Home Teachers' Resources PHYSICS ELECTROMAGNETIC INDUCTION REVISION

# PHYSICS ELECTROMAGNETIC INDUCTION REVISION

ELECTROMAGNETIC INDUCTION

1. The diagram in figure 1 shows an arrangement that may be used to investigate how electromagnetic force varies with current. Explain how the arrangement may be used for this investigation.

1. a) The free ends of a coil are connected to a galvanometer. When the north pole of a magnet is moved towards the coil, the pointer deflects towards the coil; the pointer deflects towards the right as shown. State with reason the behaviour of the pointer in the following cases.

1. i) The north pole of the magnet is held stationary near p.
2. ii) The south pole of the magnet is made to approach the coil from Q.
3. c) Two coils T and S are wound on a soft iron core as shown. T has 1000 turns while S has 600 turns and resistance of 100W

Calculate the maximum current measured by the ammeter.

1. Calculate the peak value of an alternating current which has a root mean square value of 3.0A.
2. A large sub station transformer is used to step down voltage from 11,000V to 450V.
3. i) Determine the ratio of the turns in the primary to secondary coils.
4. ii) How is the efficiency of this transformer ensured?

iii)       State one function of the core in a transformer.

1. A generator produces a peak voltage of 220v. What is the root mean square value of this voltage?
2. Name any two ways by which a transformer loses energy.
3. The Fig; Represents a transformer connected to an ac source and a resistor R. Compare the magnitudes of the:
4. i) Voltages Vp and Vs
5. ii) Currents Ip and Is
1. (a)
2. A researcher studying the behaviour of step up transformer made the following observation. ‘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 energy.
3. A transformer of 480 turns in the primary coil used to connect a 9-volt a.c. electric device to a 240V a.c. mains power supply. Calculate the number of turns in the secondary coil
4. What causes electromagnetic damping in a moving coil galvanometer?
5. State how Eddy Currents are reduced in a transformer.
6. A transformer in a welding machine supplies 6 volts from a 240V mains supply. If the current used in the welding is 30A. Determine the current in the mains.
7. A hair drier is rated 2500W, 240v. Determine its resistance.
8. A heater of resistance R1 is rated 2p watts, v volts, Determine R1/R2
9. a) State Len’s law of electromagnetic induction.                                  (1mk)
10. b) Figure 13 shows a simple microphone in which sound waves from the person talking cause the cardboard diaphragm to vibrate.

1. i) Explain how a varying current is induced in the cell when the diaphragm vibrates.
2. ii) State two ways in which the induced current (i) above can be increased.
3. c) A transformer with 1200 turns in the primary circuit and 120 turns in the secondary circuit; it produces heat at the rate of 600w. Assuming 100% efficiency, determine the:
4. i) Voltage in the secondary circuit.                               (2mks)
5. ii) Current in the primary circuit.                                   (2mks)

iii)       The current in the secondary circuit.                         (1mark)

1. An ac flows in a resistor of 100Ω. If the peak value of the voltage across the resistor is 60V. Calculate.
2. a) The rms. Voltage
3. b) The rms. Current
4. A student designed a transformer to supply a current of 10A at a potential difference of 60V to a motor from an A.C mains supply of 240V. If the efficiency of the transformer is 80%, determine the;
5. a) Power supplied to the transformer
6. b) Current in the primary coil.
7. An immersion heater rated 300W is used continuously for 45 minutes per day. Calculate the cost per week at 60cts per unit.
8. A radio transmitter directs pulses of waves towards a satellite from which reflections are received 10 millisecond after transmission. Determine the distance of the satellite from the radio transmitter. (Speed of radio waves= 3 x 108ms-1)
9. An electric bulb with a filament resistance 300W is connected to a 2v main supply, determine the energy dissipated in 2 minutes.
10. A 50w bulb is used continuously for 36 hours. Determine the cost of energy consumed at a cost of Kshs. 2 per unit.
11. a) State Lenz’s law of electromagnetic induction.
12. b) Two identical copper coils P and Q are placed close to each other as shown in figure.

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.

1. c) Figure 11 shows a diagram of a simple electric generator.

1. i) Name the parts labelled A and B.

