AIMS

State and apply the law of conservation of energy Fixed amount in closed systems Change form not create or destroy

Understand need to transform energy Explain any losses

Use systems diagrams to account for energy changes

Identify energy forms and changes within a system

Calculate energy transfers

The Law of Conservation of Energy

The conservation of energy is a fundamental concept of physics.

Along with the conservation of mass and momentum. Derived from first law of thermodynamics.

Within a closed system, the amount of energy remains constant and energy is neither created nor destroyed.

Energy can be converted from one form to another but the total energy within the domain remains fixed.

Energy Transformation

How energy can be converted to other forms is important to technologists

Some forms are directly interchangeable Dropping a stone Potential Kinetic

Others require several stages Coal burnt in a power station to produce electrical power Chemical heat kinetic electrical

Systems Approach

Systems diagrams can be used to summarise energy changes

Consider a light bulb (simplified)

Produce system diagrams for an electric motor and an electric generator

LIGHT BULBELECTIRCAL LIGHT

Energy Transformation Examples

State the energy form at points A, B, C and D

A

B

C

D

A: Potential energy

B: Kinetic Energy (linear motion)

C: Kinetic Energy (rotary motion)

D: Electrical Energy

Energy Transformation Examples

1. State the energy form at points A to H in the diagram opposite.

2. Describe the energy changes that take place within the system

+-

A

B

CD

EF

G

H

MASS

ELECTRIC MOTOR

LOUDSPEAKERMICROPHONE

ELECTRIC BULBSOLAR CELL

ELECTROLYTIC CELL(WATER)

EXPANSIONCYLINDER

A: PotentialB: ElectricalC: SoundD: ElectricalE: LightF: ElectricalG: ElectricalH: Potential

Energy ‘losses’ during transformation

We accept that energy cannot be created or destroyed

This tells us that the energy output of a system equals the energy input

HOWEVER, not all the energy is used to do USEFUL work

When a conversion takes place there is always a loss Examples are sound, friction or heat

Go back to the energy conversion diagrams for the bulb, motor and generator and add any losses to the output side

Energy Losses in a Wind Turbine

A turbine can be used to generate electricity. The generator can be connected to it in two ways.

coupled directly to vanes coupled via shafts and gears

Energy ‘losses’ during transformation

1. List the energy conversions that take place during its operation

2. Describe the energy losses in both systems

3. Which do you think is more efficient?

Worked Example A body of mass 30 kg falls freely from a height of 20

metres. Find its final velocity and kinetic energy at impact.

First calculate the initial potential energy.

EP = mgh = 30 9.8 20 = 5.88 kJ

This potential energy is converted or transferred into kinetic energy, which means that the kinetic energy at impact is equal to 5.88 kJ.

To calculate the final velocity of the body we begin by taking EK = 5.9 kJ.

Ek = ½mv² 5.88 10³ = ½ 30 v²v² = 392.4v = 19.8 ms-1

Pupil Problems

A 5 kg mass is raised steadily through a height of 2 m. What work is done and what is the body’s potential energy relative to the start?

A body of mass 30 kg is projected vertically upwards with an initial velocity of 20 m/s. What is the initial kinetic energy of the body and to what height will it rise?

A mass of 20 kg is allowed to fall freely from a certain height above a datum. When the body is 16 m above the datum, it possesses a total energy of 3,531 J. What is the starting height of the object?

Efficiency

Calculating efficiency

The efficiency of an energy transformation is a measure of how much of the input energy

appearsas useful output energy.

The efficiency of any system can be calculated using the equation:

Efficiency, = Useful energy outputTotal energy input

Worked Example

An electric lift rated at 110 V, 30 A raises a 700

kg load a height of 20 m in two minutes.

By considering the electrical energy input and the potential energy gained by the mass, determine the percentage efficiency of this energy transformation.

Worked Problem

Ee = ItV = 30 120 110 = 396 kJ

Potential energy gained is calculated as follows.

EP = mgh = 700 9.8 20 = 137.2 kJ

Percentage Efficiency = Useful Energy Output 100% Total Energy Input

= 137.2 100% = 34.65% 396

Pupil Problems

(1) An electric kettle is rated at 240 V, 10 A. When switched on it takes three minutes to raise the temperature of 0.5 kg of water from 20C to 100C.

Determine:

The electrical energy supplied in the three minutes The heat energy required to raise the temperature of the water The efficiency of the kettle.

Pupil Problems

Boxes in a factory are transferred from one floor to another using a

chute system as shown above. The boxes start from rest at the top of

the chute and during the decent there is a 40 per cent loss of energy.

The boxes weigh 10 kg each.

Calculate the velocity of the boxes at the bottom of the chute.

7 m

dm

CHUTE

Energy Audits

Your Teacher will show you how to construct an energy audit

Revision Questions

A load is raised through 6m. The load is 1kg in mass. What is the load’s potential energy at its final height?

What work is done in lifting the above load?

If the lifting system is only 60% efficient. What input energy is required to lift the load?

A kettle boils water in 1 minute. If the kettle uses 100KJ of energy boiling the water, what is the current rating of the kettle? (240V supply to kettle)

A process raises the temperature of water from 6 0C to 70 0C in 12.6 seconds. If the initial mass of water is 2kg. What energy is used in the process?

If the energy of a moving body is 20MJ and it has a mass of 300kg, what is the bodies velocity?

ENERGY USE IN HOUSEHOLDS

http://www.yenka.com/content/item.action?quick=1s3

http://www.willsmith.org/climatechange/domestic.html