Kerala Syllabus SAMAGRA SCERT SAMAGRA Question Pool for Class 12 Physics Current Electricity
The variation of resistivity (ρ) with temperature (T) of a conductor, semiconductor and super conductor are given in the figure.
a)Identify them from the graph.
b)Identify the figure in which the temperature coefficient of resistance of the material is
c)Write the equation connecting resistivity of the material with relaxation time.
The resistance value of a conductor depends on its physical dimensions.
a) Give the expression for resistance of a conductor in terms of its physical dimension.
(l length and d diameter)
A potential V is applied between the ends of two conductor of same material shown in the figure.
i) Express the resistance of the second conductor in terms of the resistance of the first conductor.
ii) Find the ratio of electric field across the two conductors.
Explain the principle of a potentiometer. How can emf of two cells be compared using potentiometer?
If the length of a copper wire has a certain resistance R, then on doubling the length its specific resistance
(a) will be doubled
(b) will become 1/4th
(c) will become 4 times
(d) will remain the same.
(d) will remain the same.
The variation of potential difference F with length l in case of two potentiometer X and Y is as shown in fig.
Which one of these two will you prefer for comparing emfs of two cells and why?
For accurate measurement the potential gradient dV/dl should be small. The slope of the graph dV/dl is
Of which material is a potentiometer wire normally made?
Alloys like manganin, eureka having high resistivity and low temperature
on what factors does the internal resistance of a cell depend?
The nature of electrode, the distance between the electrodes, the area of the electrodes inside the
electrolyte and the nature of the electrolyte, its
velocity of the charge carries.
V proportional to drift velocity. So it
Define electromotive force (emf) of a cell
Emf is the p.d between the terminals of the cell when it sends no current in the external
The voltage current variation of two metallic wires X and Y at constant temperature are as shown in fig.
Assuming that the wires have the same length and die same diameter, explain which
wire will have a larger resistivity and why?
Slope = l/R. Slope is less for Y. So its R is higher and hence p is greater because I
A given wire having resistance R is stretched so as to reduce its diameter to half of its previous value. What will be its new resistance?
R= ρl/A, A1 = A/4, l1 = 4l,R1 = 16pl/A = 16
Two wires of equal lengths, one of copper and the other of manganin have the same resistance.
Which wire is thicker?
ρ1l/A1 = ρ 2l /A2, A1/A2
A wire of resistance one ohm is stretched to double its length what is the new resistance?
R1=ρl/A1,R2=ρ2l/A/2 = 4R1 =
A wire has a resistance of 90Ω and it is cut into three pieces having equal lengths.
If these wire now connected in parallel/find the resistance of the combination so formed.
Rp=R/n = 30/3= 10Ω
What is a non-ohmic device? Give one example.
A device which does not obey Ohm's law. Semiconductor
The potential difference across a given copper wire is increased. .What happens to the drift velocity of the charge carriers?
Drift velocity v = eEτ/m=eVτ/ml or v proportionan to V. So drift velocity increases with
While performing experiments with potentiometer (or metre bridge) the jockey should not be
rubbed against the wire" Why?
if the jockey is repeatedly dragged over the wire the uniformity fo the wire may be affected.
While performing an experiment with potentiometer the deflection in the galvanometer is found to be
in the same direction, both at the starting end and the extreme end of the wire. Also it is seen that
(i) the deflection is less at the other extreme end than at the start end and
(ii) the deflection is more at the extreme end than at the starting end. Why?
i) In this case the driver cell and the p.d. to be measured (the source of e.m.f. in the secondary circuit ) are connected
What do you mean by potential gradient of a potentiometer?
The potential difference per unit length of the wire of a potentiometer is called its potential gradient.
The potentiometer wire must be long and of uniform thickness" Why?
With increase in length the potentiometer becomes more sensitive. The fall of potential is proportional to
Which is preferred for measuring e.m.f. of a cell, a voltmeter or a potentiometer" Why?
Potentiometer. Because, at balance point, the potentiometer does not draw any currentfrom the source
Why a potentiometer is called so?
Because it is used to measure potential
"Wheatstone bridge method of determination of resistance is considered to be more accurate than the voltmeter-ammeter method" Why?
Wheatstone bridge method is a null method. The resistance determined does not depend on the deflection
"Kirchhoff's laws are related to certain conservation laws" Name them.
Kirchhoff's first law is based on the law of conservation of charge and the second law is
Two wires of equal lengths, one of copper and the other or manganin, have the same resistance.
Which wire is thicker?
Resistivity ρ= RA/l Given that R and l are same for both. i.e. Area of cross-section is
It is easier to start a car engine on a warm day than on a chilly day. Why?
The internal resistance of the battery is less on a warm day due to rise in temperature; so current from the battery is more and hence
steady current flows in a metallic conductor of non-uniform cross-section. Say which of these quantities is constant along the conductor :
current, current density, electric field, drift speeds
Since the current is steady, it remains constant. All other quantities vary inversely with area of
Name the carriers of electric current in
a) bar made of silver. b) hydrogen discharge tube.c) voltaic cell.
d) lead accumulator being charged by an external supply.
e) germanium semiconductor.
f) a wire made of the alloy nichrome. g) a superconductor.
