Thermodynamics

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TemperatureHeat Transfers

Thermodynamics & HeatThermal Equilibrium

Specific Heat

Temperature

TEMPERATURE

is a measure of the

average kinetic (moving)

energy per molecule.

The hotter something is, the faster its molecules are

moving or vibrating, and the higher its temperature.

The hotter something is, the faster its molecules are

moving or vibrating, and the higher its temperature.

The more the molecules move, the more they “bump”

into each other. These collisions mean the molecules are

further apart.

How is Temperature Measured?

Temperature is measured in 3 scales:

CelsiusFahrenheit Kelvin

A thermometer is any device which, through marked scales,

can give an indication of its own temperature.

Converting between units.

The Celsius scale (°C) is used for

common temperature measurements in

most of the world. 0°C being defined by

the freezing point of water, with

additional degrees defined so

that 100°C was the boiling point of

water.

Kelvin is used in scientific calculations,

since it begins at absolute zero, meaning

it has no negative numbers.

Fahrenheit is commonly used in the

United States.

1000C 2120F

00C 320F

Temperature Scales

The lower fixed point is the

ice point, the temperature at

which ice and water coexist

at 1 atm of pressure:

00C or 320F

The upper fixed point is the

steam point, the temperature

at which steam and water

coexist at 1 atm of pressure:

1000C or 2120F

Freezing Point

Boiling Point

Temperature Conversions

Use Celsius as a bridge between K and F.

Convert 85 F° to Celsius and Kelvin

C = (°F – 32) / (1.8)

C = (85°F – 32) / (1.8)

C = (53) / (1.8)

C = 29.44 °C

K = °C + 273

K = 29.44°C + 273

K = 302.44 K

Convert 12°C to Fahrenheit and Kelvin

F = (°C x 1.8) + 32

F = (12°C x 1.8) + 32

F = 53.6

K = °C + 273

K = 12°C + 273

K = 285 K

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REVIEWING TEMP SCALES

3 Temperature ScalesCelsius Fahrenheit Kelvin

°F = °C (1.8) + 32

°C = (°F – 32) / (1.8)

K = °C + 273

°C = K – 273

3 Heat Transfers

Conduction

Convection

Radiation

Heat Transfer by Conduction

Conduction is the process by which heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move.

Conduction

Direction

From hot

to cold.

Conduction – Transferring heat by physical contact (touching)

Heat Transfer by Convection

Convection is the process by which

heat energy is transferred by the

actual mass motion of a heated fluid.

Convection

Heated fluid rises and is then

replaced by cooler fluid, producing

convection currents.

Convection –Transferring heat by fluid (flowing) motion (fluid is

liquid or gas only! )

Heat Transfer by Radiation

Radiation

Sun

Atomic

Radiation is the process by which heat

energy is transferred by

electromagnetic waves. May not be

seen, but can be felt. This type of heat

transfer does not require a medium

Radiation – Transferring heat by electromagnetic

waves (no medium is required)

Kinds of Heat Transfer

Consider the operation of a typical pot of water:

Think about how heat is transferred by:

Conduction?

Convection?

Radiation?

Heat Transfers differently in different materials

Does allow the flow

of heat (or energy)

Examples:

Metals – (iron,

copper, steel)

Does Not allow the

flow of heat (energy)

Examples:

Plastic, paper, wool,

wood.

Best to Worst Conductors Best to Worst Insulators

Solid Liquid Gas Gas Liquid Solid

Quantity of Heat

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Heat

Heat is defined as the transfer of thermal energy

that is due to a difference in temperature.

The Hotter something is.. The more energy it has.

The Unit of Heat

When we calculate heat, we measure it in

Joules.

Joules is a standard unit for energy.

Heat Capacity

The heat capacity of a substance is the heat required to raise

the temperature a unit degree. Every substance has a

different heat capacity

Lead Glass Al Copper Iron

Heat capacities based on time to heat from zero

to 1000C. Which has the greatest heat capacity?

37 s 52 s 60 s 83 s 90 s

1000C 1000C 1000C 1000C 1000C

Heat Capacity is the amount of heat that “fits” in a

substance before it increases its temperature.

• A substance with a low heat capacity – heats

up quick

• A substance with a high heat capacity can take

A LOT of heat – heats up slowly.

Something good to know:

The specific heat capacity of water

is: 4186 Jkg/C

Specific Heat Capacity

Used to solve for the heat lost or gained by a substance

Solve for Heat

Energy (J) Q = mcΔT

Q: Heat Energy (J)

m: Mass (kg)

c: Specific Heat

ΔT : Change in Temp

IF heat (Q) is positive, then it GAINED heat. IF heat (Q)

is negative, it LOST heat.

Thermodynamics & Heat

Thermodynamics is the branch of Physics that deals

with the relationships and conversions between heat

and other forms of energy.

3 Laws of

Thermodynamics

(we only cover the first two)

Energy cannot be created or destroyed,

only TRANSFORMED.

Heat Gained = Heat Lost

Law #1 : Conservation of Energy

Law #1 : Conservation of Energy

Types of Energy:

Thermal

Mechanical

Electrical

Magnetic

Sound

Solar

Chemical

Elastic Energy

Nuclear

Gravitational

Kinetic

Potential

When energy changes from one form to another, the

entropy will increase. The heat energy will always

flow from HOT to COLD.

Law #2 : Energy Flow

Entropy – A

measurement of the

disorder of a system

Entropy means disorder!

The universe wont help

you!

Thermal Equilibrium “Heat” “Equal”

“Having equal heat”

Two objects are in thermal

equilibrium if and only if they

have the same temperature.

Thermal Equilibrium

Hot coals

Cool

water

The heat energy of the coals is

transferred to the water

After some time, both

the coals and water are

the same temperature

Heat ONLY flows if the two objects have a

different temperature

What will be the final temperature of the coals/water?

100 °F

20 °FTemp: ___

Heat ONLY flows if the two objects have a

different temperature

Hmm.. Did we get it?

40 C 80 C

In which way would energy flow?

50 C 50 C

80 C

120 C

No Flow

Hand Pizza

Hand Pizza

Misconceptions

Students believe energy gets “used up” because energy is converted to

heat and is difficult to identify

Students believe heat and temperature are the same instead of heat

being a transfer of energy and temperature measuring average kinetic of

particles

Students believe that thermal energy moves from “cold areas to warm

areas instead of conforming to the 2nd law of thermodynamics

Students believe that only objects that feel hot have thermal energy

rather than all matter with temperature above absolute zero has thermal

energy