8.3 Induction of Current

Electromagnetic induction is the production of an electric current by a changing magnetic field (magnetic flux).

8.3.1 Inducing e.m.f. and currentThe induced current is produced only when there is relative motion between the conductor / coil and the magnetic field lines

(magnetic flux) that are perpendicular to each other.

E.m.f and current can be induced by:(a) moving a straight wire quickly across a magnetic field between two flat magnets.

(b) Moving a permanent magnet towards one end of a solenoid.

Each time the straight wire cuts across the magnetic field, or the permanent magnet moves towards the solenoid, a current is induced in the coil and a deflection is observed in the sensitive galvanometer.

8.3.2 Determining the direction of the induced current

Fleming’s Right Hand Rule

Activity 1: To Investigate electromagnetic induction

Action Obseravtion Inference

The wire is moved upwards

Galvanometer deflect to left

The wire is moved downwardsThe wire is move horizontallyMagnet is moved upwards

Held stationary

Discussion1. What general inference can you make if a galvanometer shows a deflection?

_________________________________________________________________________

2. Based on the results of your experiment, when will an induced current be produced?

_________________________________________________________________________

3. Relate the motion of the copper rod that produces a current in it to the direction of the magnetic field lines.

_________________________________________________________________________

Induced e.m.f by coil

1. Write a general statement on how a current can be induced in a solenoid using a magnet.

______________________________________________________________________________________________

2. Conclusion Current is __________ in a straight conductor when it moves and _______________ the magnetic field lines. Current is induced in a solenoid when there is _________ ________ between the solenoid and a magnet.

8.3.2.1 Lenz’s LawLenz’s Law: The direction of the induced current is such that its magnetic effect always opposes the change producing it.

The solenoid will always resist any movement of the magnet relative to the solenoid. When the bar magnet is inserted into the solenoid, the solenoid will try to repel the bar magnet. Therefore, the

polarity of that end of the solenoid will be the same as the bar magnet’s.

When the bar magnet is removed from the solenoid, the solenoid will try to attract the bar magnet. Therefore, the polarity of that end of the solenoid will be the opposite of the bar magnet’s.

Using the right-hand grip rule

Activity 2: Show the correct direction of the induced current when the magnet is moved in the direction shown.

What is the direction of the induced current? Determine the poles at P and Q.

8.3.3 Determining the magnitude of the induced current (Faraday’s Law)

Faraday’s Law: The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux.

If there is no relative motion between a magnet and a solenoid, there is no electromagnetic induction.

To increase the e.m.f. and current: Increase the relative motion Increase the number of turns on the coils Increase the magnetic strength Increase the cross-section area of the wire Insert a soft iron core in between the coils of the wire

Applications of electromagnetic induction

Current Generator Current generator functions by converting mechanical energy to electrical energy. Current generator works based on electromagnetic induction and uses the Fleming’s Right hand rule. Current generator is divided into: direct current generator and alternate current generator.

Direct Current Generator

Commutator: reverses the connections of the coil with the external circuit after every half cycle, so that the current in the outside circuit always flows in the same direction.

Describe how does a direct current generator work. Show the direction of movement of the coil AB and CD. Mark the direction of the induced current in the coil and the galvanometer.

Coil AB moves ___________, coil CD moves _______

When the coil rotates, its sides cut across the magnetic field lines and induced current flows in the coil from _________________

The galvanometer will deflect to ________

The sides AB and CD are moving __________ to the magnetic field and thus do not ________ the magnetic field lines.

No __________ _________ is produced at the instant.

The galvanometer returns to ______ mark.

As the coil continues to rotate, current will again be induced in the coil but its direction in now opposite to that in figure 1 which is from ____ to _____ and from _____ to ____

However, the direction of the current through the external circuit remains the same, so the galvanometer deflects to the ___________

The sides AB and CD are moving __________ to the magnetic field and thus do not ________ the magnetic field lines.

No ___________ _________ is produced at the instant.

