Electricity Year 9 Science 1. Electrical Energy Electricity is a form of energy that uses the physical property of charge. There are two types of charge:

Electricity

Year 9 Science

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Electrical Energy

Electricity is a form of energy that uses the physical property of charge. There are two types of charge: positive and negative.Electrons are very small sub-atomic particles that have a negative charge.Electricity in wires is a movement of electrons due to a voltage.This allows electrical energy to be converted into other forms like light, heat and kinetic energy (movement).

Year 9 Science

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Electrical Energy

An electrical current is a flow of charges: usually electrons.The electrons flow because they are moving down an “energy slope” caused by the voltage.A voltage gives the electrons energy. The electrons move, and this movement can do work like lighting a globe.

Scanned probe microscope images of electron flow

Year 9 Science

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Electrical Energy

In order for an electrical current to flow there needs to be:• charges that are …• … free to move under the influence of … • … a voltage (power supply or battery) through … • a complete (unbroken) pathway.

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Electrical Energy

A wired electrical pathway is called a circuit.If the circuit is unbroken it is said to be a closed circuit and a current can flow.If the circuit has a gap in it, it is said to be an open circuit and a current cannot flow.A switch opens and closes a circuit.

Year 9 Science

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Electrical Current & Voltage

A metal wire is made of atoms.Each atom has a very small centre called the nucleus.The nucleus contains protons (+ charge) and neutrons (no charge).Electrons (- charge) orbit around the nucleus.The atoms have no overall charge because the number of protons is equal to the number of electrons.Almost all of any atom is empty space.

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In the metal wire the atoms (nuclei) are locked into position and cannot move but …… some of the electrons can move from atom to atom and this means they can transfer energy in a circuit.Metals are good conductors and are said to have a low resistance.Other materials like plastic and wood do not have free electrons and are poor conductors (insulators).Insulators are said to have a high resistance.

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Electrical current: • measures the rate of flow of charge • is measured in Amps (Amperes: A) or milliamps

(mA) • is measured using an ammeter

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Electrical voltage measures • the amount of energy the charges have (or give

up)• is measured in Volts (V) or millivolts (mV).• is measured using a voltmeter

A multimeter is able to measure either voltage or current depending on the setting used.

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Batteries

Batteries are a form of portable electricity that get their energy from a chemical reaction.

chemical energy →electrical energy

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Batteries

A car battery is “wet”. It has lead plates (electrodes) dipping into an acid electrolyte.

A simple torch battery is called a dry cell.Its electrolyte is a paste so that if the cell is damaged it does not leak so much.Its electrodes are a zinc cup and a carbon rod.

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Batteries

A 1.5V torch battery is really one cell.

Several cells can be joined together to make a battery.e.g. a 9V battery is really six 1.5V cells.

=

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Batteries

Normal dry cells cannot be recharged – and their currentslowly gets smaller over time.Different types of cells use different electrodes or electrolytes.

Alkaline cells allow a greater current.

Mercury (button) cells keep a steady voltage for longer but get flat suddenly.

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Batteries

Some dry cells can be recharged.

Examples are nickel-cadmium cells …

… and lead-acid car batteries.

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The Lead Acid Battery

A lead-acid battery consists of six, 2V cells in series connection.Each cell has two lead electrodes but one is coated with lead dioxide.When the battery is being used a chemical reaction converts the lead and lead dioxide to lead sulfate which coats the surface of the two electrodes.

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The lead sulfate stays on the electrodes and the chemical reaction can be reversed as the car’s alternaltor recharges the battery while the car is running.Over time the lead sulfate starts to become harder to convert back and the battery cannot be recharged and needs to be replaced and its parts recycled.

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Discharging:Electrode 1.Lead → lead sulfateElectrode 2.Lead dioxide → lead sulfate

ChargingElectrode 1.Lead sulfate → leadElectrode 2.Lead sulfate → lead dioxide

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Fuel Cells

Fuel cells are a different cell design that do not need recharging as long as there is a continuing supply of chemicals.

