring system appliances are connected in parallel.
  1. (a) The figure below shows the inner parts of a three-pin plug.
    • Identify the pins A and B. (2 marks)
    • State the reason why the pin B is normally longer than the other two pins A and C.

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