04-05-2012, 11:40 AM
The Nature of Energy
The Nature of Energy.ppt (Size: 2.29 MB / Downloads: 36)
Thermal Energy
The kinetic energy of the molecules in a solid, liquid or gas
The more kinetic energy, the more thermal energy the object possesses
Physicists also call this the internal energy of an object
Temperature
A measure of how hot or cold something is
Measured with a thermometer
Celsius Scale sets freezing point of water at 0o and the boiling point at 100o
Fahrenheit Scale sets freezing point of water at 32o and the boiling point at 212o
Heat
Touch something hot and thermal energy enters your hand
Touch something cold and thermal energy leaves your hand
Direction of energy flow is from hot to cold
Heat is the thermal energy transferred due to a temperature difference
Energy in the Thermal System
Thermal energy that flows from areas of hot to cold
Energy in the Thermal System
whenever work is done
whenever energy is converted from one form to another
Q3. What happens to most heat that is generated?
Q4. What is the SI unit for thermal energy?
Q5. How many joules are in one calorie of heat?
Q6. What is the English unit for thermal energy?
Q7. How many foot-pounds are in one Btu?
Q8. What happens when two objects of different temperature are brought into contact?
Q9. What is the equation for calculating heat during a temperature change?
Q10. Suppose you heat a piece of metal and place it in a cup of cold water. How does the heat flow?
Q11. What is a change of state?
Q12. What is sensible heat?
Q13. What is latent heat?
Q14. Example – A sample of ice at -10oC is heated until it becomes steam at 110oC. Classify the heat changes as latent or sensible.
Q15. What are three ways that heat can be transferred?
Q16. What is conduction?
Q17. What is convection?
Q18. What is radiation?
Heating & Insulation
Insulation- A substance that reduces the transfer of thermal energy
Made of materials that do not conduct thermal energy very well
Used in walls, ceilings, and floors to keep a house warm in winter & cool in summer
Heat Engines
Heat Engine- A machine that uses heat to do work
In a heat engine, fuel combines with O2 in a chemical change to produce thermal energy (combustion)
Two types of combustion engines
External- Burn fuel outside the engine
Internal- Burn fuel inside the engine
External Combustion Engine
A simple steam engine is an example of an external combustion engine
Coal is burned to heat water & convert it to steam
The steam drives a piston which is attached other mechanisms that do work
Internal Combustion Engine
What is the function of an internal combustion engine?
Convert fuel to power
This is accomplished by:
Burning a fuel in a closed chamber
Using the expansion of the gas to force one wall (piston) of the chamber to move
Internal Combustion Engine
Requirements for Operation
Air is drawn into the cylinder
A quantity of fuel is introduced into the cylinder
Air & fuel are mixed
Fuel-air mixture is compressed
Fuel-air mixture is ignited
Burning of fuel-air mix causes a pressure increase & moves the piston
Connecting rod & crankshaft converts linear motion to rotary motion. The force on the piston is converted to torque on the crankshaft.
Products from combustion are expelled from the engine
Internal Combustion Engine
Four Cycle Engine
Intake
Intake valve opens, exhaust valve closes
Compression
During compression, the air-fuel mixture increases in temperature and reaches a point of ignition
Power
Rapid expansion of burning gas causes a rise in pressure, which forces the piston away from the cylinder head
This is the power stroke
Chemical energy is converted to torque
Exhaust
Exhaust valve opens.
Products of combustion are expelled through open valve.
Entire Process Starts Over!!