DONT JUST READ THESE NOTES, USE YOUR TEXT BOOK TO HELP IN UNDERSTANTING THE NOTES.

MAGNETIC FIELDThe space surrounding a magnetic where it produces a magnetic force is called a magnetic field.

The direction of the magnetic field at a particular place is the direction of the force it produces on a free magnetic north pole.The lines drawn showing the direction of a magnetic field is called the magnetic field lines or lines of magnetic flux or lines of force.

MAPPING OF MAGNETIC FIELDSUsing iron filingPlotting compass: when the compass is in a magnetic field the needles north pole is pulled one way, the south pole is pulled in the opposite direction and the axis of the needle lines up with the field as a result.

ELECTROMAGNETSAn electromagnet is a temporary magnet which has magnetism only when current is passing through a coil of wire wrapped around a soft iron core.

It is known that around a wire carrying current is a magnetic field these fields are represented by field lines or lines of forces

Types of magnetic fieldField due to straight wire.Field due to circular coil.Field due to solenoid.

Field due to straight wireThe direction of the magnetic field in straight wire is determined by:

Maxwell (Cork) screw rule Right hand grip rule, (thumb direction of current, finger field)

In the diagrams below the dot(.) represent current going out of pageThe X represent current going into page

.

For the first diagram with purple field line, the direction of the field as show was determined by Maxwell screw ruleThe field lines of the second diagram was also determined by Maxwell screw rule

If we were to applied the right hand grip rule then the thumb would point in the direction of the current as the fingers would give the direction of the field lines as shown below

Field due to circular coil.The magnetic field inside the loop is denseAll the lines of force inside the loop reinforce each other, since they are in the same direction i.e they become one. The magnetic field associated with the loop is effectively the same as for a bar magnet with opposite poles at opposite faces.

Magnetic field due to solenoid(a solenoid is an arrangement in which a wire is looped into a helix)The field of a solenoid increases by :Increasing the number of turnsIncreasing the current through solenoid

together to form a very strong field along the center of the solenoid.

By applying the screw rule to different parts of the solenoid the field line can be determined and that field line as shown above is equal to that of a bar magnet.

Application of electromagnetism

Electric bell: an electric bell contain an electromagnet that switches itself off and on very rapidly, moving the bell hammer as it does so.When the bell switch is pressed, current flows through the electromagnet and the hammer is pulled across to strike the gong,

the movement pulls the contact apart, which switches off the current through the electromagnet. The hammer spring back, the contact close again and the process repeat itself until the switch is released.

The telephoneThe magnetic relayAction Vehicle starter motor circuit.

Action relay: this is a switch worked by an electromagnet. It is useful if we want one circuit to control another, especially if the current is large in the second circuit. When current flow in the coil from the circuit connected to AB, the soft iron core is magnetized and attracts the L-shaped iron armature. This rocks on its pivot and closed the contact at C in the circuit connected to DE. The relay is then energized or on (diagram in text book).

Vehicle starter-motor circuit: the starter motor in a vehicle uses a very large current. By using a relay as explained above, this large current in the motor circuit is switched on by a small current in the starter switch circuit.

ELECTROMAGNEIC FORCE

A wire carrying a current in a magnetic field experiences a force, which increases with the strength of the field and the size of the current through the wire.

In the diagram above the flexible wire is in a strong magnetic field of a C-shaped magnet. When a current flows in the wire it jumps downwards as shown. If either the direction of the current or the direction of the field is reversed, the wire moves in the opposite direction.

Explanation:Both the magnet and the wire has a field line. Those due to the wire are circular as shown in diagram 1 above. The horizontal lines are those of the magnet.When both field are combined the resulting field lines is as that shown in diagram 2. As a result there are more field lines above than below the wire since both fields acts in the same direction above but in opposite below.

Without performing the experiment to determine the direction of the force, Fleming's left-hand rule can be applied.

Fleming left-hand ruleHolding the thumb and first two fingers of the left hand at right angle to each other with the First finger pointing in the direction of the Field and the seCond finger in the direction of the Current, then the Thumb points in the direction of the Thrust.

Application of electromagnetic force

Simple d.c motorA motor is a rotating device which converts electrical energy into mechanical energy. It work strictly on the principle that a wire carrying a current in a magnetic field experiences a force.

