ELECTROMAGNETIC

DATE: -

CLASS: - S. S. 3                                                                   DURATION: - 2 HRS 40MINS

TOPIC: -                                                                ELECTROMAGNET          

INSTRUCTIONAL MATERIALS: -                  DRY CELL, WOUND OF COILS

REFERENCES BOOK: -

1.       FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

2.       M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS,  Africana first publishers. 

3.       INTERNET           

PREVIOUS KNOWLEDGE: -                           Students have been familiar with gadgets uses electromagnetic principles.

OBJECTIVES: -      At the end of the lesson students should be able to: -

i.                     Explain electromagnet

ii.                    State the application of electromagnet

iii.                  State 1^st law of electromagnetic induction

iv.                 Mention the factors determine the magnitude of induced e.m.f

CONTENT: -

ELECTROMAGNETIC

Electromagnetism describes the interaction between charges, currents and the electric and magnetic fields to which they give rise. An electric current creates a magnetic field and a changing magnetic field will create a flow of charge. This relationship between electricity and magnetism has been studied extensively. This has resulted in the invention of many devices which are useful to humans, for example cellular telephones, microwave ovens, radios, televisions and many more.

Magnetic field associated with a current

In 1819, a Danish scientist and teacher named Hans Christian Oersted was demonstrating how to create a current in a wire by connecting it to a battery when he happened to notice that a magnetic compass lying near the wire would turn every time the wire was connected to the battery. He quickly realized that the current in the wire was creating a magnetic field that was affecting the compass. Magnetic field strength is commonly measured in units of Tesla, which is abbreviated T.

1 T = 1 kg/s² *A

If you hold a compass near a wire through which current is flowing, the needle on the compass will be deflected.

Since compasses work by pointing along magnetic field lines, this means that there must be a magnetic field near the wire through which the current is flowing.

The magnetic field produced by an electric current is always oriented perpendicular to the direction of the current flow.

A magnetic field is created when an electric current flows through a wire. A single wire does not produce a strong magnetic field, but a wire coiled around an iron core does.

Force on current-carrying conductor

When current-carrying conductor is placed in a magnetic field, it will experience a force when a magnetic direction is not parallel to the current direction. The magnitude of the force is maximum when the magnetic field and the current directions are mutually perpendicular to each other. The force decreases when the angle between the magnetic field and current direction is smaller than 90^o.

Factors that affect the strength of the force:

I.                    Angle between the magnetic field and the current direction.

II.                  Magnetic field strength ( stronger magnetic field – stronger  force)

III.                Amount of current in conductor ( higher current- stronger force)

IV.                Length of conductor within magnetic field (longer conductor- stronger force)

If the current direction is parallel to the magnetic field, there will no force on the conductor by the magnetic field. The magnitude of the force is maximum when the angle between the magnetic field and current direction is 90^o.

Application of Electromagnets
1. Electric bell.
                In an electric bell, the electromagnet is switched on and off very rapidly by making and breaking the contact. When you press the switch, current flows in the coil, creating an electromagnet. The electromagnet then attracts the hammer towards the gong to hit it. When the hammer moves towards the gong, the contact opens. The circuit is broken and the current stops flowing. The coil loses its magnetism and the hammer returns to its original position, completing the circuit again. In this way, the hammer hits and lifts off from the gong repeatedly making the bell ring as long as the switch is on.
2. Electromagnetic relay
                Electromagnetic relay consists of 2 circuits. Circuit 1 is a simple electromagnet which requires only a small current. When the switch is closed, current flows and the iron rocker arm is attracted to the electromagnet. The arm rotates about the central pivot and pushes the contacts together. Circuit 2 is now switched on.
Circuit 2 may have a large current flowing through it to operate powerful motors or very bright lights. When the switch is opened, the electromagnet releases the rocker arm and the spring moves the contacts apart. Circuit 2 is now switched off.
The advantage of using a relay is that a small current (circuit 1) can be used to switch on and off a circuit with a large current (circuit2 ). This is useful for two reasons:
a) Circuit 1 may contain a component such as light detecting resistor (LDR), which only uses small currents,
b) Only the circuit with a large current needs to be connected with thick wire.
3. Maglev Train
Maglev trains use magnetic levitation propulsion systems. In this system, the cable coils generate a traveling magnetic field that moves down the length of the guide way. Magnetic attraction between this field and electromagnets on the train levitates the vehicle and drags it along behind the traveling magnetic field. These trains can achieve a very high speed of 500 Kmh^-1 because there is no contact friction between the train and the rails.
This type of train is a very safe mode of transport. There is no danger of derailment because the train cannot move side way off the guide way. The braking system is also very effective. When the polarity of the traveling magnetic wave is reversed, the train is stopped without skidding. In addition, many such trains can use the same rails without fear of collision because the train can never overtake the traveling magnetic field. However, maglev transport systems have not been commercially successful because of the high cost involved in constructing new network of guide ways for the train.

