# Thermal Physics

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• Created by: CPev3
• Created on: 15-07-20 09:12

## What is the triple point of a substance?

One specific temperature and pressure at which all three phases of the substance can exist in thermodynamic equilibrium

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## What is thermodynamic equilibrium?

There is no net transfer of thermal energy between the phases

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• 0.01 oC
• 0.61 kPa
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## When one object is hotter than another:

Net flow of thermal energy from hotter object into colder object

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......Temperature of colder object increases

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............Temperature of hotter object decreases

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## When two objects are in thermodynamic equilibrium:

• No net flow of thermal energy between them

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• Objects must be at the same temperature
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## What is the Celsius scale?

For pure water

• Freezing point = x oC
• Boiling point = x + 100 oC

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For pure water when the atmospheric pressure is 1.01 x105 Pa

• Freezing point = 0 oC
• Boiling point = 100 oC

.Any object at 100 oC must be in thermal equilibrium with boiling water

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## What is the absolute temperature scale?

• Uses the triple point of water (273 K) and absolute zero (0 K) as its fixed points

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• A temeprature increase of 1 K = a temeprature increase of 1 oC

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• T (K) ≈ θ (oC) + 273
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## What is the kinetic model?

A model that describes all substances as made of atoms, ions or molecules, arranged differently depending on the phase of the substance

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## Particles in solids:

Spacing

• Strong electrostatic forces of attraction
• Packed closely together

Ordering

• Regularly arranged

Motion

• Kinetic energy
• Vibrate around their fixed positions
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## Particles in liquids:

Spacing

• Weaker electrostatic forces of attraction
• Very close together

Ordering

• Irregularly arranged

Motion

• More kinetic energy
• Flow past each other
• No fixed shape
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## Particles in gases:

Spacing

• Almost no electrostatic forces of attraction
• Far apart

Ordering

• Irregularly arranged

Motion

• Much kinetic energy
• Move randomly with different (high) speeds in different directions
• No fixed shape or volume
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## Particles in a heated solid:

Gain kinetic energy

Vibrate more vigorously

Break away from solid structure

Flow past each other

Change in phase to liquid

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## Particles in a heated liquid:

Gain kinetic energy

Enough for some to break away from the other particles

Particles which escape body of liquid become a gas

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## Density:

Affected by spacing between particles

Generally highest in solids and lowest in gases

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Solid water less dense than liquid water

Water freezes into regular crystalline pattern

Held together by strong electrostatic forces of attraction between particles

Particles held slightly further apart than in their random arrangement in liquid water

Slightly less dense

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## What is Brownian motion?

• The continuous random motion of small particles suspended in a fluid

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• Visible under a microscope
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## Pollen grains floating on water:

• Collisions betwen pollen grains and water molecules are elastic

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• Result in transfer of momentum from molecules to grains

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• Causes grains to move in haphazard ways

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• Proves that matter is made up of particles and they have kinetic energy
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## Smoke cell:

• Smoke particles in random motion

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• Caused by collisions with air molecules

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• Air molecules in random motiion

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• Mean kinetic energy of particles = that of molecules

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• HOWEVER, particles more massive and thus slower than molecules
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## What is specific heat capacity?

The energy required per unit mass to change the temperature by 1 K (or 1 oC)

c = E / (m x Δθ)

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4200 Jkg-1K-1

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## Experiment for determining c:

Find mass of block using mass balance

Connect heater in series with ammeter, power supply and variable resistor

Connect voltmeter in parallel around heater

Put heater and thermometer in block

Turn on power supply and start stopwatch

Record values for current and voltage (use variable resistor to keep current constant)

Record temperature at regular intervals for 10 mins

Turn off power pupply

Continue to record temperature for 5 mins

Plot results on θ-t graph

c = P / (m x gradient)

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## Method of mixtures:

mblock x cblockΔθblockmwater x cwater x Δθwater = E

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## What is specific latent heat?

The energy required to change the phase per unit mass while at a constant temperature

L = E / m

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## What is specific latent heat of fusion?

The energy required to change unit mass of a substance from solid to liquid while at a constant temperature

Lf = E / m

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## What is specific latent heat of vaporisation?

The energy required to change unit mass of a substance from liquid to gas while at a constant temperature

Lf = E / m

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## Experiment for determining L of f:

Connect heater in series with ammeter, power supply and variable resistor

Connect voltmeter in parallel around heater

Put heater and thermometer in funnel filled with ice cubes over beaker

Wait for rate of melting of ice cubes to be steady, then wait 10 mins (stopwatch necessary)

Find mass of water using mass balance

Turn on power supply

Record values for current and voltage (keep current constant using variable resistor)

Wait for rate of melting of ice cubes to be steady, then wait 10 mins (stopwatch necessary)

Find mass of water using mass balance

Lf = electrical energy supplied / difference between two recorded masses of water

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## Experiment for determining L of v:

An electrical heater can be used with a condenser to collect and then measure the mass of liquid that changes phase

Lf = IVt / m where m is the mass of liquid that changed phase during heating

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## θ-t graph- energy transferred calculations:

1. Heating the solid to its melting point: E = mcsolidΔθ

2. Melting the solid at a constant temperature: E = mLf

3. Heating the liquid to its boiling point: E = mcliquidΔθ

4. Boiling the liquid at a constant temperature: E = mLv

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## What is absolute zero?

• The lowest possible temperature

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• The temperature at which substances have minumum internal energy
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## What is internal energy?

The sum of the randomly distributed kinetic and potential energies of the atoms/ ions/ molecules within the substance

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## Equation for final temperature:

T = (MCTh + mcTc) / (MC + mc)

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