Thermodynamics

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# Thermodynamics - PowerPoint PPT Presentation

Thermodynamics. SASP 18 th March 2010. Energy. A can of worms – particularly at KS3 Transfer Transport Transform Stores Pathways A problem, but not for today ( Some stuff on wiki, Millar). Temperature and Heat. Light touch today, more another time (in parent language “we’ll see”)

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### Thermodynamics

SASP 18th March 2010

Energy
• A can of worms – particularly at KS3
• Transfer
• Transport
• Transform
• Stores
• Pathways
• A problem, but not for today
• ( Some stuff on wiki, Millar)
Temperature and Heat

Light touch today, more another time(in parent language “we’ll see”)

• Temperature – a measure of hotness or coldness of an object
• Heat
• Energy
• Depends upon
• Mass
• Temperature
• Nature of object (specific heat capacity)
Temperature Scales
• F,R,C,K
• oF is for old people, like pounds and ounces BUT conversion is a skill so lets not dispose of it all together
• R just for some US engineers
• oC not C. Centigrade just means that, we want Celsius, and degress at that.
• K is not oK as it is absolute. small point but important
Lets look at reality – go Macro

When I heat things, they expand

Thermal Expansion

ΔL = k L ΔT

So?

Well the amount something expands when heated depends on how long it was in the first place (L), the amount it’s temperature changes (ΔT) and something to do with the material (k) called the coefficient of thermal expansion.

Two important physics ideas
• The coefficient: A way of making an inequality into an equals. BUT the key thing here is that it is something for a material and NOT an object. Work out k for Copper and you can do the sums for any object made of copper
• The gradient: A driving force behind so much of things happening in physics. ΔT here but could be anything
• ΔK is equivalent to ΔoC but best go the K way
Let’s quantify ‘heating up’

E = m c ΔT

c is specific heat capacity of material

Units= Jkg-1K-1

What happens when state changes?

Possibly not what you might expect

Because of state change m c ΔT isn’t enough

E = m L

L is specific latent heat of fusion/vaporisation

Units= Jkg-1

Change of state

AT CONSTANT TEMPERATURE

E = m c ΔTE = m L

Q = m c ΔTQ = m L

Q = m c ΔT

Q = m L

Q = m c ΔT

Q = m c ΔT

Q = m L

Both L and c are material and not object specific quantities, much more useful.

Thermal transfer of energy

Conduction

• Transferred directly within a material
• ΔT across material is the driving force

Convection

• Transport by bulk movement
• Density, buoyancy, currents
• Free and forced, Newton, T or T5/4

• By means of electromagnetic waves
• The black body
• Stephan
Conduction
• Good conductors (metals) it’s mainly electrons
• Poor conductors it’s mainly inter-atomic collisions

We have idealised models

Because the truth is messy

Thermal conductivity

We can quantify an ideal situation

Q/t = k A ΔT/L

Q/t = Rate of heat flow

k = Thermal conductivity (Wm-1K-1)

A = Cross sectional area

An experimental value

U takes into account the reality of the situation including convection at surface and a slow moving ‘trapped’ layer

• The energy radiated per second
• Area
• Temperature
• Nature of object
• Why T and not ΔT?
• Well, we are all at it. It is just often Qin=Qout
• What comes out?
• A continuous span of wavelengths, dependent upon T
• At T < 1000K almost all IR
• At T > 1000 Visible and UV also (1700K is white hot)

• Area
• Temperature
• Nature of object

As an equation

Q/t = e σ A T4

Q/t = rate of energy emitted by radiation

e = emissivity (B=1 skin=0.7)

σ = SB constant 5.67 × 10-8 Js-1m-2K-4

A = Area

T4= Temperature K

We want to care about particles

Just not yet – stay macro.

Bulk properties = not particle

Work and heat

Work: energy transferred to a system by the application of a force (ΔW)

Heat: energy transferred not by a force and our old friend ΔT is the driving force for this (ΔQ)

Now, we are nearly ready to jump into the world of thermodynamics

Some more terms

Internal energy: Potential energy in bonds and KE of particle motion (ΔU)

Isothermal: You guessed it (ΔT=0)

Now, lets go...

0,1

Zeroth: If Q=0 then ΔT = 0

First: ΔQ = ΔU + ΔW

Signs really matter

• ΔQ = Heat entering
• ΔU = Change in internal energy
• ΔW = Work done BY body

ΔQ

ΔU

ΔW

JPJ

The mechanical equivalence of heat

2

What we normally want is ΔQ going to ΔW

This is sort of the point of most engines

But life isn’t like that and imperfect.

The second law quantifies the imperfection

η = W/Q

• η = efficiency of heat engine
• W = work done by engine
• Q = heat provided to engine