4. Telephone earpiece

ELECTROMAGNETIC INDUCTION

Electromagnetism is the effect resulting from the interaction between an electric current and a magnetic field. This effect brings about induced electromagnetic force (e.m.f) and the resulting current is called induced current.

Experiments on electromagnetic induction

Consider the following diagram

When the wire is moved up the galvanometer deflects in one direction then the opposite direction when moved downwards. When moved horizontally or held in a fixed position there is no deflection in the galvanometer. This shows that e.m.f is induced due to the relative motion of the wire or the magnet.  

Faraday’s law of magnetic induction

After considering the factors affecting the magnitude of the induced e.m.f, Michael Faraday came up with a law which states that “The induced e.m.f in a conductor in a magnetic field is proportional to the rate of change of the magnetic flux linking the conductor”.

Lenz’s law of electromagnetic induction

This law is used to determine the direction of the induced current in a conductor. It states that “An induced current flows in such a direction that its magnetic effect opposes the change through which the current has been produced”. It is applied similarly when a wire is been moved in magnetic field.

Fleming’s right hand rule.

The law states that “The first finger, the second finger and the thumb of the right hand when placed mutually perpendicular to each other, the first finger points in the direction of the field and the thumb in the direction of motion then second finger points in the direction of the induced current”. This law is also called the generator rule.

When a long, straight current-carrying wire is placed in a magnetic field, it'll experience a force proportional to the strength of the field, the amount of current, and the length of the wire. You can see how this plays out in this image:

Thus, the magnitude of the magnetic force on a current-carrying wire is given by:

Applications of electromagnetic induction

1. A.c generator/alternator– a generator is a device which produces electricity on the basis of electromagnetic induction by continuous motion of either a solenoid or a magnet. It consists of an armature made of several turns of insulated wire wound on soft iron core and revolving freely on an axis between the poles of a powerful magnet. Two slip rings are connected to the ends of the armature with two carbon brushes rotating on the slip ring.

In an external circuit the current is at maximum value at 900 and minimum value at2700. This brings about alternating current and the corresponding voltage (e.m.f) is the alternating voltage. They are used in car alternators and H.E.P.

2. D.c generator/alternator– in this case the commutators replaces the slip rings to enable the output to move in one direction. After a rotation of 1800, instead of current reversing, the connections to the external circuit are reversed so that current direction flows in one direction.

3. Moving coil microphone– it consists of a coil wound on a cylindrical cardboard which opens into a diaphragm. The coil is placed between the poles of a magnet as shown.

As sound waves hit the diaphragm, they vibrate and move the coil which produces induced current into the coil and then it flows to the loudspeakers. 

Factors affecting the magnitude of the induced e.m.f

1. The rate of relative motion between the conductor and the field – if the velocity of the conductor is increased the deflection in the conductor increases.

