Energy Resources & Energy Transfers Flashcards
What is energy stores & transfers?
Energy stores and transfer pathways are a model for describing energy transfers in a system
In physics what is system defined as ?
In physics, a system is defined as:
An object or group of objects
Defining the system, in physics, is a way of narrowing the parameters to focus only on what is relevant to the situation being observed
A system could be large or small, incorporating just one object, or a whole group of objects and their surroundings
When a system is in equilibrium, nothing changes, and so nothing happens
When there is a change to a system, energy is transferred
If an apple sits on a table, and that table is suddenly removed, the apple will fall
As the apple falls, energy is transferred
What are energy stores ?
Energy is stored in objects in different energy stores
What are the energy transfer pathways?
Mechanical
Electrical
Heating
Radiation
How is energy transferred by heating a cup of hot coffee heating up cold hands?
Describe the energy transfers in the following scenarios:
a) A battery powering a torch
b) A falling object
Part a)
Step 1: Determine the store that energy is being transferred away from, within the parameters described by the defined system
For a battery powering a torch
The system is defined as the battery and the torch
Therefore, the energy transfer to focus on is from the battery to the torch
Therefore, the energy began in the chemical store of the cells of the battery
Step 2: Determine the store that energy is transferred to, within the parameters described by the defined system
When the circuit is closed, the bulb lights up
Therefore, energy is transferred to the thermal store of the bulb
Energy is then transferred from the bulb to the surroundings, but this is not described in the parameters of the system
Step 3: Determine the transfer pathway
Energy is transferred by the flow of charge around the circuit
Therefore, the transfer pathway is electrical
Energy is transferred electrically from the chemical store of the battery to the thermal store of the bulb
Part b)
Step 1: Determine the store that energy is being transferred away from, within the parameters described by the defined system
For a falling object
In order to fall, the object must have been raised to a height
Therefore, it began with energy in its gravitational potential store
Step 2: Determine the store that energy is transferred to, within the parameters described by the defined system
As the object falls, it is moving
Therefore, energy is being transferred to its kinetic store
Step 3: Determine the transfer pathway
For an object to fall, a resultant force must be acting on it, and that force is weight, and it acts over a distance (the height of the fall)
Therefore, the transfer pathway is mechanical
Energy is transferred from the gravitational store to the kinetic store of the object via a mechanical transfer pathway
What is the principle of conservation of energy?
he principle of conservation of energy states that:
Energy cannot be created or destroyed, it can only be transferred from one store to another
This means the total amount of energy in a closed system remains constant
The total energy transferred into a system must be equal to the total energy transferred out of the system
Therefore, energy is never ‘lost’ but it can be transferred to the surroundings
Energy can be dissipated (spread out) to the surroundings by heating and radiation
Dissipated energy transfers are often not useful, and can then be described as wasted energy
What are some examples of the principle of conservation of energy?
Example 1: a bat hitting a ball
The moving bat has energy in its kinetic store
Some of that energy is transferred usefully to the kinetic store of the ball
Some of that energy is transferred from the kinetic store of the bat to the thermal store of the ball mechanically due to the impact of the bat on the ball
This energy transfer is not useful; the energy is wasted
Some of that energy is dissipated by heating to the thermal store of the bat, the ball, and the surroundings
This energy transfer is not useful; the energy is wasted
What is the principle of conservation of energy applied to a kettle boiling water?
When an electric kettle boils water, energy is transferred electrically from the mains supply to the thermal store of the heating element inside the kettle
As the heating element gets hotter, energy is transferred by heating to the thermal store of the water
Some of the energy is transferred to the thermal store of the plastic kettle
This energy transfer is not useful; the energy is wasted
And some energy is dissipated to the thermal store of the surroundings due to the air around the kettle being heated
This energy transfer is not useful; the energy is wasted
What is the principle of conservation of energy applied to a person jumping on a trampoline?
Whilst jumping, the person has energy in their kinetic store
When the person lands on the trampoline, most of that energy is transferred to the elastic potential store of the trampoline
That energy is transferred usefully back to the kinetic store of the person as they bounce upwards
Energy is transferred from the kinetic store of the person to the gravitational potential store of the person as they gain height
Some of the energy is dissipated by heating to the thermal store of the surroundings (the person, the trampoline and the air)
The useful energy transfers taking place are:
elastic potential energy ➝ kinetic energy ➝ gravitational potential energy
What is efficiency in an energy transfer?
The efficiency of a system is a measure of the amount of wasted energy in an energy transfer
Efficiency is defined as:
The ratio of the useful energy output from a system to its total energy output
If a system has high efficiency, this means most of the energy transferred is useful
If a system has low efficiency, this means most of the energy transferred is wasted
What is the equation of efficiency ?
