# Electromagnetic Induction: Short Questions Answers Physics

## Electromagnetic Induction Definition

The phenomenon in which an emf is produced in a coil (conductor) due to the change in magnetic flux linked with the coil is called Electromagnetic Induction. The emf thus produced is known as induced emf and the current that flows through the coil is called Induced Current

#### 2. What do you mean by magnetic flux?

Magnetic flux through an area is the total number of magnetic lines of force passing normally through the area of the surface. Let us consider a magnetic field of strength B acting on a small surface of area A making an angle θ with the normal on the surface. The magnetic flux passing through the given surface area is given by (Φ) = BA cosθ. If B is parallel to normal, (Φ) = BA. Magnetic flux is measured in Weber (Wb).

#### 3. Define one Weber magnetic flux.

The magnetic flux on a surface of area A due to a magnetic field of strength B normal to the surface is given by (Φ) = BA. If B = 1T and A = 1m^2, then Φ = 1Tm^2 = 1 Weber.

Thus if a uniform magnetic field of 1T acts normal to the area of 1m2, the magnetic flux on the surface is called one Weber.

#### 4. What do you mean by magnetic flux linkage?

Let us consider a coil of,n number of turns placed perpendicular to the direction of the uniform magnetic field of strength B. Let A be the area of cross-section of each turn. The same magnetic flux which passes through each turn of the coil is called Flux Linkage. The flux linkage is given by Φ = nBA. If B makes an angle θ with the normal to the plane of the coil, then the flux linkage is given by (Φ) = nBA cosθ.

#### 5. What do you mean by motional emf?

The maximum value of the induced potential difference in a conductor in motion inside the magnetic field due to Lorentz’s force on the free charges is called motional emf. If q be the free charge moving with a velocity v inside the conductor of length l, then motional emf is given E = qvl.

#### 6. Write down Faraday’s laws of electromagnetic induction. [HSEB 2065]

The results obtained from Faraday’s experiments are known as Faraday’s laws of electromagnetic induction. They are as follows:

(a) Whenever magnetic flux linked with a coil or a closed circuit is changed, emf is induced in the circuit.

(b) The induced emf continuous until the flux linked with its changes.

(c) The magnitude of induced emf is directly proportional to the rate of change of magnetic flux linked with the circuit. i.e. if E is the magnitude of induced emf, (Φ) be the change in the magnetic flux linked with the coil in time △t, then

E ∝ △Φ/△t

#### 7. State and Explain Lenz’s law? [HSEB 2069]

it states that the direction of induced current due to the change in magnetic flux through a closed-loop is always such that it opposes the change or cause which produces it. According to Faraday’s law, induced emf

(E)∝ dΦ/dt

but when Lenz law is applied to it, it will be

E ∝ -dΦ/dt

where a negative sign shows the opposing nature of induced emf. It is based on the principle of conservation of energy.

#### 8. Show that Lenz’s law is an example of the conservation of energy.

Lenz’s law states that the direction of the induced current is always such that it opposes the change which produces it. Here when the north pole of the magnet is moved towards a closed loop as shown in the figure, if the Motion induced current flows in a clockwise direction, it induces the South Pole at the near side of the given magnet which attracts the North Pole the magnet. Then without any mechanical work done, the K.E. of the magnet goes on increasing.

Also, electrical energy also induced on the coil which violates the law conservation of energy. If the current flows in an anticlockwise direction, North Pole will be induced near to the North Pole of the magnet which repels the given magnet and we have to do mechanical work on the magnet to bring it near to the coil to induce the current in it. It shows that electrical energy will be induced when mechanical work is done. It proves that Lenz’s law is an example of the conservation of energy.

#### 9. An induced current or emf has no direction of its own. Explain.

According to Lenz’s law, the induced current or emf always has a direction so that it opposes the cause or the change which produces it Hence its direction is always determined by the cause which produces it. So it does not have its own direction.

#### 10. Why is the induced emf always called the back emf?

According to Faraday’s law of electromagnetic induction, whenever there is relative motion between a magnet and a coil, there is induced emf developed in the coil. After many experiments, Lenz found that the emf is induced in such a direction, it would oppose the cause which produced it. Since this emf has the opposing nature to drag back to its cause, it is called back emf.

#### 11. What is the cause of induced emf?

When a conductor is moved in a magnetic field, all free electrons inside the conductor move with the velocity of the conductor. Then Lorentz force acts on the free electrons due to which the electrons drift along the length of the conductor as they are bound inside the conductor. The drifting of the electrons gives rise to the induced current which gives rise to the induced e.m.f.

#### 12. When the current flowing in an inductive circuit is switched off, a spark is produced. Why?

According to Lenz’s law, when the switch of an inductive circuit is made off, the current induced opposes the decay of current in the circuit which increases the emf and the electric field will be very high. This high emf or electric field can ionize the air near to it and we see the spark.

#### 13. What are the methods to induce emf?

We can induce .emf through various methods. They are as follows:

(a) By changing the magnetic field intensity.

(b) By changing the area of the closed-loop through which the magnetic flux is passed.

(c) By changing the orientation of the area with respect to the magnetic field.

#### 14. The inductance coils are made of copper. Why?

When emf is induced in a coil due to the change in magnetic flux through it, copper coils due to having low ohmic resistance makes induced current large. This is why inductance coils are made of copper.

#### 15. A copper ring is suspended by a thread in a vertical plane One end of the magnet is brought horizontally towards the ring. How will the position of the ring be affected?

