1

Electricity

A Basic Understanding

2

What you need to know

• Excess electrons on an object create a negative charge

• A shortage of electrons creates a positive charge

• The law of charges– Unlike charges attract– Like charges repel– Charged attract uncharged

3

What you need to know

• Conductors allow electric charges to move freely

• Insulators prevent charge from moving freely

• **the basic parts of all electric circuits are;– A source (battery) of electric energy– Conductors– A load– A control device.

4

ElectricityWhether we have turned on a radio or heard a massive clap of nearby thunder, electricity is a common phenomena that we have all experienced. One thing is certain—there is energy in electricity. Electricity can be harnessed to do work. We use it to make light, heat, sound. Nature also displays the massive potential energy of electricity in a bolt of lightning which can superheat air molecules to the temperature of the sun and produce an extremely loud noise.

5

Describing electricity is extremely difficult however. If it were simple, humanity would have been able to use electricity a lot earlier in our development. To compound the problem, there are a great many misconceptions about the nature of electricity that many people believe to be true but are not. Electricity Online lists some of these common misconceptions.

Electricity is not a phenomenon composed of energy.

Electricity is not made of electrons.

Electrons do not "carry" energy.

Electrons do not flow at the speed of light.

Batteries and generators do not create electricity.

6

Now we know what electricity is not—so what is it?

Electricity is anything that happens as a result of charge.

But this definition seems to just replace one problem with another. Now the question is what is charge? In many ways this is even a more difficult question. The reason is that charge is a fundamental quantity—like length or mass. Objects have a certain charge that is given by its very nature. We cannot describe charge using other properties of matter because charge is a property by itself. We cannot describe charge in terms of length or mass, in the same way that we cannot describe mass in terms of length or charge. They are separate. Objects can change their charge just like they can change their length or mass.

7

Like all fundamental properties, charges have certain characteristics. There are two opposite types of charge which we have called positive and negative charges. There is nothing “positive” or “negative” about either charge. It is just an arbitrary label that we gave them to develop the mathematics describing charges.

Charles-Augustin de Coulomb was the scientist who described the behavior of charges in what is now known as Coulomb’s Law.

8

Coulomb’s law states the following 4 things.

1. Like charges repel each other while opposite charges attract.

2. Attraction or repulsion of charges occurs in a straight line between the charges.

9

3. The size of the force gets bigger the closer the charges are are smaller the further they are apart (inversely proportional to the square of the distance). So if the charges are half as far apart the force is 4 times greater.

4. The size of the force is directly proportional to the size of the charge.

10

It then follows that when there is a charge differential (difference in charges from one place to another) then there is the potential for a force to be created. When forces are created, work can be done.

Charges move, change, and influence each other. We call this electricity.

Electricity can help us make a display at the festival of lights in Robson Square in Vancouver.

It can also transport us at tremendous speeds on Japan’s bullet train.

11

The smallest units of charge are found in the atom. The electron carries smallest unit of charge. All other charges are multiples of this basic charge.

We measure charge in coulombs (named after the scientist).

electronpercoulombs106.1electrons106

1coulomb1electrons106

1918

18

12

The proton is another basic atomic unit that carries a charge that is equal and opposite to an electron. This is where we get that a proton has a charge of +1e and an electron has a charge of –1e.

The neutron is another sub-atomic particle that has a neutral charge (0).

Protons and neutrons are relatively stationary compared to electrons. So, it is the influences of charges on moving electrons that creates electrical phenomena.

eC 1106.1 19

Because this unit is the fundamental charge size and because it is so small we call this number e.

13

Static ElectricityFriction can transfer electrons from one object to another. The swirling air and water in a pocket of the atmosphere makes a lot of friction. When electrons are collected in a cloud, a net build up of negative charge develops. This built up charge that sits on the object is called Static charge. The mass of static negative charge on the cloud can cause a charge separation with the neutral ground.

These anvil shaped thunder clouds commonly demonstrate this charge separation process.

14

The electrons on the ground disperse away from the cloud leaving a spot on the ground that is positively charged. The situation is very unstable. The electrons in the cloud are trying to get away from each other and the positive ground is attracting the electrons. However, there is no easy path for the electrons to follow to get to the ground.

