Unit 5 - Physics Flashcards
Revision for Unit 5 test
How do you convert Celsius to Kelvin?
Add 273.15
How do you convert Kelvin to Celsius?
Subtract 273.15
How do you calculate work done using distance and force?
Force multiplied by distance
How do you calculate pressure using force and area?
Force divided by area
How do you calculate efficiency?
1 - (Qout ÷ Qin)
What is an adiabatic process?
A process that involves no transfer of heat - internal energy is converted into work and vice versa.
What is an isothermal process?
A process that involves no change in temperature - all heat added is converted into work.
What is the unit for efficiency?
There is no unit for it
What is a heat engine?
Device that converts thermal energy into useful work
How quickly do isothermal processes happen?
Very slowly
What stays constant in an isothermal process?
Temperature of gas
What does the work in an isothermal process?
Engine transfer
Is insulation needed in an isothermal process?
No because heat needs to flow
How quickly do adiabatic processes happen?
Rapidly
What stays constant in an adiabatic process?
Engine heat
What does the work in an adiabatic process?
Temperature of gas
Is insulation needed in an adiabatic process?
Yes, it has to be completely thermally insulated
What is the first law of thermodynamics in an equation?
Q = △U+W (Change in internal energy + Work Done)
What is the first law of thermodynamics in words?
The heat energy supplied to a system is equal to the increase in the internal energy of the system plus the work done by the system on its surroundings
What is the second law of thermodynamics in words?
Heat always flows from a hot source to a cold sink unless we do work on a system. In a natural process, the entropy (randomness) of a system always increases. A natural process can never be fully reversed.
What is an idealised engine cycle?
An idealised engine more efficient than any that could actually be built. It describes the limits of what is possible.
Are you doing well?
Yes
What does entropy mean?
How much disorder there is in a system
Second law of physics in words?
In a natural thermodynamic process, the sum of entropies of the interacting systems increases.
Second law of physics equation
Qₕ - Q꜀ = W
Explain how a heat pump can keep us at a regular temperature at all times in the year
Heat pumps use energy to transfer heat from a cold to a hot reservoir. Heat pumps use a refrigerant/compressor, vapour and condenser so the refrigerant vapour condensed to liquid is releasing energy to the environment. This system is reversible, gives heat in winter and cools in the summer.
What is the unit for CoP?
There is no unit, it’s a ratio
How can you lower the cost of the CoP?
By keeping the temperature difference small. For example, using very large condenser coils and evaporators in a heat pump.
What is CoP?
CoP is the ratio of the energy transferred for heating the input energy. (For heat pumps and heat engines)
Can CoP be higher than 1?
Yes
What is the equation for CoP?
Thot / (Thot - Tcold)
What is thermal (heat) capacity?
The energy required to raise the temperature of an object by 1℃. The unit is JK⁻¹
What is specific heat capacity?
The energy required to raise the temperature of 1kg of a material by 1℃. The unit is JKg⁻¹K⁻¹.
What is the specific heat capacity formula?
Q=mc△T
What are the four steps in a heat pump cycle?
Compressor, condenser, expansion valve, evaporator
What steps are isothermal and which are adiabatic?
Condenser and evaporator are isothermal. Compressor and expansion valve are adiabatic
How does a heat pump differ from a heat engine?
Heat pumps draw in heat from a cold area and transfer to a warmer area. Heat pump is the reverse of a heat engine
What is it called when solid turns to liquid?
Melting
What is it called when liquid turns to gas?
Evaporation/boiling
What is called when a liquid turns to a solid?
Freezing
What is it called when a gas turns to a solid?
Deposition
What is it called when a solid turns to a gas?
Sublimation
What is called when a gas turns to a liquid?
Condensing
What is latent heat?
Latent heat is the energy needed to change the state of a substance without changing its temperature
What are the three forms of latent heat?
Fusion, vaporisation, and condensation
What is the specific latent heat equation?
Heat energy / Mass
Explain what happens to particles as they are gaining latent heat
The energy increases which is used to break bonds between particles.
Explain why sweating cools us down, by using the idea of latent heat
Sweating contains water with a high specific heat capacity. Energy from our skin is transferred to the water, and is transferred to break bonds. As particles evaporate, they transfer energy away from the skin, cooling it down.
When a bike pump is used to inflate a tyre, it gets hot. What happens to the internal energy of a gas if it is rapidly compressed, and why?
