Topic 4: Energy
4A1) Energy Transfer and Efficiency
4.2 Describe energy transfers involving the following forms of energy: thermal (heat), light, electrical, sound, kinetic, chemical, nuclear and potential (elastic and gravitational)
Potential energy means stored energy.
Thermal energy: Heat energy e.g. heaters
Light energy: Light carries light energy as it travels which turns into internal energy when it strikes an object. Internal energy is the difference between hot and cold. e.g. solar panel can generate electrical energy from the light energy from the sun
Electrical energy: Electrical currents carry electrical energy. This energy can easily be converted to kinetic or internal energy. e.g. electricity at homes
Sound energy: Sound waves carry a small amount of energy from the source of the noise. e.g. radio
Kinetic energy: Any movement energy e.g. pendulum swinging
Chemical potential energy: Any objects with atoms that are held together by forces like bonds have chemical potential energy. When the bonds are broken, energy is released. e.g. petrol, diesel
Nuclear energy: The energy in a nucleus of an atom is stored by extremely strong bonds. Some of this energy can be released. e.g. uranium by splitting the nucleus into two smaller nuclei, which is done in a nuclear power station
Elastic potential energy: This is also known as strain energy. e.g. bows when they are drawn back contain elastic potential energy before the arrow is released.
Gravitational potential energy: This is energy stored by an object being raised up in a gravitational field. e.g. if a load is raised above the ground, it will have GPE.
4.3 Understand that energy is conserved
First Law of Energy: energy cannot be created or destroyed.
Second Law of Energy: energy at the start = energy at the end
However, in nearly all energy transfers, some of the energy will end up as ‘useless’ heat.
- Battery driven toy dog (chemical potential energy) à Electrical energy à Dog moves along (kinetic energy)
WASTE: Heat and Sound energy
- A spring driven toy car (Elastic potential energy) à Car moves along (kinetic energy)
WASTE: Heat and Sound energy
- Torch with battery (chemical potential energy) à Heat and light produced (heat and light energy)
WASTE: Sound energy
- A boy pulls a catapult back (Elastic potential energy) à Released, objects moves through air (kinetic energy)
WASTE: Heat and Sound energy
4.4 Recall and use the relationship: efficiency = useful energy output / total energy input
4.5 Describe a variety of everyday and scientific devices and situations, explaining the fate of the input energy in terms of the above relationship, including their representation by Sankey diagrams
Energy Value: Wasted heat – 400J, heat to water – 1000J, sound energy – 600J
Kettle efficiency: 0.5
Energy converted to useful energy: 120J
Computer efficiency: 0.8
4A2) Conduction, Convection and Radiation
4.6 Recall that energy transfer may take place by conduction, convection and radiation
Energy will always try to flow from areas at high temperatures to areas at low temperatures. This is called thermal transfers. Thermal energy can be transferred in three ways:
- Conduction – Convection – Radiation
Conduction: The most common heat transfer mechanism in solids
As the metal heats up, the heated molecules gain energy and vibrate more. This has a knock-on effect and the surrounding molecules also start vibrating. The kinetic energy passes through the whole material and possibly materials nearby (if they are in contact). Conduction cannot take place in a vacuum because there are no particles in a vacuum to transfer the vibrations. All metals are good conductors and plastics, water and air are poor conductors.
Convection: The most common heat transfer mechanism in liquids and gases
Molecules at the bottom of the liquid heat up. The molecules at the bottom heat up and gain kinetic energy. This causes slight expansion resulting in lower density. Therefore, molecules move from a lower to higher density area (hot à cold). The cold fluid sinks and hot fluid rises, resulting in heat transfer. Convection current is how the heat energy is transferred. While the space between particles increase in a gas or a liquid (lower density), solid particles are in fixed positions and are not free to move (not fluid); therefore, convection cannot take place in a solid.
Radiation: The only heat transfer mechanism in a vacuum
When the grill gets hot, it starts to emit infrared radiation. This travels through the air. The food absorbs the radiation and heats up. Energy can travel through empty space by radiation rays, which can be reflected by mirrors like light rays. Dull black surfaces are bad radiators and good absorbers. Shiny, bright surfaces are good radiators and bad absorbers. A vacuum flask uses silvering to cut down heat transfer by radiation and uses a vacuum to cut down heat transfer by conduction and convection.