The instability of the now oppositely charged areas cause the electrons from the cloud to jump to the ground in an electrical discharge which we see as lightning. This is a common phenomena around Edmonton Alberta.

15

The negative clouds can also influence the charge of another cloud causing a cloud-cloud discharge.

Air normally doesn’t let charge flow through it.

Therefore, when the charge differential is great and a discharge occurs, heat is produced from the air’s attempts to stop the movement of charge. This massive heating causes the light and sound we notice as lightning and thunder.

16

This same process on a much smaller scale explains why you get a shock if you rub your feet on carpet and then come close to touching a metal door knob. When you touch the door knob, the electrons have a relatively easy path to get to the ground so they discharge from your body. Making a path for the electrons to get to the ground is called grounding.

But, why would electrons attempt to get to the ground?

The reason is that electrons all carry negative charge, hence they repel one another. The ground offers electrons the greatest area to spread out and stay as far away from each other as possible.

17

Other common static phenomena include rubbing a balloon in your hair and sticking it to a wall. The electrons from your hair are transferred to the balloon. When the balloon is brought close to the wall, the negative charge makes a temporary positive spot on the wall. The two are attracted to each other and the balloon sticks.

Pet hair is very hard to clean off of fuzzy fabrics because it seems to “cling” to the fabric. A little water interupts the charge separation and the hair pulls off more easily.

18

A Van De Graaff generator is a device that collects static charge on a metal sphere. The friction of the rubber band “brushes” electrons to the sphere.If you grab the sphere the electrons travel to your hair. Each strand of hair is then negatively charged and repel each other making your hair stand on end.

19

Electric CurrentStatic electricity is not really all that common. Electrons don’t usually like to gather in large stationary groups. When they do they create an unstable situation. The unpredictability of static electricity also makes it less useful to us.

Electrical current is a much more common and useful phenomena. It is a steady flow of charged particles. Some materials allow charged particles to pass through more easily than others. These materials are called conductors. Metals are good conductors of electric current.

20

The path that electric current travels is called a circuit. A Wire made of conductive material, carries the electric current. A complete circuit is require for current to flow.

+ + + + +++ + +-- - -

- - - - - - - - ---

Negative electrons flowPositive “holes” flow

Electrons flowing in this case to the right and the holes left behind flow to the left. It is a kind of paradox. The electrons need holes to flow into and the holes need the electrons to leave in order to exist. Hence, the circuit needs to be completed in order for electric current to flow.

21

Current is measured in amperes (or amps). An amp measures the rate of the flow of charge. One ampere is defined as one coulomb of charge passing through a given point in one second.

6 1018 electrons per second = 1 ampere

Most common household appliances require few amps to work.

Appliance Amps needed

60W Bulb 0.5 A

Microwave Between 5 and 8 A

Electric Kettle 13 A

Color TV 1.7 A

Electric Oven 40 A

22

This galvanometer measures small amounts of electric current. An ammeter will measure larger amounts of current.

This digital ammeter is connected to a battery, a switch, and a light. The circuit is complete so the current should flow.

23

However, the speed that the current flows is not the same as the potential energy that can be done. The electrical potential energy is a result of the difference in charge. In a battery, chemicals force electrons to move to one end making a negative and positive terminal of the battery. This is the role of batteries of all shapes and sizes.

24

If the battery is then connected to a circuit, there is a difference in the charges at different spots in the circuit line.

The term potential difference describes the energy per coulomb of charge of one point in a circuit compared to another point in the circuit. We commonly call potential difference voltage. A volt is the unit of potential difference and is given as the joules (Unit of work) per coulomb. Voltage is measured with a voltmeter.

25

The chemical reaction drives the flow of electrons through the wire in the circuit. Different substances will produce different voltages.

A battery will create a steady voltage in a circuit called direct current. A battery is a device that converts chemical energy into direct current. A solution with a dissolved salt called an electrolyte and two metal called electrodes are connected as the diagram shows.

26

A primary cell is a battery that uses the chemicals to produce electric current. Once the chemicals are use up, the cell is dead.

