Ch 6Thermal Energy

Sec 1 Temperature and Heat As you will find out in PS1, all matter

is made up of tiny particles in constant motion

Because they are moving they have KE

The faster they move, the more KE they have

Temperature is the measure of the average KE of the particles in a sample of matter

As the Temp ↑ the particles move faster and their average KE ↑

Temp is measured in kelvins (K), although we use Celsius when working in the lab

One degree of kelivn is the same as one degree of Celsius

Temp & Thermal E

Temp is an intrinsic quality: 1 kg of sand has the same Temp as 100 kg of sand—it doesn’t change with the amount

Thermal E (TE) is the total E of the particles in a material—this includes both KE and PE

KE is due to the vibrations/movement of the particles

PE is determined by forces that act between and within the particles

TE is an extrinsic quality: more mass, more TE—it does increase when the mass increases

Different kinds of matter have different TE even when mass and Temp are the same

5g of sand and 5 g of pudding at the same temp have different TE due to how their atoms are arranged

TE depends on the total E of its particles—the KE of the object itself has no effect on its TE

EX. A moving basketball at 20°C has the same TE as one sitting still

Heat Heat--TE that ALWAYS flows

from something with a higher Temp to something with a lower Temp

EX -- touch something HOT and heat is transferred to your hand making it warm

Touch something cold and heat is transferred away from your hand making it feel cold

Measured in joules—transfer of E—just like work

Now explain: How does the cooling occur when you put ice cubes in your drink?

Measuring TEMeasuring TE Different materials need different

amounts of heat to produce similar changes in their Temp

EX Out at the lake the air Temp is 36°C (HOT!) and you are hot and sweaty and decide to go for a swim so you jump in the lake and the water seems VERY cold even though it has been sunny all day

Water requires a lot more E to change it’s Temp compared to air and other substances

This amount of E is called the materials SPECIFIC HEAT—Cp– amount of E required to raise the Temp of 1 kg of material 1 degree kelvin

Copy the chart on pg 161 onto your Physics Hand-out (add Al= 920J/kgxK)

As you can see, water requires more E to raise its Temp 1 degree K, but iron is much less—metals heat up very quickly

Measuring TE cont.

You can’t measure TE directly like you can Temp with a thermometer, but you can use Cp to measure changes in TE

Q = m x ΔT x Cp

Q= change in TE m= mass ΔT= change in Temp (Tf - Ti) Cp= specific heat ΔT→positive→increase in

Temp→heat gained ΔT→negative→decrease in

Temp→heat lost

Let’s Practice!

A 3.1 kg ball of Al foil cools from 30°C to 15°C. What is its change in TE?

1. Q = m x ΔT x Cp

2. m = 3.1 kg ΔT = 15°- 30° Cp= 920 J/kg x K

3. Q= 3.1 x (-15°) x 920 J/kgxK 4. Q= -42780.0J The Al foil ball loses 42780.0 J

Whiteboards!

If a 45 kg brass sculpture gains 180,480 J of TE when its Temp increases from 28°C to 40°C, what is its approximate Cp?

A 55.0 g iron nail has been heated to 90°C, then cooled to 25°C. What is the change in TE?

How much TE does a 420 g of liquid water gain when it is heated from freezing point to boiling point?

50.0 g of water and 50.0 g of sand each absorb 200 J of solar E. What will the Temp change (ΔT) of each material be?