Vacuum Flask: Conduction, convection, radiation elimination
Conduction – Thin wall of glass (poor conductor of heat) and plastic stopper
Convection – space between inner wall and outer wall is made a vacuum
Radiation – inner walls coated in aluminium, shiny surfaces reflecting the heat back in
4.7 Describe the role of convection in everyday phenomena
Convection of air in a room:
When air particles heat up, they move apart. The bottom part of the room of hot air is less dense than the top part of the room. So the hot air rises and creates a circulation, which is convection.
Cycling to the sea in the morning:
Lee is tired after cycling to the sea in the morning due to convection current. The sun heats up the ground quickly while the sea absorbs and preserves a lot of thermal energy. The hot air from the ground rises, which brings the cold air from the sea to move towards the land. This means Lee is riding against the wind in the morning.
Cycling back home in the evening:
When Lee cycles home in the evening, he will have a difficult ride home. Since the sun is gone, the ground will cool down quickly while the sea still contains hot air. So hot air will rise from the sea and the wind will blow towards the sea, against Lee.
4.8 Describe how insulation is used to reduce energy transfers from buildings and the human body
Insulators of heat are not good at transferring heat. If a building is a well-insulated, it is difficult for heat to enter or leave the building. Ways to reduce waste energy transfers in a house:
Walls – Cavity wall insulation, Modern houses have cavity walls (two single walls separated by air cavity). Due to the presence of air, convection energy transfer can still take place. So you can fill the space up with fibre insulation.
Roof – Loft insulation, Fibre insulation placed on top of the ceiling to trap air between the fibres. This reduces energy transfer by conduction and convection.
Floors – Carpets, Carpets underlay reduces energy loss by convection and conduction. Some modern houses use foam blocks under the floors.
Draughts – Draught excluders, Cold air can get into the houses through gaps between windows and doors. Draught excluder tape can be used to block the gaps.
Windows – Double glazing, energy is transferred through glass by conduction and radiation. By having two panes of glass with a layer of glass between them reduces conduction. Radiation can be reduced by using curtains.
The same applies for the human body. In winter, if you want to stay warm, you should wear white clothes to prevent heat from leaving. In summer, if you want to stay cool, you should wear white clothes to prevent heat from entering. Mountaineers use several layers of thin clothing to fill each layer with trapped air. The aim is to surround the body with air since air is a poor conductor of heat.
4B1) Work
4.9 Recall and use the relationship between work, force and distance moved in the direction of the force: Work done (J) = force (N) x distance moved (m) (Wd = F x d)
4.10 Understand that work done is equal to energy transferred
Work done = energy transferred. 1 Joule = 1 N x 1 m (in the direction of the force)
e.g. A girl weighing 500 N climbs 40 m vertically when walking up the stairs in an office block. How much work does she do against gravity? What are the energy transfers here?
Wd = F x d
= 500 N x 40 m
= 20000 Joules = 20kJ
KE has been converted to heat energy and GPE.
4B2) GPE, KE and Conservation of Energy
4.11 Recall and use the relationship:
Gravitational potential energy (J) = mass (kg) x g (N/kg) x height (m) (GPE = m x g x h)
g = 10N/kg on earth
Example question:
50,000 J of work are done as a crane lifts a load of 400 kg. How far did the crane lift the load? (g = 10N/kg)
W = m x g
= 400 x 10
= 4000 N
W = F x d
50000 J = 4000 N x d
d = 12.5 m
4.12 Recall and use the relationship:
Kinetic energy (J) = 1/2 x mass (kg) x speed2 (m/s) (KE = 1/2 x m x v2)
4.13 Understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work
Kinetic energy given to a stone converts into gravitational potential energy as it is thrown into the air. At the top of the flight, most of the kinetic energy will have converted to gravitational potential energy. Small amount of energy will be lost as heat due to friction between the stone and the air.
4B3) Power
4.14 Describe power as the rate of transfer or energy or the rate of doing work
Power is defined as the rate of doing work or the rate of transferring energy.
“Rate” just means “divided by time”. So Power = Energy / Time or Work Done / Time.
The more powerful something is, the quicker it does a fixed amount of work.
e.g. Person A (lighter, smaller) can run up the stairs in 30 seconds. Person B can run up the stairs in 20 seconds. Who is the most powerful? Why?
Person B is more powerful. He does more work running up the stairs because he has a greater weight in a shortened time, so he is more powerful.
4.15 Use the relationship between power, work done (energy transferred) and time taken
Power = Work done / Time taken (P = Wd / t)
Example Question: A man lifts a weight of 300N through a vertical height of 2m in 6 seconds. What power does he develop?
Wd = F x d
Wd = 300 x 2