The galvanometer returns to ______ mark.

The process is ____________

Alternating Current Generator

The two ends of the coil are connected to two slip rings which rotate with the coil.

Each slip ring is always in contact with the same carbon brush.

The output current generated is an altern.ating current because the current changes direction in the external circuit each time the coil passes the vertical position.

Assume the current flow from P to Q is positive and the current flow from Q to P is negative. The current, I changes magnitude and direction after every half rotation.

Describe how an alternating current generator works. Show the direction of movement of the coil AB and CD. Mark the direction of the induced current in the coil and the galvanometer.

Coil AB moves _________, coil CD moves _________When the coil rotates, its sides cut across the magnetic field lines and induced current flows in the coil from ______ to _______ and from _____ to _____ (using Fleming’s right hand rule)In the external circuits, current flows from ____to ____

The sides AB and CD are moving _____ to the magnetic field and thus do not _______ the magnetic field lines.No ________ ________ is produced at the instant.The galvanometer returns to ______ mark.

After the vertical position, the current increases until it attains the maximum value when the coil is in a horizontal position.

Coil CD moves _________, coil AB moves _________

The direction of the induced current is from ____ to ____ and from ___ to _____

The direction of the current through the external circuit is from the brush _____ to _____ which is reversed.

The sides AB and CD are moving __ _______ to the magnetic field and thus do not _______ the magnetic field lines.

No _________ __________is produced at the instant.

The galvanometer returns to ______ mark.

The process is ____________.

8.3.4 Direct Current & Alternating Current1. A direct current is a current that flows in one direction only in a circuit.2. The magnitude of a direct current may be:

(a) constant(b) changes with time

1. An alternating current is a current which flows to and fro in two opposite directions in a circuit. It changes its direction periodically.

A direct current can flow through a resistor but cannot flow through a capacitor.

An alternating current can flow through both a resistor and a capacitor.

Both the direct current and alternating current have a heating effect on the filament of a bulb and can light up the bulb.

TUTORIAL 8.31. Diagram below shows a permanentmagnet moving towards and entering the end of a coil of wire connected to asensitive galvanometer.

Which of the following observations iscorrect?A. The magnetic field passing through the coil is unchanged.B. The induced current passes through the galvanometer in the direction from P to Q.C. The induced current passes through the galvanometer in the direction from Q to P.D. The heat energy produced in the coil is more than the work done to move the magnet.

2. Diagram below shows the arrangement of a solenoid and a magnet.

Which of the following statement is not a method to produce induced e.m.f. in the solenoid? (2006)A. The magnet is pulled out of the solenoid.B. The solenoid is moved towards the magnet.C. The magnet is moved into the

solenoidD. The magnet and the solenoid are moved at the same velocity.

4. Which diagram below shows the correct direction of the induced current in the coil when the bar

magnet is moved in the direction shown? (2005)

5. The diagram shows a coil of wire placed next to a magnet. The galvanometer pointer deflects when the magnet is pushed into the coil.

Which action will increase the deflection of the galvanometer pointer? (2004)A. The magnetic pole is reversed.B. The number of coils is increased.C. The coil is made from insulated wireD. The magnet is pushed slowly into the

7. A straight conductor is movedperpendicularly to the magnetic field.Which of the following diagrams shows the correct direction of the induced current in the conductor?

(2006)

8. Diagram below shows a square coil rotating at a steady speed in a magnetic field.

Which graph shows the fluctuation in the current flowing through the lamp starting with the coil being in the horizontal position?

3. The diagram shows a magnet oscillating in a solenoid.

Which of these actions will not increasethe deflection of the galvanometerpointer? (2003)A. Reversing the polarity of the magnet.B. Increasing the number of coils in the solenoidC. Decreasing the diameter of the solenoidD. Increasing the oscillation speed of the magnet

10. Diagram below shows a direct current generator. Part of the coil XY rotates clockwise.

Which of the following XY position of the coil which does not produce inducedcurrent? (1995)

11. Diagram 24 shows an experiment to induce current in a magnetic field.

Which movement of the copper rodinduces the maximum current? (2008)

coil.