A……………………………                                                 1mk

B……………………………                                                 1mk

1. ii) On the axis provided in the diagram below sketch the output as seen on the screen of the cathode ray Oscilloscope (CRO) when the amature is rotated starting at the position shown in figure 11.

iii)State two factors that would affect the value of the voltage output 2mks

1. d) A transformer has 400turns in the primary coil and 20 turns in the secondary coil. If the primary voltage and the primary are 120v and O.SA respectively determine the value of the secondary current. State any assumptions made.
2. a) The figure 11 (a) and 11 (b) represents a sections of simple current generators

x and y. When in action, each coil is rotated in a magnet field.

Fig 11 (a)                                                                    Fig 11 (b)

• State the name of generator x and y
• Sketch graph of the induced emf against number of revolution of each coil in y.
• State one way in which induced emf of a simple generator can be increased.
1. b) Explain why it is cheaper to transmit mains electricity at high voltages.
2. c) Four 40 w bulbs and seven 100 w 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)
1. a) The figure below shows a rigid circuit ABCD suspended by a silk thread from a support. The coil AB is made of copper.

1. When a magnet near A is suddenly pulled to the left, the circuit swings to the left.

Explain this observation.

1. ii) Give a statement of the law applied in (i) above.
2. b) Distinguish between an electric motor and an A.C generator in reference to the energy changes involved in each case.
3. c) i) State two features of the soft iron core that helps to improve the efficiency of the transformer.
4. Explain how any one of the features named in c (i) above helps to improve efficiency.
5. d) The figure below shows a coil ABCD being rotated in a clockwise direction in a magnetic field. The set-up produces alternating current. Initially the coil lies in the plane of the field as shown.

1. i) Name the features labeled.

X         …………………………

Y         ………………………..

1. ii) The terminals P and Q are connected to a cathode ray oscilloscope. Sketch a graph of the output e.m.f against time as will be seen on the C.R.O screen.

On the same axes for d(ii), sketch the graph of e.m.f against time  for an e.m.f from an accumulator.

1. a) The figure below shows the features of a cathode ray tube
2. i) Name the parts labeled A and B.

A         ……………………

B         ……………………

1. ii) Explain how electrons are produced in the tube.

iii)       State two functions of the anodes.

1. iv) State the part where the time base would be connected.
2. v) State any two adjustments that would be made on the C.R.T to create an x-ray tub
3. b) The time base calibration of the C.R.O was set at 20milliseconds per centimeters and they gain at 5 volts per centimeter. If an a.c signal input produced a wave whose peak voltage was 20V, how many centimeters did it rise.
4. c) Give a reason why a T.V screen is wider and the tube is shorter than a C.R.O.
5. a) The figure below shows a simple door alarm.

Explain how the door alarm works.                                                   (3 marks)

1. b) An electric light bulb is rated 60W 240V. What is its operating resistance.
2. c) During the manufacture of filament lamps, the bulb is filled with a mixture of nitrogen and argon. State the reason why.
3. d) The inside of a fluorescent tube is lined with a white powder. Give a reason for this.
4. e) State the material used in the manufacture of heating elements.
5. a) State Lenz’s law of electromagnetic induction.
6. b) The diagram below is a simplified illustration of an e.m.f generator.

1. i) Name the part labelled A.
2. ii) Show the direction of induced e.m.f when the coil is in the position shown in the diagram.

iii) State two ways of increasing the amount of induced e.m.f in this sate up.

1. c) Give a reason why the output from this generator can not be used to charge a battery.
2. d) State and explain any two ways by which energy losses are minimized in a transformer.
3. e) A person has a 6v bell. He hopes to operate the bell from a 240v a.c mains supply, with the help of the transformer shown in the figure.

Calculate the output voltage of the transformer when connected to the 240v mains.

1. a) State Lenz’s law of electromagnetic induction.
2. b) The figure shows two coils of insulated copper wires wound on a single soft iron core.

One coil is connected to a battery through a switch and the other is connected to a Resister through a galvanometer.

It is observed that as the switch is closed, the pointer of the galvanometer deflects momentarily.

The same as when the switch is opened.

1. i) Explain why the pointer deflects momentarily.
2. ii) State one way in which the current through R can be increased.
3. c) i)         State one way in which power is lost in a transformer.
4. ii) A transformer uses 240V ac supply to deliver 9A at 80V to a heating coil.

