# Class 12 Physics Chapter 6 Important Questions Electromagnetic Induction

1 Q:- Write the SI unit of magnetic flux. Is it a scalar or vector quantity? Ans:- The SI unit of magnetic flux is Weber (Wb). Magnetic flux is a

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Electromagnetic Induction. The best solution of the problem is to practice as many Physics Class 12 Chapter 6 Important Questions as possible to clear the doubts.

1 Q:- Write the SI unit of magnetic flux. Is it a scalar or vector quantity?
Ans:- The SI unit of magnetic flux is Weber (Wb). Magnetic flux is a scalar quantity.

2 Q:- Does change in magnetic flux linked with the circuit induced an EMF or current?
Ans:- Whenever magnetic flux linked with a circuit changes, an induced EMF must be set up in it. An induced current flows through the circuit only if the electrical circuit is complete.

3 Q:- Why is induced EMF referred to as the back EMF?
Ans:- Because induced EMF tends to oppose the change in flux for change in current flowing in a circuit.

4 Q:- What is the cause of electromagnetic induction?
Ans:- The cause of electromagnetic induction, that is, the cause of productions of an induced EMF in a circuit is the change in magnetic flux linked with that circuit. Whenever there is either an increase or a decrease in the magnetic flux, an induced EMF is generated which last so long as the change in magnetic flux continues.

5 Q:- Which laws gives the directions of induced EMF our current?
Ans:- Lenz's law gives the directions of induced EMF or current in a circuit whenever magnetic flux linked with it changes.

6 Q:- Does Lenz's law violate the principle of conservation of energy?
Ans:- No, Lenz's law does not violate the principle of conservation of energy. In fact, it supports the principle of conservation of energy.

7 Q:- What is motional EMF?
Ans:- Setting up of an induced EMF whenever a conductor or a coil is moved in a normal, uniform magnetic field. Kn motional EMF, magnetic flux changes due to change in effective area of a circuit exposed to the magnet field.

8 Q:- What are the possible methods to produce an induced EMF in a circuit?
Ans:- There are three possible methods of producing an induced EMF in a circuit. The methods are:

• (a). by changing the magnetic field B,
• (b). by changing the area A of the circuit, and
• (c). by changing the relative orientation of B and A.

9 Q:- When a coil of conducting wire is moved in a uniform magnetic field parallel to field, no induced EMF is generated in it. Why?
Ans:- When a conducting coil is moved parallel to a given uniform magnetic field, the magnetic flux linked with a coil remains unchanged. As there is a no change in magnetic flux, hence no induced EMF is produced in it.

10 Q:- A closed loop is held stationary in the magnetic field between the North and South poles of two permanent magnets held fixed. Can be hope to generate current in the loop by using very strong magnets?
Ans:- No,we cannot hope to generate current in the loop even by using strong magnets because the loop as well as magnets are held fixed. Hence, magnetic flux linked with the loop remains constant and there is no change in magnetic flux. As a result, no induced EMF of current occurs.

11 Q:- What are Eddy Currents? Why is their magnitude so high?
Ans:- Eddy currents are currents induced in a bulk conductor whenever magnetic flux linked with the conductor changes. Magnitude of Eddy Currents is generally high because electrical resistance offered by a bulk conductor is extremely small.

12 Q:- Write any two applications of Eddy currents.
Ans:- Magnetic brakes in trains, electric power meters, speedometer, inductor furnace, etc, are some of the applications of Eddy Currents.

13 Q:- Give an two methods to control or minimise Eddy currents.
Ans:- By introducing slots in a metallic conductor and by taking a laminated metal core, we can control or minimise Eddy Currents.

14 Q:- Give an applications of electromagnetic damping.
Ans:- We make use of the phenomenon of electromagnetic damping in the dead beat Galvanometer. In such galvanometer, the coil is a wound on a nonmagnetic metallic material core. When the coil oscillates on passing current through it, the Eddy Currents generated in the core oppose the motion of the coil and bring the coil to rest quickly.

15 Q:- What is self inductance of a coil?
Ans:- The self inductance of a coil is the magnetic flux linked with it when a constant unit current flows through it. Alternatively, self inductance of a coil is equal to the induced EMF in the coil when current flowing through it is changing at a unit rate with time.

16 Q:- Name the factors on which the self inductance of a solenoid coil depends.
Ans:- The self inductance of a solenoid coil depends on its dimensions, total number of turns and the relative permeability of the core of solenoid.

17 Q:- If the number of turns of a solenoid is doubled, keeping the other factors constant how does the self inductance of the coil change?
Ans:- The self inductance will becomes 4 times of its original value if all over factors remain unchanged. It is because L ∝ N2.

18 Q:- Define mutual induction.
Ans:- Mutual induction is the phenomenon according to which a back EMF is induced in a coil as a result of change in the current flowing in a neighbouring coil.

19 Q:- State the factors on which the mutual inductance between a given pair of coils depends.
Ans:- Mutual inductance of a pair of coils depends on:

• (a). the dimension of the coils,
• (b). number of turns in the coils,
• (c). relative orientations of the coils, and
• (d). magnetic permeability of the core material, if any.

