Energy Flashcards
(42 cards)
What is the equation for kinetic energy? Give units for all quantities involved.
1/2 m v^2
1/2 x mass x (velocity)^2
Energy (J), Mass (kg), Velocity (m/s)
What is the equation for elastic potential energy? Give units for all quantities involved
1/2 k e^2
1/2 x Spring constant x (extension)^2
Energy (J), Spring Constant (N/m), Extension (m)
What is the equation for gravity potential energy? Give units for all quantities involved
mgh
mass x gravitational field strength x height
Energy (J), Mass (kg), Gravitational Field Strength (N/kg), Height (m)
Define the ‘Specific Heat Capacity’ of a substance
The amount of energy needed to raise the temperature of 1 kilogram of a substance by 1 degree celsius
State the units for specific heat capacity
J/°C/kg
Joules/Degree Celsius/Kilogram
What is the definition of ‘Power’?
The rate at which energy is transferred (or rate at which work is done)
State two equations for power. Give units for all quantities involved
power = energy transferred/time
power = work done/time
Energy (J), Work Done (J), Time (s)
What is the unit of power?
Watt (w)
Two motors lift the same mass through the same height. Motor A does this in half the time of Motor B. Which dissipated the most power?
Motor A
The energy transferred is the same but the time taken is less (P=E/t)
Describe the energy changes involved when a ball is thrown upwards and then returns to its starting position. Ignore air resistance.
Upwards: KE is converted to GPE
Peak: Maximum GPE, zero KE
Downwards: GPE is converted to KE
KE(kinetic energy), GPE(gravitational potential energy)
Describe the energy transfers for a bungee jumper
When falling, the GPE is converted to KE of jumper.
As the cord tightens, KE is converted and stored as Elastic Potential Energy (EPE).
At lowest point, the jumper’s initial GPE equals the EPE stored in the cord
Explain why a bungee jumper slows down once the cord begins to stretch
kinetic energy decreses since it is converted to elastic potential energy.
Since KE is proportional to (velocity)^2, as KE decreases, so does velocity
Give examples of chemical energy stores
Food
Fuel (eg. wood, coal, petrol)
Batteries
State 4 different stores of energy
- Kinetic energy
- gravitational potential energy
- elastic potential energy
- chemical energy
State the law of energy conservation
energy cannot be created or destroyed (it can only be transferred into different forms)
State any changes in the total energy of a ball that is kicked, assuming that no external forces act.
The total energy of the system remains constant due to the conservation of energy
What is waste energy?
the energy that is not used by the device for its desired purpose
describe the energy changes that occur in a filament light-bulb
electrical energy is transferred into light and heat energy
Light is a useful energy form, heat is waste energy
State two equations to calculate efficiency
=useful output energy transfer/total input energy tranfer
=useful power output/total power output
How can the efficiency of a system be increased?
- Reducing waste output (by lubrication, thermal insulation etc.)
- Recycling waste output (eg. recycling thermal waste energy as input energy)
State the consequence for energy transfer of a material with a high thermal conductivity
the rate of energy transfer through the material is higher than for a material with a lower thermal conductivity
Do double-glazed windows have a higher or lower thermal conductivity than single-glazed windows?
lower, meaning less energy transfers through them
what key factors affect the rate of cooling of a building?
The thickness of the walls
The thermal conductivity of the walls
state three methods of reducing heat loss in a building
- double glazing
- loft and wall insulation
- thicker walls
