PHYSICS EDEXCEL UNIT 4

 Notes for Physics Edexcel unit 4 Energy resources and transfer.

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Unit 4: Energy resources and energy transfer

 

Energy stores and systems

Definition of a system: it can be defined as an object or a group of objects that work together. For example:

• Upward throwing of an object:

When a ball is thrown upwards, immediately there is kinetic energy in store. At the highest point, the ball has stored energy in form of (Gravitational Potential Energy). Before reaching its original position the b1all again has kinetic energy.

• Kettle of boiling water:

Using an electric kettle, the thermal energy in water is increased, which helps increase the temperature of water. So, the warm/hot water has more thermal energy than cold water.

• Coal burning:

The coal has chemical energy in store which is slowly converted to thermal energy which helps used for the purpose. Chemical energy is transferred to thermal energy.

• Slowing down of a car:

A moving object always has kinetic energy. So, when the brakes are applied the kinetic energy is lost and a frictional force is being applied which causes the brake to warm up and energy is lost as thermal energy to the surroundings.

•An object being dropped without bounce:

A moving ball has kinetic energy in store and when it comes to stop it has thermal energy which is a result of air resistance and once the ball has hit the ground, the energy is lost as heat to the surroundings. If there is sound, then some energy is converted to sound (acoustic) energy

•Ball accelerating with constant force.

Chemical energy stored in muscle of hand throwing the ball is transferred to the ball and the ball gains kinetic energy to accelerate with constant force.

•Two Magnets facing same pole each other.

When two magnets are held with same poles facing each other, there is a force that repels each. When they are close, they have magnetic potential energy

When magnets are released, this energy gets converted to kinetic energy.

Different form of energy include:

kinetic

•chemical	• thermal (or internal)
• gravitational	• magnetic
• electrostatic	• elastic
• nuclear

Energy is measured in Joules (J), kilojoules (kJ) and megajoules (MJ).

Law/principle of conservation of energy:

The law states that the energy can neither be destroyed nor be created, it can be transferred from one form to another.

Efficiency:

If there is waste of energy then it can be said as less efficient and when there is no waste of energy it is said to be highly efficient. Equation to be used:

Sankey diagram: a diagram used to represent transfer of energy as shown below:

Conduction and convection

Insulators:

Materials that have bad thermal conductivity are insulators. E.g.: Air, plastics and wood

Conductors:

Materials that allow faster transfer of thermal energy are known as conductors. E.g.:  metals, concrete, glass and bricks. Good conductors have free electrons which transfer the energy from one atom to the other.

 

 

 

 

Insulation:

Air, wool and fat are insulators. They keep warm-blooded animals warm. For instance, birds

Practical part: The conductivity of metals:

How to compare conductivity of different metals such as: brass, aluminium, iron and copper.

Method:

a)  Centrally place all the metals on the tripod.

b)  Place water at the hollow end of each metal.

c)   A Bunsen burner is used to heat up the metal sheets so they are heated equally. Use tongs to for heated metal sheets.

d)  Create the temperature table with temperature reached to melt the metal sheet and record the results.

 

Convection currents

Most commonly known with sea breezes. When the temperature rises, the air expands and density becomes less. Hot air rises. When air is cooled, density becomes less and it comes down. Seabirds quickly take advantage of rising currents, and without flapping their wings soar upwards.

Practical: Observing convection currents

Method

A beaker/flask is filled with water and places on a tripod on a gauze.

A crystal of potassium manganate (VII) is carefully dropped in the centre if beaker.

The beaker is slowly heated from the centre. The path of the potassium manganate (VII) is observed and it shows the direction of the current.

Preventing unwanted energy transfer

Heat is lost from homes through roof, walls, doors and windows to warm up the air outside the home. This can be reduced in the following ways:

Chimneys:

Having chimneys inside home we can reduce the wasteful transfer of energy outside the

Walls:

The transfer rate of energy from inside the home to outside depends upon 4 factors:

• Temperature difference between inside and outside of home

• Area of walls: large walls need more heat than small houses.

• Thermal conductivity of the materials with which wall is made. The higher the conductivity of material (e.g.:  glass, brick) the higher the rate of loss of energy from walls.

•Wall’s thickness: thicker walls have slower conductivity and thinner walls have high conductivity. There are two walls, in between is the cavity which is filled with form which insulates the trapped air. Air is a bad thermal conductor.

Carpet and loft insulation:

Installing a thick layer of loft increases `insulation and transfer of thermal energy is reduced. So, the heat lost by conduction to the floor and roof is reduced.

 

Double gazing

This metho reduces the energy transfer through windows. A layer of trapped gas between the two sheets of glass provides insulation.

Radiation

Third way of energy transfer of thermal energy, most commonly seen in light rays from the sun. Infrared waves from the spectrum are one which gives heat

Absorbers: Good and bad.

Radiations are absorbed well by dull and black surfaces.

Shiny and white surfaces are bad absorbers of heat. So bottom surfaces are black in colour.

Emitters: Good and bad

Black surfaces are good absorbers as well as emitter of heat radiation.

Shiny and white surfaces are bad absorbers and bad emitters of radiation.

Three forms of energy transfer: convection, conduction and radiation.

Radiation:

Experiment to investigate how different surfaces absorb radiation.

Method:

a.   Prepare two sheets of aluminium labelled A and B. Paint one surface with black and B shiny from both sides. Using candle wax stick a marble to the shiny side of both A and B.

b.   Both sheets should be placed at equal distance from the heater and heater should be turned on.

c.   Observe which marble falls off first.

Vacuum flask:

No conduction or convection take place from a vacuum flask, as there are no particles for transfer of energy. However, thermal energy can be radiated and to prevent that from happening the surfaces are silvered. A stopper is placed at the top to prevent any loss by convection.

What is work?

Definition: Work done is when a force of 1 newton moves through a distance of 1 metre, in the direction of the applied force.

The unit of work, J, Joules

Work and energy transfers

Energy transfer takes place in 4 ways :

·        By mechanical work

·        By electrical work

·        By heating

·        By radiation: mechanic (sound and shock),light and electromagnetic

Does a force always do work?

 

No not always, work done should always in the right direction and not in the wrong direction is not work done.

 

 

Energy calculations:

Gravitational potential energy:

Calculated using:

Kinetic energy:

Calculated using:

 Power

Definition: Power is rate at which the energy is transferred or rate at which work is done. Measured in watts (W), kilowatts (kW), Megawatts (MW).

Energy resources and electricity generation

Electricity generation and energy transfers:

Magnetic field induced voltage: Produced when a magnet is moved (chemical energy in hand) towards a solenoid. The changing magnetic (magnetic energy) field induces voltage (electric energy) which drives current through LED (Light Emitting Diode).

 

Energy sources:

Renewable energy: one that never runs out: e.g.: wind, water and solar power.

A non-renewable energy: limited supply. E.g.: oil, gas, coal and nuclear.