If fleminga left-hand rule is applied to the coil in the position shown, we find that aside ab (side to the left) experiences an upward force and side cd (side to the right) a downward force. These two forces form a couple, which rotates the coil in a clockwise direction until it is vertical. The brushes are then in the line with the gap in the commutator and the current stops. However because of inertia, the coil overshoots the vertical and the commutator halve changes contact form one brush to the other as a result the process continues, as the coil carries on rotating clockwise.

Loud speakersThese are device for converting electrical signals into audible sound.

Moving-coil loudspeakerscoilcircular poleshort coilstiff paper or plastic coneradial magnetic fieldAs a.c. passes through the coil, coil is pushed in & out gives out sound waves I flows backwards & forwards

A moving coil loud speaker has 3 main parts:A cylindrical permanent magnet which produces a strong radial magnetic field.A coil which is free to move short distances backward and forwards in the magnet fieldA stiff paper cone attached to the coil.

The wire in the coil lies at right angle to the magnetic field. If a current (A.C) is passed through the coil, an alternately backward or forward forces acts on it. This makes the paper cone vibrate, and sound waves are given out as a result. The nature of the sound produce depends on the frequency and amplitude of the alternating current flowing through the coil.

The current could be supplied by a single generator. Alternatively, it could be supplied by an amplifier connected to the pickup on a guitar.

Forces between current carrying wiresA wire carrying a current in a magnetic field experiences a force. In the diagram below current flow in each wire and each is in the field created by the other. A force therefore exist between them, which according to Fleming left-hand should be one of attraction if the currents are in the same direction and one of repulsion if they are in opposite direction.

Electromagnetic inductionAn electric current creates a magnetic field. The reverse process where by magnetism creates electricity was discovered by Michael Faraday and is called electromagnetic induction.

Investigating magnetic induction Using a straight wire and U-shape magnet: when the wire is connected to a galvanometer (voltmeter) and is moved across the magnetic field as shown below a small e.m.f is produced in the wire. This is called electromagnetic induction: an e.m.f has been induced. This induced e.m.f cause a current to flow and is detected by the galvanometer.

This emf is produced only when the wire is moving perpendicular to the magnetic field i.e up and down. If the wire is held stationary or moved parallel in the magnetic field no emf is produced.

2.Bar magnet and coil: in the diagram below the magnetic is pushed into the coil, one pole first, then held still inside it. The magnetic is now removed. The galvanometer show that current is induced in the coil in on direction as the magnetic moves in and in the opposite direction as it is removed. There is no deflection when the magnetic is at rest. The result is the same if the coil is moved in stead of the magnetic.

The magnitude of the emf induced in both cases above can be increased by:Moving the wire or magnet at a higher speed.Using a stronger magnet.Increasing the length of wire in the magnetic field i.e looping wire in the field several times.Increasing the number of turns in the coil (this increases the length of wire cutting through the magnetic field).

It was stated earlier that the principle of electromagnetic induction was discovered by Michael Faraday, from his work done as shown above he was able to derive a law called Faradays law of electromagnetic induction which states as follow:

Faradays lawThe size (magnitude) of the induced emf is directly proportional to the rate at which the conductor cuts magnetic field lines.

The direction of the induced current that faraday discovered was predicted by a Russian scientist called Lenz. Lenz like faraday created his own law called Lenzs law, which states:

Lenzs lawThe direction of the induced current is such as to oppose the change causing it.

Investigating Lenz's lawIn the diagram below the magnet approaches the coil, north pole first. According to lenzs law the induced current should flow in a direction which makes the coil behaves like a magnet with its top a north pole. The downward motion of the magnetic will then be oppose. When the magnet is withdrawn, the top of the coil should become a south pole and attract the north pole of the magnet, so hindering its removal. The induced current is thus in the opposite direction to that when the magnet approaches. Lenzs law is an example of the principle of conservation of energy.

If current flow clockwise that the south poleIf current flows anticlockwise then a north pole exist.As show for the 2 system above

For a straight wire moving at right angle to a magnetic field a more useful form of lenzs law can be used called Fleming's right-hand rule (dynamo rule).

