# 14 Thermal Physics

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## 14.1 Temperature

Thermal Equilibrium is where there is no net transfer of thermal energy between the phases.

Temperature is how hot an object is and is the property that determones whether or no one object is in thermal equilibrium.Is the measurement of the average kinetic energy of particles. Temperature can be measured in both celsuis and kelvin.

Celsuis Scale uses the freezing point and boiling point of water as two fixed points. Any object at 100 degrees celsuis is at thermal equilibrium with boiling water.

Absolute Temperature Scale uses the triple points of water around 0.01 degrees celsuis and absolute zero.

Temperature in kelvin = degrees celsuis + 273.

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## 14.2 Solids, Liquids and Gases

Internal Energy = Kinetic Energy + Potential Energy

The Kinetic Model describes how all substances are made up of atoms or molecules arranged differently.

Solidsatoms or molecules regularly arranged and closely packed together. Has strong electrostatic forces of attraction and can vibrate in their fixed positions around simple harmonic motion. Low Internal Energy,

Liquids - atoms or molecules are still very close together, Has more kinetic energy than a solid and can change position as the flow past each other. Medium Internal Energy,

Gases - atoms or molecules have lots of kinetic energy and free to move past each other. Gases have no electrostatic forces.

Elastic collisions are collisions when total momentum and kinetic energy is conserved.

Inelastic collisions are when kinetic energy is not conserved.

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## 14.2 Solids, Liquids and Gases

In 1905 Albert Einstein explained Brownian Motion in terms of collisions between pollen grains and millions of tiny water molecules. The collisions were elastic and resulted in a transfer of momentum from the water molecules to the pollen grains causing the grains to move in haphazard ways.

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## 14.3 Internal Energy

Internal Energy - is defined as the sum of the radomly distributed kinetic and potential energies of atoms or molecules within the substance.

By increasing the temperature of a body it will increase its internal energy. When a substance changes phase e.g from a solid to a liquid temperature or kinetic energy does not increase. The electrostatic potential energy increases significantly.

• Gases - no electrostatic potential energy.
• Liquids - electrostatic potential energy is negative.
• Solids - electrostatic potential energy will give a large negative value.
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## 14.4 Specific Heat Capacity

is the energy required per unit mass to change the temperature by 1 K OR 1 C, Has the units of J kg-1 K-1.

Energy = mass x specific heat capacity x change in temperature.

Determining specific heat capacity

Energy transferred to heater - is current x voltage x time

c = ivt/change in temperature x mass

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## 14.5 Specific Latent Heat

Specific Latent Heat is the amount of energy needed to turn 1kg of liquid into gas.

Latent Heat = Energy/Mass

When the substance changes from a solid to the liquid phase it is referred to as the specific latent heat of fusion.

When the substance changes from a liquid to the gas phase it is referred to as the specific latent heat of vapourisation.

Finding the specific latent heat of fusion

L = current x voltage x time/mass

Current is measured using the ammeter, voltage is measured using a voltmeter and time is measured using a stopwatch (how fast the ice melted). Mass is weighed using top pan balance of the melted ice.

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## 14.5 Specific Latent Heat

The specific latent heat of vapourisation

Is the energy required to change 1 kg of substance from its liquid phase to its gaseous phase at its boiling point is often considerably more thant its specific latent heat of fusion.

E = IVT/m

mass is the substance changed phase during heat.

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