Energy transfers

Question 1

What are all the different types of energy stores?

Answer 1

• Kinetic - Energy due to movement
• Gravitational - An object gains gravitational energy when lifted
• Thermal - Energy due to heat
• Elastic - Energy stored when stretched
• Electrostatic - Energy due to forces of attraction or repulsion between charges
• Magnetic - Energy due to forces of attraction or repulsion between magnets
• Chemical - Energy found in food, fuels or batteries released during a reaction
• Nuclear - Energy contained within the nucleus

Question 2

What are the different types of energy transfers?

Answer 2

• Mechanical - When force acts upon a body, e.g. collision
• Electrical - When electricity is transferred, such as from a cell to a bulb
• Heating - When heat is transferred by conduction, convection or radiation
• Radiation - When light and sound carrying energy is transferred between two points

Question 3

What is the principle of conservation of energy?

Answer 3

• Energy cannot be created or destroyed, it can only be transferred from one store to another
• This means that the total amount of energy in a closed system stays constant
• For example, the electrical energy to power a TV is transferred into light, sound and heat but none is destroyed

Question 4

What is the relationship between efficiency, useful energy output and total energy output?

Answer 4

Efficiency = (Useful energy output/Total energy output) x 100

Question 5

What are sankey diagrams and what do the different arrows mean?

Answer 5

• They are diagrams used to represent energy transfers
• The arrow pointing to the right is the useful energy out, and the arrow going down is the wasted energy
• The width of the arrow represents the amount of energy

Make sure to be precise when drawing them

Question 6

What are the three ways in which heat can be transferred?

Answer 6

• Conduction
• Convection
• (Thermal) Radiation

These processes will occur until the objects temperature matches the temperature of the surroundings

Question 7

How does conduction work?

Answer 7

• Conduction occurs when two solids are in contact with eachother and there is a temperature gradient between them
• The heated atoms will have more energy and as they vibrate they will come in contact with the atoms of the other solid, transferring energy to them
• Metals are better conductors than other materials because they are dense and due to the presence of delocalised electrons which can freely move and carry thermal energy within them

Gases and liquids can conduct, but at a much slower rate than solids as their particles are more spread out therefore they are poor conductors/insulators

Question 8

How does convection work?

Answer 8

• As a fluid (liquid or gas) is heated, it expands
• As a result, it is less dense than its surroundings so it rises up and the cooler surrounding air takes its place
• Eventually the hot fluid will cool and start to sink off to the side
• This process will repeat and a convection current will be created

Convection is the main heat transfer in liquids and gases and cannot occur in solids

Question 9

How does (thermal) radiation work?

Answer 9

• All bodies emit infrared radiation, but at higher temperatures more will be emitted
• This infrared radiation contains thermal energy and all objects are perpetually cooling down in this fashion
• Black objects will emit and absorb infrared better as oppose to white/shiny objects

Thermal radiation is the only kind of heat transfer that can occur without particles (such as in space)

Question 10

How does surface area and temperature relate to rates of emission and absorbtion?

Answer 10

• A larger temperature gradient will result in faster rates of emission and absorbtion from hot to cold
• A higher surface area will result in faster rates of emission and absorbtion from hot to cold as there is a higher surface area over which these processes can occur

Question 11

What are ways the reducing thermal energy transfers?

Answer 11

• To decrease conduction, place an insulating material between the two objects with very low thermal conductivity which is thick but with a low density
• To decrease convection, prevent a convection current from forming by restricting the free movement of the fluid (liquid or gas)

• The best insulators contain trapped air which is a very poor conductor and cannot move so prevents convection currents from forming
• Heat does not rise, hot fluids do
• Black things do not absorb heat well, they absorb infrared radiation