2011 EdExcel Unit 1

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# 2011 EdExcel Unit 1 - PowerPoint PPT Presentation

2011 EdExcel Unit 1. This PowerPoint supports Topic 5 (Generating and transmitting electricity) from the 2011 EdExcel Physics Unit 1 module. W Richards. Topic 5 – Generation and Transmission of Electricity. Electric Current.

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2011 EdExcel Unit 1

This PowerPoint supports Topic 5 (Generating and transmitting electricity) from the 2011 EdExcel Physics Unit 1 module

W Richards

Electric Current

Note that electrons go from negative to positive and are “pushed” by the voltage

+

-

e-

e-

Electric current is a flow of charge around a circuit

By definition, current is “the rate of flow of charge”

Basic ideas…

Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.

Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.

Resistance is anything that resists an electric current. It is measured in _____.

Words: volts, amps, ohms, voltage, ammeter, voltmeter

More basic ideas…

If a battery is added the current will ________ because there is a greater _____ on the electrons

If a bulb is added the current will _______ because there is greater ________ in the circuit

Electrical Power

P

V

I

Power is defined as “the rate of transferring energy” and is measured in units called “Watts” (W).

The amount of power being transferred in an electrical device is given by:

Power = voltage x current

in W in V in A

• How much power is transferred by a 230V fire that runs on a current of 10A?
• An electric motor has a power rating of 24W. If it runs on a 12V battery what current does it draw?
• An average light bulb in a home has a power rating of 60W and works on 230V. What current does it draw?
Fuels

Coal, oil and gas are called “fossil fuels”. In other words, they were made from fossils.

A “fuel” is something that can be burned to release heat and light energy. The main examples are:

Some definitions…

A renewable energy source is clearly one that can be _______ (“renew = make again”), e.g. _____, solar power etc.

A ___________ energy source is one that when it has been used it is gone forever. The main examples are ____, oil and gas (which are called ______ ____, as they are made from fossils), and nuclear fuel, which is non-renewable but NOT a fossil fuel.

Words – non-renewable, coal, fossil fuels, wood, renewed

Using non-renewable fuels in power stations

1) A fossil fuel is burned in the boiler

2) Water turns to steam and the steam drives a turbine

3) The turbine turns a generator

4) The output of the generator is connected to a transformer

5) The steam is cooled down in a cooling tower and reused

Efficiency of Power Stations

Heat

Heat

Heat

15J

Boiler

Turbine

Generator

100J

50J

5J

85J

35J

30J

Heat

Kinetic

Electrical

Pollution

When a fuel is burned the two main waste products are _____ dioxide and ________ dioxide.

Carbon dioxide is a _________ ___ and helps cause _______ _________. This is produced when any fossil fuels are burned.

Sulphur dioxide, when dissolved in ________, causes ______ _____. This is mainly a problem for ___ power stations.

Nuclear power stations do not produce these pollutants because they don’t ____ fossil fuels.

Words – sulphur, coal, global warming, carbon, acid rain, greenhouse gas, rainwater, burn

Nuclear power stations

The main difference is that the nuclear fuel is NOT burnt – it is used to boil water in a “heat exchanger”

These work in a similar way to normal power stations:

Start up times

Quick

Slow

Different power stations have different start up times:

Gas

Oil

Coal

Nuclear

Non-renewable energy sources

Coal, oil, gas and nuclear

Pollution – CO2 leads to global warming and SO2 leads to acid rain

Cheap fuel costs

Generate a lot of energy

Fuel will run out

Easy to use

Renewable energy sources

Wind, tidal, solar etc

Often depend on the weather – is it sunny???

Clean

Won’t run out

Look ugly

Energy is “dilute” – in other words, it’s very spread out

Easily accessible

Other ways of generating electricity

Can we drive the turbine directly without burning any fossil fuels?

Tidal Power

Low tide

High tide

Biofuels

Biomass can be used as a fuel in a number of ways:

• Fast-growing trees that can be ____
• Manure or other waste that can be used to release _______ (biogas)
• Corn or sugar cane that can be broken down in a fermenter to produce ______ like bio-ethanol.

Biofuels have two main advantages over traditional fuels – they are ______ and ________. However, they still release ______ _______.

