Conservation of Energy Energy is the ability to do work. When work is done energy is transferred. Energy can only be turned from one form to another. Energy.

Conservation of EnergyEnergy is the ability to do work. When work is done energy is transferred . Energy can only be turned from one form to another. Energy cannot be created or destroyed.

Consider a car running down a slope :

At the top of the slope the car has potential energy. As it runs down the slope the potential energy ( Ep ) gets transformed into kinetic energy. (Ek)

Conservation of energy 2If there are NO frictional forces to overcome then

Ep at top = Ek at bottom of slope( Expts on this Loop the Loop, Car running down ramps)

In the world outside NAT 5 Physics frictional forces act and so work must be done to overcome friction and there is always some ‘waste’ heat and sound energy produced

Coal Power Stations( Revision )

Burning coal

In boiler

Heat exchanger Turbine generator

Coal Power Station

Chemical Energy Heat Energy

( burning coal )

Heat Energy Kinetic Energy

( steam moving through pipes )

Kinetic Energy Electrical Energy

( generator turning )

Hydro - Electric Power StationWater has potential energy

Water flows through pipes

simulation

Hydro - Electric Power Station

• Potential Energy Kinetic Energy

( water flowing through pipes)

• Kinetic Energy Electrical Energy

( turbine turns generator )

Pumped Storage schemes

EnergyPumped Storage schemes

During periods of peak demand they operate as conventional hydro electric stations.

During early hours of morning the excess electrical energy generated by the coal and nuclear stations is used to pump water back behind the dam. The pumps are inefficient. This allows the coal and nuclear stations to be run at peak efficiency 24 / 7.

Expt to determine efficiency of pump storage schemes.

EnergyGeneration of electrical energy

There must be a changing magnetic field about a conductor to induce a voltage across it:Move a magnet towards and away from a wire Or move the wire so that the magnetic field changes

EnergyThe size of the induced voltage can be increased

by :1. Increasing the number of coils of wire2. Increasing the magnetic field strength3. Increasing the rate at which the field is cutNorth pole wire South pole

As the wire is moved up / down an a.c. current / voltage is induced

Model generator

Voltage is induced across stator coils.

Working Generator

The rotating coil is an electromagnet unlike the permanent magnet in a model generator. There is a voltage induced across the fixed coils ( stator coils ).To increase the size of the induced current the current flowing through the electromagnet is increased. If the coil spins faster then a bigger voltage is induced but the frequency also increases.

The stator coils are fixed. The voltage is induced across these.

The rotor is a spinning electromagnet.

Heat EnergyTemperature of a substance is a measure of the mean kinetic energy of its particles.

Units : Degrees Celcius, oC

Heat is a form of energy. The heat stored in a substance is the total kinetic energy of all its particles.

Units are Joules, J.

100cm3 of water at 1000C contains more heat energy than 10cm3 water at the same temperature.

Heat energy flows from areas of high temperature to low temperature.

Heat EnergySpecific Heat CapacityThe energy required to raise the temperature of

1 kg of a substance by 1 0C.

The same quantity of energy is released if 1 kg cools down by 10C.

Units are J kg-1 0C-1

Water has the highest specific heat capacity :

4180 J kg-1 0C-1.

i.e. 4 180 J of energy are required to raise the temperature of 1 kg of water by 10C.

Eexpts on c of water and metal blocks.

Heat Energy

TmcEh ..Eh = heat energy : J

C = specific heat capacity : J kg-1 0C-1

M = mass : kg

T = change in temperature : 0C

Heat EnergyCalculate the final temperature if 20 kJ of heat energy are absorbed by 10 kg of water at an initial temperature of 20 0C.

Eh = 20 kJ m = 10 kg

= 20 000 J

c = 4180 J kg-1oC-1

mc

ET h

.

TmcEh ..

Heat EnergyWorking

CT

xT

005.0

1004180

20000

Change of StateWhen a material changes state there is NO temperature change.

Temp

(oC )

Time ( s )

Solid to liquid

Liquid to gas

http://www.bbc.co.uk/learningzone/clips/latent-heat-and-changes-of-state/4551.html

Expts on cooling by evaporation and measure latent heat of fusion / vapourisation of water

Change of State

1. When we sweat the liquid water evaporates, the energy for this comes from our skin and we cool down.

2. Ice packs in cool boxes melt. The energy for this comes from the air and food in the cool box and it cools down.

3. Coffee makers inject steam into cold milk. The steam turns into liquid giving out energy which is absorbed by the milk, raising its temperature

http://www.bbc.co.uk/learningzone/clips/what-energy-changes-happen-during-freezing/8759.html

Change of State

Fridges :A refrigerant is pumped around the ice box. It turns from liquid into gas. The energy required for this is taken from the food / air and it cools down.

The refrigerant is pumped to the back of the fridge and compressed. When it turns from gas into liquid it gives out energy.

The fridge is a heat pump. Energy is transferred from inside the fridge to the outside.

Change of StateSpecific latent heat of fusion ,lf ,

The energy required to turn 1 kg of a solid at its melting point into 1 kg of liquid at the same temperature.Units are joules per kilogram , J kg –1

Eh = m x lf

heat energy mass latent heat of fusion

Change of State

Example

Calculate how much ice cream is melted at 0oC if it absorbs 400 kJ of heat energy.

Lf = 3.34 x 105 J kg-1 Eh = 400 kJ m =?

kgm

xxm

lxmE fh

2.1

1034.3400000 5

Change of StateSpecific latent heat of vapourisation , lv,

The energy required to turn 1 kg of a liquid at its boiling point into 1 kg of gas at the same temperature.

Units :Joules per kilogram, Jkg-1

Eh = lv x m

Heat energy latent heat of vapourisation mass

( J ) ( J kg -1) ( kg )

Change of State

ExampleAn 12 V electric heater draws a current of 2.5A. It is immersed into a beaker of boiling water. Calculate the mass of water turned into gas if it is switched on for 5 minutes.

Lv = 2.26 x 106 J kg-1.V = 12 VI = 2.5 AT = 5 mins = 300 s

Calculate power of heater then energy supplied

P = V x I = 12 x 2.5 = 30 Watts

E = P x t = 30 x 300 = 9000 Joules

This energy is used to turn the liquid into gas

Use Eh = m x lv

Eh = 9000 J

M = ?

Lv = 2.26 x 106 J kg-1

9000 = m x 2.26 x106

m = 4.0x10-3 kg or 4 g