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Section 4: Thermal Energy

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1 Section 4: Thermal Energy
Chapter 3: Matter and Energy Section 4: Thermal Energy

2 Learning Objectives Convert between Fahrenheit, Celsius, and Kelvin temperature scales. Relate energy, temperature change, and heat capacity.

3 Thermal Energy The atoms and molecules that compose matter are in constant random motion they contain thermal energy The temperature of a substance is a measure of its thermal energy.

4 Thermal Energy The hotter an object, the greater the random motion of the atoms and molecules that compose it, and the higher its temperature.

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6 Thermal Energy Heat, which has units of energy, is the transfer or exchange of thermal energy caused by a temperature difference. when a piece of cold ice is dropped into a cup of warm water, heat (thermal energy) is transferred from the water to the ice.

7 Thermal Energy Temperature, by contrast, is a measure of the thermal energy of matter (not the exchange of thermal energy). Measures the average kinetic energy of the molecules of matter.

8 Thermal Energy Both cups of water are at the same temperature…
Which has a higher average kinetic energy? Which contains more thermal energy?

9 Temperature Scales The Fahrenheit scale was set according to the following standards 0 °F to the freezing point of a concentrated saltwater solution 96 °F to normal body temperature.

10 Temperature Scales On the Fahrenheit (°F) scale water freezes at 32 °F
water boils at 212 °F Room temperature is approximately 72 °F.

11 Temperature Scales On the Celsius (°C) scale: water freezes at 0 °C
water boils at 100 °C Room temperature is approximately 22 °C

12 Temperature Scales The Kelvin (K) scale avoids negative temperatures by assigning 0 K to the coldest temperature possible, absolute zero. Absolute zero is the temperature at which molecular motion stops. On the Kelvin (K) scale, water freezes at 273 K water boils at 373 K. Room temperature is approximately 295 K

13 The Fahrenheit degree is five- ninths the size of a Celsius degree.
The Celsius degree and the Kelvin are the same size.

14 Temperature Scales We can convert between Fahrenheit, Celsius, and Kelvin temperature scales using the following formulas:

15 Practice Convert –25 °C to kelvin.

16 Practice Convert 358 K to Celsius.

17 Practice Convert 55 °F to Celsius.

18 Practice Convert 139 °C to Fahrenheit.

19 Practice Convert 310 K to Fahrenheit.

20 Practice Convert –321 °F to kelvin.

21 Heat Capacity Heat capacity: The quantity of heat (usually in joules) required to change the temperature of a given amount of the substance by 1 °C

22 Heat Capacity Specific heat capacity: the amount of heat required to raise the temperature of 1 g of any substance by 1oC Specific heat capacity has units of joules per gram per degree Celsius, J/g °C

23 Heat Capacity Specific heat capacity is an intensive property!
Specific heat describes how well an object retains heat A substance with a low specific heat is quickly heated, but also quickly cools A substance with a high specific heat takes a long time to warm up, but will also retain that heat for a longer period

24 Heat Capacity Styrofoam is a very poor conductor of heat; it is a good insulator. It has a high specific heat. Metals are good conductors of heat. They have low specific heats.

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26 Check-in: If you want to heat a metal plate to as high a temperature as possible for a given energy input, what metal should you use? (Assume all the plates have the same mass.) copper iron aluminum it would make no difference

27 Heat Capacity Calculations
q is the amount of heat in joules. m is the mass of the substance in grams. C is the specific heat capacity in joules per gram per degree Celsius. T is the temperature change in Celsius. The symbol Δ means the change in, so ΔT means the change in temperature.

28 Example Gallium is a solid at 25.0°C and melts at 29.9°C. If you hold gallium in your hand, it can melt from your body heat. How much heat must 2.5 g of gallium absorb from your hand to raise its temperature from 25.0°C to 29.9°C? The specific heat capacity of gallium is J/g°C.

29 Practice The temperature of a lead fishing weight rises from 26°C to 38°C as it absorbs 11.3 J of heat. What is the mass of the fishing weight in grams?

30 Practice A chemistry student finds a shiny rock that she suspects is gold. She determines that its mass is g. She then finds that the temperature of the rock rises from 25°C to 52°C upon absorption of 174 J of heat. Find the heat capacity of the rock and determine whether the value is consistent with the heat capacity of gold (which is listed in Table 3.4).

31 Practice A 328 g sample of water absorbs 5.78 × 103 J of heat. Calculate the change in temperature for the water. If the water is initially at 25.0°C, what is its final temperature?

32 Check-in: The heat capacity of substance A is twice that of substance B. If samples of equal mass of the two substances absorb the same amount of heat, which substance undergoes the larger change in temperature?


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