KPK Board 12th class Physics Ch 14 Electromagnetic Induction short questions answers

Similarities: Following are the similarities.

1. Both motor and dynamo have armature.
2. Both have commulator or split rings.
3. Both require magnetic field by permanent are electromagnetic.

Differences: Following are the differences.

1. A moving wire, such as the two sides of the AC generator converts mechanical energy into chemical energy. While electric motor convert electrical energy into mechanical energy.
2. In the case of wire are dynamo, it is a moved by some mechanical means in order to produce current. But in case of motor, current is supplied from outside to rotate the coil.

For a motor back emf is always smaller than the applied potential difference V. Because the back emf E depends upon the speed of the coil of motor. When potential difference is applied and the motor is just started, the back emf is almost zero and large current passes through the coil. As the speed of the motor increases, the back emf also increases and the current becomes smaller. Moreover the applied potential difference V and induced emf E are opposite in polarity, so the net emf in the circuit is.

(emf)_net  =  V-E

If R is the resistance of the coil, then the current drawn by the motor is.

I = V-E/R

This equation shows that current drawn by the motor depends upon the potential difference and back emf. Therefore back emf is always smaller from P. D and cannot be greater than V.

The following factors which limit the magnitude of the back emf.

1. The magnitude of back emf depends upon the speed of motor’s coil. It increases with increasing the speed of motor.
2. Load controls the speed of the motor. If the motor is over loaded, it slows down the speed of motor. As a result the back emf decreases and the motor draws more current. If R is the resistance of the coil, then the power dissipated in it is.

P = I²R

Due to this power dissipated in the form of heat may damage or burn out the motor.

As the rate of doing work increases the back emf decreases. Because normally sufficient current flows through the motor, exerting torque on the coil to drive the load and overcome the losses due to friction. But increasing the load on the motor, its speed decreases as a result back emf also decreases and allows the motor to draw more and more current from the circuit. The current flowing through the motor when voltage V is applied is.

I = V-E/R

Where R is the resistance of the coil and E is back emf.

IR = V-E

I²R = IV-IE

IV = I²R+IE

Where IV is the input power, I²R is the power dissipated in the coil in the form of heat and IE is power used to do work against the back emf.

Ans: From Faraday’s Law of electromagnetic induction we know that the magnitude of the induced emf is equal to the negative time rate of change of magnetic flux through the circuit. If Δφ is the change in magnetic flux in time Δt through a loop of wire, then according to Faraday’s Law we have

E= –Δφ/Δt

But for single loop we also know that

E =-L ΔI/Δt

Since    L =µ₀ln²A

So eqn. (2) becomes

E=µ₀ (ΔI) nA (N) l

E=-NΔφ/Δt

Hence E=-Δφ/Δt=-ΔI/Δt

As                           E=-Δφ/Δt

Taking dimensions of both the sides, we have

[E] = [Δφ/Δt]

[Volt] = [Weber/Se]

[J/c] =[N.m/A/Se]

[N.m/A.Se]= [N.m/A.Se]

[Kg.m.m/A.Se³]= Kg.m.m/A.Se³

[Kg.m²/A.Se³]=[ Kg.m²/A.Se³]

[ML²A^-1T^-3]=[ML2A^-1T^-3]

Thus it is clear that dimensionally correct.

Consider a rectangular coil of N turns each of area A rotating about its axis in a uniform magnetic field of flux density B. If the normal to the coil makes an angle with magnetic field B at time t, then the flux o linking each coil is given by

θ = B.A=BAcosθ

But the coil rotates with steady angular velocity w then θ is

θ = wt

θ = BA coswt

Applying Faraday’s Law we have

E = -NΔφ/Δt

E = NAwBsinwt

This emf will produce alternating current.

The currents produced due to induce emf in metals moving in a magnetic field or exposed to a changing magnetic flux are called Eddy Currents. The induced emf’s are not usually very large, but large eddy current flows through the conductor due to low resistance of the current path,

According to Lenz’s law the eddy currents must flow in a direction so as to oppose the cause which produces it. If the conductor is moving in the magnetic field, the eddy current will flow in the direction, so that the magnetic force (ILB) will oppose the motion. Hence eddy currents acts as an effective brak to the motion. The polarity produced in the sheet and direction of eddy currents have been shown.

Electromagnetic brakes or EM brakes are used to slow or stop the motion of a first moving object such as bullet train. They are using electromagnetic force, creating torque to apply mechanical resistance. Principle: The working principle of electric brakes is based on electromagnetic induction, creating eddy currents,

Construction: Electromagnetic brakes consists of two major parts

Stator: It is the stationary part of the brake. It holds disk & 16 induction coils which can be energized separately springs in groups of four.

Rotor: The rotor is made of two discs which provide the braking forces when subjected to the magnetic field of the coils of stator.

