Question

You buy a 75-W lightbulb in Europe, where electricity is delivered at 240 V.

If you use the bulb in the United States at 120 V (assume its resistance does not change), how bright will it be relative to 75-W 120-V bulbs?

Assume that brightness is proportional to power consumption.

a.2.5 X 10^-1

b.1.0

c.7.7 X 10^2

d.19

e.4.0

Answer 1

Answer:

a.    2.5 X 10^-1

Explanation:

First of all we shall calculate the resistance of the bulb of 75W , 240 V .

power = V² /R , V is potential and R is resistance

75 = 240²/ R

R = 240²/ 75

= 768 ohm .

Now this bulb is used with 120 V

current = 120 / 768

= .15625 A

power = V X I

potential x current

= 120 x .15625

= 18.75 W.

comparing it with 75 W

the ratio

18.75 / 75

= 2.5 x 10⁻¹.