# Physics Unit 5

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• Created by: stesar
• Created on: 23-11-17 18:19

## Work Done and Power

Work Done = amount of energy transferred when carried out

Work Done = Force x Distance

Power

The rate of doing something per unit time - rate is the amount of work done in a specific time period

1 Watt = 1 Joules/ Second

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## Thermodynamics

Pressure; temperature; volume; heat are all related.

The change in pressure of the gas, the conditions need to be changed. E.g to increase the volume of the gas, heat would need to be applied as it would then expand. E.g 2 = to change the pressure, weight would need to be removed.

Energy in a system = equal to work that is done +/- heat that flows through in/ out of a system

DELTA U = Q - W

Q = work done

W = heat

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## Heat Engines

Chemical energy -> Thermal energy -> Mechanical Energy

Gas heated = expand, gas is confined = increase in pressure

Pressure exerts force on surface of the pistol causing it to force

(fuel) gas heated (using spark plugs) = it expands

Gas is confined = it increases in pressure

Pressure exerts a force on the surface of the piston, causing it to move

Car then moves

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## Heat Engines

It's impossible to extract an amount of heat, Qh, from a hot reservoir and use it all to do the work. Some amount of heat must be exhausted to a cold reservoir.

W/ Qh

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## Steam Engines

Fuel (coal) heats the water, which is converted into steam = pressure is then used to drive the piston

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## Fridges and Air Conditioning

Fridge = gas is compressed, it's temperature increases. The Hot gas then transfers heat into its surrounding environment. When the compressed gas is allowed to expand, its temperature becomes colder.

Pressure = force / area
(Pa)        N     /   m2

The pressure of a gas is caused by collisions (momentum transfer) of the molecules of the gas with walls of the container.

The size of the pressure is related to how hard/ how often the molecules hit the wall.

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Equal volumes of gasses under conditions of equal temperature and pressure contain the same number of molecules.

Used to determine the number of molecules in any substance (6.023x10(23))

Kinetic Theory

Zero Energy - absolute zero = -273 degrees / 0 K

• No speed
• No collisions - no pressure
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## Boyle's Law

Physical Characteristics of Gases

• Gas will take the shape of the container and assume the volume
• Most compressible state of matter
• Will mix evenly and completely when confined to the same container
• How much lower densities than liquids and solids

Pressure in Gases

• Random motion
• Molecules hit container, they exert a force, hence a pressure on the walls in the container.

For a constant amount of gas at a constant temperature as the pressure increases, its volume will decrease.

Pressure is inversely proportional to volume e.g. pressure increases, volume decreases.

If the gas is kept at the same temperature, the average speed of the molecules is the same

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## Boyle's Law

If the number of particles are squeezed into a smaller volume, they will hit the container walls more frequently.

Each collision provides a force so more collisions per second increase the average force on the wall container walls, hence the increasing the pressure.

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## Ideal Gas Law

n = PV/ RT = will always equal 1

PV = nRT

P - atm, Torr, Pa, mmHg
V - L, m2
n - mol
R - 0.082057L*atm mol-1 k-1, 8.3145 m3 Pa mol-1 K-1
T - K (Kelvin)

A gas constant will change when dealing with different units of pressure and volume. The different units/ volume correlates accordingly with the units given.

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## Ideal Gas Law

PV = NkT

N - number of particles in a gas

k - Boltzmann gas constant (1.3807x10(-23)JK-1)

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## Energy Potential Energy

Energy is stored in an extended or compressed spring

If the material has been strained elastically (limit has not been exceeded) the energy can be recovered.

If the material has been plastically deformed, some of the work done has been

Strain - how much it extends by tensile or compressive forces
Strain - extension x original length

Stress - force applied per unit cross-sectional area of the wire
Stress = force / cross-sectional area

Young Modulus - the ratio of stress to strain for given material, resulting from tensile forces, provided Hooke's Law is obeyed.

Ultimate Tensile Strength (breaking stress) - the breaking stress of a material

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## Energy Potential Energy - Equations

Stress = force / area

Strain = extension / original length

Young Modulus = stress / strain

Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrations such as the surface.

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## Pistons

Temperature increases = more kinetic energy, so they move around more, so they are more likely to hit the walls. Pressure = how much the particles hit the walls. Therefore, increasing the temperature will increase the pressure.

Volume decreases = smaller space = particles collide more.

Both of the above result in the pressure increasing.

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## Isothermic Process

Process is the overall temperature of a system remains constant despite energy going in and out e.g. melting, evaporation, condensation, freezing

Temperature doesn't change, but the state does.

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Doesn't involve the transfer of heat or matter in or out of the system. E.g. rapid compression and expansion, sound waves, explosion = piston is compressed so that there is no heat exchange, temperature and internal energy is raised.

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## 2nd Law of Thermodynamics

All energy changes = if no energy enters or leaves the system, energy at the beginning will be more than the end.

IMPOSSIBLE TO HAVE A PERFECT ENGINE

Maximum efficiency can be achieved by the Carnot efficiency.

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## Fridge

In order to get the gas towards the back of the fridge, work must be done. Although not all of the energy would be used/ transferred as this is impossible

Efficiency

1 - Qout/ Qin

Max. Efficiency

1 - Tc/ Th

Coefficiency of Performance - Ratio to useful heating or cooling provided to work required

COP = Q / W

Q = useful energy

W = work done

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## Specific Heat Capacity

TRIANGLE Q = mcTRIANGLET

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## Specific Latent Heat of Fusion/ Vaporisation

The heat energy needed to change the state of a mass of material without changing its temperature. (breaking molecules)
Fusion = solid - liquid
Vaporisation = liquid - gas 20 of 24

## Specific Latent Heat of Fusion/ Vaporisation

EXAM QUESTION

3) Explain why a scald from heat at 100 degrees is much worse than a scald from water at 100 degrees

Steam at 100degrees is much worse than water at 100degrees due to the steam being a latent heat of vaporisation as it has more energy. As it hits the skin, it will then go back into the water, but the temperature won't change. Water is 100degrees has less energy.

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## Springs

Uses of springs

• Mattress
• Suspension
• Staplers
• Pens
• Pogo sticks

Hooke's Law

F = kx

F = force
k = force constant of the spring/ wire - gradient of the f/x graph (steepness)
x = extension of the spring/ wire

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## Springs

Hooke's Law states that the extensions produced in an object is proportional to the load producing it, providing the elastic limit is not exceeded.

Behaviour of Springs

Elastic limit - the value of stress beyond which an object will not return to its original dimensions.

The spring extends obeying Hooke's Law i.e. proportional to the force up to the start = elastic limit.

Beyond this point, the line is not straight and the springs will not return to its original dimensions.

Elastic - force put on object goes back to original shape.
Plastic - force put on object and stays the same

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## Tensile and Compressive Force

Tensile - spring gets stretched

Compressive - gets squashed = + shortened, - attraction

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