Temperature, Heat and Energy

1. Temperature, Heat and Energy It’s Hot, Hot, Hot!

2. Remember Conservation of Energy? We have been ignoring the non-mechanical or “lost” energy.

3. Energy is the property that enables a body to do work. Energy, Work and Heat Work can transfer mechanical energy into or out of a substance. Heat is the transfer of thermal energy into or out of a substance.

4. Mechanical Energy Potential Energy – results from changing position. Kinetic Energy – results from motion.

5. Internal Energy Internal energy is the total energy of all of the molecules in a substance. (This is not mechanical energy; you can’t see this.) Thermal Energy – results from the motion of atoms (thermal motion).

6. Internal Energy Suppose you put a block of metal into hot water. What happens to the energy of the block? Does the kinetic energy of the block change? NO! Does the potential energy change? NO! Does the mechanical energy change? NO! Does the thermal energy change? Yes! Does the internal energy change? Yes!

7. (Conservation of Energy Applied to Heat) First Law of Thermodynamics Heat added to a system transforms to an equal amount of another form of energy. External work done by the system Heat added Increase in internal energy = +

8. Examples of First Law of Thermodynamics External work done by the system Table does no work Balloon expands and does work by pushing air in the car aside Increase in internal energy Heat added Heat added to table by rubbing hand on the surface Heat added to balloon by placing it in a hot car Table gets warmer Balloon gets warmer = + = = + +

9. Conservation of Energy Applied to Heat Second Law of Thermodynamics When energy is transferred some of the energy can not be used to do work. Or …Natural systems tend toward a state of greater disorder.

10. Example of • Second Law of Thermodynamics Waterfalls are used to generate electricity. The electricity that it generates is not enough to pump all of the water back up to the top of the waterfall. (Can’t have perpetual motion because we always lose usable energy.)

11. EntropyHow’s this for a little chaos? Entropy is the measure of the amount of disorder in a system. Which picture shows greater entropy?

12. Temperature or Heat Temperature is the result of the average kinetic energy of the atoms and molecules in motion. Think of the block in the water. • The block is not moving, KE = 0 J • The molecules in the block are moving (vibrating), KE >0. This is thermal energy. Thermal Energy is the total KE of the atoms.

13. Heat • Energy that flows from one object to another. It is the transfer of energy. • It flows from higher temperature to lower temperature. • It is measured in joules in the SI system. • It is thermal energy in transit.

14. Heat Transfer Transfer of heat depends on temperature difference. Heat stops flowing when the temperatures are equal – Thermal equilibrium is reached. heat lost by one object = heat gained by the other

15. Heat Equation Q – heat transferred to an object m – mass c – specific heat DT – temperature change

16. c = Specific Heat The amount of energy that needs to be added to raise the temperature of a unit of mass (1kg) one degree. This means an object with a large specific heat will require a larger amount of heat to change the temperature. Examples : Water c = 4186 J/kgoC Copper c = 386 J/kgoC