ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: Studyinghard on April 11, 2011, 09:12:51 pm

Title: Thermal energy
Post by: Studyinghard on April 11, 2011, 09:12:51 pm
Jamie is stacking shelves in the local supermarket, and does physical work of 12 kJ in 5 minutes. Jamie's efficiency for this activity is 12%. What is the rate at which thermal energy is generated? (in watt).

Title: Re: Thermal energy
Post by: schnappy on April 11, 2011, 09:16:19 pm
Power=12kJ/5min = 2.4 kJ/min.

Useful work is 12% of this, so the unuseful (presuming it is all heat) is (100-12)%=88%.

2.4*0.88=2.112 kJ/min

Then just convert to J/s (Watt)
(2.112*1000)*60=126.72 W
Title: Re: Thermal energy
Post by: Studyinghard on April 11, 2011, 09:18:38 pm
Shouldnt it be 2.4/0.12
Title: Re: Thermal energy
Post by: Studyinghard on April 11, 2011, 09:31:31 pm
So does this mean that the answer to this

At what rate is heat energy lost due to sweating, if m = 0.1 kg of sweat is lost in 40 minutes? Use 2400 kJ as the total heat loss per kg of sweat or water evaporated. Answer in watt.

is 360
Title: Re: Thermal energy
Post by: Chromeo33 on April 11, 2011, 11:41:42 pm
Whoa which area of study is this? Motion?
Title: Re: Thermal energy
Post by: schnappy on April 12, 2011, 01:53:03 am
Yeah it's odd, but last year we got a question regarding efficiency. It's not motion but thermal energy as an energy loss in systems and the concepts of power, efficiency and energy are on the course.

Shouldnt it be 2.4/0.12
I don't see how.

So does this mean that the answer to this

At what rate is heat energy lost due to sweating, if m = 0.1 kg of sweat is lost in 40 minutes? Use 2400 kJ as the total heat loss per kg of sweat or water evaporated. Answer in watt.

is 360

240kJ/40min=6kJ/min=6000J/(40*60)s=2.5W.

Working looks right to me, maybe I'm doing something wrong?