This lead acid battery is a secondary cell. Electricity can be used to restore the chemicals and “recharge” the battery’s potential energy.

27

Making and Transferring ElectricityMagnetism and magnetic force is actually a result of moving charges. So electric current produces magnetic forces. Electricity and magnetism are actually two aspects of the same phenomena—electromagnetism.

An electromagnet can be created by wrapping a wire around an iron core (like a nail) and connecting the wire to a power source. A magnetic field is created. Making more wraps or increasing the current will make the magnet stronger. Wrapping the wire the other direction will reverse the poles of the magnet.

28

So electricity can create magnetism. But can magnetism create electricity?

The answer is yes. Moving a wire coil over a magnet or moving the magnet over a coil produces a current. This is called electromagnetic induction. Once again, putting more coils in the wire or moving the wire more quickly will increase the current produced. Movement in opposite directions will produce currents in opposite directions. Without motion, or if the motion of the magnet is parallel to the wire, there is no current.

29

An alternating current (AC) generator uses a wire coil that rotates between the poles of a magnet. When the coil rotates half a turn, the coils are in the opposite orientation so the current reverses direction. The current cycles in waves from one direction to the opposite.

In North America, current changes direction 120 times a second. This means 60 complete cycles in 60 seconds. We call this 60 cycle AC or 60 Hertz. So, a Hz is a measure of how long it takes for AC to cycle.

Alternating current is very useful. It travels long distances very easily compared to direct current produced from batteries.

30

A DC generator is called a dynamo. The connection to the wires is alternated with each half cycle. This is done by a split-ring commutator that insulates and reconnects the wires to the correct sources as the coil rotates. This way the current produced is all the same direction (direct current) even though the flow of electricity is not steady.

31

As the ring turns the relative connection to the coil alternates. So instead of the current alternating, the connection does. The current is interrupted each time the insulators hit the brushes.

32

An AC motor converts the electric energy to kinetic (moving) energy. These are common devices. The rotor rotates a coiled wire. Stators are stationary electromagnets. The power is fed into the coil and motion is produced from the opposing and alternating magnetic forces. The AC motor does the opposite of an AC generator.

33

The DC motor uses a split-ring communtator to alternate the polarity of the electromagnet and turn the machine. Thus the DC motor undoes the DC generator.

34

Why would we use AC if we can do all the same things with DC?

AC is easy to produce but more importantly it is much easier to transport. Because of the alternating polarity of AC, the magnetic fields generated alternate as well. A simple device, called a transformer, uses this alternation to “dial up” voltage to send the AC long distances, and then “dial down” to come into your home.

35

Electricity “flows” in wires. Wires carry electricity to our homes and to the electrical appliances that have become a part of our everyday life. We have already seen how electricity “flows” in a wire—negative charge flowing one way and positive holes flowing the other. When a complete wire path is created for the flow of electricity we call the path a circuit.

A circuit made completely out of wire is not very interesting. Usually, a circuit is interrupted by devices that control to flow of electricity or converts the electrical energy from one form of energy to another form of energy. A device that converts electrical energy from one form to another is called a load.

36

A load like a light bulb will take some of the energy from the electrons and slow them down, converting the electrical energy to heat and light.

This electric motor will convert the electrical energy into kinetic (motion) energy.

This speaker converts electrical energy into sound energy.

37

It would be extremely difficult to draw a circuit with all of the devices hooked up. Therefore, we have made a standard way to draw circuit diagrams. Here are a few of the common symbols we use and what they mean.

Wire – Carries electric current.

38

Joined Wire – Wires that cross and are connected offering a second path. Wires are usually joined with connectors.

Not Joined Wire – Wires that cross and are not connected.

39

Battery – Electrochemical cells that convert chemical energy to DC.

Cell – A single electrochemical cell (technically a battery is more than one cell).

These lemons are acting as an electrochemical cell.

40

DC Power Source – A dynamo or some other producer of DC.

AC Power Source – A generator producing AC.

This dynamo is used on a bicycle to work a headlight.

41

Fuse – A safety device that is broken when the current is too great.

This car fuse box (left) holds fuses that burn when the current is too great. The breaker box (right) flips a switch and can be flipped back.

