Physics part 1

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  • Created by: Harriet
  • Created on: 02-04-13 15:19

Energy Types

Energy is extremely important as without it, nothing would happen. Nothing would move, be visible or be hot. Energy can exist in 10 forms:

  • Kinetic Energy - movement energy depends on two factors - mass and speed
  • Gravitational Potential Energy (GPE) - energy due to height. this also depends on mass and gravity strength
  • Elastic Potential Energy (or strain energy) - the energy contained in squashed or stretched objects
  • Sound - energy of vibrating objects
  • Radient Energy - this is wave energy including light, infra-red and ultra-violet
  • Chemical energy- stored in central nucleus of an atom. It's used for nuclear power, nuclear weapons and nuclear submarines
  • Electrical Energy - energy in flowing electrons
  • Magnetic Energy - magnetic materials and electromagnets
  • Thermal energy - energy in hot objects
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Concervation of Energy Principle

There are plenty of different types of energy, but they all obey the same principle:



Energy is only useful when it can be converted from one form to another.

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Although we can't destroy eneryg we can waste it, by transferring it into non-useful forms. The less energy wasted, the more efficient that machine is.

We use Sankey diagrams (or Energy Transfer diagrams) to show the useful and wasted energy (example below).

 Efficiency (energy)             
Efficiency = useful output energy   x 100
                       total input energy               

Efficiency is either expressed as a percentage (less than 100%) or left as a number (less than 1). Usefull energy input isn't usually equal to total eneergy output, as no device is 100% efficient. Wasted energy is usually spread out as heat. Electric heaters are the one exception to this as all the energy is converted to useful heat.     

To use the formula you need to find how much energy is supplied to the machine and how much useful energy the machine delivers. Then divide the smaller one by the bigger one.      

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Power is the rate of energy transfer:

Power = Energy transferred (J)

   (w)           Time Taken (s)      

OR    Power = Voltage x Current  
           (watts)        (V)       (amps)  

1 watt = 1 Joule per second
1 KW = 1000 Joules per second

The efficiency of an electrical object can also be calculated by this equation:

Efficiency (power)

Efficiency = useful output power
                        total input power   

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Paying for Electricity

The unit used for electrical energy used on electricity bills is kilowatt hours. This is because when you're dealing with large amounts of electrical energy (e.g the energy used by a home in one week), it's easier to think of the power and time in kilowatts and hours - rather than in watts and seconds.

A Kilowatt-hour is the amount of electrical energy used by a 1KW applience left on for 1 hour

The two easy formulas for calculating the cost of electricity:

No. of units (kWh) used = Power (in Kw) x Time (in hours)      units = Kw x hours
Cost = No. of units x Price per unit                                           cost = units x price 

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Paying for Electricity page 2

Example 1
An lectricity supplier charges 14p per unit. Find the cost of leaving a 60W light bulb on for:
a) 30 minutes       b) one year

a) No. of units = kW x hours = 0.06kW x 1/2 hr = 0.03 units
    cost = units x price per unit (14p) = 0.03 x 14p = 0.42p for 30 minutes
b) No. of units = kW x hours = 0.06 kW x (24x365)hr = 525.6 units
    cost = units x price per unit (14p) = 525.6 x 14p = £73.58 for one year

Example 2
Each unit of electricity costs 14p. For how long can a 6 kW heater be used for 14p?
A 6 hours      B 1 hour       C 10 minutes       D 7 hours

The cost of 1 unit is 14p. So for 14p you can use one unit.
UNITS = POWER x TIME, so TIME = UNITS / POWER = 1 / 6 = 0.167 hours = C 10 mins

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