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Ch 6

Thermal Energy. Ch 6. 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

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Ch 6

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  1. Thermal Energy Ch 6

  2. 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 averageKE ↑ • 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

  3. Temp & ThermalE • Temp is anintrinsic 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

  4. 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?

  5. Measuring 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

  6. 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

  7. 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

  8. 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?

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