EFFICENCY OF MACHINES AND ELECTRCITY Flashcards
During energy transfers who is energy most often lost
Through heat
Why are machines useful
Because they transfer energy from one form to another
How is some of the input energy lost or wasted in a machine?
It is wasted as heat but the more efficient the machine the less energy that is lost.
How do you calculate the efficiency of a machine?
Efficiency = Useful Energy Out (or Useful Power Out)
______________
Total Energy in (or USeful Power In)
Is any device 100% efficient?
Not many machines are 100% efficient and energy in rareky equals energy out as some of the energy is wasted in heat.
The EXCEPTION to this is electrical heaters as they are 100% efficient because all electricity is converted to useful heat energy
Consider COST EFFECTIVENESS V EFFICIENCY
Energy efficient bulbs are more expensive than normal bulbs but last longer.
LED bulbs are even more efficient than ow energy bulbs and can last even longer.
However, they don’t give out as much light as the other two types of bulbs.
Sometimes WASTE ENERGY can actually be useful. Give an example of this.
HEAT EXCHANGERS reduce the amount of heat energy that is lost.
It does this by pumping cool fluid through the ESCAPING HEAT.
This fluid gains heat energy and the heat energy in the fluid can be used again.
Heat from a car engine can be used to transfer air to the air that used to warm the passenger compartment.
Check out page 22 of Revision Guide
There’s no answer!!,
Check out page 22 of Revision Guide
What are the units of electricity?
Kilowatt-hours (kWh) are units of electricity
What is the equation of energy, power & time?
ENERGY = POWER x TIME
Energy \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Power x Time
How in energy usually measured?
Joules (J)
How is power usually measured?
Watts or kilowatts
A 5 kW appliance transfers 5000 J in 1 second
What are the two formulas for calculating electricity?
No of units (kW) used = POWER x TIME
COST = No of UNITS x PRICE (per unit).
An electricity supplier charges 14p per unit
Find the cost of leaving a 60w light bulb on for for (a) 30 minutes (b) one year.
Answer (a)
No of units = kW x hours
0.06 kW x 30 mins
COST = units x price per unit 14p. =. 0.03 x 14p = 0.42p for 30 mins
Answer (b)
No of units = kW. X. Hours
0.06 kW. X one year
COST = units x price per unit
14p. = 525.6 x 14p = £73.58 for one year
What are the pros and cons for using either a battery radio or a clockwork radio?
- Both do not use mains electricity.
- Batteries can be expensive but powering a clockwork radio is free.
- Battery power can run out unless you have spare batteries. You don’t get that problem with clockwork radios.
- Annoying to have to wind up radio but don’t do this with battery power.
- Clockwork better for environment