(a), (f) and (g) electrons
(b)electrons and protons (c),(d) H+and SO4
Are the paths of electrons straight lines between successive collisions with the positive ions of the metal in the
(i) absence of electric field,
(ii) presence of electric field?
(i) straight line in the absence of electric field.
(ii) curved path in the presence of electric
When electrons drift in a metal from a lower to a higher potential, does it mean that all the free electrons of the metal are moving in the same direction?
No. The free electrons always have their random thermal velocities. The drift velocity is superposed over this.
The electron drift speed is estimated to be only a few mm/s for currents in the range of a few A. How then is current established almost the instant, a circuit is closed?
For the flow of current to start through a conductor, the electrons need not travel from one end to the other end
In a resistance box there is a resistance marked infinity. What is the length of the wire used for making this infinite resistance?
No wire is used to produce an infinite resistance. The infinite resistance is produced by an air
Why manganin is preferred for making standard resistance?
The resistance of manganin does not change with temperature because the temperature coefficient of resistance of manganin is nearly
A metal contains a large number of free electrons and they are in motion. Then why there is no current in a piece of wire which it is not connected to a cell?
The free electrons move at random. They move in all possible directions. There is a continuous changen in direction due to their
What is the charge on a wire, when a current flow through the wire?
a) State whether the following statement is correct.
‘‘If the current through the cell is from its positive to negative, against the direction of
emf, then the potential drop across the internal resistance aids the emf of the cell.’’
b) You are given two cells. Group them so that they give more voltage. Arrive at the expression for effective emf and internal resistance.
a) Kirchhoff’s second law is a consequence of
i) Law of conservation of angular momentum
ii) Law of conservation of energy
iii)Law of conservation of charge iv) None of these.
b) Find the potential difference across the resistance 2Ω in the given network.
The Figure shows the arrangement of five identical bulbsR1 ,R2 ,R3 ,R4 and R5
a) When the switch is closed will all the bulbs glow? Give reason.
b) Identify the underlying principle. Deduce the principle for a resistance network.
The Figure shows the diagram of a potentiometer.
a) Give the principle of a potentiometer.
b) The length of AB is 3m and resistance per unit length of the potentiometer wire is 4Ω/m. If E 1 = 4V, R=20Ω
and E 2 =1 V find the length of the potentiometer wire that balances E 2 .
c) If E 2 > E 1 can we get the null deflection in galvanometer. Give reason
The figure shows the V-I characteristics of two resistors R 1 , R 2 and the effective resistance of
the series and parallel combinations. (R1 >R 2 ).
a) Identify from the graph whether these resistors are ohmic or non-ohmic?
b) Match the graphs with the corresponding resistances.
a)Define electric power. What is its SI Unit?
b) Two bulbs of 50W, 220V and 100W, 220Vare given. How will you connect the bulbs so that50W, 220V bulb will glow brighter than 100W, 220V bulb.
c) When the bulb 50W, 220V is connected to an 110V supply calculate the power generated.
A student was asked to compare the resistance of two conductors using a meter bridge apparatus.
a) What is the working principle of Meter Bridge.
b) Draw the necessary circuit diagram for the experiment.
c) Write an expression for the ratio of resistances.
d)A Meter bridge wire has resistance0.2 Ω /cm. The bridge is balanced at 60cm from the
end of an unknown resistance X. If the known resistance is 2 Ω , find the current
flowing through the circuit in the balanced condition if a cell of 8V is connected in the
When potential difference is applied between the ends of conductor the electrons will flow through the conductor with a constant average velocity.
a) Name the velocity
b) A potential ‘V’is applied across a conductor of length ‘l’. Keeping the potential difference same, the length of the conductor is halved. How does the velocity of electrons change?
c) Write the colour code of the resistor of resistance (540±10%) MΩ
An electric circuit is given in the Figure
The potential difference between A and D is 40V.
a) Find the effective resistance between A and D in terms of R.
b) Calculate the potential difference between A and E.
Three resistors R 1 ,R 2 ,R 3 are to be combined as shown in the figure.
a)Identify the series and parallel combinations.
b)Which combination has lowest resistance. Arrive at the expression for the effective resistance of this.
The temperature coefficient of resistance of tungsten is 0.0045/ °C.The resistance of a tungsten coil at 30°C is 45 Ω. What will be its resistance at 70°C
Calculate the relaxation time in a metallic conductor from the following data.Conduction electrons
per m3 =5.8x1028, mass of electron 9.1 x 10-31 Kg. Resistivity = 1.6 x 10-8 Ωm
An electron of charge e is revolving around a nucleus with speed v in a circular orbit of radius r.
Calculate the equivalent current
Through a conductor 10 electrons flow per second. Estimate the current.