Grade 9 - Electrical Principles and Technologies
Section 1: Introduction
Remember, all matter is made up of atoms, and all atoms are made up of protons, neutrons and electrons. The protons are the positively charged particles and the electrons are the negatively charged particles. When the positive and negative particles are equal, the charge equals out. When there are more electrons than protons, you have a charged atom, called an ion.
Electrons spin around the outside of the nucleus and are held in that place by the force of attraction from the protons in the nucleus. However, electrons can be lost by one atom and picked up by another atom resulting in a change in the charge of both atoms. The atom that has lost the electron now becomes positively changred (because it has more protons than electrons) and the atom that gains an electron becomes negatively charged, because it now has more electrons than protons. The transfer of charged particles from one atom to the other can build a series of electrically charged atoms. Electricity refers to the movement and transfer of the energy of charged particles. This energy is used to power motors, lights, appliances, and many other devices.
Static Electricity
When two materials touch one another, electrons can move from one material to the other. This causes one maerial to become more negatively charged and the other positively charged. This transfer of electrons causes an imbalance which results in static electricity. Objects with the same electric charge repel each other and objects with opposite charge attract each other. Some materials lose electrons more easily than others while others attract electrons more easily. For example, the atoms on the human skin more readily lose electrons, becoming positively charged. The atoms on a cat's fur do not lose electrons easily, so if you pet a cat you can create static electricity.
Benjamin Franklin was the first to describe the charges as 'positive' or 'negative'. When a charged object is placed near a neutral object, the charged object can affect the overall charge of the neutral object. Like charges within the neutral object are repelled, and unlike charges are pulled toward it. This movement can result in an induced charge. An induced charge is a static charge caused by the presence of an object that itself has a net positive or negative charge. When you rub a balloon on your hair, some electrons leave your hair and are transferred to the balloon. The balloon then has a net negative charge. When you place that balloon near a wall with no net charge, the negative particles in that area of the wall are repelled. This leaves a net positive charge on the surface of the wall. The balloon and wall then attract each other, and the balloon sticks to the wall. Evidence indicates that lightning can also be produced as a result of induced charges. Storm clouds can accumulate a negative charge near the bottom of the cloud. This can induce a positive charge in the ground below the cloud. This imbalance of charges can result in the discharge called lightning, which can reach 5 km in length.
Condutors and Insulators
A conductor is a material through which an electric charge flows easily. Most conductors are made of atoms from which some electrons are likely to become unattached. Metals such as copper are the best conductors. This is why copper is commonly used in electric wiring.
Semiconductors are almost perfect conductors - they have almost no resistance to electron flow. Silicon semiconductors are used extensively to make computer microchips. The largest obstacle is to get the semiconductor to work at reasonable temperatures for practical applications. Superconductors are materials that offer little, if any, resistance to the flow of electrons.
An insulator is a material that does not allow an electric charge to transfer easily. Conductors and insulators of electric charge are very similar to conductors and insulators of heat energy.
Electrical Discharge is the movement of charges whenever an imbalance of charges occurs. The action results in neutralizing the objects. The over-charged electrons repel the electrons in the object and the positive protons attract the charged electrons causing a discharge or 'miniature lightning bolt'. There is now an electron balance. An ionizer can be used to neutralize charges on non-conductors.
If a bare wire touched the metal case of an appliance, it could become electrified and can harm people when they touch the appliance. To avoid this problem, a grounding wire is connected to the metal case of the appliance. The grounding wire connects the case to the ground and because this charge gets distributed over much of Earth, the charge on the case is then too small to cause problems.
Section 2: Circuits
Electric charges flow through conductors along different paths. Each path for electric charge is an example of a circuit. In circuits, electric charges move within wires, bulbs and other devices.
All circuit diagrams have four basic parts:
- Source - provides energy and a supply of electrons for the circuit usually a Battery
- Conductor - provides a path for the current - Wires
- Switching mechanism - controls the current flow, turning it off and on, or directing it to different parts of the circuit - Switch
- Load - converts electrical energy into another form of energy, such as a Bulb
A simple circuit consists of an energy source such as a battery, a device such as a lamp, and connecting wires. The flow of an electric charge through a circuit is called current electricity. In a circuit, energy from a source such as a battery causes an electric charge to flow through the wire. Electrons that are not strongly attached to the atoms inside the wire move, causing current electricity.
Batteries stop working when the chemical reactions inside them can no longer transfer energy to electrons and move them through the wire in this manner.
Although the movement of negatively charged electrons is most often referred to when studying current electricity in wires, current is always said to flow from the positive to the negative terminal in a circuit. This is called conventional current. This way of describing the movement of electric current originated before scientists fully understood electricity. However, it is still the way used to describe how circuits operate.
