Physics Questions Flashcards
What are the basic properties of electric charge?
- 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
State Coulomb’s law. Write its expression.
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
force between multiple charges.
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.
What are the properties of electric field lines?
- 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
What is meant by electric flux and write its expression?
Electric flux is defined as the no of field lines that cross a unit area
flux proportioanl E
-”- dS cos theta
flux = EdS
What is meant by a) electric dipole b) dipole moment.
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
State Gauss’s law.
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.
What is meant by electrostatic potential? Write its expression and assumptions taken.
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)
What is meant by equipotential surface?
what are its properties
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
What is meant by electrostatic shielding?
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.
What are the differnences between polar dielectric and non-polar dielectric?
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
State Ohm’s law and write its expression.
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
What is meant by drift velocity?
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
What is meant by mobility and write its expression.
Mobility is the magnitude of drift velocity per unit electric field
What are the limitations of Ohm’s law?
- 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
How the temperature depends on resistivity in case of conductors and semiconductors.
In conductors, resistivity increases w/ increase in temp
In semiconductors, resitivity decreases w/ increase in temp (e goes to conduction band)
State Kirchhoff’s laws.
- 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.
Write the expression for Lorent’z force?
for a charge q moving through a magnetic field B w/ a velocity of v experiences a force.
F= q[v x B]
State Biot Savart’s law and write its expression.
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
State Ampere’s circuital law and write its expression.
The line integral of magnetic field around closed loop is μo times the total current passing through the loop
∮Bdl = μo I
Briefly explain how a charged particle takes a helical path and what is pitch?
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.
What is a solenoid?
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
Mention properties of magnetic field lines.
- 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
State and Explain Gauss’s law in magnetism.
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
Define magnetic intensity. Give its S.I unit.
It is defined as the ratio of magnetic field to the permeability of free space.
H (intensty) = n i A/m
Define magnetization of a magnetic material. Mention its S.I unit
It is defined as the net magnetic moment developed per unit volume.
M= m / V A/M
