12th Class Physics Chapter 4 Electromagnetic Induction Short Question Answers

A light metalic ring is released to fall from certain height above the north pole (N-pole)of bar magnet.Because of this motion the magnetic flux passing through the ring changes an emf is produced which causes the electric current in the ring.By the application of Lenz’s law the direction of this induced current is such that it opposes the motion of the ring.Thus a N-pole must be produced towards the face of the ring opposite to the N- pole of the bar magnet and S-pole on the upper on the upper face of the ring.This is because of the clockwise direction of induced current in the ring when viewed from the a bove.

(a) When the switch in first coil is made on by closing it then current in this coil increases from zero to certain maximum value.During this time the magnetic flux in the second coil changes and thus the induced current is produced in the second coil.The right end of current carying coil facing the other coil becomes noth pole while the other end as south pole.As the closest end of the second coil is N- pole since the N- pole of first coil opposes the N-pole of the second coil so the current in the second coil will flow in anti-clockwise direction.It means that current will flow from left to right through the resistance R as shown in fig.Hence the current will be anticlockwise when switch is closed. (b)When the switch is opened the current begins to decrease from maximum to minimum in this case flux also decreases.It means according to Lenz’s law a south pole is induced on the lett produced in the opposite direction that is current will flow from right to left through resistance R.Hence the current will flow in clockwise direction when the switch is opened.

No the induced emf always opposes the cause that produces it.If magnetic flux through the circuit is increasing then the induced emf acts to decrease the magnetic flux.If magnetic flux is decreasing then the induced emf acts to increase the value of magnetic flux.Hence the induced emf does not always act to decrease the magnetic flux through the cireuit.

When the switch in the circuit is closed then electric current flows in the circuit.A magnel field is produced in the coil which changes from zero to maximum value.Due to the change of magnetic flux of the coil an induced current is produced in the metal ring.This induced current produces its own magnetic field which is opposite to the magnetic field of the coil.Both the fields repel each other.As the ring is free to move so the metal ring jumps upward due to force of repulsion suddenly. When polarity of battery is reversed even then the process is similar i.e. the ring will jump upward in the same manner.The reason is the same as given above.

In order to generate an emf and a current in the coil it must be rotated about x-axis. If the coil is rotated about z-axis then no change of magnetic flux takes place through it.If the coil is rotated about Y-axis even then the flux passing through the coil remains zero because plane of the coil is at all times parallel to the lines of magnetic field B. If the coil is rotated about X-axis then there is a change of magnetic flux passing through a coil.Hence only in this case an emf is induced in coil.

When plane of flat loop of wire is parallel to the magnetic field i.e. θ=0 no flux change will take place through the loop.It means rate of change of flux through the loop is zero.Hence no emf will be induce in the loop.Thus,
€ = ɷAbsinθ
or       € = ɷAbsin0ᵒ=0
€= 0

The magnetic force on moving electric charge in the wing is given by
F^→= -e (v^→×B^→)……………………… (1)
It means that F^→acting at right angle to V^→ is towards north and B^→ is vertically downward.The above equation (1) shows that electrons move towards left.According to right hand  rule the direction of F^→ is directed towards west.Thus the west wingtip of the plane is positively charged.

Yes an electric motor when running also acts as a generator.When an electric motor is running them its armature is rotating in a magnetic field.Due to its rotation change of magnetic flux takes place and so induced emf is produced.This is called back emf of the motor.Thus the induced emf opposes the rotation of armature.It shows that the running electric motor also acts as a generator. When the motor rotates without load it rotates quite fast then the back emf induced is quite large so the current flowing through the motor becomes very small.But if the motor drives some load then depending upon the load the speed of motor decreases and so emf induced also decreases.This makes the current flowing through the motor quite large.If the motor is overloaded beyond its limits the induced back emf in the coil can produce high current that it may burn the motor out.

Yes D.C.motor can be turned into a D.C.generators.When an electrical motor is running by passing a current through armature of motor then its armature coil is rotating in a magnetic field.Due to its rotation the magnetic flux through the coil changes and an emf is induced at the output.Thus the motor becomes a generator.Which convents electrical energy into mechanical energy. In order to convert D.C motor into D.C generator two changes are to be done. (1) The magnetic field must be supplied by permanent magnet and not by electromagnet. (2) An arrangement to rotate the coil armature should be provided.

