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Electromagnetic Induction - Physics Form 4

Background

In form two, you learnt about the magnetic effect of an electric current where a current carrying conductor placed in a magnetic field experiences a force. This force was found to be dependent on the amount of current and the strength of the magnetic field among other factors.

Objectives

By the end of the lesson you should be able to:

-Perform and describe simple experiments to illustrate electromagnetic induction

-State the factors affecting the magnitude of electromagnetic induction

- State the laws of electromagnetic induction

-Describe simple experiments to illustrate electromagnetic induction

-Explain the working of an alternating current (a.c) generator and direct current(d.c) generator

-Explain the working of a transformer

-Explain the applications of electromagnetic induction

-Solve numerical problems involving transformers.











Introduction

In this topic, we shall look at the generation of electricity from changes that take place in a magnetic field. Play the animation below and make your observations.


CASE 1: A magnet moving into a coil.

The animation below is a simulation about electromagnetic induction when a magnet is inserted into a coil. Click on the play button and make your observations.

Observation
When the magnet is moved into the coil, current flows making the pointer to move.

Explanation

When there is relative motion between the solenoid and the magnet, the solenoid cuts through the magnetic field lines (magnetic flux) which causes an induced electromotive force (emf). When there is no relative motion, the solenoid does not cut the magnetic flux hence no induced emf.



CASE 2: A coil inserted to enclose a magnet.

The animation below is a simulation about electromagnetic induction when a coil is inserted to enclose a magnet. Click on the play button and make your observations.


Observations

When the coil moves into the magnet, current flows making the pointer to move.

Explanation

As seen in both experiments, when there is relative motion between a magnet and a coil, an emf is induced in the coil resulting in an induced current which causes a deflection on the galvanometer. When there is no relative motion, no emf is induced hence the pointer goes back to zero.


CASE 2: A coil inserted to enclose a magnet.

The animation below is a simulation about electromagnetic induction when a coil is inserted to enclose a magnet. Click on the play button and make your observations.

Observations

When the coil moves into the magnet, current flows making the pointer to move.

Explanation

As seen in both experiments, when there is relative motion between a magnet and a coil, an emf is induced in the coil resulting in an induced current which causes a deflection on the galvanometer. When there is no relative motion, no emf is induced hence the pointer goes back to zero.

Factors affecting the magnitude of the induced emf in the coil.

The experiment below can be used to demonstrate the factors affecting the magnitude of the induced emf in the coil. The animation below demonstrates these factors. To make your observations click on the play button.

Observations

When the magnet moves slowly, a small emf is induced into the coil creating a small current hence a small deflection on the galvanometer. Increasing the speed of movement increases the rate of cutting the magnetic flux thereby inducing more e.m.f hence more current. The pointer therefore deflects further compared to the previous one.

Conclusion

From the demonstration, the induced emf hence current is directly proportional to the relative motion between the conductor and the solenoid. The rate of change of magnetic flux is therefore directly proportional to the magnitude of the induced emf. This relationship was first established by Faraday and is commonly known as Faraday's Law of electromagnetic induction.

Lenz's law

Faraday discovered one of the important laws of electromagnetic induction. The animation below has been used to explain and illustrate the law. Play it and make your observation.

Observation

When the magnet moves into the solenoid, the pointer moves to the left while when the magnet moves out of the solenoid, the pointer moves to the right.

Explanation

When the magnet is moving in, it induces a current in the solenoid which flows in a direction to induce a similar pole at the end of the solenoid to the approaching pole of the magnet and vice versa. This similar pole opposes the movement of the magnet which causes the induction of an e.m.f. This idea is summarized in Lenz's Law which states that "An induced current flows in such a direction as to oppose the change producing it."

Mutual induction

Mutual induction is the process of inducing an e.m.f. in a coil due to the changes in magnetic flux taking place in a neighbouring coil.


Observation

When the switch in the primary coil is closed, current is induced in the secondary coil which flows in the anticlockwise direction making the pointer to deflect to the right then goes back to zero. When the switch is opened, current induced in the secondary coil moves in the clockwise direction making the galvanometer pointer to deflect to the left then back to zero. The deflection in the second case is greater than in the first case.

Explanation

When the circuit is switched on, current time to build from minimum to maximum. This creates a gradual change in flux which causes the galvanometer pointer to deflect minimally. When the circuit is switched off, there is a rapid change in magnetic flux from maximum to zero. This causes a greater deflection in the pointer of the galvanometer

Fleming's Right Hand Rule

The relationship between the direction of induced current, the direction of the field and the motion of the conductor is summarized by the Fleming's Right Hand Rule. The animation below shows this relationship. Click on the play button to make your observations.

Observation

When the conductor moves inwards, a current is induced which flows in the direction shown. When the wire changes direction, the current flows in the opposite direction. The animation above brings a modification of Len's Law into what is called Fleming's Right Hand Rule. The rule states that "if the thumb, the first finger and the second finger of the right hand are placed mutually at right angles with the First finger pointing in the direction of the FIELD while the thuMb points in the direction of MOTION of the conductor, then the seCond finger points in the direction of the induced CURRENT".