20 Q:- In an AC generator, can we maintain armature coil fixed and rotate the field magnet for producing an EMF?
Ans:- Yes, we can keep the armature coil fixed and rotate the field magnet so as to produce induced EMF. It is because even now flux linked with coil will change and an induced EMF is set up in it. In practice, most commercial generators make use of this technique.

21 Q:- An induced EMF has no directions of its own. Explain why.
Ans:- As per Lenz's law the direction of induced EMF for current in such that it opposes the very cause which produces the EMF. If in a circuit current (flux) is increasing, the induced EMF or current tends to oppose the growth of current. On the other hand, if current is failing in a circuit, the induced EMF or current tends to oppose the fall in current. Thus, it is self-evident that in induced EMF has no definite directions of its own.

22 Q:- From where does the energy for induced current come from?
Ans:- The electrical energy due to productions of induced EMF for current comes at the expense of an equal amount of mechanical energy. When a magnet is brought towards a coil or a coil is rotated in a uniform magnetic field or a conductor road is moved in a uniform magnetic field, etc., some mechanical work has to be done on the magnetic or coil or conductor rod, etc. As a result of this, mechanical work done, an electrical emf current is induced.

23 Q:- Can a system of two coils have mutual inductance without having self inductance? Is reverse true?
Ans:- No, a system of two coils cannot have a mutual inductance without having self inductance of two coils individually. However, two coils may have their own self inductance L1 and L2 without having a mutual inductance if their coupling coefficients K = 0.

24 Q:- A short but powerful bar magnet is moved towards a coil and consequently, an EMF and an electric charge are induced in the coil. How will the magnitude of induced EMF and charge be affected if motion of the magnet is (a) slow, and (b) fast?
Ans:- The magnitude of induced EMF increases on increasing the speed of moment of a bar magnet towards the coil. If motion is slow, the induced EMF is small but on moving the magnet at a faster rate, the magnitude of the induced EMF increases. However, the magnitude of induced charge remains same because it depends on the total flux change.

25 Q:- An iron ball falling vertically through the hollow region of a thick cylindrical shell made of copper experiences a retarding force. What can you conclude about the iron bar? Give reason.
Ans:- The iron bar must be a magnet. When this magnetized iron bar falls through the copper cylindrical sell, it causes a change in magnetic flux linked with the shell and consequently, Eddy Currents are set up in the shell. These eddy currents oppose the motions of the iron bar. As a result, the iron bar experiences a retarding force.

26 Q:- A metallic pole falls down vertically in the plane of magnetic meridian. What is the EMF induced between its ends?
Ans:- No EMF is induced between the ends of the metallic pole. We know that no motional EMF is produced when any two out of B, v and l are parallel to each other. In present question, no external magnetic field is present and induced EMF can be produced only due to the Earth's magnetic field. However, vertical component (Bv) of the earth's magnetic field is parallel to velocity velocity v and horizontal component (BH) is parallel to the length l of the pole. Hence, neither of the two can produce induced EMF in the pole.

27 Q:- A metal rod supported horizontally in North South directions is allowed to fall freely. What will be the potential difference between the two ends of the rod? Give reason too.
Ans:- No potential difference exists between the two ends of the rod as explain in above question.

28 Q:- Two metallic wheels of same size but one having eight spokes while the other having 12 spokes are rotated in a uniform normal magnetic field B with same angular speed ω. In which wheel, more emf will be produced between the axle and the rim of the wheel, and why?
Ans:- In both wheels, same emf will be produced between the axle and Rim of the wheel. It is because various spokes in a wheel are connected in parallel and net EMF remains same as due to single spoke only.

29 Q:- Generally, a mild Spark is produced in the switch off an electrical fan when it is put off. explained why?
Ans:- The armature coil of the motor of an electric fan has a high value of self inductance (L). When the fan is switched off, electric current flowing through armature and consequently, magnetic flux linked with the armature coil suddenly falls. As a result of this change in magnetic flux, an EMF is generated across the ends of the coil, which causes a mild Spark in the air gap of switch.

30 Q:- Are induced EMFs in a coil having same magnitude at the time of Make as well as break of its electrical circuits? Give reason.
Ans:- Induced EMF at the time of breaking of electric circuit is generally more than that produced at the time of make. It is because at the time of break when switch is put off, a large air resistance is present in the circuit. Hence, electric current falls at a faster rate than rise of current at the time of make. So, rate of change of magnetic flux and consequently, the magnitude of induced EMF is more at break.

31 Q:- Two identical coaxial circular loops carry equal currents circulating in the same direction. What will happen to current in each loop if the loops approach each other?
Ans:- As two coaxial circular loop carry equal currents circulating in the same direction, magnetic fields due to these currents are in same sense and are added up. As the loops approaches each other, the field and hence flux linked with each loop increases. So, in accordance with the Lenz's law, the induced current developed in each loop tries to decrease the magnetic flux. It means that the induced current in each loop will be opposite to the initial current flowing through it. As a result, the current in each loop will decrease as the loop approach each other.