Saturday, 1 April 2017

electromagnetism notes in physics

Alternating and Direct Current

Generators can produce alternating or direct current.
Direct Current (DC) is a current which flows in one direction only. All batteries produce direct current.
Alternating Current (AC) is one which flows in two opposite directions alternately. This means that the direction of current flowing in a circuit is constantly being reversed back and forth.
The electric current supplied to our homes is alternating current. This comes from power plants that are operated by the electric company.
The types of current can be differentiated by looking at the pattern produced on Oscilloscope.

The pattern above is produced by Alternating Current. Direct current will produce straight line only.
AC can be converted to DC by using AC/DC converter. This is used by many equipments such as computers.

Application of Electromagnetic Induction

AC and DC generatorsA generator produces electrical energy by electromagnetic induction. Generators or dynamos of various sizes are used for different purposes.
The current produced from a small dynamo is used to light up a bicycle lamp. Huge generators are used in power plants to generate electricity for mass usage. In all cases the basic operating principle is the same i.e. the mechanical energy is used to move a conductor through a magnetic field to produce electrical energy.
A generator is basically the inverse of a motor. A generator consists of many coils of wire wound on an armature that can rotate in a magnetic field. The axle is turned by some mechanical means (e.g. falling water, steam,wind etc) so that the coil cuts the magnetic field lines as it rotates.

Uses of Electromagnets

Uses of Electromagnets

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.

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.

Maglev Train

Maglec trains use magnetic levitation propulsion systems. In this system, the cable coils generate a traveling magnetic field that moves down the length of the guideway. 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 sideway off the guideway. 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 guideways for the train.

Lenz's law

Lenz's Law states that the direction of the induced e.m.f is such that its magnetic effect always oppose the change producing.
Let's  say a North pole of a magnet is approaching a coil. The direction of the induced e.m.f is such that a north pole is created at the end of the coil facing the magnet. This will cause the magnet to be repelled, thereby opposing the motion of the magnet. However, when the magnet is moved away from the coil, a south pole is produced to attract the magnet back.
Lenz's law is an example of the principle of conservation of energy. Work must be done to move the magnet against the repulsion or attraction of the induced magnetic poles in the coil. This work is then converted to electrical energy in the form of induced e.m.f.
Another method to determine the direction of the induced e.m.f is by using Fleming's right-hand rule. This rule is usually applied for straight conductors cutting through a magnetic field.

Faraday's Law of Electromagnetic Induction

Faraday's law of Electromagnetic Induction states that:

"The Magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux experienced by the conductor".

When a bar magnetic moves in a solenoid (coiled wires),there is a change in the magnetic flux linkage through the coil. An induced e.m.f is produced in the coil. The magnitude of the e.m.f increases when:

-the relative motion between the magnet and the coil is increased.
-the number of turns on the coil is increased.
-the cross sectional area of the coil is increased.

The direction of the induced current can be determined by using the Lenz's Law.

Induced Current and Induced Electromotive Force

The production of an electric current by a changing magnetic field is called electromagnetic induction.

The induced current is produced only when there is a relative motion between the conductor/coil and the magnetic field lines.

The induced current is produced when:
a)a conductor cuts across a magnetic flux.
b)there is a change of magnetic flux linkin  a coil or a circuit.

Relative motion

1. There is a relative motion between two objects if the two objects are getting closer or further apart.
2. When two objects are moving at the same speed in the same direction, there is no rleative motion because the distance between the two object does not change.

Induced current and Induced Electomotive Force



1. The electromotive force is required to drive the current in  a closed circuit.
2. Induced electromotive force is produced between the ends of the moving conductor or the solenoid.
3. When a galvanometer is connected to form a closed circuit, the pointer of the galvanometer moves when the magnet is directed back and forth inside the solenoid.
4. The induced e.m.f is responsible for driving the current flowing the closed circuit through the galvanometer. The greater the induced e.m.f., the greater the induced current.
5. The magnitude of the induced e.m.f and direction of the induced current can be determined by application of the laws of electromagnetic induction.



Electromagnetic Induction

An electric current can produce a magnetic field, but a magnetic field can also produce electric current.
This is discovered by Michael Faraday and Joseph Henry independently in 1831.
Electromagnetism induction is the complementary phenomenon to electromagnetism. Besides producing a magnetic field from electricity, we can also produce electricity from a magnetic field.
The difference is that, a steady magnetic field is produced by a solenoid or wire carrying electric current. whereas in electromagnetic induction, it requires relative motion between the magnet and the coil to produce an induced current.
REMEMBER: in order for electromagnetic induction to happen, there must be a relative motion between the magnet and the coil to produce current. That is, either the magnet or the coil shall move relative to each other.


The force on a current-carrying conductor in a magnetic field

The force on a current-carrying conductor in a magnetic field

A moving coil loudspeaker is an application of the force acting on a current-carrying conductor in a magnetic field. When a varying electrical signal is sent to the coil, the coil is pushed in and out. This makes the cone vibrate, creating sound waves.


Magnetic Force 

If we put two magnets near to each other, their magnetic fields will interact. Interact means that the magnets will experience forces on them as like poles will repel and unlike poles attract. It follows then that a wire in a field of a permanent magnet will experience a force when current flows through it. The magnetic field generated around the wire will interact with the field around the magnet. The two fields will produce a force.


