P7.2 Flashcards
Power equation
Power = Work done / time
What unit do electricity companies measure your energy usage in
kWh, kilowatt-hour
What is kWh a measure of
measure of energy NOT POWER
Define 1kWh
1kWh is the amount of energy transferred by a1kW appliance, in 1hour
P = E/t = 18kWh/2h = 9kW
Cost = (kWh’s used) x (cost/kWh) = 18kWh x 10p/kWh = £1.80.
Suggest and explain whether using higher - power devices is cheaper, more expensive or costs the same as using lower - power devices = CLASSIC 6 MARKER
Higher power devices transfer electrical energy faster to other forms.
In the same time, they will use more energy.
So, they will cost more to run.
However, if the higher power of the device makes itmore efficient at doing its job (e.g. – higher power hair dryer dries hair faster)…
… it won’t need to be on for as long a period of time…
… so may actually end of using less energy and so costing less money to use.
3 Advantages of electrical energy
- is very easily transferred into other forms.
- is very easily transferred over long distances.
- is portable.
What is a wasted energy transfer when current flows
Whenever an electric current flows, some energywill always be transferred as heat to the surroundings. = THIS IS WASTED
Actually try this one - took me some time but I understand it now
Mr Sara is answer -
When does thermal energy transfer occur
Thermal energy transfer will always occur when there exists a temperature difference between two points/media.
Definition of temperature
Temperature is a measure of the average kinetic energy of particles (in a substance).
What is a system
A system is a group of bodies interacting.
When is the only time the energy of a system stays constant
when the system is closed.
Define a closed system
one which has no external forces actingon it (no forces from bodies outside the system).
A car travelling at a constant speed will have a constant kinetic energy.Thus, kinetic energy is transferred to the thermal (store) energy of the surroundings at the same rate as the chemical potential (store) energy of the fuel is transferred to kinetic energy of the car.
What are the three main ways heat energy is lost from a house
- Walls
- Roof
- Windows
3 main ways to minimise heat loss from your house
- cavity wall insulation
- loft insulation
- double glazing
How does double - glazed windows work
Traps air and stops it moving
How does cavity wall insulation work
Traps air and stops it moving
How does aluminium foil between a radiator panel and the war minimise heat loss
Reflects radiation back from the wall
What does the rate at which heat energy is transferred through a wall depend on
- Temperature difference between the two sides of the wall (temperature gradient)
- Area of walls
- Thickness of the walls
- Material of the walls(constant called Thermal Conductivity)
How does temperature gradient effect the rate at which heat energy is transferred through a wall
The greater the temperature difference between the two sides of the wall… the greater the rate of heat energy transfer.
How does area of a wall effect the rate at which heat energy is transferred through a wall
The greater the area of the walls…the greater the rate of heat energy transfer.
How does wall thickness affect the rate at which heat energy is transferred through a wall
The thicker the wall,the slower the rate of heat energy transfer.
How does thermal conductivity effect the rate at which heat energy is transferred through a wall
The lower the thermal conductivity of the wall, the lower the rate of heat energy transfer.
Definition of thermal conductivity
The heat energy transferred, per second, through a 1m thick wall, of area 1m2, when a temperature difference of 1oC exists across it.
Units of thermal conductivity
W/mK
K = Kelvin
Don’t understand this but maybe you do
Law of conservation of energy
Energy can never be created nor destroyed.
Energy can only ever be transferred from one form to another.
What is useful energy
Energy in the place we want it and in the form we can use it.
What is wasted energy
Energy that is not useful energy.
What is efficiency
a measure of how much of the input energy of a device, is transferred to useful energy.
Efficiency formula
Why do mechanical devices ALWAYS waste energy in the form of heat
This is owing to the force of friction which alwaysacts between moving parts/surfaces.
What is a Sankey diagram - draw example
How to reduce friction
Lubricate moving parts
How to reduce the heating up of a wire
Use wires with as little resistance as possible
How to reduce air resistance
Streamline shapes
How to reduce sound caused by machinery
Tighten loose parts to reduce vibration
Suggest how to improve the efficiency of an oven
Insulate the oven to ensure minimise wasted energy transferred to the surroundings as heat.(heat energy is useful only when transferred to the oven itself)
Explain why using LEDs might not reduce your electricity bill
LEDs are more efficient then incandescent bulbs…
… so waste less energy by transferring less to heat energy in the surroundings.
A room lit by LEDs will feel colder than a room lit by incandescent bulbs.
Thus, you may need to turn on the heating.
Any electricity you may have saved in powering LEDs (as opposed to less efficient incandescent bulbs) will now just be used instead in heating the house.
