INDIAN SCHOOL MUSCAT
CLASS : XII Physics Worksheet 3 (2014 – 15)
3.Current Electricity
Section A .Conceptual and Application Type Questions
1. A toaster produces more heat than an electric bulb when they are connected in parallel .Which of
them have greater resistance?
2. A Nichrome and copper wire of same length, and same radius are connected in series, when current
is passed through them which of them will get heated more? (2001)
3. What is the relation between current density J, electric field E and conductivity σ?
4. What is the effect of increasing the temperature on relaxation time and mobility of electron in a
Conductor?
5. The drift speed of electron is very small of the order of mm/s, then how does an electric appliance
start working as soon as it is switched on?
6. If the current through a variable resistor is constant, draw a graph between Potential difference V
and the resistance R.
7. Which is more sensitive a 10m or a 4m length of potentiometer? Justify.
8. Is it possible to have a terminal P d greater than the emf of a cell? Justify.
9. Sketch graphs to show the variation of resistivity of i) copper ii) Nichrome iii) silicon with
temperature. (2004, 2005, 2006)
10. What are the advantages of Carbon resistors?
11. A potential difference V is applied to a conductor of length L, diameter of D. How are the following
i) the electric field E, the drift speed vd, the resistance R affected when a) V is doubled b) L is doubled
c) D is doubled?
12. How does the internal resistance of a cell vary with temperature?
13. Why the internal resistance of a i) cell must be very low ii) high tension supply must be high? (1995 , 1997)
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14. A cell of emf ε and internal resistance r is connected across a variable resistance R. Plot graphs to
show variation of i) ε with R ii) terminal p.d of the cell V with R.
15. Explain the variation of conductivity with temperature for i) metallic conductor ii) ionic conductors.
(2004, 2008)
16. Two heater coils made up of same material are connected in parallel across the mains. If the length
and diameter of one of the coils is double those of the other, which one of them will produce
more heat?
17. A piece of semiconductor s in series with a variable resistor R and a source of constant voltage V
and an ammeter A .Would you increase or decrease the value R to keep the ammeter A reading
constant, when the semiconductor S is heated? Give reason. (2004)
Section B. Numerical Problems
1. Calculate the resistivity of a conductor in which a current density of 2.5 A/m2 exist, when an
electric field of 15V/m is applied on it.
2. A copper wire is stretched to make it 0.1 % longer .What is the percentage change in resistance?
3. What is the i) drift velocity of electrons ii) current density in a copper conductor having a cross sectional
area of 5x 10-6 m2, if the current is 10A. n= 8 x 1028/m3
4. An electron beam has an aperture of 10-6m2 .The total number of electrons moving through any
perpendicular cross section per second is 6.0 x 1016.Calculate the current density in the beam .(2000)
5. At room temperature (27◦C) the resistance of a heating element is 100Ω. What is the temperature at which
its resistance will become 117 Ω?
take α = 1.70 x 10-4 ◦ C-1.
6. A battery of emf 12V is connected to a resistor of 105Ω through an ammeter of resistance 5Ω. If 100mA
current flows through the battery, find the internal resistance of the battery. (1995, 2006)
7 i) What is the current in 2Ω resistor in the circuit?
ii) What is the terminal voltage of each cell?
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10 V, 3Ω
12 V , 2Ω
2Ω
8 .Find the minimum number of cells, each of emf 1.5V and internal resistance 10Ω required to produced an
electric current of 1.5A through a resistance of 30Ω.
9. A 10m long wire AB of uniform cross section and 20Ω resistance is used as a potentiometer wire. This wire is
connected in series with a battery of 5V and a resistor of 480Ω.Calculate i) the potential gradient of the
potentiometer wire ii) the unknown emf of a cell which is balanced at 600cm length of the potentiometer.
(1998)
10.
(a)
(b)
Two primary cells of emfs E1 and E2 with E1 > E2, are connected in series in the secondary circuit of a
potentiometer of length 5m connected in series to a standard cell of emf 5V and negligible resistance
which maintains a steady current as in figure (a) and (b). If the balancing lengths of the potentiometer
in both the cases are 350cm and 50cm respectively, and then calculate the value of emf of the two cells.
(2004)
11. Two metallic wires of the same material have the same length but cross sectional areas in the ratio 1: 2.
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They are connected in i) series ii) parallel. Compare the drift velocities of electrons in the two wires in
both the cases. (2008)
12. Two metallic wires of the same material have lengths in the ratio 1:2 and diameters in the ratio of 2:3 are
connected in series with an accumulator. Calculate the ratio of potential drop across the two wires.
13. Two identical cells, whether they are joined in series or parallel, give the same current when connected
to a load resistance of 1 Ω .Find he internal resistance of each cell.
14. Find the charge on the capacitor shown in figure.
6µF
10Ω
20Ω
2V
15.. Calculate the currents I1, I2, and I3 in the electrical circuit given below.
16. A wire of resistance 32Ω is melted and drawn into a wire of half its original length .Calculate the resistance
of the new wire and the percentage change in resistance?
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