Flemings right-hand ruleHold the thumb and firs tow fingers of the right hand at right angles to each other with the First Finger pointing in the direction of the field and the thuMb in the direction of Motion of the wire, then the seCond finger points in the direction of the induced Current.

The left hand rule applies when current causes motion (motor rule)The right hand rule applies when motion causes a current. (dynamo effect)

Simple a.c Generator A simple a.c generator works on the principle of electromagnetic induction. It consist of a rectangular coil between the poles of a C-shape magnet, the ends of the coil are joined to two slip rings on the axle, against which carbon brushes press. When the coil below is rotated, one side moves upwards through the magnetic field, the other side moves downwards and an emf is induced in the coil as a result. This emf causes a current to flow in the coil and outside circuit. The rotating coil one quarter of a turn later as it passes through the vertical position, the induced emf and current have fallen to zero because the two sides of the coil are now travelling parallel to the magnetic field and no field lines are therefore being cut.

When the coil complete half a turn and is horizontals again, the current in the outside circuit now flows in the opposite direction because the side of the coil moving upward (B) is now moving downward as maximum field lines are being cut. A quarter turn later the coil is vertical with side A uppermost, and the induced emf and current is zero again when the coil is vertical. After this the direction of the emf is reversed because during the next half rotation, the motion of B is directed upwards and A downwards.

An alternating emf is generated which acts first in one direction and then in the other; it would cause a.c to flow in a circuit connected to the brushes.

Transformers

If two coils are close together, then a changing current in one coil (the primary) sets up a changing magnetic field at the site of the other (the secondary) this effect or emf is know as mutual induction.

The device which works primarily upon the principle of mutual induction is called a transformer in that it produces a large alternating emf form a small one, or a small alternating emf form a large one.

Principle of operation of a transformerSwitching on the current in the primary sets up a magnetic field and as the field lines grow outwards from the primary they cut the secondary. And emf is induced in the secondary until the current in the primary reaches it steady value. When the current is switched off in the primary the magnetic field dies and the field line again is cut by the secondary coil inducing an emf in it.

As explained above when the primary coil is switched on and off an induced current flow alternately backwards and forwards through the secondary. This same effect is produced more simply when an a.c is allowed to pass through the primary coil .

In the diagram below the primary coil is connected to a low voltage a.c supply and the secondary is connected to a small bulb. As current flow backward and forward in the primary, it sets up an alternating magnetic field in the core. This induces an alternating emf and current in the secondary coil hence the bulb lights.

Types of transformersThere are 2 types of transformersStep-down transformerStep-up transformer

If the number of turn in the primary coil is more than that in the secondary coil then the transformer is a step-down i.e if there are fewer turns on the secondary than primary.If the number of turns on the primary is less than that on the secondary then the transformer is a step-up i.e the secondary has more turn than primary coil.

Based on the number of turns for the coils of the transformer it is possible to make the alternating emf (a.c voltage) in the secondary different form the voltage across the primary.The equation which links the a.c voltage across the primary and secondary and the number of turns on each

The equation which links the a.c voltage across the primary and secondary and the number of turns on each and current associated is as follow

Vs / Vp = Ns / Np = Ip / Is

Energy losses in trnasformersEnergy is lost by 3 main factorsResistance of the winding: the winding of copper wire do have some resistance and heat is produce by the current in them. Eddy current: the iron core is in the changing magnetic field of the primary and current, called eddy current are induced in it which cause heating.Leakage of field lines: all the lines produce by the primary may not cut the secondary especially if the core has and air gap or badly designed.

Efficiency of transformersTransformers are not 100% efficient however they are at least 99% efficient. This efficiency depends on the ability to prevent energy loss which is generally by heat. To overcome this some transformer uses oil as a coolant. To reduce energy loss by eddy current the core is laminated in that it is made f sheets which are insulated and have high resistance.

Resistance of the copper wire which causes heat is reduce by being coated with a layer or insulating varnish.And to reduce leakage of field the primary and secondary coil is wounded on each other.

Advantage of using a.c to transfer electrical energy The a.c can easily be step up or step down with the transformerPower can be sent at high voltage and low current to reduce power loss or at low voltage and high current if neededIt is also easy to convert a.c into d.c

DONT JUST READ THESE NOTES, USE YOUR TEXT BOOK TO HELP IN UNDERSTANTING THE NOTES.