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Energy
The Nature of Energy
What is energy?
Electrical Energy
Energy is the ability to cause a change or the ability to do workWhat are the different forms of energy?
Solar Energy
Nuclear Energy
Heat Energy
Mechanical Energy
The energy of different objects may fall into two main categories:
Kinetic EnergyKinetic Energy and Potential EnergyPotential Energy
Kinetic EnergyKinetic Energy is the energy of a moving object
Potential EnergyPotential Energy is stored energy. This can be thought of in two ways. Stored energy in our bodies or in a battery is called chemical potential energychemical potential energy. Stored energy due to the position of an object above the surface of the earth is gravitational gravitational potential energy.potential energy.
Calculating Kinetic Energy
Kinetic EnergyKinetic Energy depends upon two factors: the mass of the object and its velocity.
Mass is measured in kg and velocity is measured in m/s
When the units are multiplied, the result is kg m2/s2
Since a Newton (N) is a kg m/s2, therefore kg m2/s2 is the same as a N-m, which is the same as a joule (J). Hence, the unit of kinetic energy is the joule.
Practice ProblemsHow much energy does a soccer player with a mass of 75 kg and a velocity of 5.5 m/s possess?
KE = ½mv2 KE = ½(75 kg)(5.5 m/s)2
KE = 1.1 X 103 kgm2/s2 1 kgm/s2 = 1N
KE = 1.1 X 103 Nm 1 Nm = 1 Joule (J)
KE = 1.1 X 103 Joules (J)
Solve the following:What is the velocity of a car with a mass of 675 kg and a kinetic energy of 1690 joules?
KE = ½mv2 2KE = mv2 2KE = v2
m
2KEm
= v v = 2(1690J)675 kg
v = 2.24 m/s
2( ) 2( )
Calculating Gravitational Potential Energy
Gravitational Potential Energy (GPE) depends upon three factors: the mass of the object, the height of the object from the surface of the earth, and the gravitational acceleration constant (on earth = 9.8 m/s2)
Notice how the units multiply :
mass (kg) x m/s2 x height (m). Hence the unit is kg m2/s2
which is the joule (J).
Let’s do a practice problem…
Potential EnergyCalculate the energy possessed by a rock on the side of a cliff. The rock has a mass of 237 kg at an elevation of 45 meters.
PE = mgh PE = (237 kg)(9.8 m/s2)(45m)
PE = 1.0 X 105 kgm2/s2
PE = 1.0 X 105 Nm
PE = 1.0 X 105 Joules
What is the Law of Conservation of EnergyLaw of Conservation of Energy?
Energy cannot be created nor destroyed….but….
Energy can be transformed from one type to another.
In this diagram, electrical energy is converted into thermal (heat) energy and light energy.
There are many energy transformations in an automobile.
The explosion of the gasoline involves the release of chemical energy.
The chemical energy causes the pistons in the engine to move (mechanical energy)
The moving pistons affect many moving parts including a generator which produces electrical energy which keeps the battery charged.
The moving pistons and other moving parts also produce heat energy.
Objects can convert potential energy to kinetic energy or kinetic energy to potential energy.
The diagram below illustrates this relationship.
In nuclear reactions some mass is converted into energy.
The major source of energy is the SUN.
In photosynthesis plants convert sunlight into stored energy in the plants.
Animals and humans eat the plants to get energy.
The human body “burns” this stored energy in units called Calories.
One Calorie is equal to 4.18 J.
The human body stores energy in Calories and will utilize “burn” this energy as needed.
What is Thermal Energy?
Particles of matter are in constant motion. This motion relates directly to the state of matter of the object (solids, liquids, or gases).
TemperatureTemperature affects how fast these particles move. The higher the temperature the faster the particles move. Moving particles possess kinetic energy.
TemperatureTemperature is defined at the average kinetic energy of the particles of an object.
Thermal EnergyThermal Energy is the sum total of all of the energy of the particles of an object.
Thermal energyThermal energy and temperaturetemperature are related though DIFFERENT.
TemperatureTemperature is the average kinetic energy of the particles of an object.
Thermal energyThermal energy is the total amount of energy of the particles of an object.
A bathtub full of water at 100oF has more thermal energy than a thimble of water at 100oF. The temperature is the same but the total amount of energy is different. The bathtub has more energy.
What is heatheat?
HeatHeat is thermal energy that flows from something at a higher temperature to something at a lower temperature.
What is specific heatspecific heat?
Specific heatSpecific heat is a property of matter which determines how readily a material is to absorb heat and change temperature.
Specific HeatSpecific Heat is defined as the amount of heat energy needed to raise the temperature of 1 kg of a substance by 1 oC or 1 K.
Did you notice that water has a very high specific heat, whereas iron has a low specific heat?
Water requires a lot of heat energy to raise its temperature.
Metals require little energy to raise its temperature.
Thermal energy can be calculated using the following formula:
Try this one!! Answer = 376 J/kg K
Did you do this problem?
Transferring Thermal EnergyTransferring Thermal Energy
How is Thermal Energy Transferred? YOU MUST KNOW THIS!!!!!
ConductionConduction – direct contact
ConvectionConvection – through a fluid
RadiationRadiation – by electromagnetic waves
What is heat transfer by conductionconduction?
Heat transfers as particles of an object increase their collisions as heated. These collisions transfer the heat energy through the object by colliding with adjacent particles.
What is heat transfer by convectionconvection?
Heat is transferred through a substance through currents. This occurs in fluids (liquids AND gases)
ConvectionConvection currents are caused by heating of a liquid or gas, the liquid or gas rises, then cools and falls. This occurs in the mantle of the earth
And in the atmosphere.
Most of our weather patterns are the result of convection currents in the atmosphere.
What is heat transfer by radiation?
Radiation is heat transfer by electromagnetic waves. These wave may pass through all states of matter and also through NO matter – such as the vacuum of space.
This energy is often called radiant energy.
Radiant energy from the sun travels through the vacuum of until it reaches the earth.
How is heat flow controlled?
Insulators – a material which does not allow heat to pass through it easily. Some animals have
good insulation to survive severe winters.
Buildings and houses are insulated so that heat does not pass out of (winter time) and into (summer time).
Using heat – How do we use heat in our lives?
Forced-Air Systems – a fuel is burned in a furnace and a fan circulates the heat in the house
Radiator Systems – closed metal containers that contain hot water or steam. The thermal heat is transferred to the air and circulated by convection currents.
Electric radiators – heat metal coils which transfer the thermal heat to the surrounding air.
Using heat to do work – the internal combustion engine
Automobile engines are usually four-stroke engines since each four-stroke cycle converts chemical/thermal energy to mechanical energy.
Refrigerators, air conditioners, and heat pumps – how do they work?
A coolant is circulated through pipes by a compressor. When the pipes get small and narrow the coolant compresses and changes to a liquid giving off heat energy. When the pipes get larger, the coolant changes from a liquid to a gas (evaporates) causing the area to absorb heat and get cooler
The human coolant - In-built cooling system
The human body has a cooling system which function automatically – perspiration – sweat.
Like a refrigeration system, the evaporation of the sweat from the surface of the body causes energy to be absorbed from the skin, causing the skin to feel cooler.
No sweat!!!
