electromagnetism

Phenomenon of generating current e.m.f. by changing magnetic fields is called Electromagnetic Induction

Magnrtic flux = B.A= BACos weber

An e.m.f. is induced in a coil, when amount of magnetic flux linked with the coil changes with time.

Ist law: whenever the amount of magnetic flux linked with a circuit changes, an emf is induced in the circuit.  This induced emf last so long as the change in magnetic flux continues.

IInd law: the magnitude of emf induced in a circuit is directly proportional to the rate of changr of magnetic flux linked with the circuit.

Lenz law: the polarity of the induced emf is such that it opposes the change in magnetic flux responsible for its production.

Various methods of producing Induced E.M.F

1. By changing the magnitude of magnetic field B,

2. By changing the area A, i.e., by shrinking or stretching or changing the shape of the coil.

3. By changing angle between the direction of B and normal to the surface area Ai.e., changing the relative orientation of the surface area and the magnetic field.

Eddy currents are the currents induced in the bulk pieces of conductors when the amount of magnetic flux linked with the conductor changes. They are basically the currents induced in the body of a conductor dur to chand in magnetic flux linked with the conductor.

We can reduce the eddy currents but we can not eliminate them.

Some application of eddy currents

1. Electro-magnetic damping
2. Induction furnace
3. Magnetic brakes
4. Electric Power Meters
5. Induction Motor

To minimize the eddy currents, the metal core to be used in an appliance like dynamo, transformer, choke coil, motor etc. is taken in the form of thin sheets. large resistance between the thin sheets confines the eddy currents to the individual sheets.

Self induction is the property of a coil by virtue of which, the coil opposes any change in the strength of current flowing through it by inducting an emf in itself.

A wire cannot act as an inductor as magnetic flux linked with wire of negligible area of cross section is zero.  the wire has to be in the form of a coil to serve as an inductor. Further, self induced emf appears only during the time the current is changing.

Mutual Induction is the property of two coils by virtue of which each opposes any changes in the strength of current flowing through the other by developing an induced emf.

Conceptual points

When the conductor is moved in a direction parallel to magnetic field, amount of magnetic flux linked with the conductor does not change.

Change in magnetic flux always induce emf but the current will induce only when the circuit is complete.

1. AC can be transmitted over long distances using step up transformers. The loss of energy is negligible. DC cannot be transmitted as such.
2. The AC voltages can be easily varied using transformer.
3. The AC can be easily converted into DC.
4. the magnitude of AC can be reduced using a choke coil, without involving loss of energy.
5. the AC is easier and cheaper to generate than DC. The AC generators are usually more robust and their efficiency is high.

Drawbacks of AC are:

1. It is more dangerous to work with AC at high voltages. The moment the insulation is faulty, one gets severe shock.
2. The shock of AC is attractive, whereas that of DC is repulsive.
3. There are certain phenomena like electroplating electro-refining, electro-typing etc. where AC cannot be used. In such cases, DC is needed.
4. The AC is transmitted more from the surface of conductor than from inside. Therefore, several fine insulated wires are required for the transmission of AC.

The AC can be converted into DC with the help of a rectifier while DC can be concerted into AC with the help of an inverter. The AC cannot produce chemical effects of current e.g. electrolysis; electroplating etc. This is due to large inertia of heavy ions which cannot follow the frequency of AC.

Time constant of RC circuit indicates how fast or how slow willbe the charging and discahrging of a capacitor through a resistor.

Time constant of RL circuit tells us how fast or how slow is the growth /decay of current in the RL circuit. Low value of time constant indicates that the growth and decay are fast.

Capacitor blocks DC and not AC.

We can measure DC by an ordinary ammeter, but not the AC because average value of AC over a complete cycle is zero. The DC and AC both can be measured by a hot wire instrument because both AC and DC produce heat, which is proportional to square of the current.

The metal detector works on the principle of resonance in AC circuits.

The voltage drops across the inductor or capacitance in a series LCR circuit be greater than the applied voltage. This is because these villages are not in phase and they cannot be added like ordinary numbers.

Power is dissipated in an AC circuit in resistance only.

Peak vale of 220V = √2* 220= 311 V

When an AC is fed to a moving coil galvanometer it shows no deflection because such a galvanometer measures average vale of current, which is zero in case of AC. Galvanometer is an instrument used to detecting and measur ing very small electric currents.

Adnittance of an AC circuit is the reciprocal of impedence (Z) of the circuit.

Wattless current is that which involves no consumption of power per cycle for its maintenance in the circuit.

We can use a capacitor instead of coke coil for reducing AC because average power/cycle in ab ideal capacitor is also zero.

In an inductor current rise only ecponentially.

At a very high frequency of AC a capacitor behaves as a pure conductance.

The average power consumed in a circuit consisting of resistanceless inductance is zero.

An inductor acts as a conductor for DC.

Electrical devices

1. An AC generator/dynamo is a machine which produces alternating current energy from mechanical energy. Based on electromagnetic inductance. Designed by Tesla

The direction of current inducted is given by fleming right hand rule.

The frequency of AC generated depends only on does of rotation of the coil.

2. DC motor: it converted direct current energy from a battery into mechanical energy of rotation.

Principle: when a coil carrying current is held in a magnetic field, it experiences a torque, which riste the coil.

3. A transformer is an electrical device which is used for changing the A’C voltages.

Principle: a transformer is based on the principle of mutual induction. Impedance matching is yet another important function of the transformer.

A transformer is essentially an a.c. device. It cannot work on d.c. A transformer changes a.c. villages/currents. It does not affect the frequency of a.c. When a.c. voltage is raised n times, the corresponding alt current reduces to 1/m times.

Transformer used as village regulator. A step down transformer is used for welfing purpose.

4. Choke coil is an electrical device used for controlling current in an AC circuit. If we use a resistance R for the sane purpose, a lot of energy would be wasted in the form of heat etc.

Conceptual points

1. There is no device that can control dc without any energy loss.
2. The transformer core is laminated to minimise loss of energy due to Eddy currents. By doing so, the magnitude of eddy currents is reduced considerably and hence the energy loss is minimised.
3. For stepping down dc voltage, we may use an ohmic resistance only such as in potential dividing arrangement.
4. A capacitor can be used insteadof a choke coil for controlling ac, as it also involves no dissipation of energy.
5. Primary coil in step down transformer has large no. of turns.
6. We can convert an ac generator into dc generator by replacing slip ring arrangement by split ring arrangement.
7. The function of motor starter is to provide maximum resistance when motor is switched on, so that current remains low even when induce emf is zero. It is a variable resistance.
8. Oil in transformer provide cooling as well as insulation.
9. The sharpness of tuning at resonance is measured by Q factor or Quality factor of the circuit. Q=(1/R)* (√L/C)
10. The total effective resistance of RLC circuit is called Impedance i.e. .