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p7 (energy at home (can be transferred usefully eg radiators and wasted eg…
p7
energy at home
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loft insulation : fibreglass wool lair on the loft floor and ceiling reduces energy loss from the house by conduction and convection
hot water tank jacket : reduces conduction, keeps water hot
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draught proofing : strips of foam and plastic around doors and windows stops hot air going out, reduces convection
cavity walls/cavity insulation : two layers of bricks w a gap between to reduce convection, energy also transferred across the gap by convection, squirting insulating foam into the gap traps pockets of air to minimise convection
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elastic objects
eg springs, squashed by moving object, work is done on it and energy is transferred from it's kinetic energy store to it's elastic potent energy store
as spring pushes object back out, spring transfers energy back to the kinetic energy store of the object
same energy transfers take place if the object was stretching the spring and the spring pulled the object back in
energy transferred to the elastic potent energy store of a stretched or squashed spring = 0.5 x spring constant x extension squared
power ratings
amount of energy transferred to a device is the same as the work done by the battery or mains when a current flows
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the power rating is how much energy is transferred to the appliance per second (the rate of energy transfer)
energy transfer can be measured in kilo-watt hours, kWh, the amount of energy transferred to a 1000 W device per hour
cont
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energy still lost from the walls by radiation, if there are any spaces where air is not trapped, there will be some convection
thicker walls, lower rate of energy transfer. thermal conductivity of the material also effects, high thermal conductivity, high rate of energy transfer
friction
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changing shape of an object, eg making a car more streamlined, reduces friction from air resistance
electrical circuits
when device is plugged into a socket in the wall, connected to the mains ie the national grid, energy transferred electrically from mains to device
battery has energy in its chemical energy store, energy can be transferred electrically to the devices in a circuit
some energy is wasted, lost during transfer eg energy is transferred to the thermal energy stores of the wires by heating