Topic 8: Energy - forces doing work Flashcards
8.1 Describe the changes involved in the way energy is stored when systems change
systems = single object/ group of objects
when a system changes: energy is transferred: into/away from system, between different objects in a system or between different types of energy stores
whenever a system changes - some energy is dissipated and stored in less useful ways
The efficiency of a transfer is the proportion of the total energy supplied that ends up in useful energy stores
8.2 draw diagrams to represent energy transfers
any arrows going off main arrow is energy lost from the system
8.3 what’s a closed system
where there are energy transfers in a closed system there is no net change to the total energy in that system
8.4 how can energy of a system be changed through work done by forces?
- A box is lifted up - the box is the system.
- Work is done against gravity when the box is lifted.
- This causes energy to be transferred to the box’s kinetic and gravitational potential energy stores.
(if the box was dropped - gravitational force would do work to transfer energy from the box’s GPE store to its kinetic energy store)
8.4 how can energy of a system be changed in electrical equipment?
- electrical devices work by transferring energy between different energy stores
e.g. electric irons transfer energy electrically from the mains power supply to the thermal energy store of their metal plates
electric toothbrush
= system
* transfers energy electrically from chemical energy store of battery to kinetic energy store of its bristles
* some of the energy is transferred out of systems to surroundings by sound and by heating
8.4 how can energy of a system be changed in heating?
- pan of water heated on a gas stove
- when system = pan of water:
energy transferred to system by heating to the thermal energy stores of the pan and the water - increases their temp - when the system = stove and pan
energy transferred from the chemical energy store of the gas to the thermal energy stores of the pan and water - increasing temp
8.5 when is work done
when a force moves an object through a distance, WORK is DONE on the object and energy is transferred
8.6 work done equation
E = F x D
work done (joule J) = force (newton, N) x distance moved in direction of force (metre, m)
8.7 describe the changes in energy involved when a system is changed by work done by forces
when you push something along a rough surface (e.g. carpet) you are doing work agaisnt frictional forces
- energy is being transferred to the kinetic energy store of object - starts moving
- some energy also being transferred to thermal energy stores: friction
- causes overall temperature of object to increase
8.8 change in gravitational potential energy equation
△GPE = m x g x △h
change in gravitational potential energy (joule, J) = mass (kg) x gravitational field strength (N/kg) x change in vertical height (m)
8.9 kinetic energy store equation
KE = ½ x m x v²
kinetic energy (J) = ½ x mass (kg) x speed² (m/s²)
8.10 how is energy dissipated so that it is stored in less useful ways?
a force doing work often = rise in temp
- as energy is dissipated to the thermal energy stores of the moving object and its surroundings
- means that the process is wasteful and the efficiency of process is reduced
(mechanical processes become wasteful when they cause a rise in temp so dissipating energy in heating the surroundings)
8.11 what happens to mechanical processes when they cause a rise in temperature
become wasteful as dissipating heat into the surroundings
8.12 define power and use an example to explain the definition
the rate of energy transferred
the larger the power of an object the more work it does per second
e.g. if an electric heater has a power of 600W - it transfers 600J of energy per second.
A 1200W heater transfers twice as much energy per second and so would heat a room quicker than the 600W heater.
8.13 power equation
P = E/t
P - power in watts (W)
E - energy in joules (J)
t - time in seconds (s)
efficiency equation
total energy input