Efficiency is represented as a percentage
The equation for efficiency is:
efficiency = useful energy output divided by total energy output x 100%
Total energy output is equal to total energy input due to the principle of conservation of energy
total energy input = total energy output
Total energy output is the sum of the useful energy output and the wasted energy
total energy output = useful energy output + wasted energy
The blades of a fan are turned by an electric motor. In one second, 300 J of energy is transferred electrically from the mains supply. 85 J is wasted due to friction and sound.
Calculate the efficiency of the motor.
Step 1: List the known quantities
Total energy input = 300 J
Total wasted energy = 85 J
Determine total energy output
Due to the conservation of energy:
total energy input = total energy output
Therefore, total energy output = 300 J
Calculate the useful energy output
total energy output = useful energy output + wasted energy
useful energy output = total energy output − wasted energy
useful energy output = 300 − 85 = 215 J
Substitute these values into the equation for efficiency
efficiency space equals fraction numerator space useful space energy space output over denominator total space energy space output end fraction space cross times space 100 percent sign
efficiency space equals fraction numerator space 215 space over denominator 300 end fraction space cross times space 100 percent sign
efficiency space equals space 72 %
What are Sankey diagrams?
Sankey diagrams are visual representations of energy transfers
Sankey diagrams are characterised by the splitting arrows that show the proportions of the energy transfers taking place
The different parts of the arrow in a Sankey diagram represent the different energy transfers:
The left-hand side of the arrow (the flat end) represents the energy transferred into the system
The straight arrow pointing to the right represents the energy that ends up in the desired store; this is the useful energy output
The arrows that bend away represent the wasted energy
The width of each arrow on a Sankey diagram is proportional to the amount of energy being transferred
As a result of the conversation of energy:
Total energy in = total energy out
Total energy in = Useful energy out + Wasted energy
An electric motor is used to lift a weight. The Sankey diagram below represents the energy transfers in the system.
Step 1: State the conservation of energy
Energy cannot be created or destroyed, it can only be transferred from one store to another
This means that:
total energy in = useful energy out + wasted energy
Step 2: Rearrange the equation for the wasted energy
wasted energy = total energy in – useful energy out
Step 3: Substitute the values from the diagram
500 – 120 = 380 J
What is Conduction, convection and radiation?
Energy is transferred by heating and radiation via the processes of:
Conduction
Convection
Radiation
What is Conduction, conductors and insulators?
Conduction is the main method of energy transfer by heating in solids
Metals are extremely good thermal conductors
A material is a good conductor if it transfers energy by heating
Non-metals are poor thermal conductors whilst liquids and gases are extremely poor thermal conductors
Poor conductors are called insulators
A material is a good insulator if it does not transfer energy by heating
Insulators are used to prevent energy transfer by conduction
Materials containing small pockets of trapped air are especially good at insulating because air is a gas and hence a poor conductor
The air is trapped, so it cannot move and form a convection current, therefore energy transfer by conduction occurs, but it happens very slowly since air is a gas
When a substance is heated, the atoms start to move around (vibrate) more
As they do so they bump into each other, transferring energy from atom to atom
What is convection ?
Convection is the main way that thermal energy is transferred through liquids and gases
Convection cannot occur in solids
What is convection in currents?
When a fluid (a liquid or a gas) is heated:
The molecules push each other apart, making the fluid expand
This makes the hot fluid less dense than the surroundings
The hot fluid rises, and the cooler (surrounding) fluid moves in to take its place
Eventually, the hot fluid cools, contracts and sinks back down again
The resulting motion is called a convection current
Exam Tip
If a question refers to thermal energy transfers and a liquid or gas (that isn’t trapped) then make sure your answer mentions that convection currents will probably form!
What is thermal Radiation ?
All bodies (objects), no matter what temperature, emit infrared radiation
The hotter object, the more infrared radiation it radiates in a given time
What are the colours of an object affects how well it emits and absorbs thermal radiation?
How does Conduction, convection and radiation in a mug of coffee take place ?
For a mug of hot coffee:
Energy is transferred by radiation from the surface to the mug to the surroundings
Due to the infrared radiation being emitted from its surface
All objects (above 0 K) emit infrared radiation, but the hotter an object is, the more IR radiation it emits
Energy is transferred by heating from the surface of the coffee to the surroundings
The most energetic particles of the coffee evaporate setting up a convection current
Energy is transferred by heating from the bottom of the mug to any surface it is in contact with, such as a table
This energy transfer happens by conduction
Objects will continue to lose heat until they reach thermal equilibrium (equal temperature) with their surroundings
For example, a mug of hot coffee will cool down until it reaches room temperature
Exam Tip
If a question refers to the colour of something (black, white or shiny) then the answer will probably have something to do with thermal radiation!
