Chapter 6

Thermal Energy

6 – 1 Temperature and Thermal Energy

Temperature

Do not use “HOT” or “COLD” to describe temperature.

Temperature – a measure of the average value of the kinetic energy of the molecules. The Mountain Dew in Mr. Gill’s bottle is

not moving, but the atoms that make it up are moving. We are measuring the kinetic energy of the atoms.

The greater the kinetic energy, the greater the temperature.

Measuring Temperature

We cannot measure the kinetic energy of each and every atom in a material.

We use a thermometer to measure the expansion and contraction of a material. Used to use mercury, now we use

alcohol.

Temperature Scales Fahrenheit, Celsius, Kelvin Water :

Freezing Point -

Boiling Point -

0 C

100 C

32 F

212 F

273K

373K

F C K

Converting

9

5

Kelvin = 273 + C

Kelvin Scale

0 K is the lowest possible temperature. It is called Absolute Zero.

Thermal Expansion When an object warms up :

The atoms move faster The atoms tend to spread out This causes the object to expand.

When an object cools off : The atoms move slower The atoms tend to move closer together This causes the object to contract

( shrink )

The amount of expansion or contraction depends upon the material and the change in temperature. Liquids tend to expand more than

solids

Thermal Energy

Thermal Energy – the sum of the potential and kinetic energy of the atoms in an object. Kinetic Energy comes from the

movement of the atoms. Potential Energy comes from the

attractive force between the atoms.

Thermal Energy vs. Temperature

Thermal Energy and Temperature are different. One does not affect the other.

If you have a glass of water and a pitcher of water, both at room temperature, which has more Thermal Energy ? Pitcher because it has more atoms.

6 – 2 Heat

Heat

Heat is not temperature.

Heat – The thermal energy that flows from something with a higher temperature to something with a lower temperature.

Place your hand on your desk. Your hand has a higher temperature. Thermal Energy flows from your hand

to your desk.

If you were to place an ice cube into a glass of water… Does the ice cube cool the drink ? Does the drink warm the ice cube ? By warming the ice cube, the drink

loses thermal energy and is cooler.

Transferring Thermal Energy

3 Ways Thermal Energy is Transferred :1. Conduction2. Convection3. Radiation

Conduction

Conduction – the transfer of thermal energy through matter by direct contact of particles. Object # 1 to Object # 2 Throughout an entire object

Collisions The particles collide with one another. Momentum is transferred from one

particle to the next.

Efficiency

1. Solids work best because their particles are packed the closest together.

2. Liquids3. Gases

Conductors The ability to easily transfer thermal

energy. Metals

Loosely held electrons make metals work well.

Silver, Copper, Aluminum, Gold

Insulators

Do not easily allow the transfer of thermal energy between particles. Wood, Plastic, Glass, Fiberglass

Convection Convection – the transfer of thermal

energy by the bulk movement of matter. Requires a Fluid Fluid – any material that flows.

Liquids and Gases

Convection Currents

The rotation caused by warmed fluids rising and cooled fluids sinking. Winds ( weather ) Ocean Currents

Radiation Radiation – the transfer of thermal

energy in the form of waves. Radiant Energy – energy that travels in

the form of radiation.

Heat Absorption

Why is it that the pavement is hotter than grass ?

The change in temperature of a material as it absorbs heat depends upon the material it is made of.

Specific Heat

Specific Heat – the amount of thermal energy needed to raise the temperature of 1 kg of a substance 1 °C.

The greater the specific heat, the more energy is needed to change the temperature of a substance. Water has a very high specific heat.

Thermal Pollution Thermal Pollution – the increase in

the temperature of a body of water caused by adding warmer water.

Some electric power plants and factories that use water to cool machinery produce hot water as a by-product.

If the hot water is placed back into its source, it will raise the temperature of the water nearby.

Effects of Thermal Pollution

Higher water temperature requires fish to use more oxygen.

Warmer water contains less dissolved oxygen.

Fish and other organisms can die.

Reducing Thermal Pollution

Cooling towers are used to bring water temperatures back down closer to the temperature of the main body of water.

6 – 3 Engines and Refrigerators

Heat Engine Heat Engine – A device that converts

thermal energy into mechanical energy. Engines in cars, trucks, motorcycles, tractors.

Mechanical Energy – The sum of the kinetic and potential energy of an object. Differs from thermal energy because this is

the energy of the object, not the energy of the atoms that make it up as in thermal energy.

Internal Combustion Engine

The fuel is burned on the inside of the engine in a combustion chamber.

Most cars have an engine that has 4 or more cylinders.

A mixture of fuel and air is injected into the cylinder and ignited by a spark.

The burning fuel produces a gas. The expansion of this gas pushes

the piston down. Each cylinder contains a piston

that moves up and down. The up and down motion of the

piston turns a crankshaft that turns the wheels.

Heat Mover

Heat Mover – A device that moves thermal energy from one location, having a lower temperature, to another location having a higher temperature. A refrigerator, air conditioner, freezer. A refrigerator absorbs thermal energy

from food and carries it to outside the refrigerator.

Air conditioners work under the same principles as a refrigerator. Thermal energy is absorbed from

inside the building and transferred to outside the building.

A heat pump can move thermal energy either into or out of a building.