THERMAL ENERGY AND HEAT+ CONSERVATION OF ENERGY

Lesson 3

THERMAL ENERGY AND HEAT

Thermal energy and heat play significant roles in our lives from the furnaces that heat our homes to winds generated by the uneven heating of the Earth’s surface. Even most of the food that we consume is converted into thermal energy.

THERMAL ENERGY

The total kinetic energy and potential energy of the atoms or molecules of a substance.

Depends on: mass, temperature, nature and state of matter

HEAT

A measure of the energy transferred from a warm body to a cooler body because of a difference in temperature.

TEMPERATURE

A measure of the average kinetic energy of the atoms or molecules of a substance.

EXAMPLE 1

- Consider two samples of water: 100 g at 50 °C and 500 g at 50° C50° C

50° C

50° C

EXAMPLE 1

The two samples have the same temperature, but the bigger sample contains more thermal energy because there’s more of it. If the samples were mixed, no heat transfer would occur because they are the same temperature50° C

50° C

50° C

EXAMPLE 2 -

Consider two other samples of water: 500 g at 50 °C and 500 g at 90° C

50° C 90° C 70° C

EXAMPLE 2 -

The masses are the same but the warmer sample has more thermal energy because the water particles have more motion, that is, the average kinetic energy of the molecules is greater at a higher temperature. When the two samples are mixed, heat would transfer from the 90 °C sample to the 50° C. 50° C 90° C

70° C

METHODS OF HEAT TRANSFER

There are three methods that heat can transfer from a warmer body to a cooler body: Radiation, Conduction, and Convection.

Radiation - the emission of energy as electromagnetic waves. When radiant energy encounters particles of matter, it may be reflected or absorbed. Absorbed energy increases the motion of particles.

An increase in kinetic energy increases the temperature of the matter.

Any substance at a higher temperature than its surroundings will emit radiant energy, usually as infrared radiation. The warmed matter then transfers some of its thermal energy to substances at lower temperatures or re-emits it as IR.

CONDUCTION

- The transfer of thermal energy through direct contact between the particles of a substance, without moving the particles to a new location.

Thermal energy transfer by conduction usually takes place in solids.

During conduction, particles with more kinetic energy transfer some of their energy to neighbouring particles with lower kinetic energy increases the kinetic energy of the neighbouring particles. Metals are the best heat conductors as their electrons can vibrate more freely than those of other substances.

CONVECTION

- The transfer of thermal energy through the movement of particles from one location to another. Thermal energy transfer by convection usually occurs in gases and liquids.

During convection, the movement of the particles forms a current, which is a flow, from one place to another in one direction. Liquid water has a high heat capacity which means that it takes a lot of energy to increase the temperature of a mass of water.

Since Earth’s surface is over 70 percent water, water has a large effect on Earth’s climate. Therefore, regions closer to large bodies of water tend to experience more moderate weather conditions than regions farther from them.

HTTP://WWW.YOUTUBE.COM/WATCH?V=VZE_IPEDUJC

CONSERVATION OF ENERGY

When energy is transformed from one form to another form the energy is said to be conserved. None of the energy disappears and no new energy suddenly appears.

THE LAW OF CONSERVATION OF ENERGY

When energy changes from one form to another, no energy is created or destroyed.

This law applies to all energy transformations. In ideal situations, just as in ideal machines, no energy is lost to friction.

However, some energy is usually needed to overcome friction. This results in the production of thermal energy and, sometimes sound energy.

Energy-transformation equations are more complete when they include thermal energy and in some cases sound energy as well.

Consider the operation of a pile driver. The overall goal of the energy transformation is the work done on the pile.

The energy-transformation equation is listed below.

Echem W1 + EChem Eg EK + Etherm W2 + Etherm + Esound

Echem = chemical potential energy released in the burning of fuel in the engine

W1 = Work done by the force on the hammer to raise it to Max Eg

Etherm = Thermal energy resulting from heat losses in the engine and from friction forces.

Eg = Max gravitational potential energy of the hammer at its highest position

Ek = Max kinetic energy of the hammer right before it hits the pile

W2 = Work done by the hammer as it pushes downward on the pile

Esound = Sound energy resulting from the operating the engine, and the collision of the hammer and the pile.

QUESTIONS

Explain the difference between thermal energy, and the temperature of a metal coin.

A parent places a baby bottle containing 150 mL of milk at 7 ° C into a pot containing 550 mL of water at 85 ° C. Qualitatively compare the average kinetic energy of

the milk and the water. Qualitatively compare the thermal energy of the

milk and the water. Is there a stage at which the heat will stop

transferring from the water to the milk? Explain

Explain your answer to each question. Would it be better to place an electric heater near

the floor or near the ceiling? Would it be better to place an air conditioner vent

near the floor or ceiling? Why are fans placed near the ceilings in tall

rooms? A ball is dropped vertically from a height of

1.5 m and bounces back to a height of 1.2 m. Does this violate the law of conservation of energy? Explain.

A 0.20 kg ball is held at rest 2.2 m above the ground, and then it is dropped. Apply the law of conservation of energy to determine the ball’s speed after it has fallen.

A) 1.1 m and B) 2.2 m. Ignore air resistance. C) As the ball falls, what type of energy

does its gravitational potential energy change into?