If 10% of the energy taken from the supply is lost in the transformer itself, What is the current in the primary winding.

1. A transformer has 8000 turns in its primary coil and 200 in its secondary coil. The voltage in the primary coil is 240V.

(a) Calculate the voltage in the secondary coil.

(b) If the current in the primary coil is 3A while that in the secondary is 100A, determine the efficiency of the    transformer.

(c) State any two reasons why the efficiency of the transformer is less than 100%.

1. (a) State Faraday’s law of electromagnetic induction.

(b) The figure below shows a simple transformer.  Study it and answer the questions that follow.

(i) Explain why the core is a continuous loop.

(ii) Give a reason as to why the core is laminated.

(iii) State and explain which coils are thicker.

• State one difference and one similarity between a step up transformer and an induction coil.
• State two advantages of the use of alternating voltage for the transmission of electrical energy.
• In the circuit below, the e.m.f of the battery is 2.1V and has an internal resistance of 0.5W.

(i)  Determine the effective resistance in the circuit.

(ii)Determine the ammeter reading when the switch is closed.

1. (a) 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:
1. Voltage in the secondary circuit.
2. Current in the primary circuit.
• Current in the secondary circuit.
1. State the type of transformer represented above.
• (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. (a) State Lenz’s Law of electromagnetic induction

(b)             Figure 6 shows a simple microphone in which sound waves from the person talking cause the cardboard diaphragm to vibrate

• Explain how a varying current is induced in the coil when the diaphragm vibrates

(ii)   State one way in which the induced current in (i) above can be increased

(c)      A transformer with 1200 turns in the primary circuit and 120 turns in the    secondary circuit has its primary circuit connected to a 400V a.c source. It is found that when a heater is connected to the secondary circuit, it produces heat at the rate of 600w. Assuming 100% efficiency, determine the:

(i) Voltage in the secondary circuit

(ii) Current in the primary circuit

(iii) The current in the secondary circuit

1. a) State two ways of increasing the force on a coil.
2. b) Figure 9 shows a current carrying coil in a magnetic field. The direction of the current and the resulting force are shown. Study the figure and answer the questions below.

1. i) Label the poles of the magnets.
2. ii) Explain the purpose of the split ring commutator in the principle of the D.C motor shown in the diagram.
3. (a) The two free ends of a coil are connected to a center – zero galvanometer. When the north pole of magnet is moved towards the coil, the pointer deflects in the direction shown in figure 2.

Figure 2

State with a reason the behavior of the pointer in the following cases:

1. The north pole of the magnet is held stationary near P.
2. The north pole of the magnet is made to approach the coil from end Q
3. b) State Faraday’s law of electromagnetic induction.
4. c) A transformer supplies 12V when it is connected to 240V supply of electricity. The output of the transformer is connected to 12V 36W bulb. The current drawn from the supply by the transformer is 0.5A. Calculate:
5. i) The input power of the transformer.       ii)        The current drawn from the transformer.                                                                                    iii)       The output power of the transformer                                                                                        iv)       The efficiency of the transformer
6. a) State Lenz’s law of electromagnetic induction.
7. b) The figure below shows a simple electric generator.

(i) Name the parts labeled P and Q.

(ii)             The e.m.f. generated as the coil rotates is represented in the graph below.

iii) Give reasons for the changes in the emf as the coil rotates from 0 to 90 and 90to 180

(c)  Explain how energy losses in a transformer are reduced by having:

(i)        a soft-iron core;

(ii)       a laminated core

1. d) A transformer with 1200 turns in the primary circuit and 120 turns in the secondary circuit has its primary circuit connected to a 400 V a.c. source. It is found that when a heater is connected the secondary circuit, it produces heat at the rate of 600 W, assuming its 100% efficient, determine the:
2. i) Voltage in the secondary circuit;
3. ii) Current in the primary circuit;

iii)       The current in the secondary circuit;

1. (a) In the figure below the bar magnet is moved into the coil.

State and explain what is observed in the galvanometer

(b)       State two ways in which energy is lost from a transformer and explain each

1. (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:-
1. The voltage in the secondary circuit.
2. The current in the primary circuit.
3. The current in the secondary circuit.

1. a) State the Faraday’s Law of Electromagnetic Induction. (1 mk)
2. b) Coil carrying a large alternating current is placed close to a iron ring suspended freely on a silk thread as shown in the diagram below.
 iron ring

(i) Explain why the ring is repelled continuously.