No it is not possible.It is against the law of conservation of energy in actual practice input power in not equal to output power.Electric motor and generator are elector – mechanical machines as such their efficiency is always less than 100%. So when a motor rums a generator the electrical output is always less than input.Hence a combination of motor and generator can never produce more energy out than its energy input.

If a suspended magnet is made free to oscillate in a horizontal plane then magnetic field changes in a region close to it.Thus the magnetic flux passing through the metallic plate placed below it change.Due to this change of flux eddy currents are produced inside the metal.According to Lenz’s law these eddy currents oppose the cause which produces it i.e. opposes the motion of the bar magnet.Due to which the oscillations of the magnet are strongly damped.

No a step up transformer cannot step up power.According to an ideal transformer In pup power= output power.But in actual transformer there is a loss of energy in the coils.Therefore output power is always less than the input power.Hence it shows that step – up transformer cannot increase the power level.Of course it can increase the voltage level. (b) In transformers the electric power or energy is transferred from primary coil secondary coil due to magnetic field.The change of magnetic flux through one coil is linked with the other secondary coil and emf is produced.Thus the magnetic flux produces power at the secondary coil and emf is produced.Thus the magnetic flux produces power at the secondary coil of a transformer if power is provided at its input.

(a) If the secondary circuit is open then output power will be zero.Because output power is always slightly smaller than the input power therefore a very small value of current is being drawn by a primary coil of transformer from A.C. mains. (b) When the secondary circuit is closed the output power will be increased.As we know that input power is equal to output power i.e. Input power = output power Therefore the transformer will draw large current from the A.C.mains to increase the primary power.Hence greater current is needed in primary to equalize power in secondary coil.

No a transformer cannot  step – up energy but energy is conserved.In an efficient  transformer if we  ignore  power losses.
Power input = Power output
V_pI_p = V_sI_s
It means  power remains constant so it cannot step up energy.

i) By moving the wire in a magnetic field. ii) By changing the current through a circuit merely by it. iii) By moving the magnet near the wire.

Yes a current carrying coil can be used as a compass.When current carrying conductor is suspended freely then the component of the earth’s magnetic field being perpendicular to the plane of the coil will exert a torque on the coil and rotate the coil so that the plane of the coil and the earth’s magnetic field become parallel to each other.Since the direction of earth’s field is along the North – South therefore the plane of the current – current – carrying coil will settle down along the North – South direction.Hence it can act as compass.

The induced e.m.f. of a generator is given by € = NBAɷsinα Where ɷ is the speed of rotation of its coil.If ɷ is increased the output voltage will also increase in the same proportion.

i) Both motor and generator have armature. ii) Both have split rings or commutator segment. iii) Both require magnetic field by permanent magnet or electromagnet. iv) Both have carbon brushes touching the split rings. Differences Between Them:- i) An electric generator coverts mechanical energy into electrical energy while an electric motor converts electrical energy into mechanical energy. ii) In case of generator the coil is rotated by some mechanical means in order to produce the current.In case of motor the current is supplied to the coil from the main for its rotation.

A.C. and D.C. Generator:- If the dynomo has sliprings it is an A.C.generator and if it has commutator it is a D.C.generator.

Whenever the current passes through any coil it produces a magnetic field and produces an emf in the coil.This emf tries to oppose the external cause which produces it and this fact is in accordance with the lenz’s law.Due to this fact when the current in the coil is increased ,the resulating induced emf will be opposite to that of battery.Thus if the current is decreased the induced emf will increase.This is the reason that the induced emf is sometimes called as back emf.

In order to retard the motion of a fast moving automobile a strong magnetic field should be applied across the wheels.The consequent change in magnetic flux induces emf which by Lenz’s law opposes the cause i.e. motion.Thus the motion is retarded.

The self – inductance are used as inductors in A.C circuit. They behave as resistors because when the current in any coil increases the inductor opposes it and decreases the current to a required level.

The principle of electric generator is based upon the Faraday’s law of electromagnetic induction.In this an armature is rotated externally in a magnetic field causing a motional induced emf in the armature coil.This induced emf is directly proportional to the rate of change of magnetic flux by Faraday’s law i.e.

Difference:- i) Alternating current is that which changes its direction several times during one second while D.C. does not change its direction and travels straight. ii) An A.C can travel a long distance while D.C cannot travel long distance. iii) An A.C consists of both positive and negative half cycles of currents while D.C consists of only positive half cycles of current.