42

Transformer – Device that dials up or dials down voltage.

Ground – A path for electrons to go to the earth.

43

Lamp – Converts electrical energy to light and heat.

Heater – A device that converts electrical energy to heat mostly and light.

44

Motor – Device that converts electrical energy to kinetic energy (motion).

Speaker – Device that converts electrical energy to sound.

45

Switch – Opens and closes the circuit. Controls electrons to flow or not flow.

Two Way Switch – Decides the path the current flows.

46

Resistor – A device that restricts the flow of electricity.

Charged particles moving through the resistor are slowed down. Resistance is measured in ohms (Ω).

This resistor gives 100ohms of resistance and helps protect delectate speaker components.

47

Variable Resistor – A device that restricts the flow of electricity in varied amounts. Sometimes called a rheostat.

48

G

Voltmeter – Measures the potential difference (V) of a particular spot in the circuit.

Ammeter – Measures the speed of the current (A) of a particular spot in the circuit.

Galvanometer – Measures the speed of the current (mA) of a particular spot in the circuit. Usually used for small amounts of current.

Ohmmeter – Measures the resistance of current (Ω) of a device.

49

The loads in a circuit offer resistance to the flow of current. Ohm’s law relates the voltage (V) to the current (I) and resistance (R).

IRV To find the resistance, we need 1) the potential difference across a load, 2) the current in the circuit, and 3) the equation solved for resistance.

50

A series circuit has one way the current can flow. No other routes can be taken. Each load is placed one after the other down the line in series.

These lights would not be very bright since all three offer resistance to the current in the line.

51

A parallel circuit has more than one way for the current to flow. The current has a choice of routes to take. Each load is placed on a parallel line in the circuit.

All of these lights would be just as bright as one light in series since the resistance in each individual branch is only as large as one light.

52

In your home, wire is usually drawn in a bundle of wires called cable. There are 3 wires that are used. Two are called “live” wires.

They are a white insulated neutral wire and a black insulated hot wire. The third wire is either uninsulated copper or green insulated ground wire (green = ground). The potential difference from white to black in North America is 120V for normal outlets and 220V for items that require more power like ovens and dryers.

53

A home circuit is basically a combination of series circuits connected in parallel circuits. When there is a surge in any line the breaker “pops” cutting the power to that line and avoiding a build up of heat in the wire.

54

Microchips are now a common part of our lives. On a microchip are microcircuits. Instead of using large switches, microcircuits us transistors.

Voltage through the transistor will control the current in tiny microscopic wires.

55

Electrical Energy and PowerA fundamental law of physics states that energy cannot be created or destroyed. Electrical generators DO NOT create energy. They produce electricity. To do this they need to convert energy from one form to electrical potential energy. In the case of generators, kinetic energy (motion) is used to produce electric current. An electric motor takes the electrical potential energy in the current and creates motion. In this way, generators and motors do opposite jobs. Batteries take chemical energy and produce DC. There are many ways to produce electricity. The one we pick depends on many factors and requires at least a basic understanding of all of the processes.

56

The efficiency of any device is its the ability to convert one form of energy into another useable form. None of the energy is actually “lost” but it may be converted into an energy form that is non-usable. We usually calculate the efficiency with a percentage.

%100inputenergyTotal

outputenergyUsefulefficiency

Energy is measured in Joules. A joule is defined as the energy needed to exert one newton of force over a one meter distance. A newton is approximately the force exerted by the mass of a small apple. So, one Joule of energy is about the energy required to lift a small apple one meter high.

57

In electricity, energy can be calculated using the voltage and charge since voltage is energy per coulomb. We get the following equation.

timePowerEnergy PtE

Energy = Charge Voltage

CVE The number of joules used in a second is the number of watts used. We call the amount of work in watts power. From this we get the following equation.

From this we also get the following formula to calculate power in electric current.

VoltageCurrentPower IVP

58

Here is a summary of our formulas.

VoltageCurrentPower IVP

timePowerEnergy PtE

Energy = Charge Voltage CVE

%100inputenergyTotal

outputenergyUsefulefficiency

59

The need for power in our society is obvious. In 2001 Canada used a total 504.4 billion KWh (1 KWh = 3.6 MJ) which makes us the greatest per person consumer of electricity in North America and number 3 on the world list. Our usage of power is actually visible from space during the blackout of 2003. How can we make all our power?

