Urgent! Help needed please with Science homework!

[stellsie]

My DD (Year 9) has been given homework which needs to be in on Monday. She has been given 2 websites to look at to find how the amount of water heated affects the temperature rise in the water.

We have looked at the websites but are both just confused! Quote DD "I'm only Year 9, not at university!!".

Worst case I will email her Science teacher and apologisse for DD not doing homework but she and I just didnt get it!

But first I thought I would try on the wonderful mumsnet to see if some kind person could explain it to us!

Thanks so much

hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html
www.kentchemistry.com/links/Energy/SpecificHeat.htm

and apologies for spelling apologise wrong

The temp rise is inversely proportional to the amount of water, so:

You'll need twice the heat input to raise the temp of twice the amount of water by 1 degree

Or to put it another way:

The same heat input (say, buring up one candle under the pan!) might raise the temp of 1 kg of water by 1 degree, 2 kg of water by 0.5 degrees, or 500 g of water by 2 degrees.

The key thing she needs to mention is the difference between heat energy, (that is the energy put in by the kettle, gas under the saucepan etc.) and temperature which is kind of an average of the amount of energy each of the particles of water heated have. (If it was my day they'd have wanted you to mention the specific heat, but I doubt she needs to nowadays.)

thank you Lancelottie. So..... if DD has to explain - say, why does the water in a bath take so long to cool down compared to a cup of tea? The cup of tea cools down quicker because there is less of a rise in temperature? I'm sorry I really dont understand I'm sure you are explaining it well, I just dont get it!

both those websites talk about specific heat mummytime
still dont get it though!!

god give me strength (that's probably what you and Lancelottie are saying)

DeltaH = m c deltaT

deltaT = (delta H/c) /m

Assuming the same amount of heat (eg from electrical immersion heater or bunsen burner) and with the specific heat being a constant, then the temperature will rise linearly as mass decreases and vice versa. That means, if you double the mass, you will half the change in temperature, etc.

There is a brilliant selection of videos on Youtube called Eureka. The one of heat vs temperature is very easy to understand (although it is Canadian rather than British, so they talk about molecules rather than particles).

The cup of tea cools down slower, because the cup has a small surface area, the bath has a large one (and if its a posh stand alone cast iron bath it is losing heat from all sides, where the ceramic cup is a poor heat conductor). If you measured it using a thermometer, this should be true. However it also shows how bad the human body is at measuring tempreture, because we expect a cup of tea to be hotter than a bath.
It will be even faster for the bath if the bathroom fan is on, due to convection being increased.

The cup has a large surface area for its volume.