ASSIGNMENT WORK
Class : xii
Subject : physics
(CURRENT ELECTRICITY)
1. Establish the relation between drift velocity of electrons and electric
current.
2. How many electrons constitute a current of 1 ampere?
3. Deduce ohm’s law by using basic idea of drift velocity.
4. What is a thermistor? Write one application of the thermistor.
5. You have three resistances of value R each. How will you connect these
3𝑅
for obtaining a total resistance of
? Show this connection by a
2
diagram.
6. Give the nature of V – I graph for ohmic and non – ohmic circuit
elements. Give one example of each.
7. Is there any connection of solar winds or cosmic rays with electric
current?
8. Is Ohm’s law a universal law ? Support your answer with examples.
9. Is the temperature coefficient of resistance always positive ?
10.State and explain Kirchhoff’s laws.
11.What is Wheatstone bridge? State and prove the principle of
Wheatstone bridge.
12.Show that a potentiometer is an ideal voltmeter.
13.Explain the principle of potentiometer. How will you compare e.m.f. of
two given primary cells by using a potentiometer? Explain with proper
circuit diagram.
14.Sometime the balance point in the potentiometer is not obtained.
Suggest the reasons for it.
15.A potentiometer of longer wire is more sensitive. Explain.
16.What do you understand by a potential divider circuit? Explain it with
the help of an example.
17.If 0.8 mol of electrons flow through a wire in 40 min. What are (a) the
total charge that passes through the wire and (b) the amount of current
flowing through the wire? [Ans.7.71× 𝟏𝟎𝟒 C; 32.13 A]
18.A cylindrical metallic wire is stretched to increase its length by 5%.
Calculate the percentage change in its resistance.[Ans. 10.25%]
19.The carbon resistor has coloured strips with sequence brown black,
brown and gold. What is the value of the resistor? [Ans. (101±5%)]
20.A set of n identical resistors, each of resistance R ohm, when connected
in series have an effective resistance X ohm and when the resistors are
connected in parallel, their effective resistance is Y ohm. Find the
relation between R, X and Y. [Ans. XY = R2]
21.A uniform wire has resistance 12 . It is bent in the form of an
equilateral triangle. Find the effective resistance between any two
corners of the triangle. [Ans. 2.7  ]
22.An electric heater and an electric bulb are rated 500 W, 220V and 100W,
220V respectively. Both are connected in series to a 220V a.c. mains .
Calculate the power consumed by (i) the heater and (ii) electric bulb.
[Ans. 13.98W; 69.89W]
23.Calculate the number of electrons moving per second through the
filament of an electric lamp of 60W operating at 220V. [Ans. 1.7
× 𝟏𝟎𝟏𝟖 ]
24.A battery of e.m.f. E and internal resistance r gives a current of 0.5 A
with an external resistor of 12 and current of 0.25 A. Calculate (i)
internal resistance of the cell and (ii) e.m.f. of the cell. [Ans. 1; 6.5 V ]
25.Four identical cells, each of e.m.f. 2V, are joined in parallel providing
supply of current to external circuit consisting of two 15 resistors
joined in parallel. The terminal voltage of the cells, as read by an ideal
voltmeter is 1.6V. Calculate internal resistance of each cell. [Ans. 7.5]
26.Two cells of e.m.f. 1.5V and 2.0V having internal resistance 1 ohm and 2
ohm respectively are connected in parallel so as to send the current in
the same direction through an external resistance of 5 ohm. (a) Draw the
circuit diagram. (b) Using Kirchhoff’s laws, find the current through each
branch of the circuit and potential difference across 5 ohm resistance.
[Ans. 0.0294A; 0.265A; 0.294A;1.47V]
27.In a Wheatstone bridge network P and Q, the ratio arms are
approximately equal. When R = 500 ohms, the bridge is balanced. On
interchanging P and Q, the value of R for balance is 505 ohm. Find the
value of S and ratio P : Q. [Ans. 502.5; 1 : 1.005]
28.In a potentiometer arrangement, a cell of e.m.f. 1.20V gives a balance
point at 30.0 cm length of wire. If this cell is re;laced by another cell and
the balance point shifts to 65.0 cm, what is the e.m.f. of the second cell?
[Ans. 2.6V]
29.A 10 m long wire of uniform cross section and 20  resistance is used in
a potentiometer. The wire is connected in series with a battery of 5V
along with an external resistance of 480. If an unknown e.m.f. E is
balanced at 6 m length of the wire. Calculate the potential gradient of
the potentiometer wire and value of unknown e.m.f. E. [Ans. 0.02 V/m;
0.12V]
30.A potentiometer wire is 100 m long and a constant potential difference
is maintained across it. Two cells are connected in series first to support
one another and than in opposite direction. The balance points are
obtained at 50 cm and 10 cm from the same end of the wire in the two
cases. Find the ratio of e.m.f.’s [Ans. 3:2 ]