Physics Questions Flashcards

1
Q

What are the basic properties of electric charge?

A
  • charges are scalar quantities, and can be added or subtracted
  • like charges repel, unlike harges attract
  • polarity of charges differentiates the 2 types of charges
  • conductors : easy flow of charges ; insulators: high resistance to flow of charges
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2
Q

State Coulomb’s law. Write its expression.

A

Couloumb’s law states that the force between two charges is proportional to the productuct of magnitude of the two charges and inversely proportional to the square of distance between them acting along the line of the 2 charges.

F=k|q1 q2| / r^2 where k = 9*10^9 Nm2/C2

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3
Q

force between multiple charges.

A

Force on any charge due to multiple charges is equal to the individual sum of all forces on charge due to other charges. Individual forces are unaffected due to presence of other charges.

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4
Q

What are the properties of electric field lines?

A
  • start from + ve and end at -ve. if its a single charge may start or end at infinity
  • field lines never intersect
  • do not form any closed loops
  • in a charge-free region, E field lines can be taken as continuous curves w/o breaks
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5
Q

What is meant by electric flux and write its expression?

A

Electric flux is defined as the no of field lines that cross a unit area

flux proportioanl E

-”- dS cos theta

flux = EdS

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6
Q

What is meant by a) electric dipole b) dipole moment.

A

An electric dipole is a pair of equal and oppositve charges seperated by a distance of 2a. (total charge=0)

Dipole moment is the product of magnitude od either charge and the distance between them

p = q2a

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7
Q

State Gauss’s law.

A

Gauss’s law states that the total electric flux inside hypothetical closed surface (gaussian) is equal to 1/Eo times the net electric charge in that closed surface.

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8
Q

What is meant by electrostatic potential? Write its expression and assumptions taken.

A

the work done in bringing (w/o acceleratin) a unit positive test charge from infinity to a point inside the electric field of source charge

assumptions:

  • test charge is taken as point charge (it doesnt disturb the original configuration.
  • we apply ext force Fext just enough to counter the repulsive force
  • ⇒ no net force or acceleration of charge

w = - int r to p ( Fext dr)

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9
Q

What is meant by equipotential surface?
what are its properties

A

it is a surface with a constand potential at all points of surface

v = kq/2

For any charge configuration, equipotential surface through a point is normal to the electric field at tht point.

Properties:

  • All points have same potential
  • WD in moving charges inside surface = 0 (by def of potential , Wd= W/q)
  • two equipotential surfaces do not intersect
  • electric field is always perpendicular to the eq. surface
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10
Q

What is meant by electrostatic shielding?

A

Electric field is 0 inside cavity of a conductor. Whatever be the charge and field configuration outside, any caviy in a conductor remains shielded from outside electric influece. This is known as electrostatic shielding.

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11
Q

What are the differnences between polar dielectric and non-polar dielectric?

A

molecules are polar ; molecules are non-polar
centre of positive charge and centre of negative charge are seperated ; centre of both positive and negative charges coincide
Non-zero intrinsic dipole momnt when E=0 ; Zero intrinsic dipole moment
eg: h2o, co, hcl ; eg: h2,o2,co2

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12
Q

State Ohm’s law and write its expression.

A

Ohm’s law states that the potential across the ends of a conductor is directly propotional to the current flowing across it provided that the temperature and physical conditions remains the same. V=IR where R is a constant known as resistance of conductor

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13
Q

What is meant by drift velocity?

A

Drift velocity is known as the average velocity with which free electrons get drifted towards the ends of conductor due to influece of electric field

relaxation time is the average time between successive collision between electron with flixed atoms/ions in conductor

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14
Q

What is meant by mobility and write its expression.

A

Mobility is the magnitude of drift velocity per unit electric field

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15
Q

What are the limitations of Ohm’s law?

A
  • does not apply so conductors such as diodes when electricity passes through gases
  • not applicable when physical conditions arent constant
  • relation between I and V is not unique, more than 1 value of V can be there for same current
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16
Q

How the temperature depends on resistivity in case of conductors and semiconductors.

A

In conductors, resistivity increases w/ increase in temp

In semiconductors, resitivity decreases w/ increase in temp (e goes to conduction band)

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17
Q

State Kirchhoff’s laws.

A
  • Junction Law: at any junction, the sum of current entering the jounction should be equal to the sum of current leaving the junction
  • Loop law: the algebraic sum of changes in potential around any closed loop involving resistors and cells is 0.
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18
Q

Write the expression for Lorent’z force?

A

for a charge q moving through a magnetic field B w/ a velocity of v experiences a force.

F= q[v x B]

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19
Q

State Biot Savart’s law and write its expression.

A

The strength of magnetic field dB due to a small current element dL carrying a current I at a point P, distance r from the element, is proportional to l, dl, sintheta and inversely proportional to the square of the distance between them.
dB ∝ I dL sintheta / r^2

dB = μo I dl sintheta/ 4π r^2

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20
Q

State Ampere’s circuital law and write its expression.

