Give me your 3 questions about finding power(physics)

Question

It's my assignment to search for questions regarding in finding power in class. I think many smart people are here. So the best questions (but not too hard) are will be chosen as the best answer. You can add your solution. Hit your best shot!

Answer 1

I have a 10 watt and a 20 watt light bulb (both rated at 100 volts). i hook them up in series with a 100 volt supply, what is the power used? what is the power used when they are connected in parallel?

you first need to find the resistance in the bulbs

10 = 100 * i(10)

20 = 100 * i(20)

100 = 1/10 * r(10)

100 = 1/5 * r(20)

so the 10watt bulb has a resistance of 1000ohms and the 20 watt 500ohms

now resistance in a  series circuit is r1 + r2 so 1500ohms

v = ir   100 = i * 1500

i=1/15

p = i*v

p = 100/15 or 7.5watts

now for parallel resistance is 1/rt = 1/r1 + 1/r2

1/x = 1/1000 + 1/500

1/x = 3/1000

x = 1000/3

100 = i * 1000/3

i = 300/1000

i = 3/10

p = iv

p = 100*3/10

p = 30