Efficiency of machines Flashcards
When are useful devices only useful?
Because they can transform energy from one form to another.
When useful devices transform energy from one form to another what happens?
Some of the useful input energy is always lost or wasted, usually as heat.
When is the device said to be more efficient?
The less energy that is wasted from it.
What does an energy flow diagram look like?
—> useful device —>
|
1st arrow = energy input
2nd arrow = useful energy output
3rd arrow = wasted energy
What is the equation for working out the efficiency of a device?
Efficiency = useful energy or power out / total energy or power in
What is efficiency given as?
A decimal or x 100 to give a percentage.
What is the formula triangle in case you don’t need to find the efficiency?
Efficiency x total in
Why is useful energy input not usually equal to total energy output?
Because no device is 100% efficient and the wasted energy is usually spread out as heat.
What is the exception to the no device is 100% efficient rule?
Electric heaters.
Why are electric heaters usually 100% efficient?
Because all the electricity is converted to useful heat.
What does the entire energy output (useful and wasted) by a machine eventually end up as?
Heat.
What is useful energy?
Concentrated energy.
Why is the wasted energy called wasted heat?
Because we can’t do anything useful with it.
What does the wasted heat do?
It is transferred to cooler surroundings, which become warmer. As this happens the energy becomes less concentrated, it dissipates.
What happens as the wasted energy dissipates?
The total amount of energy stays the same, as it is still there.
What happens as the wasted energy comes increasingly spread out?
It can’t be easily used or collected back in again.
When choosing appliances what do you need to think about?
Cost effectiveness and efficiency.
What can waste energy sometimes be?
Useful.
How can waste energy sometimes be useful?
By using heat exchangers that reduce the amount of heat energy that is lost.
How do heat exchangers reduce the amount of heat energy that is lost?
- They pump a cool liquid through the escaping heat.
- The temperature of this fluid rises as it gains heat energy.
- The heat energy in the fluid can then be converted into a form of energy that’s useful again, in the old or a new device.
What is an example of a heat exchanger?
The heat from a car’s engine can be transferred to the air that is used to warm the passenger compartment.
