Page 197 - NCERT Science Class 10 English Medium

Solution
The total energy consumed by the refrigerator in 30 days would be
400 W × 8.0 hour/day × 30 days = 96000 W h
= 96 kW h
Thus the cost of energy to operate the refrigerator for 30 days is
96 kW h × Rs 3.00 per kW h = Rs 288.00

Q U E S T I O N S

?
1. What determines the rate at which energy is delivered by a current?
2. An electric motor takes 5 A from a 220 V line. Determine the power of
the motor and the energy consumed in 2 h.

What you have learnt

n A stream of electrons moving through a conductor constitutes an electric current.
Conventionally, the direction of current is taken opposite to the direction of flow of
electrons.
n The SI unit of electric current is ampere.
n To set the electrons in motion in an electric circuit, we use a cell or a battery. A cell
generates a potential difference across its terminals. It is measured in volts (V).

n Resistance is a property that resists the flow of electrons in a conductor. It controls
the magnitude of the current. The SI unit of resistance is ohm (Ω).
n Ohm’s law: The potential difference across the ends of a resistor is directly
proportional to the current through it, provided its temperature remains the same.
n The resistance of a conductor depends directly on its length, inversely on its area of
cross-section, and also on the material of the conductor.

n The equivalent resistance of several resistors in series is equal to the sum of
their individual resistances.
n A set of resistors connected in parallel has an equivalent resistance R given by
p
1 1 1 1
= + + + ...
R R R R
p 1 2 3
n The electrical energy dissipated in a resistor is given by
W = V × I × t
n The unit of power is watt (W). One watt of power is consumed when 1 A of current
flows at a potential difference of 1 V.
n The commercial unit of electrical energy is kilowatt hour (kWh).
1 kW h = 3,600,000 J = 3.6 × 10 J.
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