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Eddy currents - class-XII

Description: Electromagnetic induction and alternating currents
Number of Questions: 18
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Tags: electromagnetic induction physics electromagnetic induction and alternating currents
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Which of the following does not use the application of eddy current?

  1. Electric power meters

  2. Induction furnace

  3. LED lights

  4. Magnetic breaks in trains


Correct Option: C
Explanation:

C. LED light.

Because in the given option, Electric power meter, Induction furnace, and magnetic break in trains are the application of eddy current.

Induction Furnace make use of :

  1. Self induction

  2. Mutual induction

  3. eddy current

  4. none of these


Correct Option: C
Explanation:

Inductance Furnace makes use of Eddy current. 

Identify the wrong statement

  1. Eddy currents are produced in a steady magnetic field

  2. Eddy currents can be minimized by using laminated core

  3. Induction furnace uses eddy current to produce heat

  4. Eddy current can be used to produce breaking force in moving trains


Correct Option: A
Explanation:

Eddy currents comes in picture only when there is change in linked magnetic flux. 

They can be created when a conductor is moving through a magnetic field, or when the magnetic field surrounding a stationary conductor is varying i.e. anything which results in the conductor experiencing a change in the intensity or direction of a magnetic field can produce eddy currents.

Thus  $A$ is the correct option.

Identify the wrong statement.

  1. Eddy currents are produced in a steady magnetic field.

  2. Eddy currents can be minimized by using laminated core.

  3. Induction furnace uses eddy current to produce heat.

  4. Eddy current can be used to produce breaking force in moving trains.


Correct Option: A
Explanation:

Eddy currents comes in picture only when there is change in linked magnetic flux. Thus statement A is wrong.

Eddy currents travel in

  1. closed paths.

  2. nonconductive materials.

  3. axially when testing rod with an encircling coil

  4. perpendicular to the test surface


Correct Option: A
Explanation:

When there is a change in magnetic flux associated with a large piece of metal then currents are induced in that metal these currents are travelling in a closed circular path,  similar to Whirlpools or eddies in water. Such currents are known and Eddy Currents

Hysteresis loss and eddy current losses are used in

  1. induction heating of steel

  2. dielectric heating

  3. induction heating of brass

  4. none of the above


Correct Option: A
Explanation:

There is a magnetic loss occur in ferromagnetic material like iron and steel in form of heat these losses are used for induction heating and cooking. For example $\rightarrow$ heating of transformer.

Which of the following is not an application of eddy currents.

  1. Induction heater

  2. Conductivity meters for non-magnetic metals

  3. Skin effect

  4. Coating Thickness Measurements


Correct Option: C
Explanation:
Eddy currents are the currents induced in the body of a conductor when the amount of magnetic   flux linked with it changes
(i)    Induction furnace (heating effect of eddy current)
(ii)   Dead-beat galvanometer (electromagnetic damping)
(iii)  Electric brakes
(iv) Diathermy ( Strong 50 MHz A.C. passed through human body, also called deep heat treatment )
(v)  Induction motor
(vi) Car-speedometer
(vii)Energy meter

Silicon steel used in lamination mainly reduces

  1. Hysteresis loss

  2. Eddy current losses

  3. Copper loss

  4. All of the above 


Correct Option: D
Explanation:

Silicon steel used in lamination mainly reducesHysteresis loss,Eddy current losses,Copper loss. In order to minimize the eddy currents more,magnetic core is laminated . The hysteresis coefficient of silicon steel is less, so hysteresis loss can be reduced. used mainly because of the following advantages: low hysteresis loss. low copper loss due to lower grain resistance.

In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are 

  1. friction and windage losses

  2. copper losses

  3. hysteresis and eddy current losses

  4. none of the above 


Correct Option: C
Explanation:

In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are hysteresis and eddy current losses The losses that can occur in a material are: Iron losses: Iron loss is caused by the alternating flux in the core and consists of hysteresis and eddy current losses. of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.)

Eddy currents are produced in a metallic conductor when

  1. The magnetic flux linked with it changes

  2. It is placed in a changing magnetic field.

  3. It is placed in a magnetic field.

  4. Both A and B


Correct Option: D
Explanation:

Eddy currents are produced when the magnetic flux passing through the metal object continuously changes. This may happen due to many reasons:
1) The object is placed in a region with changing magnetic field.
2) The object continuously moves in and out of the magnetic field region (may be uniform or non uniform).

A magnet is dropped down an infinitely long vertical copper tube

  1. The magnet moves with continuously increasing velocity and ultimately acquires a constant terminal velocity

  2. The magnet moves with continuously decreasing velocity and ultimately comes to rest

  3. The magnet moves with continuously increasing velocity but constant acceleration

  4. The magnet moves with continuously increasing velocity and acceleration


Correct Option: B
Which is the correct formula for calculating the power lost due to eddy currents per unit mass for a thin sheet or wire?? Where $P$ is the power lost per unit mass $(W/kg)$, $B _p$ is the peak magnetic field $(T)$, $d$ is the thickness of the sheet or diameter of the wire $(m)$, $f$ is the frequency $(Hz)$, $k$ is a constant equal to 1 for a thin sheet and 2 for a thin wire
  1. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f^2}{6k\rho D}$

  2. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f}{k\rho D}$

  3. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f^2}{6k\rho D^3}$

  4. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f}{6k\rho D^2}$


Correct Option: A
Explanation:

(A)$P=\dfrac { { \pi  }^{ 2 }{ B } _{ p  }^{ 2 }{ d }^{ 2 }{ f }^{ 2 } }{ 6k\rho D } \ m{ L }^{ 2 }{ T }^{ -3 }=\dfrac { \left( M{ T }^{ -2 }{ A }^{ -1 } \right) ^{ 2 }\left( { L }^{ 2 } \right) \left( { T }^{ -2 } \right)  }{ M{ L }^{ -3 } } \ m{ L }^{ 2 }{ T }^{ -3 }=M{ L }^{ 2 }{ T }^{ -3 }\ m{ L }^{ 2 }{ T }^{ -3 }\neq m{ L }^{ 2 }{ T }^{ -2 }$ where A = amphere, T = Time, L = Length ,M = mass.

similarly C and D not matches so option A is correct .

A transformer core is laminated to 

  1. reduce hysteresis loss

  2. reduce eddy current loss

  3. reduce copper loss

  4. reduce all of the above loss


Correct Option: B
Explanation:

The transformer core is laminated to break eddy currents to form loop and hence reducing the power loss due to heat generated due to eddy current.


Answer-(B)

Eddy currents are used in

  1. electrolysis

  2. making a galvanometer dead beat

  3. electroplating

  4. to increase the sensitivity of galvanometer


Correct Option: B
Explanation:

In general the coil of galvanometer oscillates about it's equilibrium due to rotational inertia which consumes some time. To avoid this coils is bound over  a metallic frame or plate oscillates in a magnetic field the eddy currents generated in the frame or plate oppose the motion and bring the frame to rest as the oscillations die out quickly. This is known as making galvanometer dead beat. 

The working of magnetic braking of trains is based on

  1. Steady current

  2. Eddy current

  3. Alternating current

  4. Pulsating current


Correct Option: B
Explanation:

 It works the same as a disk eddy current brake, by inducing closed loops of eddy current in the conductive rail, which generate counter magnetic fields which oppose the motion of the train.

Read the following statements and answer whether the given statement is true or false.

Eddy current involves loss of energy in the form of heat.

  1. True

  2. False


Correct Option: A
Explanation:

Eddy currents (also called Foucault currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor due to Faraday's law of induction. 

Eddy current involves loss of energy in the form of heat. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer. 
The statement is true.

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