# Talk

## Year 8 Physics homework help

(7 Posts)
littlenicky61 Wed 18-Jan-17 17:44:36

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 Wed 18-Jan-17 17:47:12

sorry it come out jumbled

eg lightbulb row should be

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

Blueemeraldagain Wed 18-Jan-17 17:50:58

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

Efficiency is useful energy/total energy X 100

Veterinari Wed 18-Jan-17 17:53:06

Yes sounds right OP

Blueemeraldagain Wed 18-Jan-17 17:53:51

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 Wed 18-Jan-17 17:54:27

You have the right method

littlenicky61 Wed 18-Jan-17 18:04:25

Thanks so much : )

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