Thermal Energy & Thermodynamics

Heat Heat – The transfer of thermal

energy from one object to another because of a temperature difference.

Flows spontaneously from hot objects to cold objects

Heat Heat – The transfer of thermal

energy from one object to another because of a temperature difference.

Flows spontaneously from hot objects to cold objects

Temperature Measure of how hot or cold an

object is compared to a reference point.

Celsius Scale: Boiling and freezing points of water

Kelvin Scale: Absolute zero

Zeroth Law of Thermodynamics

If objects A and B are separately in thermal equilibrium with a third object, C, then A and B are in thermal equilibrium with each other.

Allows a definition of temperature

Temperature from the Zeroth Law Two objects in thermal equilibrium

with each other are at the same temperature

Temperature is the property that determines whether or not an object is in thermal equilibrium with other objects

Thermometers Used to measure the temperature of an

object or a system Make use of physical properties that

change with temperature Many physical properties can be used

volume of a liquid length of a solid pressure of a gas held at constant volume volume of a gas held at constant pressure electric resistance of a conductor color of a very hot object

Celsius Scale Temperature of an ice-water mixture is

defined as 0º C This is the freezing point of water

Temperature of a water-steam mixture is defined as 100º C This is the boiling point of water

Distance between these points is divided into 100 segments or degrees

Kelvin Scale When the pressure of a gas goes to

zero, its temperature is –273º C This temperature is called absolute zero This is the zero point of the Kelvin scale

–273º C = 0 K To convert: TC = TK – 273

The size of the degree in the Kelvin scale is the same as the size of a Celsius degree

Pressure-Temperature Graph

All gases extrapolate to the same temperature at zero pressure

This temperature is absolute zero

Some KelvinTemperatures Some

representative Kelvin temperatures

Absolute zero has never been reached

Comparing Temperature Scales

Temperature Determined by the kinetic energy

of the particles in an object.

More thermal energy = faster motion of particles

Faster moving particles lose energy to slow moving particles in collisions

Thermal Energy Depends on:

Mass Temperature Phase

Thermal Energy vs. Temperature Which has more thermal energy, a

beaker of hot water or the water in my water bottle?

Thermal energy depends on mass

Kinetic Theory of Gases The number of molecules in the

gas is large and the average separation between them is large compared to their dimensions

The molecules obey Newton’s laws of motion, but as a whole they move randomly

Kinetic Theory of Gases – cont. The molecules interact only by

short-range forces during elastic collisions

The molecules make elastic collisions with the walls

The gas under consideration is a pure substance, all the molecules are identical

Specific Heat The amount of energy needed to raise

the temperature of one gram of a material by one degree Celsius.

Water – 4.18 J/g·oC Plastic (polypropylene) – 1.84-2.09

J/g·oC Iron – 0.449 J/g·oC

Specific Heat

In setting up an aquarium, the heater transfers 1,200kJ of heat to 75,000g of water. What is the increase in the water’s temperature?

A Consequence of Different Specific Heats Water has a high

specific heat compared to land

On a hot day, the air above the land warms faster

The warmer air flows upward and cooler air moves toward the beach

Heat Compared to Internal Energy Important to distinguish between

them They are not interchangeable

They mean very different things when used in physics

Internal Energy Internal Energy, U, is the energy

associated with the microscopic components of the system Includes kinetic and potential energy

associated with the motion and position of the atoms or molecules

Heat Heat is the transfer of energy

between a system and its environment because of a temperature difference between them The symbol Q is used to represent the

amount of energy transferred by heat between a system and its environment

Units of Heat Calorie

An historical unit, before the connection between thermodynamics and mechanics was recognized

A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C .

A Calorie (food calorie) is 1000 cal

Units of Heat, cont. US Customary Unit – BTU BTU stands for British Thermal Unit

A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63° F to 64° F

1 cal = 4.186 J This is called the Mechanical

Equivalent of Heat

Calorimeter One technique for determining the

specific heat of a substance A calorimeter is a vessel that is a

good insulator which allows a thermal equilibrium to be achieved between substances without any energy loss to the environment

Calorimetry Analysis performed using a calorimeter Conservation of energy applies to the

isolated system The energy that leaves the warmer

substance equals the energy that enters the water Qcold = -Qhot Negative sign keeps consistency in the sign

convention of ΔT

Phase Changes A phase change occurs when the physical

characteristics of the substance change from one form to another

Common phases changes are Solid to liquid – melting Liquid to gas – boiling

Phases changes involve a change in the internal energy, but no change in temperature

