Energy

The Nature of Energy What is energy? Electrical Energy Energy is the ability to cause a change or the ability to do work What 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 EnergyPotential Energy Kinetic Energy and Potential Energy Kinetic Energy Kinetic Energy is the energy of a moving object Potential Energy chemical potential energy gravitational potential energy. Potential 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 energy. Stored energy due to the position of an object above the surface of the earth is gravitational potential energy.

Calculating Kinetic Energy Kinetic Energy Kinetic 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 m 2 /s 2 Since a Newton (N) is a kg m/s 2, therefore kg m 2 /s 2 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 Problems How much energy does a soccer player with a mass of 75 kg and a velocity of 5.5 m/s possess? KE = ½mv 2 KE = ½(75 kg)(5.5 m/s) 2 KE = 1.1 X 10 3 kgm 2/ s 2 1 kgm/s 2 = 1N KE = 1.1 X 10 3 Nm1 Nm = 1 Joule (J) KE = 1.1 X 10 3 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 = ½mv 2 2KE = mv 2 2KE = v 2 m 2KE m = v v = 2(1690J) 675 kg v = 2.24 m/s 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/s 2 ) Notice how the units multiply : mass (kg) x m/s 2 x height (m). Hence the unit is kg m 2 /s 2 which is the joule (J). Let’s do a practice problem…

Potential Energy Calculate 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/s 2 )(45m) PE = 1.0 X 10 5 kgm 2 /s 2 PE = 1.0 X 10 5 Nm PE = 1.0 X 10 5 Joules

Law of Conservation of Energy What is the Law 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). Temperature Temperature affects how fast these particles move. The higher the temperature the faster the particles move. Moving particles possess kinetic energy. Temperature Temperature is defined at the average kinetic energy of the particles of an object.

Thermal Energy Thermal Energy is the sum total of all of the energy of the particles of an object. Thermal energytemperature Thermal energy and temperature are related though DIFFERENT. Temperature Temperature is the average kinetic energy of the particles of an object. Thermal energy Thermal energy is the total amount of energy of the particles of an object. A bathtub full of water at 100 o F has more thermal energy than a thimble of water at 100 o F. The temperature is the same but the total amount of energy is different. The bathtub has more energy.

heat What is heat? Heat Heat is thermal energy that flows from something at a higher temperature to something at a lower temperature. specific heat What is specific heat? Specific heat Specific heat is a property of matter which determines how readily a material is to absorb heat and change temperature. Specific Heat Specific Heat is defined as the amount of heat energy needed to raise the temperature of 1 kg of a substance by 1 o C 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 Energy How is Thermal Energy Transferred? YOU MUST KNOW THIS!!!!! Conduction Conduction – direct contact Convection Convection – through a fluid Radiation Radiation – by electromagnetic waves

conduction What is heat transfer by conduction? 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.

convection What is heat transfer by convection? Heat is transferred through a substance through currents. This occurs in fluids (liquids AND gases) Convection Convection 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).

What are some other uses of insulation?

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!!!