Year 8 Physics homework help

[littlenicky61]

Can anyone help check my daughter has understood this correctly - she been off ill so has missed the actual lesson but is trying to do the homework - I have blocked out all memories of Physics so no real help.

Question is : Look at the table below that shows 4 different devices . Use the law of conservation of energy and the equation for efficiency to complete the table :

Device Useful energy Wasted energy Total energy Efficiency

Lightbulb 5 ? 25 ?

Kettle ? 500 2000 ?

Television 2500 2500 ? ?

Car 100 ? 400 ?

She is thinking that total energy - useful energy + wasted energy
and efficiency will be useful energy divided by total energy the multiplied by 100 ?

so for lightbulb eg

useful energy is 5 so wasted energy will be 20 and efficiency will therefore be 5 divided by 25 then multiplied by 100 ? so efficiency will equal 20% is that right - if not can someone point her in right

Thanks so much

[littlenicky61]

sorry it come out jumbled

eg lightbulb row should be

useful energy = 5
wasted energy = ?
Total energy = 25
Efficiency = ?

[Blueemeraldagain]

The law of conservation is never created or destroyed it just transforms.

Efficiency is useful energy/total energy X 100

[Veterinari]

Yes sounds right OP

[Blueemeraldagain]

Ha. Energy is never created or destroyed.

For the lightbulb

Wasted would be 20
Efficiency would be 5/25 = 0.2
0.2X100 = 20%

And 5 is 20% of 25 (or 1/5 of 20)

[Joinourclub]

You have the right method

[littlenicky61]

Thanks so much : )