Yes, the position of the ring is affected when it is suspended by a thread in a vertical plane and one end of the magnet is brought horizontally towards the ring. When the magnet is brought near to the ring, the magnetic flux through the ring increases which makes the current flow through the ring. As a current flowing ring is now in a magnetic field, it experiences the torque and it rotates.

#### 16. Will emf be induced between the ends of a conductor if it is moved at a right angle to a uniform magnetic field?

When emf is induced by moving a conductor in a magnetic field, it is called the motional emf and its magnitude is given by E = Blv sinθ where B, l, v, and θ be the magnetic field strength, length of the conductor, the velocity of the conductor and the angle between the direction of motion of the conductor and the magnetic field respectively. When it is moved perpendicular to the field, θ = 90° and E will be maximum and given by E = Blv.

#### 17. What is the cause of emotional emf?

When a conductor is moved in a magnetic field with a velocity v, we can consider that the free electrons in it, are moving with the same velocity in the same direction and they experience Lorentz force which is given by F = -e (v x B ) where e be the charge of each electron and B be the strength of the magnetic field. Hence the electrons gain a motion in the direction of this force. It induces the emf which is known as emotional emf. Hence, the main cause of emotional emf is the Lorentz force experienced by the free electrons of the conductor.

#### 18. When a conductor is moved parallel to a uniform magnetic field, emf will not be produced. Why?

The Lorentz force experienced by a free electron in a conductor when the conductor is moved with the velocity v in a magnetic field of strength B is given by F = Bev sin θ. When the conductor is moved parallel to the magnetic field θ becomes zero i.e. F becomes zero. Since the Lorentz force is zero, the motional emf will be zero.

## Electromagnetic induction applications

#### 19. A sheet of copper is placed between the poles of an electromagnet with the magnetic field perpendicular to the sheet. When it is pulled out, a considerable force is required, and the force required increases with speed. Explain.

When a sheet of copper is placed between the poles of an electromagnet with the magnetic field perpendicular to the sheet and it is pulled outside, the magnetic flux through it is changed. Due to the flow of current in it, a force is exerted on it which is in opposite direction to the applied force. The magnitude of this force is directly proportional to the ou speed with which it is pulled. Hence a force is required to pull it t and it increases as its speed increases.

#### 20. Two identical loops, one of copper and another of aluminium are rotated with the same speed in a uniform magnetic field. Do they have the same induced (a) emf (b) current?

The induced emf depends upon the rate of change of magnetic flux through it. Since both of the given loops are identical, they will have the same induced emf. But the copper has low ohmic resistance than aluminum, copper loop has a greater current than aluminum.

#### 21. A lamp connected in parallel with a large inductor glows brilliantly before going off when the switch is put off. Why?

When the switch is made off a large emf is induced in the inductor which opposes the decay of emf which increases the p.d. across the lamp and the lamp connected in parallel with it glows brilliantly.

#### 22. A lamp connected in series with a larger inductor does not light up full brilliance immediately when the switch of the circuit with a source is made on why?

When the switch is made on the current through the circuit increases gradually. Then an emf is induced in the inductor which opposes the growth of current. Hence the bulb does not light up full brilliance immediately. After the steady current is reached, there will be more emf induced in the inductor and the brilliancy of the lamp does not change.

#### 23. A small resistor is generally connected in parallel with a current-carrying coil of an electromagnet. Why?

When the current through the coil is switched off, an emf is induced in it which opposes the decay of emf and a strong electric field is set up which ionizes the air near to it. So spark is produced. But if a small resistor is connected in parallel to it, it provides a low resistance path for the current and sparking can be avoided.

#### 24. If a loop of conducting material is moved with a constant velocity well inside a uniform magnetic field perpendicular to the field, will the current be induced in it?

If a loop of conducting material is moved with a constant velocity well inside a uniform magnetic field perpendicular to the field, there will not be induced current in it because to induce a current, emf should be induced but the emf induced depends upon the rate of change of magnetic flux linked with it. In this case, flux through the loop remains constant. Hence there will be no induced current in the loop.

#### 25. What do you mean by A.C. generator or A.C. dynamo or alternator?

An electrical device that converts mechanical energy into electrical energy whose magnitude and, direction change with respect to time is called an A.C. generator or A.C. dynamo or alternator.

#### 26. What is the principle used in the A.C. generator?

In A.C. generator uses the principle of electromagnetic induction which states that when the coil is rotated in a uniform magnetic field so that the flux through its continuous changes, an emf is induced in it.

#### 27. What do you mean by D.C. generator or D.C. dynamo?

An electrical device that converts mechanical energy into electrical energy whose direction does not alter with respect to time is known as a D.C. generator or D.C. dynamo.

#### 28. What is the principle used in D.C. dynamo?

D.C. dynamo is based on the principle of electromagnetic induction which states that when the magnetic flux linked with a coil is changed, an emf is induced in the coil.

#### 29. What do you mean by self-induction?

When the current or magnetic flux through a coil is changed, an emf is induced in it. The phenomenon according to which an opposing emf is produced in the coil as a result of a change in current or magnetic flux through it is called the Self Induction. Since this property of the coil opposes the growth or decay of current through it, it is also called the Inertia of Electricity.

#### 30. How is self-induction caused? Explain.

When a current flows through a coil, a magnetic field is set up around it. When the current flowing through the coil is varied using a rheostat, the flux linked with the coil changes. Due to the change in the flux through the coil, an emf is induced across it according to Faraday’s law. The direction of this induced emf is such that it opposes its cause (current) according to Lenz’s law. If the current is increasing, induced emf is in the opposite direction of current and if the current is decreasing, the en if and current will have the same direction. As the emf induced has the opposing nature to its cause, the emf is called back emf.