Question

Three equal resistors connected across a source of e.m.f.
together dissipate 10 watt of power. What will be the power
dissipated in watt if the same resistors are connected in parallel
across the same source of e.m.f.?
(a) 10/3 (b) 10 (c) 30 (d) 90

Answer 1

Explanation:

first case the resistances are in series so equivalent resistance, R

S

=R+R+R=3R

Thus, power P=

R

S

V

2

10=

3R

V

2

V

2

=30R

In parallel, the equivalent resistance is R

P

=(1/R+1/R+1/R)

−1

=R/3

Power =

R

P

V

2

=

R/3

30R

=90W

Answer 2

Explanation:

first case the resistances are in series so equivalent resistance, R

S

=R+R+R=3R

Thus, power P=

R

S

V

2

10=

3R

V

2

V

2

=30R

In parallel, the equivalent resistance is R

P

=(1/R+1/R+1/R)

−1

=R/3

Power =

R

P

V

2

=

R/3

30R

=90W