# Physics

• Created by: Sorrel
• Created on: 14-05-13 20:34
efficiency =
USEFUL (energy/power out) / TOTAL (energy/power in)
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efficiency can be written in the form of either
a decimal or a percentage
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the law of the conservation of energy
energy cannot be created or destroyed
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good emitters and absorbers of thermal radiation
dark, matte surfaces
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poor emitters and absorbers of thermal radiation
light, shiny surfaces
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in terms of infra-red radiation, the hotter an object gets...
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the only form of energy transfer that does not need particles
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specific heat capacity is ...
the amount of energy required to raise the temperature of 1kg of a substance by 1 degree celcius
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energy transferred in joules (E) =
m x c x temp. (Energy transferred in joules x mass in kg x specific heat capacity in j/kgdegreesC x temperature change in dgrees C
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conduction takes place in...
a solid
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convection takes place in...
fluids - liquids and gases
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define conduction
the particles of a solid gain energy when heated, so they vibrate and collide with each other and transfer the heat energy by making those new particles move with energy
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define convection
the particles gain energy when heated, so they move faster and thus move further apart, thus making them less dense - this means that they rise causing cooler particles to then gain more energy, so a circular current is made
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define evaporation
the particles gain energy, so move further and further apart, and eventually they break off from the main body of liquid to become a gas
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payback time =
cost of insulator / annuel fuel saving
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define U values
they measure how effective a material is as an insulator
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the lower the U value, the .... the insulator
better
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what is sublimation
from a solid straight to a gas
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the power of a device is measured in...
watts or kilowatts
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power in watts =
joules per second
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energy transfered in kilowatt hours (kW h) =
power in kilowatts x time in use in hours
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total cost for running =
number of kilowatt hours used x cost per kilowatt hour
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power stations produce electricity at a voltage of
25 000 volts
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energy is supplied to home at a voltage of
230 volts
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the network of cables that connect power stations to homes and buildings
the national grid
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the national grid's voltage
132 000 volts (or more)
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what does a step-up transformer do?
this increases the voltage from 25 000 volts to 132 000 volts before it is transported via pylons and cables
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why does a step-up transformer increase the voltage?
because a higher voltage means a lower current, and it is the current that releases the energy, so a higher current means more energy lost
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what does a step-down transformer do?
used in local sub-stations to decrease the voltage down from 132 000 volts to 230 volts so that the appliances in our homes can handle the voltage
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## Other cards in this set

### Card 2

#### Front

efficiency can be written in the form of either

#### Back

a decimal or a percentage

### Card 3

#### Front

the law of the conservation of energy

### Card 4

#### Front

good emitters and absorbers of thermal radiation

### Card 5

#### Front

poor emitters and absorbers of thermal radiation