Latent Heat During a phase change, the amount of

heat is given as Q = ±m L

L is the latent heat of the substance Latent means hidden L depends on the substance and the nature

of the phase change Choose a positive sign if you are adding

energy to the system and a negative sign if energy is being removed from the system

Latent Heat, cont. SI units of latent heat are J / kg Latent heat of fusion, Lf, is used for

melting or freezing Latent heat of vaporization, Lv, is

used for boiling or condensing

Sublimation Some substances will go directly

from solid to gaseous phase Without passing through the liquid

phase This process is called sublimation

There will be a latent heat of sublimation associated with this phase change

Graph of Ice to Steam

Methods of Heat Transfer Need to know the mechanisms

responsible for the transfer Methods include

Conduction Convection Radiation

Conduction The transfer of thermal energy

with no transfer of matter.

Between particles in the same material

Between materials that are in contact with each other.

Conduction The transfer can be viewed on an

atomic scale It is an exchange of energy between

microscopic particles by collisions Less energetic particles gain energy during

collisions with more energetic particles Rate of conduction depends upon the

characteristics of the substance

Conduction example The molecules vibrate

about their equilibrium positions

Particles near the stove coil vibrate with larger amplitudes

These collide with adjacent molecules and transfer some energy

Eventually, the energy travels entirely through the pan and its handle

Conduction, cont. In general, metals are good conductors

They contain large numbers of electrons that are relatively free to move through the metal

They can transport energy from one region to another

Conduction can occur only if there is a difference in temperature between two parts of the conducting medium

Thermal Conductors and Insulators Thermal Conductor – A material that

easily conducts thermal energy Metal

Thermal Insulator – A material that does not conduct thermal energy well. Wood Air Rubber

Convection The transfer of thermal energy

when particles of a fluid move from one place to another

Convection Current – Fluid that circulates in a loop as it heats up and cools down.

Convection example Air directly above

the flame is warmed and expands

The density of the air decreases, and it rises

The mass of air warms the hand as it moves by

Convection Current Example The radiator warms

the air in the lower region of the room

The warm air is less dense, so it rises to the ceiling

The denser, cooler air sinks

A continuous air current pattern is set up as shown

Global Ocean Currents

Radiation The transfer of energy by waves

moving through space.

All objects radiate energy

The higher an object’s temperature, the more energy it radiates.

Radiation Radiation does not require physical

contact

All objects radiate energy continuously in the form of electromagnetic waves due to vibrations of the molecules

Radiation example

The electromagnetic waves carry the energy from the fire to the hands

No physical contact is necessary Cannot be accounted for by conduction

or convection

Applications of Radiation Clothing

Black fabric acts as a good absorber White fabric is a better reflector

Thermography The amount of energy radiated by an object

can be measured with a thermograph Body temperature

Radiation thermometer measures the intensity of the infrared radiation from the eardrum

1st Law of Thermodynamics Energy is conserved.

Law of Conservation of Energy

First Law, Equation If a system undergoes a change

from an initial state to a final state, then U = Uf – Ui = Q + W Q is the energy transferred to the

system by heat W is the work done on the system U is the change in internal energy

First Law – Signs Signs of the terms in the equation

Q Positive if energy is transferred to the system by

heat Negative if energy is transferred out of the system

by heat W

Positive if work is done on the system Negative if work is done by the system

U Positive if the temperature increases Negative if the temperature decreases

2nd Law of Thermodynamics Thermal energy will flow from a

warm object to a cold object.

To get energy to flow from cold to warm, additional energy must be inputted into the system.

3rd Law of Thermodynamics Absolute zero cannot be reached.

-273.15oC All molecular motion stops

End of the universe!

Thermal Expansion The thermal expansion of an object is a

consequence of the change in the average separation between its constituent atoms or molecules

At ordinary temperatures, molecules vibrate with a small amplitude

As temperature increases, the amplitude increases This causes the overall object as a whole to

expand

Linear Expansion For small changes in temperature

, the coefficient of linear expansion, depends on the material

o o oL L T or L L T T

Applications of Thermal Expansion – Bimetallic Strip

Thermostats Use a bimetallic strip Two metals expand differently

Since they have different coefficients of expansion

Area Expansion Two dimensions

expand according to

is the coefficient of area expansion

,2

oA A t

Volume Expansion Three dimensions expand

For liquids, the coefficient of volume expansion is given in the table

3,solidsfortVV o

Unusual Behavior of Water

As the temperature of water increases from 0ºC to 4 ºC, it contracts and its density increases

Above 4 ºC, water exhibits the expected expansion with increasing temperature

Maximum density of water is 1000 kg/m3 at 4 ºC