Generators

In form two, we learnt about an electric motor in which a current carrying conductor in a magnetic field experiences a force which causes motion. In a generator, motion of a conductor in a magnetic field produces an electric current. There are two types of generators namely: An alternating current (a.c) generator and a direct current (d.c) generator.

Simple a.c generator

Click on the play button of the animation below to observe how a simple a.c generator works.

Observation

The curve moves from the origin to the maximum when the coil becomes horizontal having completed a quarter turn. When the coil passes the horizontal and goes towards the vertical completing a half a cycle, the current decreases to zero. As the cycle continues to three quarter cycle, the current changes direction up to a maximum in the negative direction. As the coil completes the cycle, the current moves back to zero.

Explanation

When the coil is vertical (perpendicular to the field), the wires of the coil do not cut the magnetic field lines resulting in zero induced current. As the coil rotates to the horizontal position, it starts to cut across the magnetic field lines and an e.m.f is induced in the coil. In this quarter, the number of field lines being cut increases from zero to a maximum causing the current to increase to a maximum. When the coil passes the horizontal position going back to the vertical completing a half cycle, the induced current decreases to zero.

This is because, the number of magnetic field lines being cut by the coil decreases to zero at the vertical position. As the coil continues to rotate in the third quarter, the current increases again from zero to a maximum in the opposite direction and then reduces from a maximum to zero in the last quarter cycle as per Fleming's Right Hand Rule. This process is repeated with current changing direction every half cycle.

Simple d.c generator

The main difference between an a.c and a d.c generator is that the a.c generator has slip rings while the d.c generator has a split ring commutator. Play the following animation to observe how it works.

Observation

Starting with the coil in a vertical position with the edge ab uppermost, the induced e.m.f and current through resistor R increases from zero to a maximum when angle theta(O)increases from zero(0)degrees to ninety (90) degrees,then decreases to zero when the coil is vertical with edge ab lowest.

When the vertical position is passed, the half rings interchange brushes making the induced currents in loops ab and cd to change direction. Since the brushes have been interchanged, the direction of the current through the resistor remains the same. The induced current increases from zero to a maximum when the coil is horizontal once again. So long as the direction of rotation of the coil is the same, one of the terminals ('b' in this case) always acts as the positive terminal while the other acts as the negative terminal.

Explanation

When the coil is vertical (perpendicular to the field), the wires of the coil do not cut the magnetic field lines resulting in zero induced current. As the coil rotates to the horizontal position, it starts to cut across the magnetic field lines and an e.m.f is induced in the coil. In this quarter, the number of field lines being cut increases from zero to a maximum causing the current to increase to a maximum.

When the coil passes the horizontal position going back to the vertical completing a half cycle, the induced current decreases to zero. As the coil continues to rotate in the third quarter, the current increases again from zero to a maximum in the same direction since the half rings have been interchanged. The current then reduces from a maximum to zero in the last quarter cycle.

Factors affecting the magnitude of induced emf

The following factors affect the magnitude of induced emf:

  • The speed of rotation of the coil or magnet.
  • The number of turns of the coil.
  • The strength of the magnetic field.
  • Winding the coil on a laminated soft iron core.

Play the animation below and observe.

Transformers

A transformer is a device that transfers electrical energy from one circuit to another by electromagnetic induction between the two coils. One of the coils is named the primary coil while the other is the secondary coil. A transformer changes the magnitude of the input voltage / current. The primary coil is connected to an alternating power supply. As the magnetic field in the primary coil changes due to the alternating current, it induces an e.m.f in the secondary coil which in turn causes an induced current.

Types of transformers

There are two main types of transformers namely, step up and step down transformers.

Step up and step down transformers

There are two main types of transformers namely step up and step down transformers.Play the animation below to observe the major differences between the two.

Observations

In a step up transformer, the number of turns in the primary coil is less than that in the secondary coil. In a step down transformer, the number of turns in the primary coil is more than in the secondary. A step down transformer steps down the input voltage while a step up transformer steps up voltage.

Discussion

The output voltage depends on the input voltage and the number of turns in the primary to that in the secondary. For an ideal transformer, the ratio of the primary to the secondary voltage is equal to the ratio of the primary to the secondary turns commonly known as "the turns ratio".

Energy loses in a transformer

Since a transformer is a machine, it cannot be 100 % efficient and the following energy loses are normally experienced in normal situations.

Eddy currents

The alternating magnetic flux in the soft iron core as a result of the a.c in the primary coil causes a reverse current called eddy current. This causes a heating effect in the core leading to energy loses. This is normally minimized by laminating the core.

Flux leakage

Not all the magnetic flux in the primary coil is linked to the secondary coil. This causes a reduced induced e.m.f in the secondary coil. This is minimized by winding the secondary coil over the primary coil.

Hysteresis loss

The repeated magnetization and demagnetization of the core due to the alternating current causes a heating effect. This is minimized by use of a soft iron core.

Efficiency of a transformer

A transformer is a machine in which power is introduced at the primary coil and delivered into the output circuit through the secondary coil. The efficiency of a transformer is expressed as: Efficiency = power output/power input X 100

= Power in secondary coil/power in the primary coil X 100

= (Is x Vs) / (Ip x Vp) X 100

The efficiency of a transformer is never 100 % because some of the energy is lost in form heat

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