A

The line integral of magnetic field around closed loop is μo times the total current passing through the loop

∮Bdl = μo I

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21
Q

Briefly explain how a charged particle takes a helical path and what is pitch?

A

When a charged partical enters a uniform magnetic field at an angle theta, component of velocity is vcos theta which is parallel to field and v sin theta be perpendicular to field.

Now cos theta will be perpendicular and v sin theta will describe a circular path. The combined effect of those two is responsible for helical path of change,

The distance travlled in direction of field in time interval equal to time period/ one revolution is known as pitch of helix.

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22
Q

What is a solenoid?

A

a solenoid is an insulated copper wire wound closely in the form of helix , whose radius is negligible in terms of its length and is used to produce a uniform magnetic field.

B = μo N I

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23
Q

Mention properties of magnetic field lines.

A
  • inside magnet, start from S to N, outside from N to S
  • they never intersect
  • denser they are, more the magnetic field at that area
  • form closed loops
  • direction of magnetic field line at any point is represented by drawing a tangent at that point
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24
Q

State and Explain Gauss’s law in magnetism.

A

The net magnetic flux through any closed surface is always 0.

sigma B dS = 0

→ isolated magnetic pole does not exist

→ only exist as magnetic dipoles

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25
Q

Define magnetic intensity. Give its S.I unit.

A

It is defined as the ratio of magnetic field to the permeability of free space.

H (intensty) = n i A/m

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26
Q

Define magnetization of a magnetic material. Mention its S.I unit

A

It is defined as the net magnetic moment developed per unit volume.

M= m / V A/M

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27
Q

Define magnetic intensity. Give its S.I unit.

A

It is the ratio of magnetisation devolped in material to the applied magnetic intensity.

χ = M / H

μr = 1+χ

where μr is relative permeability

and χ is magnetic susceptibility

28
Q

Differentiate between paramagnetic and diamagnetic and ferromagnetic substances.

A

FOR PARAMAGNETIC
-weakly attracted by magnets
-acquires weak magnetisation in the direction of magnetic field
-As soon as field is removed, magnetism is lost
-Tends to move slowly from weaker to stronger part of field in a non-uniform B
-When temperature increases above Curie Temp, turns to diamagnetic

29
Q

Lenz’s Law

A

The polarity of induced emf is such that is tends to produce a current which oppose the change in magnetic flux that produced it.

30
Q

Faraday’s laws of EMI

A

emf is generated in a current carrying loop when places in a uniform magnetic field.

31
Q

Working of AC generator

A

When the axis of coil is perpendicular to the uniform magnetic field, the magnetic flux in the coil changes and it induced an emf across its ends.

32
Q

What is self-inductance?

A

Emf is induced in a coil due to change in current through same coil. its a scalar quantity (H - henry) (similar to inertia)
L= μo n^2 A l
where l- length
also called back emf since it opposes any current in circuit

33
Q

What is mutual inductance?

A

Emf induced in secondary coil due to change in current in primary coil.
e= M dI / dt
where l = current
si unit- H (henry)

34
Q

What is alternating current?

A

Current that varies periodically wrt to time reversing directions every half cycle.

35
Q

Define time period and frequency of an AC

A

Time taken by ac to comple one cycle of variation.
No of times ac quantity changes its direction in circuit per second

36
Q

What is phase and instantaneous value of an AC?

A

fraction of time elapsed since the current or voltage last passed through zero value in positive direction
Value of ac voltage/current at any instant of time t

37
Q

Mean value of voltage and current in AC?

A

Mean of instantaneous valus of voltage or current taken over a half cycle
v mean = 2v(m) / pi = 0.637 v(m)
where v(m) is the maximum value of voltage in a half cycle

38
Q

Define rms values of effective value of current and voltage

A

Square root of mean of squares of instantaneous values of voltage or current taken over a complete cycle
Vrms = vm / root 2

39
Q

What are the factors on which impedance depends on?

A

Impedance is defined as the effective opposition offered by the LCR circuit to the flow of current.
it depends on resistance, capacitance and inductance
It also depends on frequency of AC

40
Q

What is electrical resonance?

A

The value of particular frequency of AC where the current in the LCR circuit is maximum and impedance of ac is minimum.
ωo = 1/√ LC
and ν o = 1/( 2π √ LC )

41
Q

What is power dissipation and power factor?

A

Power is a rate at which electric energy is dissipated.
P avg = Vrms Irms cos ϕ
cos ϕ is known as the power fact
cos ϕ = R / Z

42
Q

What is wattless current?

A

Current through a pure inductor or capacitor is wattless/ idle current.

43
Q

What is band-width? Mention Q-factor in terms fo band width

A

It is the difference between the two half-power frequencies
Q= resonant frequency vo/ Bandwidth frequency dvo
Q= vo/ v2-v1

44
Q

What is a transformer?

