Download current electricity worksheet (theory)

Buds Public School, Dubai
NAME:
PHYSICS
GRADE-12
Topic - Current Electricity
__worksheet_1(Theory)___________________________________________
1. A steady current flows in a metallic conductor of non-uniform cross-section.
Which of these quantities is constant along the conductor : current, current
density, electric field, drift speed ?
2. The emf of a cell is always greater than its terminal voltage. Why? Give reason
3. Using the concept of drift velocity of charge carriers in a conductor, deduce the
relationship between current density and resistivity of the conductor.
4. Define the term current density of a metallic conductor. Deduce the relation
connecting current density (J) and the conductivity σ of the conductor, when an electric
field E, is applied to it.
5. State the working principle of a potentiometer. Explain, with the help of a circuit
diagram, how the emf of two primary cells are compared by using a potentiometer.
6. Draw V - I graph for ohmic and non-ohmic materials. Give one example for each.
7. Define the term 'resistively' and write its S. I. Unit. Derive the expression for the
resistively of a conductor in terms of number density of free electrons and relaxation
time.3
8. State the principle of potentiometer. Draw a circuit diagram used to compare the
e.m.f. of two primary cells. Write the formula used. How can the sensitivity of a
potentiometer be increased?
9.(i) How does the resistivity of (i) a conductor and (ii) a semiconductor vary with
temperature? Give reason for each case.
(ii)Why alloys like constantan and manganin are used for making standard resistors ?
10. Define the term 'temperature coefficient of receptivity'. Write its S I unit. Plot a graph
showing the variation of receptivity of copper and nichrome with temperature.
11. Explain how electron mobility changes for a good conductor when (i) the
temperature of the conductor is decreased at constant potential difference and (ii)
applied potential difference is doubled at constant temperature
12. 4 cells of identical emf E, internal resistance r, are connected in series to a variable
resistor. The following graph shows the variation of terminal voltage of the combination
with the current output:
(i) What is the emf of each cell used?
(ii) For what current from the cells, does maximum power dissipation occur in the
circuit?
(iii) Calculate the internal resistance of each cell.
13. Write the mathematical relation for the resistivity of a material in terms of relaxation
time, number density and mass and charge of charge carriers in it. Explain, using this
relation, why the resistivity of a metal increases and that of a semi-conductor decreases
with rise in temperature
14. A voltage of 30 V is applied across a carbon resistor with first, second and third
rings of blue, black and yellow colours respectively. Calculate the value of current, in
mA, through the resistor.
15.
What may be the two possible faults in the circuit that could result in this
obsevation ?
If the galvanometer deflection at the end B is (i) more, (ii) less, than that at the end
A, which of the two faults, listed above, would be there in the circuit ?
Give reasons in support of your answer in each case.
15.The given figure shows a network of resistances R1, R2, R3 and R4
16. On what principle does a metre bridge work ? Draw a circuit diagram and explain
how this device can be used for determination of an unknown resistance. Write the
necessary precautions to minimize the error in the result
17. Write the principle of working of a potentiometer. Describe briefly, with the help of a
circuit diagram, how a potentiometer is used to determine the internal resistance of a
given cell.
18. Show that the electric field at the surface of a charged conductor is given by
Where σis the surface charge density and n∧
surface in the outward direction

19. A resistance R is connected across a cell of emf and internal resistance r. A
Potentiometer now measures the potential difference between the terminals of the cell
as V. Write the expression for 'r' in terms of , V and R.

20. (i)Define the terms (i) drift velocity, (ii) relaxation time.
(ii) A conductor of length L is connected to a dc source of emf . If this conductor is
replaced by another conductor of same material and same area of cross-section but
of length 3L, how will the drift velocity change ?
21.Draw a plot showing the variation of resistivity of a (i) conductor and (ii)
semiconductor, with the increase in temperature.
How does one explain this behaviour in terms of number density of charge carriers and
the relaxation time?


22. A variable resistor R is connected across a cell of emf and internal resistance r as
shown in the figure. Draw a plot showing the variation of (i) terminal voltage V and (ii)
the current I, as a function of R.
23. A potential difference V is applied across a conductor of length L and diameter D.
How is the drift velocity, vd, of charge carriers in the conductor affected when (i) V is
halved, (ii) L is doubled and (iii) D is halved ? Justify your answer in each case.
24. In the potentiometer circuit shown, the null point is at X. State with reason, where
the balance point will be shifted when
(a) resistance R is increased, keeping all other parameters unchanged;
(b) resistance S is increased, keeping R constant.
25. Use Kirchhoff’s rules to obtain conditions for the balance condition in a Wheatstone
bridge.
26. (a) Deduce the relation between current I flowing through a conductor and drift
velocity _d of the electrons.
(b) Figure shows a plot of current ‘I’ flowing through the cross-section of a wire
versus the time ‘t’. Use the plot to find the charge flowing in 10s through the
wire.
27.(i) Why is potentiometer preferred over a voltmeter for comparison of emf. of cells ?
28.(i) Plot a graph showing variation of voltage vs the current drawn from the cell.
How can one get information from this plot about the emf of the cell and its
internal resistance ?
(ii) Two cells of emf’s E1 and E2 and internal resistance r1 and r2 are connected in
parallel. Obtain the expression for the emf and internal resistance of a single
equivalent cell that can replace this combination ?
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