P1 Energy Flashcards
Name all energy stores
Chemical energy-energy stored in chemical bonds
Kinetic-energy of a moving object
Gravitational store- energy due to an object’s position in a gravitational field
Elastic store- energy stored in a stretched or compressed string
Thermal store- energy in a warm object
Magnetic store-energy in magnetic fields interacting with each other
Electrostatic store-energy in charged objects interacting with each other
Nuclear store-energy stored in the nucleus of an atom
Name all energy transfers with an exmaple
Sound-speakers,voice
Heat-kettle boiling,fire and heater
Electricity-boiling
Light-sun,light bulbs
Forces-any force that makes something move
Conservation of energy and example
Energy cannot be created or destroyed only transferred usefully from one to another,stored and dissipated
When a rubber band is stretched,its elastic potential energy store increases
When a object is lifted,its gravitational potential energy store is increased
Sankey diagram example
-energy supplied is shown at the start of arrow
-the useful energy is show by the arrow pointing straight ahead
-‘wasted’ energy is shown by the arrow curving away from the useful energy
What happens when an object is falling down
-when an object falls and gains speed, its store of gravitational potential energy decreases and its kinetic energy store increases.
-when a falling object hits the ground without bouncing back, its kinetic energy store decreases. Some or all of its energy is transferred to the surroundings-the thermal energy store of the surroundings increases, and energy is also transferred by sound waves.
What is a closed system
A system in which no energy transfers take place out or into the energy stores of the system
Energy can still be transferred but overall change is always zero.
Energy can be transferred between energy stores within a closed system. The total energy us always the same,before and after
What happens in an open system
An open system can lose or gain energy when interacting with the open world
What is work and notes on work
Work is the amount of energy transferred when a force moves (in the direction of the force)
-the amount of energy transferred to the object is equal to the work done to it.
Energy transferred=work done
The work done by an object depends on the size of the force and the distance moved. One joule of work is done when a force of one newton causes an object to move a distance of one metre in the direction of the force.
What is the equation for work
Work done= force x distance
W=F x D
Work=J
Force=N
Distance=M
What energy transfers occur when a bungee jumper jumps of a platform
-when rope is slack, energy is transferred from the gravitational potential energy store to the kinetic energy store as the jumper accelerates forwards to the ground due to the force of gravity
-when the rope tightens it slows the bungee jumper’s fall. This is because the force of the rope reduces the speed of the jumper. The jumper’s kinetic energy store decreases and the rope’s elastic potential energy store increases as the rope stretches. The jumper comes to a stop as all energy from kinetic energy store has been transferred into the elastic potential energy store of rope
- after bottom, the rope recoils and pulls jumper back. As jumper rises the energy in elastic potential energy store of rope decreases until rope becomes slack. After the rope becomes slack, and at top of ascent, the bungee jumper’s kinetic energy store decreases to zero. The bungee jumper’s gravitational potential energy store increases throughout ascent.
Explain the energy transfers in a fire fuel such as coal to produce heat
-coal has a huge store of chemical energy (energy with chemical bonds)
- when coal is burned the chemical energy is transferred by heating to the thermal energy store of the coal
-hot coals and flames transfer energy to the surroundings by heating and radiation
Energy transfer examples of a driver slowing down 15m/s to rest
-the moving car has kinetic energy
-the brakes mechanically do the work on car to slow it down
-the kinetic energy of the far decreases and the thermal energy store of the brakes increase
Friction at work notes
-work done to overcome friction is mainly transferred to the thermal energy store
-if you rub your hands vigorously,they become warm. Your muscles do work to overcome the friction between your hands. The work done is transferred as energy that warms your hands
-brake pads on a vehicle become hot if the brakes are applied for too long. Friction between the brake pads and the wheel discs opposes the motion of the wheel. The force of friction does work on the brake pads and wheel discs. As a result, energy is transferred from the kinetic energy store of vehicle to thermal energy store of the brake pads and wheel discs. This makes them become hot and transfer energy by heating to the thermal energy store of the surrounding air.
Defenition of gravitational potential energy
Is the energy stored by an object due to the gravitational field of the earth.
Notes on gravitational potential energy
-When an objects is raised the energy in its gravitational potential energy increases. This increase is equal to the work done on it by lifting force to overcome the gravitational force on the object.
- when an object moves down, the energy in its gravitational potential energy store decreases. This decreases is equal to work done by the gravitational force acting on it.
Formula for gravitational potential energy increase or decrease
Change in objects gravitational potential energy store=weight x change in height
C=Joules
W=Newtons
H=metres
Gravitational potential energy formula
Gravitational potential energy= mass x gravitational field strength x height
GPE=mgh
GPE= joules
Mass= Kg
Gravitational field= N/kg
Height= m
Definition of kinetic energy
Kinetic energy is the energy an object possesses due to its motion.