A switch can control the flow of a charge in a circuit. When the switch is opened, the flow is halted. The circuit is incomplete and is then called an open circuit. When the switch is closed, the electric charge resumes its motion. When current flows once again, the circuit is called a closed circuit.
Direct and Alternating Current
Direct current, or DC, refers to current that flows in one direction. Batteries provide DC, as do solar-powered cells. The very first commercial electric power stations also used DC.
Alternating current, or AC, refers to the electric charge that does not flow through the circuit in one direction. AC power is transmitted when the charge changes direction, moving back and forth at regular intervals. The main advantage of AC is that this type of current can be transported over long distances with far greater efficiency.
Resistors
Resistors lower the amount of electric charge that flows through a device. Lights and other devices connected in a circuit act as resistors, because they too reduce current flow. A light bulb converts electrical energy to both heat and light.
Measuring Current
The steady flow of charged particles is called electrical current. The flow continues until the energy source is used up, or disconnected. The rate at which an electrical current flows is measured in amperes (A). This flow varies from a fraction of an ampere to many thousands of amperes, depending on the device. An instrument used to measure very weak electric current is called a galvanometer. Larger currents are measured with an ammeter.
Measuring Voltage
Electrical energy is the energy carried by charged particles. Voltage is a measure of how much electrical energy each charged particle carries. The higher the energy of each charged particle, the greater the potential energy. Also called 'potential difference', the energy delivered by a flow of charged particles is equal to the voltage times the number of particles. Voltage units are volts (V), and for safety purposes, the voltage of most everyday devices we commonly use is relatively low, while industries and transmission lines are relatively high. A simple way to measure voltage is with a voltmeter. [red to positive (+) and black to negative (-)] Some voltmeters can measure a wide range of voltages. These multi-meters should be used with caution, so that the sensitive needle is not damaged (by testing a low range with high voltage).
Types of Circuits
In a series circuit, there is only one path along which current electricity can flow. Each battery supplies more energy that causes electric charge to flow. The light bulbs receive the sum of the energy that comes from the two batteries. This energy is measured in volts (V). Because each battery has 1.5V, the two batteries together deliver a total of 3V. A higher voltage causes more electric charge to move through the light bulbs.
The total resistance is the sum of resistances of the individual devices such as bulbs. For example, two identical light bulbs together in a series circuit will have twice the resistance of either bulb by itself. The voltage of current electricity from a normal wall outlet is about 120V. Each small bulb on a strand of lights operates on about 2.5V. This means that the voltage from a wall socket is about 50 times the voltage that a single bulb requires. To provide the correct voltage to each bulb, each strand could only have 50 bulbs. If there were more, then each bulb would not receive enough voltage to light up.
Parallel Circuits
In a parallel circuit, there are multiple paths along which current electricity can flow. For example, in a string of lights wired in a parallel circuit, when one bulb burns out, there are other paths along which electric charge can flow to all the other bulbs. Parallel circuits are used everywhere that we use electricity, including homes, stores, and offices. If any one device on the circuit burns out, the other devices on the circuit will keep working.
Short Circuits
A short circuit is a path for current electricity that has little or no resistance. Current flowing in a short circuit can reach dangerously high levels and will also generate heat. A fuse is a device that prevents dangerous levels of current from continuing to flow through a circuit. A fuse contains a piece of metal that melts if it is heated. This melting breaks, or opens, the overloaded circuit.
House Wiring: Practical wiring in the home uses parallel circuits. The voltage across each load is the same, and by turning on one appliance in the circuit, the energy will not be reduced to the other devices. Caution – current through wires connected to the source increases whenever another branch in the circuit is closed.
Section 3: Resistance
Resistance is a measure of how difficult it is for the electrons to flow through a conductor. Resistance also converts electric energy into other forms of energy. Generally, it can be said that conductors have low resistance and insulators have high resistance. The standard unit for resistance is ohm (Ω). Resistance can be measured directly with an ohmmeter, but a multi-meter is used more often to measure resistance.
Calculating Resistance: Electrical resistance is calculated by finding the ratio of the voltage across the load (V) to the current through the load (I). This is called Ohm’s Law. R = V / I. The more resistance a substance has, the greater the energy gain it receives from the electrons that pass through it. The energy gain is evident in heat and light energy (light bulb filament, wire in a toaster).
Different resistors are used for different applications, especially in electronics. There are many styles, sizes and shapes. The major application for resistors is to control current or voltage to suit the specific needs of other electrical devices within the same circuit. The two most common resistors are the wire-wound and carbon-composition types. The colored strips on a resistor usually indicate the level of resistance and quality.
To change electron flow gradually, a variable resistor, or rheostat is used (a dimmer switch, volume control knob).
