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Heat Capacity.

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Presentation on theme: "Heat Capacity."— Presentation transcript:

1 Heat Capacity

2 Heat In This is comparable to work done on a system.
Temperature change is due to heat (Q). If an object’s temperature increases it gains heat. If an object’s temperature decreases it loses heat. This is comparable to work done on a system. Heat into a system

3 Heat Capacity The ratio of the change in heat to the change in temperature is the heat capacity (C). Depends on material Depends on the mass Measured in J/K

4 Specific Heat The ratio of the change in heat to the change in temperature is the heat capacity (C). Depends on material independent of the mass Measured in J/kg-K Material Specific heat Mercury 140 J/kg-K Copper 390 J/kg-K Steel 500 J/kg-K Granite 840 J/kg-K Aluminum 900 J/kg-K Wood J/kg-K Ice J/kg-K Water J/kg-K

5 Take a Shower You are last to use the shower today and the temperature in the water heater has dropped to 18 C from the normal temperature of 50 C. The heater holds 150 kg and has a 50 kW heating coil. How long should you wait before starting your shower? Find the heat required. DQ = mcDT = (150 kg) (4200 J/kg-K) ( 32 C) DQ = 2.1 x 107 J Find the time from the power, and set the heat to the work. P = DW/Dt t = DQ / P (2.1 x 107 J) / (5 x 104 W) t = 4000 s = 1.1 hr

6 Equilibrium Temperature
Two systems in thermal contact will adjust to reach the same temperature - thermal equilibrium If they are thermally insulated, all the heat goes from the hot system to the cold system. 1 2

7 Doing Dishes A 1.5 kg aluminum pan is heated to 180 C then placed into a sink with 8 kg water at 20 C. If no water boils, what is the equilibrium temperature? Setup the equation for an unknown equilibrium temperature T. Solve for T. T = 26 C

8 Heat of Transformation
As a system reaches the point of a phase change heat no longer changes the temperature. The heat is used to change the phase. heat of fusion heat of vaporization

9 Latent Heat The heat of transformation per unit mass is called the latent heat (L). Measured in J/kg It takes as much energy to melt 1 g of ice as it does to raise the temperature from 0 C to 80 C. Material (f or v) Latent Heat Mercury (f) 11.3 kJ/kg Lead (f) kJ/kg Uranium (f) 82.8 kJ/kg Copper (f) 205 kJ/kg Water (f) 334 kJ/kg Oxygen (v) 213 kJ/kg Water (v) 2300 kJ/kg

10 Summer Cooler 200 g of ice at -10 C is added to 1.0 kg of water at 15 C. Is there enough ice to cool the water to 0 C? Find the amount of heat needed to bring the ice to 0 C and melt it. Qi = mcDT + mLf Qi = 70.9 kJ Find the heat needed to cool the water down to 0 C. Qw = mcDT Qw = 62.8 kJ Yes, there’s enough ice.


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