Unit 7: Heat and Heat Transfer

1.  Matter and heat

a.  Matter only changes state with the addition or subtraction of THERMAL ENERGY!

b.  This means you need heat to change the state a matter is in.

c.  A change of state is a physical change. It DOES NOT change the identity of the substance.

2.  Physical and chemical changes

a.  A change of state is a physical change.

i.  They do not change the stuff that the matter is made out of.

ii.  Examples: melting, freezing, boiling, vaporization, evaporation

b.  Chemical changes—materials combine to form one or more new substances.

i.  Clues to a chemical change occurring: a CHANGE in odor, A CHANGE in color, burning, bubbling

3.  Changes of state

A.  Phase change, or a change of state, is a physical changes. The stuff that makes of the substance (the molecules) are still the same, they just take a different form. A chemical changes causes the molecules to change.

B.  The molecules are moving fastest at number 1 because it is a gas. And at higher temperatures

C.  Melting, Freezing, Boiling, Vaporization, and Condensation are all phase changes/physical changes

D.  This substance melts at 273 K It also freezes at 273 K_.

E.  Melting and Freezing occur at the same temperatures. If you add energy then the substance is melting (changing from a solid to a liquid); if you subtract energy, then the substance is freezing (changing from a liquid to a solid ).

F.  At numbers 2 and 4 on the graph above, the temperature, a measure of the average kinetic energy, is constant. Why does it not change during the phase change? What is the energy being added used to overcome? The energy added does not change the temperature, instead it is being used to overcome the attractive forces between particles and change the arrangement of the particles within the object.

4.  What is Heat?

a.  It is the transfer/ movement of energy from one object to another

b.  Heat only occurs when there are 2 or more objects present.

c.  You CAN NOT have heat with only one object!

5.  Heat Transfer---There are 3 _ types of heat transfer.

a.  Radiation—The transfer of energy through space or matter (media) in the form of an electromagnetic wave (infrared waves)

i.  Examples: sunlight, body heat, warming up next to a fire, the warmth in a greenhouse.

b.  Conduction—the transfer of energy from one object to another through direct contact

i.  Examples: the handle of a metal spoon warms as it sits in a bowl of hot soup

ii.  This type of transfer works great with conductors like metal, but poorly with insulators like plastic.

1.  Conductors—materials that easily transfer energy like metal and glass. These get hot quickly, but they also cool down quickly.

2.  Insulators—bad conductors. It is a material that resists the transfer of energy. They take a very long time to get hot, and they stay hot for a long time! Wood, plastic and rubber are examples of insulators.

c.  Convection—The transfer of thermal energy by the movement of a liquid or gas (fluid).

i.  Examples: warm air heating systems, boiling water, convection ovens

ii.  In a warm-air heating system, warm air is circulated through the building using convection currents You can see this in a pot of water boiling.

iii.  Convection currents are caused by thermal expansion. As the air gets warmer, it spreads out and becomes less dense so it rises, the rising air is replaced by cooler air. The cool air begins to warm so it rises. Once it’s a the top, it begins to cool, and become more dense and sink.

6.  Heat Sort

THERMAL ENERGY / TEMPERATURE / HEAT
Total energy of particles in a substance / Measure of the average kinetic energy of the particles in a substance / The transfer of energy between objects at different temperatures
Expressed in joules (J) / Expressed in degrees Fahrenheit (°F), degrees Celsius (°C), and Kelvin (K) / The amount of energy transferred is expressed in joules (J) or calories (cal)
Varies with the mass and the temperature of a substance / Does not vary with the mass of a substance / Varies with the mass, specific heat capacity, and temperature change of a substance.

7.  Applications of heat

a.  Heat engines—use heat to do work . Fuel is burned causing the expanding gases (thermal expansion) to push pistons and propel the machine.

i.  Examples: cars, trains, boats and steam boats, planes

b.  Heat Pump—moves energy from an area of low temperature to an area of high temperature. Requires mechanical work.

i.  Examples: refrigerators, air conditioners, heat pumps