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P1 - Conservation and Dissipation of Energy 2 (P1.9 - Energy and Power…
P1 - Conservation and Dissipation of Energy 2
P1.7 - Energy and Efficiency
weight is measured in newtons - 1kg = 10N
energy is measured in joules - energy needed to lift 1N by 1m = 1J
energy supplied to the device is called input energy
useful energy transferred by the device is called useful output energy
input energy (J) = useful output (J) + energy wasted (J)
EFFICIENCY = USEFUL OUTPUT ENERGY (J) / TOTAL INPUT ENERGY (J)
no device can be more than 100% efficient because you can get more energy from a machine than you put into it
IMPROVING EFFICIENCY
if friction between moving parts causes heating , lubricate the moving parts to reduce friction
if the resistance of a wire causes the wire to get hot when a current passes through it , use wires with as little electrical resistance as possible
if air resistance causes a force on a moving object that opposes and the energy is transferred to the surroundings, streamline the moving objects to reduce air resistance
if sound is created by machinery , cut out noise by tightening loose parts etc
P1.8 - Electrical Appliances
light bulb
useful
light emitted from glowing filament
wasted
heat to surroundings
electric heater
useful
heat to surroundings
wasted
light emitted from glowing element
electric toaster
useful
heating bread
wasted
heating toaster case and surroudings
electric kettle
useful
heating water
wasted
heating kettle itself
hairdryer
useful
kinetic energy of air driven by fan
heating air flowing past heater filament
wasted
sound of fan motor
heating hairdryer itself
electric motor
useful
kinetic energy of objects driven by motor
gpe of objects lifted by motor
wasted
heating motor
sound generated by motor
clockwork radio
Trevor Baylis created this in early 1990s
when you turn a handle you wind the clockwork spring in it and increase the epe of the spring when the spring unwinds energy from the epe store is transferred to kinetic energy and it turns a small electric generator in the radio
it doesnt need batteries or mains electricity
one disadvantage is that you have to wind up the spring every time the store of energy has been used
P1.9 - Energy and Power
the energy supplied to a motor per second is the power supplied to it
the more powerful the motor is the faster it works and the faster the rate at which is transfers energy
power is measured in watts or kilowatts
1 kW = 1 J/s
POWER (W) = ENERGY TRANSFERRED TO APPLIANCE (J) / TIME TAKEN FOR ENERGY TO BE TRANSFERRED (s)
an appliances useful output power is the useful energy per second transferred by it
its total input power is the energy per second supplied to it
EFFICIENCY = (USEFUL POWER OUT / TOTAL POWER IN) X 100%
power wasted = total power in - useful power out