Anything that moves.
Formula for kimetic energy
Kinetic energy= 1/2 x mass x speed*2
KE=(mv*2) / 2
KE= J
M = kg
V = m/s
Kinetic energy notes
-small mass, not fast, low kinetic energy
-big mass, real fast , high kinetic energy
Falling objects convert Ep into Ek so (kinetic energy gained=potential energy lost)
And some of this kinetic energy is transferred to heat and sound.
Elastic potential energy notes
Hookes law = The extension of a stretched spring is directly proportional to the force applied
- an object may change shape or stretch if a force is applied
-any object that can go back to its original shape after force removed is an elastic object
-work is done to an elastic object to change it shape and stored in elastic potential energy store
-elastic potential energy is converted to kinetic energy when the force is removed and object returns to original shape.
Elastic poential energy formula
Elastic potential energy= 1/2 x spring constant x extension*2
EPE=(ke*2) / 2
EPE= J
K = n/m
e = (m)
Difference between wasted and useful energy
-useful energy is energy transferred to where it is wanted in the way that it is wanted
-wasted energy is the energy that is not usefully transferred.
Dissipation of energy
-friction is a force exerted on an object sliding across another object
-friction transfers energy as heat
- heat energy is transferred to the surroundings which becomes slightly warmer
-as energy dissipates it gets less and less useful
-this dissipated energy is referred as wasted energy.
Energy spreading out examples
Wasted energy dissipated to surroundings- gears of a car get hot because of friction when the car is running. Energy transferred from kinetic energy of gear box to thermal energy store of gear box and surrounding air
Useful energy is eventually transferred to surroundings- useful energy applied to turn wheels of car us transferred from kinetic energy store of wheels to thermal energy store of tyres by heating-increasing thermal energy store of tyres.
Energy becomes less useful more it spreads out- energy supplied to heat rooms will be eventually transferred to surrounding air
What is the 2 formula for efficiency
Efficiency= useful output energy transferred / total input energy supplied
Efficiency= useful power out / total power in
Mo energy transfer can be 100% efficient
How to improve efficiency of objects
Friction between moving parts causing heating- lubricate the moving parts to reduce friction
Resistance of a wire causes the wire to get hot when current passes through - in circuits, use wires with as little electrical resistance as possible
Air resistance on moving object- streamline the shapes of a moving object to reduce air resistance
Sound created by machinery- cut out noise by tightening loose parts to reduce vibration
Electrical appliances-useful and wasted energy
Light bulb:
Useful- light emitted from the glowing filament
Wasted-energy transfer from the filament heating the surroundings
Electrical heater:
Useful-energy heating the surroundings
Wasted- light emitted from the glowing element
Electric toaster:
Useful- energy heating bread
Wasted-energy heating the toaster case and the air around it
Electric kettle:
Useful-energy heating water
Wasted-energy heating the kettle itself
Hairdryer:
Useful-kinetic energy of the air driven by fan/ energy heating air flowing past the heater filament
Wasted- sound of fan motor (energy heating the motor heats the air going past it)/ energy heating the hairdryer itself
Electric motor:
Useful- kinetic energy of the objects driven by the motor/gravitational potential energy of objects lifted by the motor
Wasted-energy heating the motor and energy transferred by sound waves generated by motor
Clockwork radio-electrical appliance
When you turn the handle of a clockwork radio, you wind up a clockwork spring in it, and increase the elastic potential energy of the spring. When the spring unwinds, energy from elastic potential energy store is transferred to its kinetic energy store and it turns a small generator in the radio. It does not need batteries or main electricity. People in remote areas can use without having to walk miles for replacement battery. But they do have to wind up spring every time it store of energy has been used
Choosing an electrical appliance
An electrical appliance is designed for a particular purpose and should waste as little energy as possible
Power notes-formula
The more powerful an appliance is, the faster the rate at which it transfers energy
Power= energy/time
P=E/T
P=watts
E=joules
T=seconds
Muscle power
Weightlifter:
-a 30kg dumbbell has a weight of 300n. Raising it by 1m would increase its gravitational potential energy store by 300j. A weightlifter could lift it in about 0.5 seconds. The rate of energy transfer would be 600j/s. So weightlifter’s power output would be about 600w in total.
Efficiency and power
For any appliance:
-its useful power output is the useful energy per second transferred by it.
-its total power in is the energy per second supplied to it.
Efficiency= useful energy transferred by device/ total energy supplied to it X. 100
Efficiency= useful power output/total power in x 100
Wasted power
Power wasted=total power in-useful power out
Calculating cost of electricity
No. Of units (kwh) used = power (in kw) x time (hours)
Cost= no,of units x price per unit