If the question involves a vacuum (empty space), then remember to mention radiation! Because conduction and convection require particles to transfer energy!
Core practical 8: investigating thermal energy
Experiment 1: investigating conduction
Aim of the experiment
The aim of the experiment is to investigate the rate of conduction in four different metals
Variables
Independent variable = Type of metal
Dependent variable = Rate of conduction
Control variables:
Size and thickness of metal strips
Amount of wax used
Identical ball bearings
What is the method of investigating conduction ?
- Attach ball bearings to the ends of each metal strip at an equal distance from the centre, using a small amount of wax
- The strips should then be turned upside down and the centre heated gently using a bunsen burner so that each of the strips is heated at the central point where they meet
- When the heat is conducted along to the ball bearing, the wax will melt and the ball bearing will drop
- Time how long this takes for each of the strips and record in a table
- Repeat the experiment and calculate an average of each time
What is the analysis of investigating conduction ?
Order the metals according to their thermal conductivity
The first ball bearing to fall will be from the rod that is the best thermal conductor
This is because materials with high thermal conductivity heat up faster than materials with low thermal conductivity
Experiment 2: investigating convection
Aims of the experiment
The aim of the experiment is to investigate the rate of convection of potassium permanganate crystals in two different temperatures of water
Variables:
Independent variable = Temperature of water
Dependent variable = Rate of convection
Control variables:
Amount of water in beaker
Size of Bunsen burner flame
Size of potassium permanganate crystal
What is the method of investigating convection ?
- Fill the beaker with cold water (not too full) and place it on top of a tripod and heatproof mat
- Pick up the crystal using forceps and drop it into the centre of the beaker – do this carefully to ensure the crystal does not dissolve prematurely
- Heat the beaker using the Bunsen burner and record observations
- Repeat experiment with hot water and record observations
What is the analysis of investigating convection ?
Energy is initially transferred from the Bunsen flame through the glass wall of the beaker by conduction
The water in the region of the Bunsen flame is heated and the space between the water molecules expands, therefore, the water becomes less dense and rises
This causes the dissolved purple crystal to flow upwards with the water
Meanwhile, when the water at the top of the beaker cools, there is less space between the water molecules and the water becomes denser again and falls downwards
The process continues which leads to a convection current where energy is transferred through the liquid
The dissolved purple crystal follows this current which can be clearly observed during this experiment
It should be observed that the convection current is faster in hot water
This is because the higher the temperature, the higher the kinetic energy of the water molecules
Therefore, in hot water, the water molecules and the the molecules of potassium permanganate move around the beaker faster
Experiment 3: investigating radiation
Aims of the experiment
The aim of the experiment is to investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface
Variables
Independent variable = Colour
Dependent variable = Temperature
Control variables:
Identical flasks (except for their colour)
Same amounts of hot water
Same starting temperature of the water
Same time interval
What is the method of investigating radiation ?
- Set up the four identical flasks painted in different colours: black, grey, white and silver
- Fill the flasks with hot water, ensuring the measurements start from the same initial temperature
- Note the starting temperature, then measure the temperatures at regular intervals, e.g. every 30 seconds for 10 minutes
What is the analysis of investigating radiation ?
All objects emit infrared radiation, but the hotter an object is, the more infrared waves are emitted
The intensity (and wavelength) of the emitted radiation depends on:
The temperature of the body (hotter objects emit more thermal radiation)
The surface area of the body (a larger surface area allows more radiation to be emitted)
The colour of the surface
Most of the energy lost from the beakers will be by heating due to conduction and convection
This will be equal for each beaker, as colour does not affect energy transferred by conduction and convection
Any difference in energy transferred away from each beaker must, therefore, be due to infrared radiation
To compare the rate of energy transfer away from each flask, plot a graph of temperature on the y-axis against time on the x-axis and draw curves of best fit
What is insulation ?
Insulation reduces energy transfers from both conduction and convection
The effectiveness of an insulator is dependent upon:
The thermal conductivity of the material
The lower the conductivity, the less energy is transferred
The density of the material
The more dense the insulator, the more conduction can occur
In a denser material, the particles are closer together so they can transfer energy to one another more easily
The thickness of the material
The thicker the material, the better it will insulate
Insulating the loft of a house lowers its rate of cooling, meaning less energy is transferred to the surroundings (outside)
The insulation is often made from fibreglass (or glass fibre)
This is a reinforced plastic material composed of woven material with glass fibres laid across and held together
The air trapped between the fibres makes it a good insulator
The gaps or cavities between external walls are often filled with insulation
This is called cavity wall insulation
This is often done by drilling a hole through the external wall to reach the cavity and filling it with a special type of foam which is made from blown mineral fibre filled with gas
This lowers the conduction of heat through the walls from the inside to the outside
What is work done and energy transfer?