They are used in space expeditions but are very expensive.

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Combining Cells

If a circuit needs a higher voltage the cells are connected ‘end to end and positive to negative’ and this is called a series connection.

A torch uses a series connection.

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Circuit Diagrams

Circuit diagrams use symbols to show how a circuit is connected.Straight lines are drawn between circuit components to represent wire connections.

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Power supply: two or more 1.5V cells in series

Load: A globe - its filament gets white hot and converts electrical energy into heat and light energy.

Metal strips: act as wires, a switch and a spring (to hold the cells in good contact).

Circuit Diagram for a Torch Year 9 Science

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Circuit components (cells, lamps, etc.)may be connected in a line (series) or side to side (parallel).A series connection provides greater voltage at the same current.

Series and Parallel Connections Year 9 Science

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A parallel connection provides greater current at the same voltage.


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When globes are connected in series the energy of the electricity is shared among the globes.The globes glow less brightly.If one globe is removed this opens the circuit and all globes go out.


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When globes are connected in parallel the flow of charge is shared among the globes.The globes glow do not lose any of their brightness because the voltage across each does not change.If one globe is removed the other globes do not go out because they are still connected directly to the power supply.


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Moving charges provide current electricity.Stationary charges are referred to as static electricity.Electrons and their negative charge can transfer from object to object due to friction.

An object can become negatively charged by having an excess of electrons.An object can become positively charged by having a deficiency of electrons.

Like charges repel, unlike charges attract.

Static Electricity Year 9 Science

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Materials vary in their attraction for electrons: electron affinity.When two objects are rubbed together some electrons will move to the object with greater electron affinity.The triboelectric series shows the electron affinity of different materials.


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Most positive (+)

Air

Human Skin

Asbestos

Rabbit fur

Glass

Nylon

Wool

Silk

Aluminium

Paper

Cotton

Wood

Amber

Rubber Balloon

Hard Rubber

Polyester

Polystyrene

Cellophane tape

Polythene

Teflon

Silicone Rubber

Most Negative (-)

When two materials are rubbed together the material that is lower in the series tends to become negatively charged as electrons transfer to it.The material that is higher in the series tends to become positively charged as electrons transfer from it.


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Most positive (+)

Air

Human Skin

Asbestos

Rabbit fur

Glass

Nylon

Wool

Silk

Aluminium

Paper

Cotton

Wood

Amber

Rubber Balloon

Hard Rubber

Polyester

Polystyrene

Cellophane tape

Polythene

Teflon

Silicone Rubber

Most Negative (-)

E.g. When a rubber balloon is rubbed with rabbit fur, the balloon becomes charged negatively while the rabbit fur becomes charged positively.


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Most positive (+)

Air

Human Skin

Asbestos

Rabbit fur

Glass

Nylon

Wool

Silk

Aluminium

Paper

Cotton

Wood

Amber

Rubber Balloon

Hard Rubber

Polyester

Polystyrene

Cellophane tape

Polythene

Teflon

Silicone Rubber

Most Negative (-)

Exercise 2For each of the above examples state which material electrons move to and which they move from.

Exercise 1State the charge on each object when:a) Amber is rubbed with woolb) Glass is rubbed with silkc) Polyester is rubbed with nylon

An object can become charged negatively by being rubbed by a material that is lower on the tribolelectric series.If a negatively charged pencil is brought near a piece of paper it can pick it up.


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In this case the negatively charge pencil repels negative charges on the surface of the paper.This makes the surface of the paper positive.The negative pencil then attracts the positive surface of the paper.This principle is used in photocopiers and laser printers.


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A water circuit for a swimming pool is similar to an electrical circuit.

Electrical Quantities Year 9 Science

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Electric current is a measure of the amount of electric charge passing a given point every second.Current is measured using an ammeter and the unit of current is the Ampere (A) or the milliampere (mA).Ammeters are always connected in series.

