p1 - conservation and dissipation of energy Flashcards
law of conservation
energy cannot be created, nor destroyed. it can also be transferred from store to store
explain the energy changes in a torch
1) chemical energy store in the battery
2) energy transferred by electric current in the wires
3) energy transferred to the surroundings; either by light waves or via an increase in the thermal energy store of the surroundings
energy transfers in a closed system
no net change of the total energy but transfers take place within the system due to the conservation of energy
dissipation
energy transferred to the surroundings
useful energy
energy being transferred to where it is required
wasted energy
energy that is not usefully transferred
define power
the rate at which energy is transferred/ rate at which work is done
power equation (linking energy and time)
power (watts, W) = energy transferred (joules, J) / time (s)
power equation (linking work done and time)
power (watts, W) = work done (joules) / time (s)
energy efficiency equations
efficiency = useful output (J) / total input (J)
efficiency = useful output energy transfer/ total input energy transfer
percentage energy efficiency
efficiency = useful output/ total input x 100
what is a sankey diagram?
summarises all the energy transfers taking place in a process
describe energy changes in a filament light bulb
1) transfer of electrical energy through wires
2) energy transferred through light waves (useful)
3) increase in the thermal store of the surroundings (wasted)
how can the efficiency of a system be increased?
- reducing waste output via thermal insulation
- recycling waste output
what is kinetic energy?
energy in a moving object
kinetic energy equation
KE (J) = 1/2 x mass (kg) x velocity^2 (m/s)
1/2 m v^2
elastic potential equation
elastic potential energy (J) = 1/2 x spring constant (N/m) x extension^2 (m)
1/2 k x^2
gravitational potential equation
GPE (J) = mass (kg) x gfs (N/kg) x height (m)
mgh
two motors lift the same mass through the same height. motor A does it in half the time. which dissipates the most power?
motor A dissipates more power. the energy transfer is less but the time taken is less
describe the energy change when a ball is thrown upwards and then returns to its starting position
- upwards: the KE is converted into GPE
- at its peak: maximum GPE, no KE because the ball is no longer moving
- downwards: GPE is converted back into KE
describe the energy transfers in a bungee jump
- when the person falls: the GPE is converted into KE
- when the cord tightens: that KE is converted and stored as EPE
- when the lowest point is reached: the EPE stored in the cord = the jumper’s initial GPE
why does a bungee jumper slow down once the cord begins to stretch?
- KE decreases as it is converted into EPE
- KE is proportional to velocity^2 so as one decreases so does the other
examples of chemical energy stores
food, batteries, fuel (wood, coal, petrol)
examples of energy stores
chemical, gravitational potential, kinetic, elastic potential, magnetic, electrostatic
what is the spring constant?
the measure of a spring’s stiffness
the greater the spring constant…
the greater the force required to stretch or compress
watt conversions: 1kW (kilo) and 1mW (mega)
1kW = 1000 watts
1mW = 1,000,000
watt = joule conversion
1 watt = 1 joule per second
why can’t an object reach max velocity?
- not all GPE is converted into KE, much is wasted to the thermal energy store of the surroundings
what is elastic potential energy?
a force acting on an object which may cause it to change shape
how is EPE?
- the stored until the force is removed and the deformed object returns back into its original shape
what is GPE?
the energy stored in an object that is raised above the ground
the object has the potential to fall due to gravity
what is work done?
- when an object is moved by a force work is done
- energy is transferred to the object
- energy transferred to object = the work done on the object
work done equation
work done (J) = force applied (N) x distance moved (m)
what is friction?
when a force is applied, friction is what resists movement
examples of ways in which energy can be transferred
mechanically, electrically, via light and sound waves, heating, infrared radiation
weight equation
weight = mass (kg) x gfs (N/kg)