A

It is a device used to increase or decrease ac voltage. It works on the principle of mutual induction. It consistis of two coils, primary and secondary wound on same laminated core. Ac to be altered is applied across primary coil and altered voltage is taken across secondary coil.

45
Q

Explain working of a transformer.

A

As the current through primary coil varies, the magnetic flux linked to secondary coil also changes. As a result, the alternating emf of same frequency is induced across secondary.
The magnitude of this emf depends upon:
- voltage across primary
- no of turns in both coils
- Ns/Np = Vs/Vp = T (turn ratio)

step-up: T>1 ; Vs,Ns > Vp,Np
Step-down: T<1 ; Vs,Ns < Vp,Np

46
Q

Explain different cources if energy loss in transformer.

A

Copper loss: when current flows in both coils, heat is released due to joules heating effect due to finite resistance of wire.
Flux leakage loss: some flux leaks into the air between the coils, and hence there is no compleate flux linkage between them.
Eddy current loss: when transformer is placed in non-uniform magnetic field, eddu currents are set up and heating effect is produced by these currents
Hystereisis Loss: due to repeated magetisation and demagnetisation of core during operation, heat is liberated

47
Q

How to minimise energy loss in transformer?

A

Copper loss: use thick copper wire
Flux Leakae: wind coil over one another
Eddy Current loss: use laminated cores
Hystereisis: use material of low hystereisis loss

48
Q

Advantages of AC over DC

A
  • ac can be easily stepped and can be transmitted over long distance w/o much power loss
  • easily converted in dc
  • have higher efficiency than dc
49
Q

What is a wavefront?

A

It is the surface of constant phase

50
Q

What are spherical, cylindrical and plane wavefront?

A

In a homogeneous medium, wavefront emerging from small distance from a point source of light is spherical.
For a linear source at small distance, emerging waveronts are cylindrical wavefront.
At large distances, a small portion of either spherical/cylindrical wavefronts can be considered a plane wavefront.

51
Q

Behaviour of plane after it passes through a thin prism

A

The glass being a denser medium, decreses the speed of light in glass. the lower part of the wavefront travelling through greater thickness of glass gets delayed resulting in a tilt in the emerging wavefront.

52
Q

Behaviour of plane wave when it passes through convex lens

A

The central part of the incident wave thravels through thickest portion of the lens and gets delayed the most. the emergent wavefront hus has a depression at the centre and the wavefront becomes spherical and converges to a point F.

53
Q

What are coherant sources?

A

Sources which emit waves which are in phase / has a constant phase difference
eg: Lloyd’s mirror

54
Q

What is interference of light?

A

Modification in the distribution of light energy when 2 or more light waves superpose

55
Q

Conditions for interference.

A
  • 2 sources must be coherent
    -waves from the 2 sources must travel in same direction or must intersect at small angle.
    -two slits must be as narrow as possible
  • interefering waves must have nearly same amplitude
    -2 independant monochromatic sources cannot proudce interference
56
Q

Fringe width and wavelength

A

directly proportional to wavelength of incident light (red>violet)

57
Q

Frindge width and intensity

A

does not depend

58
Q

fridge wifth in water

A

decreases in water

59
Q

What is diffraction?

A

bending of light around the edges/corners of an obstacle and hance the intrusion of shadow region by light.

60
Q

Diffraction not encountered daily. Why?

A

Since wavelength os light is much smaller than most obstacles (since diffraction is only prominent if dimernsions of obstacle is comarable to wavelength of light)

61
Q

Difference b/w Interference and Diffraction

A

INTERFERENCE DIFFRACTION
Due to superposition of waves from independent coherent sources Due to suprposition of wavlets starting from different points of same wavelength

good contrast b/w bright and dark fringe poor contrast b/w bright and dark fringe

fringes are of equal width fringes are of unequal width

intensity of bright fringes remain constant intensity of bright fringes increase with decrease in order

Condition for maxima =
path diff, del x= nλ
phase diff, ϕ = 2npi
Condition for maxima=
path diff, del x= (2n+- 1)λ / 2
phase difference,ϕ =(2n+-1)pi

62
Q

What confirms the transverse wave nature of light

A

Polarization

63
Q

What is polarization of light?

A

Phenomenon by whcih vibrations of waves on a screen are confined to a single plane.

64
Q

Define pass axis.

A

When polarized light is incident on a polaroid, the light wave is linearly polarized with the electric vector oscillating along a direction perpendicular to aligned molecules. This direction is the pass axis of polaroid.

65
Q

State Malu’s law

A

The intensity of polarised light transmitted through analyser varies as the squares of the cosine of the angle between the plane of transmission of the analyser and the plane of the polariser.

66
Q

Uses of polaroids.

A

polarizing sunglasses, view 3D pictures, improve colour contrast in old oil paintings, control intensity of light entering airplanes, wind screen of cars to reduce headlight glare