6. Diagram below shows an electric generator whose output terminals are connected to a cathode-ray oscilloscope.

Which of the following diagrams showsthe trace on the oscilloscope when thecoil is rotating at uniform speed? (2003)

(ii) the wire is moved vertically upwards at a higher speed.

____________________________(iii) The wire is held stationary in between the poles of the magnet. ____________________________

13. The south pole of a bar magnet ismoved towards a solenoid that itconnected to a sensitive centre-zerogalvanometer.

(a) Mark on the solenoid:(i) the end that will become a south pole.(ii) The direction of the induced current.(b) State three ways to increase the induced current in the solenoid ___________________________ ___________________________ ___________________________

14. Diagram 4 shows a bar magnet is hung on a spring that attached to a retortstand. The bar magnet is displaceddownwards and released so it canoscillate through a solenoid which isconnected to a centre-zero galvanometer

(a) What is meant by electromagnetic Induction …………………………………………

[1 mark](b) When the bar magnet move towards

9. Diagram below shows a solenoid. Which of the following rules may be used to determine the direction of the current in the coil? (2007)

A. Fleming’s Left Hand RuleB. Fleming’s Right Hand RuleC. Right hand Screw RuleD. Right Hand Grip Rule

Diagram 6.2(a) State the physical quantity that is

represented by the deflection ofthe galvanometer.

………………………………………[1 mark]

(b) Based on Diagram 6.1 and Diagram 6.2:(i) State two differences that can be

observed from Diagram 6.1 and Diagram 6.2.

……………………………………… ……………………………………… ………………………………………

[2 marks](ii) Based on the answer in (b)(i), explain why these differences occur? ................................................... ................................................... ...................................................

[2 marks](c) Name the physics law involved in 6(b)(ii). .......................................................

[1 mark](d) Explain why the direction of current in the solenoid is upward as shown in Diagram 6.1 and Diagram 6.2? ...................................................... ……………………………….

[2 marks]

12. A straight thick copper wire that isconnected to a sensitive centre-zerogalvanometer is moved verticallydownwards in a magnetic field.

(a) Mark on the wire the direction of the induced current that flows through it.(b) Name the rule that is used to work out the direction of the current.________________________________(c) Describe the effect on the inducedcurrent if:(i) the wire is moved vertically upwards at

a low speed, ____________________________15. The diagram shows a pendulum bob attached to the end of copper wire. The bob is allowed to swing between north and south pole of two magnets.

(a) When the switch is closed and thebob oscillates inside the magneticfield ,the pointer the centre-zerogalvanometer deflects to the left andthen to the right continuously.Explain why this observation happens?__________________________________________________________________________________________

(b) State the type of the current produced.

(c) Plot a graph current-time to relatethe observation in(a)

(d)(i) Based on the diagram above, at what positions the reading of the galvanometer is zero._____________________________(ii) Give the reason for your answer in (c)(i).______________________________(e) What types of energy possesses by the bob at point M?

the solenoid, the galvanometer pointer deflect for a while. (i) Explain why the galvanometer pointer deflects? ……………………………………… ………………………………………

[2 marks] (ii) State the polarity of end A and end

B of the solenoid. A: ………………………………… B: …………………………………

[2 marks]

15. Diagram 6.1 and 6.2 show a bar magnet is pushed into a solenoid at a speed of 2 ms-1.

Diagram 6.1

16. The diagram shows the structuraldesign of a generator.

(a) Name the principle used in the generator.______________________________(b) Mark the directions of current flow

in WX and YZ when the side WX of the coil is displaced in downwards direction.

(c) Draw a graph to show the variation with time of the current in conductor YZ.

_______________________________(f) (i) Compare the time taken by the pendulum to stop between the switch is opened with the switch is closed. ______________________________

(ii) Give the reason for your answer in f(i) ________________________________