(ii)             State and explain what would be observed when a direct current is used instead of an alternating current.

1. c) The diagram below is a simplified illustration of an E.M.F. generator.
 Direction of motion

(i) Show the direction of induced current through R when the coil is in the position shown in the diagram.

(ii)             State and explain three ways of increasing the amount of induced current in this set up.

(iii) On the axes below, sketch a graph to show how potential difference across R varies with time. The coil is initially horizontal.

 Time (s)
1. d) State and explain any two ways by which energy losses are reduced in transformation.
2. e) The figure below shows a step – down transformer connected to a 240V mains socket. The primary coil P has 4000 turns while the secondary coil has 200 turns. The efficiency of the transformer is 60% and a current of 50A flows through P. Calculate the current through S.

(e) The figure below shows a simple generator and the arrows indicate the direction of induced current.

(i) What type of generator is shown above ?

(ii)          State two ways of increasing the brightness of the bulb.

(iii) If the bulb was replaced by a CRO sketch on the axes provided a graph to show the variation of output emf with position of the coil starting from vertical position.

1. a) State Lent’s law of electromagnetic induction.
2. b) The secondary coils of a step down transformer has 500 turns and primary has 15000turns
3. If the voltage in primary is 3600vfind the voltage in secondary.
4. If the current in primary is 3.0A find the current in secondary.
5. A generator at Kindaruma can supply 375MW of electric power. If the generator is 85% efficient. Find
6. The rate which falling water must supply energy to the turbine.
7. If the water falls a height of 22m what is the mass of the water that passes through the turbine each second.
8. Explain how energy loss in a transformer is minimised.
9. The figure below shows a section of a flexible wire carrying current perpendicularly out of the paper.

The wire moves in the direction shown as current passes through it.

(a)  (i) Label the polarities of the magnets A and B.

(ii) Explain the behaviour of the flexible wire.

(b)            The figure below shows an induction coil used to step up voltage.

(i) State the difference between the induction coil and a step-up transformer.

1. ii) Explain how voltage is stepped up by the induction coil.

(iii) The voltage is stepped up from 12V to 15kV. Determine the ratio of the secondary to primary coils in the induction coil.

(iv)The function of the capacitor is to eliminate sparking at the contacts. State why sparking occurs at the contacts.

(v) State how the capacitor eliminates sparking.

1. (a) State three possible ways in which power is lost in a transformer.

(b)             State Faraday’s law of electromagnetic induction.

 X

(c)  The figure below shows a conductor XY moving in a region of uniform magnetic field.

 Motion
 Y

(i)   Indicate the direction of the induced current in the conductor and state the rule used in arriving at the answer.

(ii) Suggest two ways of increasing the magnitude of the induced current in the conductor.

(d ) State two differences between a direct current  and an alternating current  generator

1. Figure 12 shows a coil A B C D being rotated in a clockwise direction in a magnetic field. The set up produces alternating current. Initially the coil lies in the plane of the field as shown.

(i) Name the parts X & Y

(ii)             The terminals P and Q are connected to a Cathode Ray Oscilloscope (CRO).

(c) Sketch a graph of e.m.f against time.

(d)             State the TWO factors that influence magnitude of induced e.m.f.

(e) The same set-up can be used to produce a varied direct current (d.c).

Describe how to modify the set-up in order to obtain direct current.

(f) Sketch a graph of e.m.f against time for the varying direct current e.m.f.

1. a) State Lenz’s law of electromagnetic induction.
2. b) The diagram below is a simplified illustration of an e.m.f generator.

1. i) Name the part labelled A.
2. ii) Show the direction of induced e.m.f when the coil is in the position shown in the diagram.

iii) State two ways of increasing the amount of induced e.m.f in this sate up.

1. c) Give a reason why the output from this generator can not be used to charge a battery.
2. d) State and explain any two ways by which energy losses are minimized in a transformer.
3. e) A person has a 6v bell. He hopes to operate the bell from a 240v a.c mains supply, with the help of the transformer shown in the figure.

Calculate the output voltage of the transformer when connected to the 240v mains.

1. a)         State Lenz’s law of electromagnetic induction.
2. b) The figure shows two coils of insulated copper wires wound on a single soft iron core.