2. The strength of the magnetic field – a stronger magnetic field creates a bigger deflection

3. The length of the conductor – if the length is increased in the magnetic field the deflection increases.

Eddy currents

They are composed of loops of current which have a magnetic effect opposing the force producing them. When a copper plate with slits is used the loops are cut off and hence the effective currents are drastically reduced and so is the opposing force.

Practically eddy currents are reduced by laminating metal plates. Armatures of electric generators and motors are wound on laminated soft iron cores. The lamination slices, which are quite thin are glued together by a non-conducting glue and this reduces eddy currents to an almost negligible value. Eddy currents are useful in moving coil meters to damp the oscillations of the armature when the current is switched off.

Mutual induction

Mutual induction is produced when two coils are placed close to each other and a changing current is passed through one of them which in turn produces an induced e.m.f in the second coil. Therefore mutual induction occurs when a changing magnetic flux in one coil links to another coil.

Applications of mutual induction

1. The transformer- it converts an alternating voltage across one coil to a larger or smaller alternating voltage across the other. Since H.E.P is lost through transmission lines therefore it is stepped down before it being transmitted and stepped up again at the point of supply lines. In a step up transformer the number of turns in the secondary coil (Ns) is higher than the number of turns in the primary coil (Np). In a step down transformer the primary coil has more turns than the secondary coil. The relationship between the primary voltage and the secondary voltage is given by;

Np / Ns = Vp / Vs.The efficiency of a transformer is the ratio of power in secondary coil (Ps) to power in primary coil (Pp), therefore efficiency = Ps / Pp × 100%.

Examples

1. A current of 0.6 A is passed through a step up transformer with a primary coil of 200 turns and a current of 0.1 A is obtained in the secondary coil. Determine the number of turns in the secondary coil and the voltage across if the primary coil is connected to a 240 V mains.

Solution

Np / Ns = Vp / Vs = Ip / Is = Ns = (0.6 × 200) / 0.1 = 1200 turns

Vp = 240 V hence Vs = (240 × 1200) / 200 = 1440 V

2. A step-up transformer has 10,000 turns in the secondary coil and 100 turns in the primary coil. An alternating current of 0.5 A flows in the primary circuit when connected to a 12.0 V a.c. supply.

a) Calculate the voltage across the secondary coil

b) If the transformer has an efficiency of 90%, what is the current in the secondary coil?

Solution

a) Vs = (Ns / Np) × Vp = (10,000 × 12) / 100 = 1200 V

b) Power in primary = Pp = Ip × Vp= 5.0 × 12 = 60 W

Efficiency = Ps / Pp × 100% but Ps = Is Vs

Is = (60 × 90) / (1200 × 100) = 0.045 A

Energy losses in a transformer.

Loss of energy in a transformer is caused by;

i) Flux leakage– this may be due to poor transformer design

ii) Resistance in the windings–it is reduced by using copper wires which have very low resistance

iii) Hysteresis losses– caused by the reluctance of the domains to rotate as the magnetic field changes polarity.

Reduced by using materials that magnetize and demagnetize easily like soft iron in the core of the transformer.

iv) Eddy currents– reduced by using a core made of thin, well insulated and laminated sections.

Uses of transformers

1. Power stations – used to step up or down to curb power losses during transmission

2. Supplying low voltages for school laboratories

3. Low voltage supply in electronic goods like radios, TVs etc.

4. High voltage supply in cathode ray oscilloscope (CRO) for school laboratories.

 PRESENTATION

Step I: The teacher revises the previous topic.

 Step II: The teacher introduces the new topic.

Step III: The teacher explains the meaning and the uses of electromagnet.

Step IV: The teacher explains electromagnetic induction and its uses.

Step V: The students mention factors affecting the magnitude of the induced e.m.f .

Step VI: The teacher leads the students to solve problem on transformer.

 EVALUATION

The teacher evaluates the lessons by asking these questions:

1.       Explain electromagnet

2.        State the application of electromagnet

3.       State 1^st law of electromagnetic induction

4.       Mention the factors determine the magnitude of induced e.m.f

ASSIGNMENT

 Read about alternating current.