60

Most of our power is produced in plants like this thermo-electric plant where we burn a fossil fuel to make AC.

61

These power plants burn coal to generate heat, create steam, and move the turbine.

The majority of the smoke from the stacks are steam and carbon dioxide. Sulfur and nitrogen compounds are also produced.

62

Burning fossil fuels creates a lot of CO2(g) that is released into the atmosphere as well as the acid rain components SO2(g) and NO2(g). As well, mining for coal (Alberta’s primary fuel source for making electricity) can be very harmful to the environment. Mining is also potentially dangerous. Extracting, refining, and transporting oil also has its pitfalls.

63

A light bulb will create resistance in electric current to superheat a thin filament in a bulb that glows producing light and heat. We can also reverse the process and use light to create electrical energy. Many of us have used solar calculators. The solar panel at the top collects light and produces a weak electric current. Solar panels on a house roof can make electricity for the house during daylight hours. Batteries are charged so that the house has electricity available at night.

64

There are two main types of solar power plants. This one is a photovoltaic plant. Each panel is a larger version of the panel on your calculator. Electricity is created directly.

This plant uses large curved mirrors that concentrate the sun’s light and heat to superheat special liquids. The heated liquids then produce steam and turn a turbine. These plants are called solar thermal plants.

65

Solar generators are very expensive and require large sections of land. Battery technology has also not improved to the point where we can use solar power effectively at non-sunny times.

Solar energy does allow us another way to not need as much electricity.

A passive solar design includes many windows to allow light and heat to offset the needs of the building. Long overhangs also shade the building from the high and hot midday sun.

66

This wind farm in Alberta, near Pincher Creek, is used to create electricity. The turning of the blades spins the turbine in this case creating the needed motion for the electric generator. Wind powered turbines require wind to work and also require vast areas of land and don’t produce very much power.

67

This CANDU nuclear power plant in Pickering Ontario provides a great amount of electrical energy. There are in fact 8 nuclear reactors here producing 540 MW of power each.

Nuclear power is created through a process called fission. This is where uranium (or plutonium) is atomically altered to make other elements.

The atomic particles that break off from the process are very energetic and release a lot of energy. Nuclear energy is ten million times more efficient than burning coal (by mass).

68

The CANDU reactor has rods that can be added and taken out while the reactor core is operating. Many computerized and manual controls help prevent a reaction from continuing too fast which generates too much heat causing a melt down.

69

Normal uranium is a mixture of two isotopes of uranium—U-238 and enriched U-235. U-235 is more radioactive and used more often in reactors. The CANDU reactors are different because they use normal uranium in their reactors and nuclear fuel is constantly added and wastes taken out. Nuclear wastes (plutonium) can then be recovered and re-used as fuel. To this point, much of the waste is not recovered.

There is a great political debate over the safety of nuclear energy considering the potential destructive capability of a nuclear plant. You can find strong views supporting and opposing nuclear power.

70

Hydroelectric dams use the movement of water to turn a turbine. There is little pollution made but holding back the water causes flooding.

Flooding alters the marine and land ecosystems drastically. As well, hydro dams must be near large bodies of moving water.

71

Geothermal energy uses naturally heated underground water from geysers. Emissions are around 50 times less than that of a fossil fuel burning plant.

This steam plant uses very hot steam that naturally occurs to turn the turbine and generate electricity.

This binary plant uses less hot water to heat a hydrocarbon with a low boiling point.

Just like hydroelectric power, geothermal power is restricted to certain locations.

72

There are environmental and economic pros and cons to each source of power. Each province generates its power in different ways. Alberta and Saskatchewan use mostly fossil fuels. Ontario uses mostly nuclear. Quebec, Newfoundland and Labrador, and BC use mostly hydroelectric. Overall, about 60% of Canada’s power is generated through hydroelectric dams.

Fossil fuels are considered non-renewable resources and will be depleted some day. It is therefore necessary for use to continue to look at different and innovative ways to produce and conserve electricity.