When work is done on the gas by the surrounding to compress it, the overall internal energy will increase.
When a gas in a deodorant can is sprayed, it rapidly cools. What happens to the internal energy of a gas if it is rapidly expanded, and why?
The gas does work in order to spread out and push air out of the way. Thus it transfers internal energy to do work, and this comprises only kinetic energy, this decreases and so does the temperature.
Fridges can transfer heat from a cold area to a warmer area. Explain why this does not violate the second law of thermodynamics
This is not a natrual process because it is powered by electricity
What is the purpose of the evaporator coils?
They transfer heat from the warmer internal fridge environment to the colder coolant. This adds latent heat to the refrigerant, evaporating it.
Explain why a good refrigerant needs to have a low boiling point (around -30℃)
It needs to be able to reach boiling point when absorbing small amounts of energy from warmer (but still cold) food an air in the fridge, so that it can start absorbing latent heat quickly
Why does a good refrigerant need to have a low specific heat capacity?
This helps a rapid increase in temperature for small additions of overall energy
Why does a good refrigerant need to have a high latent heat of vaporisation?
At boiling point the refrigerant needs to absorb a large amount of latent energy transferred from the food, in order for it to carry heat away from the internal of the fridge to keep things cool
What is plastic deformation?
A permanent deformation
What causes plastic deformation?
Excessive force
What is elastic deformation?
When a deformed object goes back to its original shape once force has stopped being applied
What is length in terms of Hooke’s law?
Original length before force is added
What is extension in terms of Hooke’s law?
The difference between the original length and the new length once force is added
Why do Hooke’s law graphs always have a positive correlation?
Extension is directly proportional to force
How do you convert mass into a force in spring constant questions?
Mass x 9.81
What is meant by a plastic material?
A material that cannot return to its original shape after being stretched
How can the energy stored in a spring be obtained from a Hooke’s Law Graph?
The area under the curve shows the elastic potential energy in the spring
Why does the temperature of a gas increase when it is compressed?
When we compress a gas, work is being done on the gas. Work done creates energy and this means there’s more energy in the gas which causes the temperature to increase
What is tensile stress?
Tensile stress describes the force an object experiences per unit area when being stretched
What is the tensile stress equation?
Force divided by area
What are the units for tensile stress?
Nm⁻² (Newtons per metre)
What is strain?
Strain is the ratio of extension to original length
What is the equation for strain?
Change in length / Original length (△x / L )
What is young’s modulus?
Young’s modulus, E, is the ratio of stress to strain
What does young’s modulus show?
How much an object will extend when a force is applied
What is the equation for Young Modulus
E = Stress / Strain
Is stress directly proportional to strain for a spring?
Yes, provided the elastic limit hasn’t been reached
How is Young’s modulus obtained?
From the gradient of a stress/strain graph
How do you calculate gradient on a graph?
Rise ÷ Run
What is density?
Mass per unit volume
What is the equation for density?
Mass ÷ Volume
What are the two units for density?
kg/m³ g/cm³
What are the units of Young Modulus?
N/m²
Why are materials brittle?
Materials are brittle because they absorb relatively little energy before they fracture
Can ductile metals be stretched into a wire?
Yes because they can deform under tensile stress
Why can malleable materials be hammered or rolled into a sheet?
Because they can deform under compressive stress
What is creep?
Creep is a gradual extension and deformation under a constant load
What are examples of creep?
Old leap pipes sagging over time, heavy trolley wheel flattening, steel screws becoming loosing
What is fatigue caused by?
Fatigue is caused by the continual loading and unloading of a material
What is an example of fatigue?
Bending a paper clip and continuously bending the paper clip until it breaks
Do rubber and elastic bands show elastic hysteresis?
Yes
What does the centre of a hysteresis loop represent?
The area in the centre of the hysteresis loop is equal to the energy lost by the material
Why does temperature increase during elastic hysteresis?
Energy is lost by the material in stretching, which results in its temperature increasing
What is the law of conservation of energy?
Total energy of isolated system is constant
What is meant by thermal equilibrium?
The condition under which two substances in physical contact with each other exchange no heat energy
What is thermal capacity?
The amount of heat energy required to raise the temperature of one kilogram of the substance by one degree Celsius
What does the ‘W’ in the first law of thermodynamics stand for?
Work done
The temperature of the air in the barrel increases as the tyre is pumped up. The friction is the pump is negligible. Explain why the temperature of the air in the barrel increases.
