Energy and Efficiency & The Usefulness of Electrical Appliances Powerpoint (P1.2 and P1.3)

A presentation I made for personal use, but that I thought might be useful for some of you. I apologise if any of the notes are brief; originally they were only meant for my use, but hope it helps you with your revision :)

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Slide 1

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Physics: P1
Energy and Efficiency
The Usefulness of Electrical Appliances…read more

Slide 2

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Energy Transfers
· Energy can be transferred, stored or
dissipated, but not created or destroyed.
· Most devices take in one form of energy and
transfer it in another. Electric lamp
· Only part of the energy is usefully transferred;
the rest is wasted.
· Wasted energy becomes spread out and
warms its surroundings; it is difficult to use
for further energy transfers.
· Example: a light bulb transforms electrical
energy into light, but most is wasted as heat
· Sankey diagrams show the relative
proportions of energy using arrows; the width
of the arrows show the proportion of energy
they represent.
· Electrical appliances transfer useful energy
but some is wasted.
· Example: a kettle transfers heat energy to the
water and some is wasted as sound and as
heat to the kettle and the air.
· Example: a motor transfers kinetic energy and
some is wasted in the form of heat and sound.…read more

Slide 3

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Energy efficiency
· Refers to the proportion of energy that is usefully transferred.
· The greater the proportion, the more efficient and cost-
effective a device is.
· Payback time is calculated by the amount you pay for a more
efficient device, divided by how much you save per year.
· Efficiency is expressed as a percentage or decimal.
· It is calculated using this equation:
­ (Useful energy out ÷ total energy in) x 100
· Example: Only a quarter of energy supplied to a television is
transferred as useful light and sound energy.
· The rest is wasted, so the television is only 25% efficient.…read more

Slide 4

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Transferring Electrical Energy
· Most energy transferred to houses and
industry is electrical.
· Electrical energy is easily transferred into
heat, light, sound and kinetic energy.
· The amount of energy transferred by an
appliance depends on how long it runs
for and its rate of energy transfer.
· Without a reliable source of energy we
have power cuts.…read more

Slide 5

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Energy Calculations
· Energy is measured in joules (J).
· Power is the rate of energy transfer, and is
measured in watts (W) or kilowatts (kW).
· 1 watt = 1 joule per second.
· To calculate the amount of energy transferred:
· E=pxt
­ E = energy in joules or kW hours
­ p = power in watts or kilowatts
­ t = time in seconds or hours
· The cost of electrical energy is:
­ No. Or kW hours x cost per kW hour…read more


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