In commercial generators armature can be rotated by the following means. i) By fuel engine using diesel gasoline petrol etc. ii) By rotating huge turbine machines through water fall such as Terbella and Mangala dams. iii) By rotating the fans of a turbine through wind.

i) In D.C generator slip rings are replaced by split rings. ii) A.C generator converts mechanical energy into electrical energy while D.C generator converts electrical energy into mechanical energy. iii) The output of a D.C generator is unidirectional.The output of A.C.generator changes its direction.

The working principle of a D.C motor is the torque experienced under the affect of couple of magnetic field.In doing so electrical energy fed to the motor coil is converted into mechanical energy.

The back emf in motors is almost zero.Since less the back induced emf more will be the current flowing through it.Thus at the start the current will be large due to small back emf.However when motor speeds up back emf also increases and opposes the external voltage due to which current in the motor decreases.

The induced emf depends upon the rate of charge of flux through the coil and speed with which the coil rotates.If the speed of rotation of the coil increased the output voltage will increase.

Such a transformer in which number of turns in the secondary coil N_s are greater  than number  of turns on the primary coil N_p is called  a step – up  transformer.For every  step  up transformer the voltage  across  the secondary V_s is always  greater than the input  primary voltage V_p.Step down transformer:- Such a transformer in which number of turns in the secondary  coil Ns are less  than number of turns on the primary coil Np is called a step – down transformer.The voltage  across the secondary Vs is always  less  than the voltage  Vp  at the primary.

(1) The high power voltage through grids are stepped up from about 1100 v to 33000 V at power gonerating stations.These high voltages cannot be directly used at cities for domestic use.The high voltage can be reduced by the use of step – down transformer according to the desired requirement of the people at their homes or in cities. (2) In Pakistan the high voltages are stepped down to 220 volts for domestic supply. (3) Television and radio sets may require several secondary transformers for their complete operation. (4) An electric bell can operate at 9 volts by kowering a high voltage of 220 v to 9 v by a step – down transformer.

The eddy currents are the main cause of generating heating effect in transformer due to which lot of electrical power is lost.It can be prevented by properly inserting lamination material between the different layers of transformer which stops the eddy currents to flow from one layer to the other.

Power loss can be minimized by the following steps. i) Selecting laminated sheets of material whose hysteresis loop area is very small. ii) The resistance sheets should be properly laminated to stop the flow of eddy currents. iii) The resistance of a material of wires must be minimum. iv) The coils must be so designated and coupled as to produce maximum change of flux between the pair of coils.

It states that the direction of the induced current is always so as to oppose the change which causes the current.

When a conductor is moved in a magnetic field an emf or voltage is produced in the conductor and the current is generated due to the effect of induced emf.Such phenomenon is called as an electromagnetic induction.

A device which is used to increase or decrease the value of alternating emf or voltage is called transformer.

The efficiency of a transformer can be increased by reducing the power losses.It can be done by taking the following steps. i) Loop Area:- Core is assembled from the laminated sheets of a material whose hysteresis loop area is very small. ii) Insulation:- The insulation between the sheets of core should be perfect to stop eddy currents. iii) Resistance of primary and secondary coils:- The material of wires of primary and secondary coils must be such that their electrical resistance is very small as may be possible. iv) Flux Coupling:- Primary and secondary coils should be wound in such a way that flux coupling between them is maximum.

There are two main causes of power loss which are i) Eddy currents. ii) Magnetic hystersis. i) Eddy currents:- Due to the change of flux through a soild conductor the induced currents set up within the body of a conductor in a direction perpendicular to the flux are known as eddy currents. ii).Magnetic Hysteresis:- The energy spent in magnetization and demagnetization of core is called hystersis loss. The loss of energy during each cycle of A.C.can be decreased by using such a material for the care whose hysteresis loop is of small area.

Self – inductance depends upon the following factors. i) Area of cross – section of the coil. ii) Number of turns of the coil. iii) The rate of change of current. iv) The material of the coil.

This is the unit of self – inductance and mutual inductance.It can be. i) In the case of self – inductance:- It is defined as the ratio of average emf in the coil to the time rate of change of current in the same coil. ii) In the case of Mutual inductance:- It is defined as the mutual inductance of the pair of coil in which a rate of change of current of one ampere per second in the primary causes an induced emf of one volt in the secondary coil.

When we pull the wire across the magnetic field of the magnet we do work against the magnetic force arising from the interaction of the original magnetic field and that of the induced current.We use some energy in doing this work.This energy is the source of induced current thus electro – magnetic induction is exactly according to the law of conservation of energy.