The air is compressed when the user of the pump pushes down on the pump so the volume decreases which makes the air pressure increase. Work is also being done on the gas so the air particles have increased kinetic energy from the moving piston
How does the compressor affect the refrigerator’s efficiency as a heat pump?
The compressor heats up when on. It also compresses the vapour to form a liquid. This means lost energy is wasted when it runs so it is not very efficient because of the wasted energy
How does the condenser coils affect the refrigerator’s efficiency as a heat pump?
Heat is radiated from condenser coils which wastes energy and heat is conducted which reduces efficiency
How does the expansion device affect the refrigerator’s efficiency as a heat pump?
The expansion device liquid adiabatically to form a gas. Gas leaving is cooler than liquid entering. This is the most efficient part of the refrigerator
How do the evaporator coils affect the refrigerator’s efficiency as a heat pump?
There’s a change of state which needs heat energy input and heat energy is taken from the inside of the refrigerator to produce vapour. This means the efficiency depends on how fast heat can be extracted from the coils.
What two states are fluids?
Liquids and gases
What can a fluid do when flowing?
Transmit pressure, transfer heat, deliver a substance to a new location
What is laminar flow?
Slow gentle flow of fluid where the motion occurs in one direction
What are streamlines?
Parallel lines of flow
What is turbulent flow?
Turbulent flow is when obstacles/sharp objects create turbulence in fluid flow and the drag forces become larger
What do the streamlines look like with turbulent flow?
They mix and cross
What is the correlation between speed of fluid and turbulence?
The faster a fluid flows, the more likely there is to be turbulence
What does the closeness of streamlines show us?
Speed of flow. The closer the lines are, the faster the fluid
What is viscosity?
Viscosity is a measure of how much a liquid resists flowing
What is low viscosity?
Flows with little resistance. Liquid is ‘thinner’. Example: water
What is high viscosity?
Flows with resistance. Liquid is ‘thicker’. Examples: tar, syrup
What are the streamlines going to look like for a high viscosity flow?
Far apart and slower (Laminar flow unless turbulence is mentioned)
What are the streamlines going to look like for a low viscosity flow?
Close apart
What is the effect of temperature on viscosity?
Viscosity reduces at higher temperatures because there are fewer forces between layers
What is viscous drag?
Viscous drag is the resistance to the flow of a fluid. It is a kind of internal friction between the layers.
What is the correlation between viscous drag and velocity?
As the velocity increases, the viscous drag increases
Is there more viscous drag in turbulent flow than in lamina flow?
No
Liquids are incompressible. Why?
Due to less intermolecular force of attraction, they acquire the shape of the container in which they are kept. The volume of the liquid doesn’t change with pressure
What is volume flow rate?
Volume flow rate is the mass of fluid flowing per second
Is volume flow rate constant for an incompressible liquid?
Yes
What is the link between the width of a pipe and the fluid flow rate?
If a pipe narrows, the fluid flow rate increases
Car electrical systems use copper. Copper is described as being ductile. Explain the term ductile.
Copper can be pulled/stretched without breaking therefore can be made into wires because these layers of atoms can slide over each other
Roofing lead is used as a building material. Explain how creep can cause roofing lead to become damaged
Creep is deformation due to mechanical stress. In a roof it’s caused by weather changes. This produces a change in length/shape over time
Give the relationship between speed of fluid & internal pressure
The higher the speed, the lower the internal pressure
What does Bernoulli’s principle state?
An increase in the speed of a fluid occurs simultaneously with a decrease in pressure/potential energy
What is a Newtonian fluid?
A fluid that follows Newton’s law of viscosity
In terms of non-newtonian fluids, why do people shake a ketchup bottle?
Because it becomes less viscous over time when shaken, so when the ketchup is too thick people shake it to make it thinner.
The fact that modern matte paints stay on the wall when you paint it shows they must be shear thinning. Explain.
When being brushed it’s very runny so it slides easily onto the walls. When you stop brushing it it stops being runny so it doesn’t slide down the wall.
Why does quicksand appear to be solid until you step in it?
The viscosity decreases with a rise in the rate of shear strain. There’s a rise in the rate of shear strain when you step onto quicksand which is why the viscosity decreases, revealing that it is not a solid.
What is viscous drag?
Viscous drag is the drag force felt by an object moving through a fluid due to the viscosity of the fluid
