ENERGY

Joanna Modupeh Hodasi (JAMHodasi)Physics department, Rm 55

INTRODUCTION

Lightning is the breakdown of air by strong electric fields and is a flow of energy. The electric potential energy in the atmosphere changes into thermal kinetic energy, light, and sound, which are other forms of energy.

INTRODUCTION

• Energy is one of the most fundamental parts of our universe.

• We use energy to do work. – Energy lights our cities. – Energy powers our vehicles,

trains, and planes. – Energy warms our homes, cooks

our food, plays our music, gives us pictures on television.

– Energy powers machinery in factories and tractors on a farm.

• Energy from the sun gives us light during the day. It dries our clothes when they're hanging outside.

• It helps plants grow. Energy stored in plants is eaten by animals, giving them energy.

• Everything we do is connected to energy in one form or another.

INTRODUCTION• But where does energy come from? • There are many sources of energy. In for this section of the

course, we will look at the energy that makes our world work. Energy is an important part of our daily lives.

• The various forms of energy include: – Electricity – Biomass Energy - energy from plants – Geothermal Energy – Fossil Fuels - Coal, Oil and Natural Gas – Hydro Power and Ocean Energy – Nuclear Energy – Solar Energy – Wind Energy – Transportation Energy

INTRODUCTION

• We will also look at how energy is stored, and transferred. Finally, we'll look at the various forms of energy and its effect on the environment.

INTRODUCTIONCOURSE OBJECTIVESBy the end of this section of the course, students will be able to

– understand what energy is and define it.– Differentiate between the type of energy, the various forms of energy

and their sources.– know how energy is stored and transformed from one form into

another.– understand and know the effect of energy on the environment.

METHOD OF PRESENTATION– Through power point presentation– Through discussions– Demonstrations

WHAT IS ENERGY?

WHAT IS ENERGY?

Energy causes things to happen around us. • The sun radiates light and heat energy which helps plants to

grow. • At night, lamps in our home use electrical energy to light our

rooms. • When a car drives by, it is being powered by gasoline, a type

of stored energy. • The food we eat contains energy which we use to work and

play. • We can say then that: Energy is the Ability to Do Work.

WHAT IS ENERGY?

• So an object’s energy, or the energy of a system, is a measure of its potential or ability to do work.

• If we measure the amount of work done on a body/system, we have the amount of energy of the body/system.– Eg. If you were to push a car from the Physic department to the V.C.’s

lodge, you would say that you have done work. You have exerted energy (may be even sweating).

• The amount of work done in this situation is given by multiplying the force used in pushing the car, by the distance covered.

Work = Force x distance

WHAT IS ENERGY

UNITS OF ENERGY• One of the basic measuring blocks is called a Btu. This stands

for British thermal unit . – Btu is the amount of heat energy it takes to raise the

temperature of one pound of water by one degree Fahrenheit, at sea level.

– One Btu equals about one kitchen match stick. – It takes about 2,000 Btus to make a pot of coffee.

• Energy can also be measured in J, joules. A thousand joules is equal to a British thermal unit. – 1,000 joules = 1 Btu

WHAT IS ENERGY

• One joule is the amount of energy needed to lift something weighing one pound(2.2kg) to a height of nine inches (22.5cm).– So, if you lifted a one-pound piece of meat from the floor to the

top of a table (27 inches), you would use about 3 joules of energy.

– A piece of buttered toast contains about 315 kilojoules (315,000 joules) of energy. With that energy you could: • Jog for 6 minutes • Bicycle for 10 minutes • Walk briskly for 15 minutes • Light a 60-watt light bulb for 1-1/2 hours

WHAT IS ENERGY?

• The Newton-meter (Nm), the work done by 1 N( weight) in moving a distance of 1m is also a measure of energy.

• Other units of energy are the calorie (Cal) and kilocalorie (kcal).

• The units are related as follows:– 1 J = 1Nm– 1Btu = 1000J– 1 cal = 4.2J

WHAT IS ENERGY?

WORKED EXAMPLEA tow truck pulls a car 5km along a horizontal stretch of road. The tension in the rope towing the car is 850N. What is the amount of energy the truck used to pull the car?

solutionEnergy is a measure of the work done.Work = Force x distance = 850N x (5x1000)m = 4250000Nm = 4250000J

= 4250KJ = 4250Btu– (1000J = 1KJ = 1Btu)

WHAT IS ENERGY

• Energy comes in various forms.

– Heat (thermal energy) – Light (radiant energy) – Motion (kinetic energy) – Electrical energy– Chemical energy– Nuclear energy – Gravitational energy – Biomass Energy (energy from plants) – Geothermal Energy – Fossil Fuels (Coal, Oil and Natural Gas) – Hydro Power and Ocean Energy – Solar Energy – Wind Energy

WHAT IS ENERGY?