Working: Electric brakes operate through electrical activity but transmit torque mechanically. When voltage is applied to the coils of the stator, they create magnetic field and become electromagnet. The armature attached to the shift of the rotor is rotating in the magnetic field of the coils producing induced emf and eddy currents. As the armature does so, it squeezes the inner and outer together. When the discs are squeezed torque is transmitted from the hub into the machine frame stopping and holding the shaft. When the voltage is removed from the brake, the magnetic field is off, so the armature is free to rotate with shaft.

Uses: Electromagnetic barks are used in bullet train, speed record cars and even now in air crafts.

When a bar magnet is dropped lengthwise along the metal tube, the magnetic flux through the pipe increases. The change of magnetic flux, linked with metal tube due to motion of magnet induces an emf in the tube. According to Lenz’s Law the polarity of the induced emf will be such that the induced current in the tube will make the tube end opposite pole of the magnet. Hence the repulsive force will oppose the motion of the magnet. When both upward magnetic force due to induced current and downward gravitational force become equal, the magnet will fall with terminal constant speed.

F_m= F_g

ILB=GM₂m/r²=mg

ILB=mg

On the other hand when a bar magnet is dropped in a cardboard, no induced emf is produced, as cardboard is non-ferromagnetic. So the magnet will fall freely under the action of gravity with constant acceleration “g”.

1. The induced currents circulating inside a piece of metal moving in a magnetic field or exposed to a changing magnetic field are called eddy currents. The induced emf is not usually very large but large eddy currents are flowing through the conductor due to low resistance.
2. Features of Transformer: The changing magnetic flux through the iron core of transformer induces eddy current in the core. This eddy current causes the loss of power and heating the core of transformer. To minimize energy loss through eddy currents the iron core of the transformer is made up by means of lamination sheets which are insulated from each other and stops the flow of eddy currents.

Induction cook top is a electrical device used for cooking and heating food.

Principle: Induction cook tops work on the principle of electromagnetic induction and using the following three basic concepts.

1. Every electric current produces magnetic field around it.
2. Alternating current produces fluctuating magnetic field.
3. The changing magnetic field causes induce emf and induce current in a conductor placed in it which is also known as Faraday’s Law.

Construction & Working

1. Induction cookers are similar to other cook tops having a distinct zone where one can place cooking pots.
2. Inside the cooking zone, there is a metal coil. When power is turned on and alternating current flows through the coil produces a changing magnetic field above and below all around it.
3. The Changing magnetic field produced by the coil penetrates into an iron pan placed on the cooking zone. This changing magnetic field induces eddy current inside the pan due to which power (I’R) is dissipated in the pan in the form of heat. This turns the pans into heat. Cooking pot or fry pan is placed on the heater where heat flows directly into the food containing fats and water. In this way cook tops are used for cooking food.

The induced emf produced when the armature of the motor rotates in a magnetic field is called back emf or counter emf in the motor. When armature of a motor is rotated in a magnetic field by applying potential difference v, an induced emf is produced. This induced emf opposed the potential difference which is running the motor. So the net emf is

(emf) net = V-E

The back emf is zero when the armature is at rest. The magnitude of back emf increases with increasing the speed of armature which ultimately increasing the eddy currents.

(b) It is advantageous for the armature to rotate radially to magnetic field, so that to provide strong magnetic field in the vicinity of the coil. Therefore large torque is applied to rotate the armature.

Torque = NIAB

1. When armature of the motor rotates across the magnetic field by the applied potential difference V, according to principle of electromagnetic induction induced emf E is produced in the coil. But V and E are opposite in polarity and the net emf in the circuit is V-E. When the motor is just started back emf is almost zero and large current passes through the coil. As the angular speed of the armature increases the back emf also increases and the current becomes smaller and smaller.
2. When the motor is started, the armature is at rest and back emf is zero. The maximum current passes through the armature given by, according to ohm law’s is

I = V/R

where R is the resistance of the coil. So the maximum torque acts on the coil.

Torque = NIAB

Under the action of this torque, the angular velocity is reached to maximum value.

A transformer is an electrical device which steps up or steps down alternating current voltage working on the principle of electromagnetic induction. There are two main causes of power losses regarding the core of the transformer which should be reduced by using proper material.

1. Eddy Current: The constantly changing magnetic flux induces eddy current in the core of transformer which causes heating effect. This effect is reduced by having laminated core of soft iron as shown.
2. Hysteresis: The energy spent in magnetization and demagnetization of the core of transformer is called hysteresis loss. This loss of energy during each cycle of A.C can be reduced by using such a material for the core whose hysteresis loop is of small area. To make a good transformer core, laminated soft iron would be required.

When an electric current is increasing in magnitude flowing from A to B, in the wire as shown, the magnetic field produced due to current in wire links with coil. As the current increases, the magnetic flux changes and induces current I in the coil. The direction of the induced current I in the coil can be determined by using Fleming right hand rule. According to this rule the induced current in the coil will flow clockwise.