When work is done on an object, energy is transferred
The amount of energy transferred (in joules) is equal to the work done (also in joules)
energy transferred (J) = work done (J)
Work is done when a force is used to move an object over a distance, and energy is transferred from the person to the box
If a force acts in the direction that an object is moving, then the object will gain energy (usually to its kinetic energy store)
If the force acts in the opposite direction to the movement, then the object will lose energy (dissipated to the surroundings, usually by heating)
What are some examples of work done?
Work is done on a ball when it is lifted to a height
The energy is transferred mechanically from the ball’s kinetic energy store to its gravitational potential energy store
Work is done when a bird flies through the air
The bird must travel against air resistance, therefore energy is transferred from the bird’s kinetic store to its thermal store and dissipated to the thermal store of the surroundings
How do you calculate work done ?
The amount of work that is done is related to the magnitude of the force and the distance moved by the object in the direction of the force
To calculate the amount of work done on an object by a force, the work done equation is used:
W = f × d
Where:
W = work done in joules (J) or newton-metres (N m)
F = force in newtons (N)
d = distance in metres (m)
A car moving at speed begins to apply the brakes. The brakes of the car apply a force of 500 N which brings it to a stop after 23 m.
Answer:
Step 1: List the known quantities
Distance, d = 23 m
Force, F = 500 N
Step 2: Write out the equation relating work, force and distance
W = F × d
Step 3: Calculate the work done on the car by the brakes
W = 500 × 23
W = 11 500 J
What is the gravitational potential energy equation?
he amount of energy in the gravitational potential store of an object can be calculated using the gravitational potential energy equation:
G P E = m x g x h
Where:
GPE = gravitational potential energy, in joules (J)
m = mass, in kilograms (kg)
g = gravitational field strength in newtons per kilogram (N/kg)
h = height in metres (m)
What is gravitational field strength?
. The gravitational field strength (g) on the Earth is approximately 10 N/kg
.The gravitational field strength on the surface of the Moon is less than on the Earth
.This means it would be easier to lift a mass on the Moon than on the Earth
.The gravitational field strength on the surface of the gas giants (eg. Jupiter and Saturn) is more than on the Earth
.This means it would be harder to lift a mass on the gas giants than on the Earth
How does the GPE change with height for a ball being thrown up in the air and when falling down?
A man of mass 70 kg climbs a flight of stairs that is 3 m higher than the floor.
Gravitational field strength is approximately 10 N/kg.
Calculate the increase in energy transferred to his gravitational potential store.
Step 1: List the known quantities
Mass of the man, m = 70 kg
Gravitational field strength, g = 10 N/kg
Height, h = 3 m
Step 2: Write down the gravitational potential energy equation
GPE = m x g x h
Step 3: Calculate the gravitational potential energy
GPE = 70 x 10 x 3
GPE= 2100J
What is energy in an object’s kinetic store defined as ?
The amount of energy an object has as a result of its mass and speed
This means that any object in motion has energy in its kinetic energy store
The diagram shows a rollercoaster going down a track.
The rollercoaster takes the path A → B → C → D.
The rollercoaster begins at a height of 15 m above the ground and ends at ground level.
Breaking to stop the ride begins after it passes position D.
The mass of the rollercoaster is 100 kg.
Calculate the maximum speed of the rollercoaster at position D. Ignore any frictional effects before passing point D.
Answer:
Step 1: List the known quantities
Height, h = 15 m
Mass, m = 100 kg
Gravitational field strength, g = 10 N/kg
Why is time a important consideration when it comes to power?
Time is an important consideration when it comes to power
Two cars transfer the same amount of energy, or do the same amount of work to accelerate over a distance
If one car has more power, it will transfer that energy, or do that work, in a shorter amount of time
What are power ratings ?
Power ratings are given to appliances to show the amount of energy transferred per unit time
Common power ratings are shown in the table below:
Calculate the work done if an iron of power 2000 W is used for 5 minutes.
What are non - renewable energy resources ?
Non-renewable energy resources include:
Fossil Fuels (coal, oil and natural gas)
Nuclear fuel
What is Hydroelectric dam?
What is Fossil fuel electricity generation?
What is Geothermal electricity generation?
Give some examples of energy resources ?
compare advantages and disadvantages of energy resources?
Tidal
yes
Tides are very predictable, so energy can be produced at regular intervals
Few suitable locations (estuaries). Can harm aquatic life and disrupt shipping
Geothermal
Yes
Reliable
Not many suitable locations. Can release methane (a greenhouse gas) in the extraction process.
Solar
Yes
Produces no pollution or greenhouse gases. Good for electricity production in remote areas.
Non-reliable. Only generates electricity when the Sun is shining. Each solar cell only produces a small amount of electricity so many cells are needed.
