Electromagnetism Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

on increasing current what happens to deflection of compass needle

A

deflection increases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

from magnetic field lines around a wire, what do we observe (with straight wire)

A
  1. magnetic field lines form concentric circles around the wire, with their plane perpendicular to the straight wire and with their centres lying on the wire
  2. when direction of current in wire is reversed, pattern of iron fillings does not change, but direction of deflection of compass needle gets reversed
  3. on increasing current, field lines become denser and iron filings get arranged in circles up to a larger distance from the wire, showing that the magnetic field strength as increased and it is effective up to a larger distance
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

right hand thumb rule

A

if we hold the current carrying conductor in our right hand such that the thumb points in the direction of flow of current, then the fingers encircle the wire in direction of magnetic field lines

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

from magnetic field lines around a wire, what do we observe (with a coiled wire)

A
  1. in the vicinity of wire at P and Q, magnetic field lines are nearly circular
  2. within space enclosed by wire, magnetic field lines are in same direction
  3. near the centre of loop, magnetic field lines are nearly parallel to each other, so magnetic field is nearly uniform in a small space at the centre
  4. at centre, magnetic field lines are along the axis of loop and normal to its plane
  5. magnetic field lines pass through loop in the same direction (loop acts as dipole)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

when does magnetic field line get denser in coiled wire

A
  1. strength of current in loop is increased
  2. number of turns in loop is increased
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

clock rule to determine polarity at faces of loop

A

looking at the face of the loop, if the current in wire around that face is in anticlockwise direction, the face has north polarity, while if current at that face is clockwise, face has south polarity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

from pattern of magnetic field lines of solenoid, it is found that

A
  1. magnetic field lines inside the solenoid are nearly straight and parallel to the axis of solenoid (uniform)
  2. lines become denser on increasing current in solenoid
  3. magnetic field is increased if number of turns in solenoid is increased
  4. magnetic field is also increased if soft iron core is placed along the axis of solenoid, because soft iron has HIGH MAGNETIC PERMEABILITY
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

similarities between current carrying solenoid and bar magnet

A
  1. magnetic field lines of a current carrying solenoid are similar to bar magnet
  2. when suspended freely, both rest in north-south direction
  3. both acquire magnetic properties and attract iron filings
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

dissimilarities between solenoid and bar magnet

A
  1. strength of magnetic field due to solenoid can be changed by changing current in it, but bar magnet’s strength cannot be changed
  2. direction of magnetic field can be changed in solenoid by reversing direction of current in it, but not for bar magnet
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

define electromagnet

A

an electromagnet is a temporary strong magnet made by passing current in a coil wound around a piece of soft iron. it is an artificial magnet

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

when is bar shaped electromagnet used

A

relay (switching device)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

use of horse shoe magnet

A

dc motor
ac generator
electric bell

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

why can’t ac current be used in electromagnet

A

polarity at ends change 50 times in each second

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

why is steel preferred over soft iron in making a strong permanent magnet

A

steel has higher magnetic retentivity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

uses of permanent magnets

A

galvanometer, ammeter, voltmeter
magnetic compass

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

advantages of electromagnet over a permanent magnet

A
  1. strong magnetic field
  2. strength of magnetic field can be easily changed
  3. polarity can be changed by reversing direction of current in solenoid
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

uses of electromagnet

A
  1. lifting and transporting heavy iron scrap, girders, plates.
  2. loading furnaces with iron
  3. separating iron pieces from debris and ores, where iron is impurity
  4. removing iron pieces from wounds
  5. scientific research, to study the magnetic properties of a substance in a magnetic field
  6. electric bell, telegraph, microphone
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Lorentz force

A

a charge moving in a magnetic field, in a direction other than the direction of the magnetic field, experiences a force named Lorentz force. this can produce motion in a conductor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

experimentally, what is observed during Lorentz experiment

A
  1. when no current flows in wire, no force acts on wire and wire does not move
  2. when current is passed through the wire, force acts on the wire in a direction perpendicular to both the direction of current and direction of magnetic field. conductor moves towards the observer/ normal to the plane of paper in outward direction
  3. if direction of current through wire is reversed by interchanging the terminals of battery, direction of movement of wire is also reversed
  4. on reversing direction of magnetic field, direction of movement of wire is reversed
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

during Lorentz experiment, what if wire is short

A

when length AC is in a direction such that current in it is in the direction parallel to direction of magnetic field, no force acts on the wire and it does not move