One coil is connected to a battery through a switch and the other is connected to a Resister through a galvanometer.

It is observed that as the switch is closed, the pointer of the galvanometer deflects momentarily.

The same as when the switch is opened.

1. i) Explain why the pointer deflects momentarily.
2. ii) State one way in which the current through R can be increased.
3. c) i)         State one way in which power is lost in a transformer.
4. ii) A transformer uses 240V ac supply to deliver 9A at 80V to a heating coil.

If 10% of the energy taken from the supply is lost in the transformer itself, what is the current in the primary winding?

1. The figure below shows a conductor moving in a region of uniform magnetic field.

(i)        Mark on the diagram the direction of induced current.

(ii)       Apart from increasing the field strength, state two other ways in which the magnitude of the induced current can be increased.

1. b) Two coils are placed near each other as shown in the figure below:
• State and explain what will be observed if the switch is opened.

(c)       The figure below shows a transformer with 400 turns in the primary coil and 1000 turns in the secondary coil.

(i)        Calculate the potential difference across BC

(ii)       State one assumption made in calculating the potential difference across BC.

1. (a)Figure 12 shows two circuits close to each other

When the switch is closed, the galvanometer shows a reading and then returns to zero. When the switch is then opened, 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

1. An ideal transformer has 500 turns in the primary circuit and 2000 turns in the secondary circuit. When the primary circuit is connected to a 200V a.c source, the power delivered to a resistor in the secondary circuit is found to be 1000W.
1. Giving a reason, compare the thickness of the wires used in primary and secondary circuit.
2. Determine the current in the secondary circuit.
• Determine the current in the primary circuit.
1. A transformer of 600 turns in the primary circuit and 9000 in the secondary is connected to a 240V mains supply. The current in the secondary circuit is 0.15A. What is the current in the primary circuit? Assume 100% efficiency.
2. (a)The figure below shows two circuits close to each other.

When the switch is closed, the galvanometer shows a reading and then returns to zero. When the switch is opened, the galvanometer shows a reading in the opposite direction and then returns to zero. Explain these observations.

(b)An ideal transformer has 500 turns in the primary circuit and 2000 turns in the secondary circuit. When the primary circuit is connected to a 200V a.c. source, the power delivered to a resistor in the secondary circuit is formed to be 1000W.

(i) Giving a reason compare the thickness of the wires used in the primary and secondary        circuit.

(ii) Determine the current in the secondary circuit

(iii) Determine the current in the primary circuit.

1. a) Sate Lenz’s law of electromagnetic induction
2. b) The figure below shows two circuits close to each other.

 Ώ

When the switch is closed, the galvanometer shows a reading then returns to zero. When the switch is open, the galvanometer shows a reading in the opposite the direction and then returns to zero. Explain theses observations.

1. c) A transformer is connected to a 12.0V, 30.0W lamp from the 240V main. If the transformer is 75% efficient, determine the mains current.
2. d) The figure below an a.c generator

 A

 Coil
 B

1. i) Label the parts A and B
2. ii) Explain how this type of generator works.
3. (a) State one difference between a transformer and an induction coil.

(b) State two energy losses in a transformer.

(c) A transformer has 10,000 turns on its secondary coil and 100 turns on its primary coil. An alternating current 5.0A flows in the primary circuit when it is connected to a 12V a.c supply.

(i) State the type of transformer

(ii) Calculate the power input to the transformer.

(iii) Calculate the E.M.F across the secondary coil.

(iv) Determine the maximum current that could flow in a circuit connected to the secondary coil if the transformer is 90% efficient. (Same E.M.F in secondary as you have calculated above)
(v) In transmitting power why is it necessary to step it up before transmission. Explain.

1. (a) State one cause of energy losses in a transformer and explain how it can be minimized.

(b) A transformer is designed to supply a current of 7.5A at a potential difference of 100V to a motor from an a.c supply of 240V. If the efficiency of the transformer is 85%;

Calculate

(i) The power supplied to the transformer.

(ii) The current in the primary coil

(c) The figure below shows a cross —section of a bicycle dynamo. The wheel is connected by an axle to a permanent cylindrical magnet and is rotated by the bicycle tyre.

(i) Explain why the bulb lights.

(ii) How can the bulb be made brighter?