Words – alcohols, cleaner, burnt, renewable, methane, carbon dioxide

Solar Energy

Solar panels – convert sunlight directly into electricity. Sunlight knocks electrons loose from the crystal structure and the loose electrons form an electric current. The amount of power depends on the area of the panel and the light intensity.

Heating for homes – these pipes carry water that absorbs heat energy and transfers it to the house.

Geothermal Energy

Geothermal energy can be used in _______ areas such as ______. In a geothermal source cold water is pumped down towards ____ _____. The water turns to steam and the steam can be used to turn ______. In some areas the _____ rising at the surface can be captured and used directly.

Words – steam, Iceland, volcanic, turbines, hot rocks

Electromagnetic Induction

N

• The direction of the induced current is reversed if…
• The wire is moved in the opposite direction
• The field is reversed
• The size of the induced current can be increased by:
• Increasing the speed of movement
• Increasing the magnet strength
Electromagnetic induction
• The direction of the induced current is reversed if…
• The magnet is moved in the opposite direction
• The other pole is inserted first
• The size of the induced current can be increased by:
• Increasing the speed of movement
• Increasing the magnet strength
• Increasing the number of turns on the coil
AC Generators

N

S

Voltage

Time

Other generators

A dynamo works by the same principle.

1) How can you make its output bigger?

2) How can you reverse the direction of its output current?

Large-scale production of Electricity

A generator at Drax power station in England

DC and AC

V

DC stands for “Direct Current” – the current only flows in one direction:

Time

1/50th s

AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)

230V

T

V

The National Grid

Step up transformer

Step down transformer

Homes

Power station

Electricity reaches our homes from power stations through the National Grid:

If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much ______ loss by the time electricity reaches our homes. This is because the current is ___. To overcome this they use devices called transformers to “step up” the voltage onto the power lines. They then “____ ____” the voltage at the end of the power lines before it reaches our homes. This way the voltage is _____ and the current and power loss are both ____.

Words – step down, high, power, low, high

Power Lines

Here’s my new shed. I want to connect it to the electricity I my house. Should I use an overhead cable or bury the cable underground?

Transformers

Voltage across primary (Vp)

No. of turns on primary (Np)

Voltage across secondary (Vs)

No. of turns on secondary (Ns)

Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil.

Words – alternating, magnetic field, induce, step up, voltage

We can work out how much a transformer will step up or step down a voltage:

Some example questions
• A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil?
• A step-down transformer has twice as many turns on the primary coil than on the secondary coil. What will be the output (secondary) voltage if the input voltage is 50V?
The Cost of Electricity

A 3kW fire left on for 1 hour uses 3kWh of energy

A 1kW toaster left on for 2 hours uses 2kWh

A 0.5kW hoover left on for 4 hours uses __kWh

A 200W TV left on for 5 hours uses __kWh

A 2kW kettle left on for 15 minutes uses __kWh

Electricity is measured in units called “kilowatt hours” (kWh). For example…

The Cost of Electricity

To work out how much a device costs we do the following:

Cost of electricity=Power (kW)xtime (h)xcost per kWh (p)

For example, if electricity costs 8p per unit calculate the cost of the following…

• A 2kW fire left on for 3 hours
• A 0.2kW TV left on for 5 hours
• A 0.1kW light bulb left on for 10 hours
• A 0.5kW hoover left on for 1 hour

48p

8p

8p

4p

Reducing Energy Consumption

60W older bulb, roughly 70p, to be banned in the EU from 2012.

25W “energy efficient” light bulb, £7.30 on Amazon

• Which one is more cost-effective?
• Jane wants to replace all the bulbs in her house with energy-efficient ones. If she has 10 light bulbs in her house calculate the following:
• How much will it cost her to buy the bulbs?
• What will the total power consumption be reduced by?
• If she uses the bulbs for 5 hours per day and electricity costs 10p per unit how much money will she save?
• How long will it take her to repay the cost of the bulbs?
Energy and Power

E

P

T

The POWER RATING of an appliance is simply how much energy it uses every second.

In other words, 1 Watt = 1 Joule per second

E = Energy (in joules)

P = Power (in watts)

T = Time (in seconds)

Some example questions
• What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds?
• What is the power of an electric fire that transfers 10,000J of energy in 5 seconds?
• Farhun runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating?
• How much energy does a 150W light bulb transfer in a) one second, b) one minute?
• Shaun’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson?
• Damien’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day?

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