Law of Conservation of Energy

• States that energy can neither be created or destroyed or that the total amount of energy in a system remains constant.

• We can measure the energy in terms of calories.

Calories

• One calorie is defined as the amount of energy needed to raise the temperature of 1 gram of water 1oC

• Abbreviated as (cal)

• Dietary calories (C) = 1000cal or 1kcal = amount of energy released from foods

Heat Energy• Heat - a flow of energy from a hot object

(higher energy) to a cold object (lower energy).  – The most familiar form of energy – All matter contains some heat energy.– The net energy transferred from or to an

object as a result of a temperature difference which results in a change in internal energy

Review Heat Energy

The heat energy of a substance is determined by how active its particles (atoms and molecules) are.

A hot object is one whose atoms and molecules are excited and show rapid movement.

A cooler object's molecules and atoms will be less excited and show less movement.

What is Heat Energy?

• As heat is added to an object the temp. increases

• The added heat becomes part of the object’s internal energy

Heat travels from a hot object (higher energy) to a cold object (lower energy).

So how does heat travel from one object to another?

By Convection, Conduction and Radiation.

Heat Transfer by Convection

• Convection occurs only in liquids and gases. 

• We call liquids and gases fluids. 

• It cannot happen in solids.  • It needs particles to be free

to move about.

• A radiator in a room heats up the room by convection (NOT radiation).

Heat Transfer by Convection

• Look at the engine at the left.

• Water carries heat from a hot engine through a pump and delivers it to a radiator, whose duty is to give up heat to the air.

• The process in which heat passes through a solid substance is called conduction. 

• Metals are good conductors of heat.  • Non-metals are generally bad conductors

of heat. • Liquids and gases are bad conductors of

heat as well. • A bad conductor of heat is called an

insulator.

Heat Transfer by Conduction

Ability of a metal to conduct heat?

• What is an element of metal made of?

Atoms that vibrate

Free electrons that move around

• They are constantly vibrating or oscillating against each other.

• Through these collisions, the heat energy is conducted

• The flow of heat from a hot part of a body to a cooler part, without transfer of matter.

• Can also take place from one body to another, provided the two bodies are in contact, and a temperature difference exists between them.

• A pot on a hot plate is heated by conduction from the stove

surface, via the underside

of the pot.

Heat Transfer by Conduction

Heat Transfer by Radiation

• The easiest way to explain heat transfer by RADIATION is by the heat we feel from the sun or a light bulb.

• RADIATION of heat is the transfer of heat energy through matter or space as electromagnetic waves.

• Examples :– Electromagnetic waves called infra-red radiation that is

emitted from earth.– Ultraviolet radiation that is emitted from the sun and

causes sunburns

– Microwaves that cook food

Review

• Heat flows from hot objects to cold objects.

• Heat flow depends on the temperature difference.

• Heat is not temperature.

• Heat passes by conduction, convection, or radiation.

• Radiation is caused by infra-red radiation

Determining the amount of heat gained or lost by a substance

Step 1• You will need the following formulas for

your calculations:– where T refers to the change in

temperature

– Tf refers to the final temperature and

– Ti refers to the initial temperature

T = T - Tf i

Determining the amount of energy lost or gained

Step 2– where Qcal refers to the heat gained (or lost)

expressed in calories– mass of water is expressed in grams – (1 mL water = 1 gram) T refers to the change in temperature

– Answers are expressed in calories or cal

Q ca l (m a ss o f wa ter)( T)

Q ca l (m a ss o f wa ter)( T)

45 gm sample of water is heated from 24.0ºC to 82.0 ºC

How much heat was gained by the cold water?

∆T = 82.0 – 24.0 = 58.0 ºC

Qcal = 45 g x 58.0 ºC = 2610 = 2600cal

T = T - Tf i

• If a sample of hot water and cold water are mixed together…

• Based on the Law of Conservation of Energy what would you expect?

Q ca l (m a ss o f wa ter)( T)

110.5 gm sample of hot water has an initial temp of 61.8ºC. 250.0 gm of 1.8ºC water is added to it and the final temperature is 6.8ºC

1 - How much heat was gained or lost by the by the cold water?

2 - How much heat was gained or lost by the warm water?

How Do We Measure the Changes in Heat Energy?

Temperature

What is Temperature?

• The measure of the average kinetic energy of the particles (atoms and molecules) of a substance– Kinetic energy is the energy an object posses

because of it’s motion

How do we measure temperature?

With a thermometer.

1. A thermometer is an instrument that utilizes the physical property of thermal expansion for the purpose of accurately determining temperature

What is thermal expansion?

1. The property of a substance to expand with an increasing temperature and

contract with a decreasing temperature

2. The expansion and contracting allows for the quantitative measure of temperature by using a thermometer.

Thermometer

• Most common is the bulb type

• As the substance gets hotter, the particles in the liquid mover faster and spread out

• This causes the red liquid to move up.

• Gives us a quantitative reading.

3 Different Temperature Scales