• Energy can be classified into two types: – Stored energy is called potential energy. – Moving energy is called kinetic energy.

For example, the food you eat contains chemical energy, and your body stores this energy until you use it when you work or play.

• With a pencil, try this example to know the two types of energy. • Put the pencil at the edge of the desk and push it off to the floor. The moving

pencil uses kinetic energy. • Now, pick up the pencil and put it back on the desk. You used your own

energy to lift and move the pencil. Moving it higher than the floor adds energy to it. As it rests on the desk, the pencil has potential energy. The higher it is, the further it could fall. That means the pencil has more potential energy.

• Potential energy is related to the distance the object is from the ground or a reference point. We can calculate potential energy from the following formula:– POTENTIAL ENERGY = Weight Of Object x Height Above Ground

P.E. = mgh[Weight (W )= mass(m) x acceleration due to gravity (g)]

• Kinetic energy which is due to motion, depends on the speed of object and is given the following formula:– KINETIC ENERGY = ½ x Mass of object x Square of speed

K.E. = ½ Mv2

WHAT IS ENERGY ?• WORKED EXAMPLE

A mango hanging on a tree 4.3m above ground falls at a speed of 5ms-1 . The weight of the mango is found to be 0.5kg. How much potential energy did it possess, and what is its kinetic energy? (g = 9.8ms-2 )

Solution:P.E. = mgh = 0.5kg x 9.8ms-2 x 4.3m = 21.07J

K.E. = ½ mv2 = ½ x 0.5kg x (5ms-1 )2 = 6.25J

HOW IS ENERGY STORED?

HOW IS ENERGY STORED?

• Energy cannot be created or destroyed, but it can be saved in various forms.

• Storing energy allows humans to balance the supply and demand of energy. Energy storage systems in commercial use today can be broadly categorized as: – mechanical– electrical– chemical– biological – thermal

HOW IS ENERGY STORED?

• One way to store it is in the form of Electrochemical energy in a battery. – Inside the battery, a reaction between the chemicals takes

place and electrons flow from the negative to the positive terminals when connected through a circuit. This produces electricity.

– Batteries can be stored for a long time and still work because the chemical process doesn't start until there is a closed circuit.

HOW IS ENERGY STORED?

• Food - (biological energy storage)

– Plants, like grass, corn, etc, convert the sun's energy through photosynthesis into stored chemical energy. This energy stored in the plant cells is used by the plant to grow, repair itself and reproduce itself.

– Cows and other animals eat the energy stored in the grass or grain and convert that energy into stored energy in their bodies. When we eat meat and other animal products, we in turn, store that energy in our own bodies. We use the stored energy to walk, run, ride a bike or even read a page on the Internet.

HOW IS ENERGY STORED?• Mechanical

– Compressed air energy storage (CAES) – Flywheel energy storage – Hydraulic accumulator – Hydroelectric energy storage – Spring – Gravitational potential energy (device)

• Thermal – Ice Storage – Molten salt – Cryogenic liquid air or nitrogen – Seasonal thermal store – Solar pond – Hot bricks – Steam accumulator – Fireless locomotive – Eutectic system

Storage methods• Chemical

– Hydrogen – Biofuels – Liquid nitrogen – Oxyhydrogen – Hydrogen peroxide

• Biological – Starch – Glycogen

• Electrochemical – Batteries – Flow batteries – Fuel cells

• Electrical – Capacitor – Supercapacitor – Superconducting magnetic energy storage

(SMES)

HOW IS ENERGY TRANSFORMED OR TRANSFERRED?

• Lets look at our falling pencil.• When the pencil is on the table it has potential energy.• When dropped, it obtains kinetic energy.

– As it starts to drop, the potential energy reduces as the height reduces and since it is moving (downwards), its kinetic energy increases.

• If we neglect air resistance, the amount of potential energy the pencil had before the fall, is equal in quantity to the amount of kinetic energy at the end.

• The total energy of the pencil is unchanged!• THE LAW OF CONSERVATION OF ENERGY: The total energy of

all participants in any process remains the same throughout the process.

• Energy can not be created or destroyed.• That means it can be transformed (changed) from one form

into another form or transferred from one place to another. The total amount stays the same no mater what!

• There are some devices, energy converters, that transform energy from one form into one or more forms.– Light bulb converts electrical energy into radiant energy and thermal

energy.– When you talk on the phone, your voice is transformed into electrical

energy, which passes over wires (or is transmitted through the air). The phone on the other end changes the electrical energy into sound energy through the speaker.

• Examples continued– A television changes electrical energy into light and sound energy.– A car uses stored chemical energy in gasoline to move. The engine

changes the chemical energy into heat and kinetic energy to power the car.

– Food is stored energy. It is stored as a chemical with potential energy. When your body uses that stored energy to do work, it becomes kinetic energy.