Q1. What is electrical resistivity? Derive its SI unit. In a series electrical circuit comprising a
resistor made up of a metallic wire, the ammeter reads 100 mA. If the length of the wire is
doubled, how will the current in the circuit change? Justify your answer.
Q2. Derive an expression for the equivalent resistance of three resistors R1, R2 and R3
connected in
a) Series b) Parallel
Q3. Calculate the total cost of running the following electrical devices in the month of
September, if the rate of 1 unit of electricity is ₹ 6.00.
(a) Electric heater of 1000 W for 5 hours daily.
(b) Electric refrigerator of 400 W for 10 hours daily
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Answers
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Explanation:
1. The electrical resistivity is the electrical resistance per unit length and per unit of cross-sectional area at a specified temperature. The SI unit of electrical resistivity is the ohm⋅metre (Ω⋅m). It is commonly represented bythe Greek letter ρ, rho.
2.Parallel Connection : In parallel connection of resistors there is same potential difference at the ends of the resistors. Hence the voltage in the circuit is equal to V. Let I1, I2 and I3 be the currents flowing through R1, R2 and R3 resistors respectively. Hence, we can write I = I1 + I2 + I3.
3. a P
1
=
1000
W
=
1
k
W
t
1
=
5
h
b. =
400
W
=
0.4
k
W
t
2
=
10
h
E
2
=
P
2
×
t
2
×
n
E
2
=
0.4
k
W
×
10
h
×
30
=
120
k
W
h
(
150
+
120
)
k
W
h
=
270
k
W
h
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