Force produced by the combined magnetic field
“Catapult force”


The magnadur or slab-shaped magnets produce a uniform, parallel magnetic field around itself. When the two fields are combined, the pattern produced by iron filings indicates a complex field pattern, if free to move, it will be catapulted from the stronger field towards the weaker field or a neutral point.

The “catapult force” acts perpendicular to both the current and the magnetic field. If the wire carrying current is horizontal along the y-axis, and the magnetic field is horizontal along the x-axis, the force on the wire is vertical, up or down along the z-axis. Fleming’s left-hand rule neatly sums up this observation.

Fleming’s left-hand rule

Fleming’s left-hand rule says that if you hold the thumb and the first two fingers of your left hand at right angles, the thumb gives the direction of the force, the first finger shows the direction of the magnetic field (which is taken from north to south) while the centre finger points in the direction of the current (which is from the positive terminal to the negative terminal of the battery)

Turning force on a current-carrying coil in a magnetic field

Ammeter

The pointer of the ammeter is attached to a coil in a magnetic field. The higher the current through the meter, the farther the coil turns against the springs holding it. Thus the pointer moves farther along the scale, showing a bigger current.

Direct current motor

The catapult is used to make a simple electric motor. A wire is pushed in the opposite direction if the direction of the current through it is reversed.

The turning effect on a current-carrying coil in a magnetic field is used to make simple electric motors. This is why the turning effect is also known as the motor effect. The coil will turn in the opposite direction if the direction of the current through it is reversed. This principle is used in the electric drill to insert and remove screws when a coil is placed in a magnetic field, a magnetic force acts on each side of the coil. This produces a turning effect on a coil.

In a motor, the wire is wound around a central block called an armature. A spindle through the armature allows it to rotate. The current flows in opposite directions on each side of the armature, so one side is pushed while the other is pulled. This makes the armature to rotate.

After the coil has completed half a rotation, the current flowing on the side of the coil which is next to the north pole of the magnet is in the opposite direction to the first half of the coil switches direction. The same thing happens to the other side. As a result, the coil will start turning in the opposite direction producing a see-saw effect.

A commutator used in a DC motor to produce complete rotations of the coil. When the coil is in the upright position, there is no turning force trying to push it round. It is at this point that the commutator swaps over the contacts.

If the coil is already spinning, its inertia will carry it through this upright position. When the contacts are reconnected, the commutator has reversed the current. So the side of the coil that was being pulled up before is now being pushed down and vice versa. As a result the coil keeps spinning in the same direction.

FACTORS AFFECTING THE SPEED OF ROTATION OF AN ELECTRIC MOTOR

The speed of rotation of the coil can be increased by:
- increasing the current
- using a stronger magnet
- increasing the number of turns on the coil


 

 

Primary and Secondary Current in Transformers

Although we can make the secondary voltage greater than the primary voltage in a transformer, this does not mean energy is created. Energy is still conserved.In an ideal transformer, there is no loss of energy. Hence,
Power supplied to the primary coil = Power used in the secondary coil
That is,
VpIp = VsIs
Vp = Primary voltage , Ip = Primary current
Vs = Secondary voltage, Is = Secondary current
(Remember, Power, P = IV)
Which brings us to the ratio of current which is
Is / Ip = Vp / Vs
This means that if the voltage is stepped-up, the current in the secondary coil is stepped-down by the same ratio.
Comparing the transformer equation.
Vs / Vp = Ns / Np
We ultimately get
Is / Ip = Np / Ns
(Ns = number of coils in secondary coil)
(Np = number of coils in primary coil)

Transformers

In electric power distribution systems it is important to convert a small AC voltage to a larger one or vice versa. The device used for these conversions is the transformer. Transformers are found in many electronic devices nowadays, such as television and mobile phone.Principle of a Transformer
A transformer consists of two coils of wire known as the primary and secondary coils which are not connected directly to each other. The two coils are usually wound separately on a soft iron core. The purpose of the soft iron core is to provide a magnetic field linkage in the secondary coil.

When an alternating current flows in the primary coil, an induced magnetic field is produced in the soft iron core. This constantly changes the magnetic flux linkage in the secondary soil. Therefore, an alternating e.m.f induced across it.
A voltage is generated across the secondary soil only when there is a change in the flux linkage through the secondary coil. Thus the current in the primary coil must keep on changing as in the case of an alternating current. For this reason, a transformer can only function with an AC input.
The frequency of the secondary voltage is the same as that of the primary voltage.  The magnitude of the secondary voltage, however, depends on the ration of the number of turns of the primary and secondary coils.









Step Up and Step Down Transformers

In a transformer, the secondary voltage can be more or less than primary voltage. The voltage is dependent on the number of turns in each coil. In an ideal transformer, the relationship between the voltages and the ratio of the turns of the primary and secondary coils is given as below:

Where:
Vs = Secondary voltage
Vp= Primary voltage
Ns = Number of turns in secondary coil
Np = Number of turns in primary coil
From the above equation, if Ns is greater than Np, the Vs is greater than Vp. In this case we have a step-up transformer. If Ns is less than Np, we have a step-down transformer, where the secondary voltage is less than the primary voltage.

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