Electrical Quantities - Current Year 9 Science

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Exercise What is the reading on the following ammeters:

a)


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b)


36

c)


37

d)


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Electrical voltage is a measure of the amount of electrical energy gained or lost by electric charge as it moves through the circuit.Voltage is measured using an voltmeter and the unit of current is the Volt (V) or the millivolt (mV).Voltmeters are always connected in parallel.

Electrical Quantities - Voltage Year 9 Science

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Exercise What is the reading on the following voltmeters:

a)


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b)


41

c)


42

d)


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Conductors have very little resistance to electric current.This resistance is still measurable though.Insulators have a very large resistance.

Electrical Quantities - Resistance Year 9 Science

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As electrons move through a circuit they collide with atoms in the wires etc. This causes them to lose energy.Electrical resistance is a measure of how difficult it is for electrons to move through a circuit.

Electrical Quantities - Resistance Year 9 Science

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Resistors are used in electric circuits to control the voltage and current.The volume control on a radio is a variable resistor.

Other types are fixed resistances.

Controlling the Flow Year 9 Science

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A variable resistance can be used to control light dimmers.When the resistance in the variable resistor increases the resistance of the whole circuit increases and this reduces the current through the globe.The amount of energy available to the globe reduces as well – the globe glows less brightly


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The symbol for a variable resistor can be any of the following:


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Exercise 3Draw a circuit diagram that includes a power supply, a globe, a switch and a variable resistor all connected in series.


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An electrical current always faces some opposition to its flow.This opposition (resistance) is observed as a heating of components or wires.If too much current flows through a circuit there is danger of either melting of plastic insulation, sparking and a resulting fire or the melting of components.In order to prevent this there are two devices that can open a circuit if the current gets too high.


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1. A fuse is a length of low melting point metal alloy of a certain thickness that melts when it gets to a certain temperature produced by a rated current.

http://en.wikipedia.org/wiki/Image:Electrical_fuses,_plug-in_type,_different_sizes.jpeg

http://en.wikipedia.org/wiki/Image:Fuses.jpg


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2. A circuit breaker is a device that disconnects the circuit if the current exceeds a certain rated value. This is a non-destructive way of automatically opening a circuit and may be reset manually.

Circuit breakers are effectively electromagnetic or thermal switches.

Measuring the Flow Year 9 Science

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4351


9536


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6589


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9126


57

4442


58

6050


59

1634


60

6231


61

5561


62

3336

Electrical Power Year 9 Science

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Electricity is a means off supplying energy that can be used for various purposes.The energy can be used quickly or slowly.Power is a measure of how quickly energy is converted from one form to another.

Power is measured in Watts (W) or kilowatts (1000W).

Energy EP = =

Time t


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Electricity is a means off supplying energy that can be used for various purposes.The energy can be used quickly or slowly.


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In a circuit the power can be determined by multiplying the current by the voltage.

P = VI

where V = voltage

I = current

Exercise 4Determine the power of an electrical heater that operates at a voltage of 240V and a current of 5A.What is this in kilowatts?If the heater was switched to its highest setting of 2400W, what current would it draw?

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Resistance is measured in Ohms () and is related to voltage and current by Ohm’s Law:

Example 1A torch globe carrying a current of 200mA with a voltage drop of 3 volts has a resistance of:


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VI =

R

V 3R = = = 15

I 0.200

VR =

IV = RI

Example 2An appliance carrying a current of 4A with a voltage drop of 240V, has a resistance of:


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V 240R = = = 60

I 4


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Exercise 4A globe carries a current of 2A when a voltage of 6V is applied. Determine the resistance of the globe.

Exercise 5A length of conductor has a resistance of 5. What current will flow through the conductor when a voltage of 10V is applied?

Exercise 6An electrical motor carries a current of 4A and has a resistance of 62.5. What voltage exists across the motor?

Circuit breakersReading an electricity meterElectrical powerGlobe wattage

To do Year 9 Science

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Electricity Year 9 Science 1. Electrical Energy Electricity is a form of energy that uses the physical property of charge. There are two types of charge: