energy - forces doing work Flashcards
topic 8
Describe the changes involved in the way energy is stored when systems change (what happens when a system changes)
- when a systme changes, ENERGY IS TRANSFERRED
- it can be trasnferred
a) into or away from the system
b) between different objects in the system
c) between different types of energy stores
how is some energy stored whenever a system changes
some energy is dissipated and stored in less useful ways
explain that where there are energy transfers in a closed system there is no net change to the total energy in that system
– a closed system has a net change in energy of 0
- because law of conservation of energy
what is a closed system
- a system treated completely on its own
- without any energy being exchanged to or from the surroundings
- net change in energy = 0
Identify the different ways that the energy of a system can be changed
a) through work done by forces
- a box is lifted up off the floor (making it a system)
- work is done as the box is lifted against gravity
- causing energy to be transferred to the box’s KINETIC and GPE stores
Identify the different ways that the energy of a system can be
changed
b) in electrical equipment
- electrical devices work by transferring energy between different energy stores
Identify the different ways that the energy of a system can be
changed in electrical equipment ex electric toothbrush
(electric toothbrush is a system)
- it transferes energy electrically from chemical energy store of its battery to the kinetic energy store of its bristles
- some energy is transferred out of system by sound and heating
Identify the different ways that the energy of a system can be
changed in electrical equipment ex hair dryer
(hair dryer is a system)
- transfers energy into system electrically from mains supply to kinetic energy store of fan inside it
- transfers energy electrically to the thermal energy store of the heating element and some is transferred away by sound
Identify the different ways that the energy of a system can be
changed
c) in heating (when system is the pan of water
- pan of water heated on a gas camping stove
- when system is the pan of water => energy is transferred into system by ehating to the thermal enegry stores of the pan and the water (increasing their temperature)
Identify the different ways that the energy of a system can be
changed
c) in heating (when system is the camping stove and the pan)
- energy is transferred from the chemical energy store of the gas
- to the thermal energy stores of the pan and water (increasing their temperature)
describe how to measure the work done by a force
E = F * d
one joule of work is done when a force of one newton causes an object to move a distance of one metre
what is energy equal to
work done
what is the equation to work out how much work has been done
work done (E, J) = force (F, N) * distance moved in the direction of the force (m)
what happens when a force moves an object through a distance
work is done on the object and energy is transferred
Describe and calculate the changes in energy involved when a system is changed by work done by forces
- a force doing work can cause a change
ex it often causes a rise in temperature, energy dissipated to, thermal energy stores of moving object, surroundings - this means that the process is wasteful and the efficiency of the process is reduced
- efficiency equation
Recall and use the equation to calculate the change in
gravitational PE when an object is raised above the ground
change in GPE = m * g * change in height
Recall and use the equation to calculate the amounts of energy
associated with a moving object:
kinetic energy = 0.5 * m * (v squared)
explain how in all system changes energy is dissipated so that it is stored in less useful ways
- some input energy is always dissipated or wasted
- often to thermal energy stores
- energy that isn’t useful gets dissipated
Explain that mechanical processes become wasteful when they cause a rise in temperature so dissipating energy in heating the surroundings
- force done on objects in mechanical processes causes energy to be transferred mechanically
- a force doing work = a rise in temperature
-> energy is dissipated to thermal energy stores
define power
the RATE OF ENERGY TRANSFERRED
example of power as a rate of energy transferred
- larger object does more work per second
ex electric heater has a power of 600W so transfers 600J of energy every second
a 1200W heater would transfer twice as much energy per second so would heat up the room quicker than a 600W heater
equation linking power , work done and time taken
power = work done / time taken
(W) = (J) / (s)
what is one watt equal to
one joule per second (J/s)
what is the equation for efficiency
efficiency = (useful energy transferred by the device) / (total energy supplied to the device)
