Thermal Energy, Specific Heat and Heat TransferChapter 6

Temperature and Thermal EnergyWhat’s the difference?Temperature IS NOT HOW HOT

OR COLD SOMETHING IS!Temperature is defined as the

measure of the average kinetic energy of the individual particles in an object/substance. ◦What is kinetic energy? ◦What does average mean?

TemperatureThe higher the average KE, the higher the

temperatureWhich substance has a lower temperature?

Temperature and Thermal EnergyWhat’s the difference?

Thermal energy is defined as the total energy of all the particles in an object/substance.◦What differences do you see in these

definitions?

Temperature and Thermal Energy

TEMPERATURE IS a measure of the average kinetic energy of the molecules in a substance.

Thermal energy is defined as the total energy of all the particles in a substance

Adding thermal energy to a substance increases the average kinetic energy of the molecules and therefore causes a rise in temperature.

Higher temperature = faster molecular motionLower temperature = slower molecular motion

Temperature VS Thermal EnergyYou have two containers with water at

90ºCOne container has a mass of 10 g, and

the second has a mass of 5 g. ( there’s more substance in one container than another!)

They are at the same temperature, but the

10 gram sample has more thermal energy

TE depends upon mass, but temperature does not.

CELSIUS SCALE is used to measure temperature in the metric system.

CELSIUS FAHRENHEIT boiling 100 degrees point 212 degrees

(water)

human 37 degrees body 98.6 degrees

temp

freezing 0 degrees point 32 degrees

(water)

A Question to Consider…

The water in both of the beakers below is at a temperature of 30 degrees Celsius. Which has more thermal energy? Explain your answer.

50 mL100 mL

BA

Heat

a. The flow of thermal energy from one object to another.

b. Heat always flows from warmer to cooler objects. Ice gets

warmer while hand gets

cooler

Cup gets cooler while hand gets

warmer

3 Types of Heat Transfer

ConductionConvectionRadiation

Conduction Conduction heat transfer is the flowing of heat

energy from a high-temperature object to a lower-temperature object.

Why does metal feel colder than wood, if they are both at the same temperature?

Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

Convection

Water movement

Hot water rises

Cooler water sinks

Convection current

Cools at the surface

Radiation

The third method of heat transfer

How does heat energy get from the Sun to the Earth? There are no particles

between the Sun and the Earth so it CANNOT travel by conduction or by convection.

?RADIATION

Radiation The transfer of energy by electromagnetic waves.

Examples:

SunFireLight bulb

Specific Heat

a. Some things heat up or cool down faster than others.

Land heats up and cools down faster than water

b. Specific heat is the amount of heat required to raise the temperature of 1 kg of a material by one degree (C or K).

1) C water = 4184 J / kg C

2) C sand = 664 J / kg C

This is why land heats up quickly during the day and

cools quickly at night and why water takes longer.

Why does water have such a high specific heat?

Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds

and do not need as much energy to break them.

water metal

How to calculate changes in thermal energy

Q = m x T x Cp

Q = change in thermal energy

m = mass of substance (kg or g)

T = change in temperature (Tf – Ti)

Cp = specific heat of substance (J/kgC or J/gC)

Calculating Specific HeatIt takes 487.5 J to heat 25 grams of copper from 25

°C to 75 °C. What is the specific heat in Joules/g·°C?

Q = m x T x Cp

Thermal ExpansionThe increase in the volume of a substance because

of an increase in the temperature.

As a substance gains thermal energy the particles move faster and spread out causing the substance to expand.

Other Thermal Expansion ExamplesExpansion joints on the highwayThermostatsHot Air Balloons

Read p. 278 – 279 and describe in a few sentences how each of these examples is related to thermal expansion.

Expansion Joints and Thermal ExpansionEngineers leave

space in the road to allow for thermal expansion of the pavement during the summer time.

During the summer the road expands and closes these gaps. In the winter the road contracts and gaps appear.

Thermostats and Thermal ExpansionA bimetallic strip is made up of two types of metal that gain thermal

energy at different rates.As the bimetallic strip gains thermal energy it begins to expand and

because the two metals do not expand at the same rate the strip will bend.

As it bends it works to open and close the circuit to turn the heater on and off.

Hot Air Balloons and Thermal ExpansionWhat types of objects

float in water?Objects less dense

than water float in water.

This is the same reason hot air balloons float!

The gas in the balloon is heated and becomes less dense than the air around the balloon so the balloon begins to float!