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
2
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
3
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.
Year 9 Science
4
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
5
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.
Year 9 Science
6
Electrical Current & Voltage
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.
Year 9 Science
7
Electrical Current & Voltage
Electrical current: • measures the rate of flow of charge • is measured in Amps (Amperes: A) or milliamps
(mA) • is measured using an ammeter
Year 9 Science
8
Electrical Current & Voltage
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.
Year 9 Science
9
Batteries
Batteries are a form of portable electricity that get their energy from a chemical reaction.
chemical energy →electrical energy
Year 9 Science
10
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.
Year 9 Science
11
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.
=
Year 9 Science
12
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.
Year 9 Science
13
Batteries
Some dry cells can be recharged.
Examples are nickel-cadmium cells …
… and lead-acid car batteries.
Year 9 Science
14
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.
Year 9 Science
15
The Lead Acid Battery
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.
Year 9 Science
16
The Lead Acid Battery
Discharging:Electrode 1.Lead → lead sulfateElectrode 2.Lead dioxide → lead sulfate
ChargingElectrode 1.Lead sulfate → leadElectrode 2.Lead sulfate → lead dioxide
Year 9 Science
17
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.
Year 9 Science
18
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.
Year 9 Science
19
Circuit Diagrams
Circuit diagrams use symbols to show how a circuit is connected.Straight lines are drawn between circuit components to represent wire connections.
Year 9 Science
20
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
21
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
22
A parallel connection provides greater current at the same voltage.
Series and Parallel Connections Year 9 Science
23
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.
Series and Parallel Connections Year 9 Science
24
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.
Series and Parallel Connections Year 9 Science
25
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
26
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.
Static Electricity Year 9 Science
27
Static Electricity Year 9 Science
28
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.
Static Electricity Year 9 Science
29
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.
Static Electricity Year 9 Science
30
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.
Static Electricity Year 9 Science
31
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.
Static Electricity Year 9 Science
32
A water circuit for a swimming pool is similar to an electrical circuit.
Electrical Quantities Year 9 Science
33
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
34
Exercise What is the reading on the following ammeters:
a)
Electrical Quantities - Current Year 9 Science
35
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
39
Exercise What is the reading on the following voltmeters:
a)
Electrical Quantities - Voltage Year 9 Science
40
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
44
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
45
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
46
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
Controlling the Flow Year 9 Science
47
The symbol for a variable resistor can be any of the following:
Controlling the Flow Year 9 Science
48
Exercise 3Draw a circuit diagram that includes a power supply, a globe, a switch and a variable resistor all connected in series.
Controlling the Flow Year 9 Science
49
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.
Controlling the Flow Year 9 Science
50
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.
Controlling the Flow Year 9 Science
51
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.
Electrical Power Year 9 Science
63
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
Electrical Power Year 9 Science
64
Electricity is a means off supplying energy that can be used for various purposes.The energy can be used quickly or slowly.
Electrical Power Year 9 Science
65
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?
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:
Controlling the Flow Year 9 Science
67
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:
Controlling the Flow Year 9 Science
68
V 240R = = = 60
I 4
Controlling the Flow Year 9 Science
69
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?