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

what does magnitude of force acting on current carrying wire placed in magnetic field, in direction perpendicular to its length, depend on

A
  1. force is directly proportional to current I flowing in wire
  2. force F is directly proportional to strength of magnetic field B
  3. fore is directly proportional to length of wire within magnetic field
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

unit of magnetic field B

A

N/ A.m
OR
tesla (T)
OR
weber/meter^2 (Wb/m^2)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Fleming’s left hand rule

A

stretch forefinger, central finger, and thumb of your left hand mutually perpendicular to each other. if forefinger indicates direction of magnetic Field, and central finger indicates Current direction, thumb will indicate direction of motion of conductor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

define electric motor

A

a device which converts the electrical energy into the mechanical energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

principle of electric motor

A

on passing electric current through a conductor placed normally in a magnetic field, a force acts on the conductor as a result of which the conductor begins to move and thus mechanical energy is obtained

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

when is deflecting couple on coil of dc motor max

A

when plane of coil is parallel to the direction of magnetic field, while the deflecting couple is zero

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

when is deflecting couple on coil of dc motor 0

A

when plane of coil is perpendicular to magnetic field

28
Q

speed of rotation of coil of dc motor depends on

A

magnitude of deflecting couple formed by the forces acting on arms of the coil (I, B, Area of coil, number of turns in coil)

29
Q

how to increase speed of rotation of coil in dc motor

A
  1. increase strength of current in coil
  2. increase the number of turns in the coil
  3. increase area of the coil
  4. increase strength of magnetic field
30
Q

function of split ring commutator

A

when coil passes from vertical position, the direction of current through the coil has to be reversed as to keep the direction of the rotating couple unchanged
split ring reverses direction of current in the coil so that the coil continues to rotate in same direction

31
Q

electromagnetic induction

A

whenever there is a change in the number of magnetic field lines linked with a conductor, an emf is developed between the ends of the conductor which lasts as long as there is a change in the number of magnetic field lines through the conductor

32
Q

what is implied by induction experiment

A

it is the relative motion between the solenoid and magnet which produces a current, irrespective of whether the magnet moves or the solenoid moves

33
Q

Faraday’s three conclusions

A
  1. a current flows in a coil only when there is a relative motion between the coil and the magnet
  2. the direction of current is reversed if the direction of motion (or polarity of magnet) is reversed
  3. current in coil can be increased by…
34
Q

how is current in coil increased during Faraday’s experiment

A
  1. rapid motion of magnet/ coil
  2. use of a strong magnet
  3. increasing the area of cross section of coil and by increasing the number of turns in the coil
35
Q

define electromagnetic induction

A

phenomenon in which an emf is induced in the coil if there is a change in the magnetic flux linked with the coil

36
Q

Faraday’s laws of electromagnetic induction

A
  1. whenever there is a change in magnetic flux linked with a coil, and emf is induced. the emf lasts as long as the magnetic flux linked with the coil changes
  2. magnitude of emf induced is directly proportional to rate of change of magnetic flux linked with the coil
37
Q

induced emf=

A

number of turns in the coil * change in magnetic flux of each turn/ time in which magnetic flux changes

38
Q

for a given coil and magent, electromagnetic induction depends on

A
  1. rate of change of magnetic flux linked with each turn
  2. number of turns in the coil
39
Q

how can direction of induced emf be determined

A
  1. Lenz’s law
  2. Fleming’s right hand rule
40
Q

Lenz’s law

A

the direction of induced emf (or induced current) is such that it opposes the cause which produces it

based on law of conservation of energy

41
Q

difference between Fleming’s left and right hand rule

A

left: gives direction of force on a current carrying conductor placed in a magnetic field
right: gives direction of induced current in a conductor moving in a magnetic field

42
Q

define ac generator

A

a device which converts mechanical energy into the electrical energy using the principle of electromagnetic induction

43
Q

principle of ac generator

A

coil is rotated in a magnetic field
due to rotation, magnetic flux linked with the coil changes and therefore and emf is induced between the ends of the coil
thus, a generator acts as a source of current in an external circuit containing load when it is joined between the ends of its coil

44
Q

what happens when coil rotates in the magnetic field between the poles of field magnet

A

an induced emf is produced between ends of coil
emf changes polarity as well as magnitude in each rotation of the coil, so it is called an ALTERNATING emf

45
Q

in ac, when does magnitude of induced emf become max

A

when magnetic flux linked with coil reduces to 0 from maximum value.
this happens when plane of coil lies in direction of magnetic field

46
Q

in ac, when does magnitude of induced emf become 0

A

when magnetic flux linked with coil is maximum
happens when the plane of coil is normal to the magnetic field

47
Q

maximum value of emf in ac induced can be increased by

A
  1. increasing number of turns in coil
  2. increase area of cross section of coil
  3. increasing speed of rotation of coil
48
Q

advantages of ac over dc

A
  1. it is cheaper and easy to generate
  2. higher efficiency
  3. easy to change ac into dc
  4. magnitude of ac voltage can easily be increased or decreased by the use of step up or step down transformer
  5. ac can be transmitted over a long distance without much loss in energy in the line wires
49
Q

define transformer

A

a device by which the magnitude of an alternating emf can be increased or decreased

it does not affect the frequency of alternating voltage. frequency remains unchanged

49
Q

similarities between ac generator and dc motor

A
  1. in both, a coil rotates in a magnetic field between the pole pieces of a powerful electromagnet
  2. both , there is a transformation of energy from one form to another form
50
Q

principle of transformer

A

works on the principle of electromagnetic induction and makes use of two coils having different number of turns (primary coil and secondary coil)

51
Q

primary coil and secondary coil in transformer

A

primary: alternating emf of which magnitude is to be changed, is applied across this coil
secondary: the appliance in which output is to be obtained is connected across this coil

when there is a change in magnetic field due to varying current in the primary coil, magnetic flux linked with the secondary coil also changes due to which an induced varying current of same frequency but of different magnitude flows in secondary coil

52
Q

why transformer cannot be used with dc

A

if current in primary coil is constant, then the magnetic field due to primary coil remains constant and there does not occur any change in the magnetic flux linked with the secondary coil and hence no emf is induced in the secondary coil

53
Q

purpose of core in transformer

A

to provide maximum magnetic flux linkage between the coils
made from thin rectangular laminated sheets of soft iron of T and U shape, placed alternately one above the other, and insulated from each other by a pain/varnish coating over them

54
Q

Turns ratio, n=

A

number of turns in secondary coil/ number of turns in primary coil

55
Q

function of lamination of core in transformer

A

prevents loss of energy due to induced currents in the core

56
Q

advantage of using closed core in transformer

A

gives a closed path for the magnetic field lines and therefore almost all the magnetic field lines, caused due to current in primary coil, remain linked with secondary coil and loss of energy is minimum

57
Q

why is core of transformer made with soft iron

A

loss of energy due to hysteresis in core is minimised

58
Q

in a transformer, magnitude of emf induced in secondary coil depends on

A
  1. ratio of number of turns in secondary coil to number of turns in first coil
  2. magnitude of emf applied in primary coil
59
Q

ideal transformer

A

when there is no energy loss, output power= input power

60
Q

step up transformer

A
  1. the transformer used to change a low alternating voltage to a high alternating voltage (same frequency)
  2. number of turns of secondary coil is more than number of turns in the primary coil (n>1)
  3. current in P coil > current in S coil, so P coil wire is kept thicker
61
Q

step down transformer

A
  1. used to change a high alternating voltage to a low alternating voltage of same frequency
  2. number of turns in secondary coil are less than number of turns in primary coil (n<1)
  3. current in S coil> current in P coil, so S coil wire is thicker
62
Q

why is thick wire used when there is more current

A

reduces resistance, hence reduces loss of energy as heat
loss of energy due to heat is called copper loss

63
Q

use of step up transformers

A
  1. transmition of electric power at power generating station to step up voltage
  2. television picture tube
  3. wireless sets
  4. X-ray machines to provide a high accelerating voltage
64
Q

use of step down transformer

A
  1. electric bells, night electric bulbs, mobiles
  2. power sub stations to step down voltage before distribution to consumers