Power cables are composed of many thin copper stands, separated in groups by paper insulation, and covered by a rubber insulation material, which reduces resistance and heating in the cable, while still making it flexible enough to handle.
Section 4: Electrical Energy Transformation
Energy transformation, also known as energy conversion, is the process of changing energy from one form to another. The four most common forms of energy are: chemical (potential or stored energy stored in chemicals), electrical - energy of charged particles, transferred when they travel from place to place, mechanical - energy possessed by an object because of its motion or its potential to move, and thermal - kinetic energy of a substance.
Electricity and Heat
A thermocouple is a device that can convert thermal energy into electrical energy. It consists of two different metals (bimetal) joined together that conduct heat at slightly different rates. When heated, the difference in conduction results in electricity flowing from one metal to the other. The basic principle of the thermocouple was discovered by Thomas Johann Seebeck in 1821, and was named the Seebeck Effect. Thermocouples are useful for measuring temperatures in areas that are difficult to access or too hot for a regular liquid-filled thermometer.
Ovens and heaters do the opposite. They convert electrical energy into thermal energy. A thermo-electric generator is a device based on a thermocouple that converts heat directly into electricity without moving parts. Several thermocouples connected in a series are called a thermopile. Thermopiles are extremely reliable, low-maintenance devices and are often used in remote locations for emergency power generation.
Electricity and Motion
The piezoelectric effect produces sound by converting electricity into motion (vibrations). When a piezoelectric crystal, such as quartz, or Rochelle salt is connected to a potential difference, the crystal expands or contracts slightly. Material touching the crystal experiences pressure, creating sound waves or vibrations.
Motion to Electricity: A barbeque spark lighter uses the piezoelectric effect in reverse. When a crystal or Rochelle salt is compressed or pulled, a potential difference is built up on the opposite sides of the crystal. Conductors then take this through a circuit to produce electric energy (a spark).
Electric actuators most commonly pair with motors to provide linear or rotary motion. Together they convert electricity into kinetic motion.
Electricity and Light
An incandescent resistance filament (load) glows white-hot when electricity is passed through it. In fluorescent tubes a gas glows brightly and when crystals are struck together they can produce light. LED’s (light-emitting diodes) are solid –state components that use a fraction of the power. When connected to a semiconductor chip in the right direction, they will produce light and last for many years.
Solar panels, containing photovoltaic cells can convert light into electrical energy. The photovoltaic (PV) cells, or solar cells, are made of semiconductor materials, such as silicon. When light is present, the material, breaking electrons loose – allowing them to flow freely, absorbs some. This current is drawn off by metal contacts on the top and bottom of the cell and then used in devices such as calculators, heater, or emergency telephones. Individual solar cells are combined in modules, to form arrays to produce larger amounts of electric current.
Section 5: Portable Power
Electrochemical Cells
An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic cells or galvanic cells and those that generate chemical reactions, through electrolysis for example, are called electrolytic cells.
Two metal electrodes are surrounded by an electrolyte. These cells supply a steady current. The chemical reaction in a cell releases free electrons, which travel from the negative terminal of the cell, through the device, which uses the electricity, and back to the positive terminal of the cell. The chemical reactions within the cell determine the potential difference (voltage) that the cell can create. Several cells connected in series produces a higher voltage, and is commonly referred to as a battery, which is a sealed case with only two terminals.
A primary cell is one in which the reactions will not continue after the reactants are used up. Wet cells use a liquid electrolyte. Wet cells are 'wet', because the electrolyte is a liquid (usually an acid). Each electrode (zinc and copper) reacts differently in the electrolyte. The acidic electrolyte eats away the zinc electrode, leaving behind electrons that give it a negative charge. The copper electrode is positive, but it is not eaten away. Electrons travel from the negative terminal (attached to the zinc electrode) through the device and on to the positive terminal (attached to the copper electrode).
Dry cells referred to as 'batteries', are called dry cells, because the chemicals used in them are a paste. The dry cell is made up of two different metals, called electrodes in an electrolyte. An electrolyte is a paste or liquid that conducts electricity because it contains chemicals that form ions. An ion is an atom or group of atoms that has become electrically charged through the loss or gain of electrons from one atom to another. The electrolyte reacts with the electrodes, making one electrode positive and the other negative. These electrodes are connected to the terminals.
A secondary cell uses chemical reactions, which can be reversed. These are referred to as rechargeable batteries. Rechargeable cells use an external electrical source to rejuvenate the cell. The reversed flow of electrons restores the reactants in the cell. The most common reactions that are efficient enough to be used for these types of cells are Nickel Oxide and Cadmium (NiCad). The reactants are restored, but the electrodes will eventually wear out over time.
Fuel Cells: combine hydrogen and oxygen without combustion. Electricity, heat and pure water are the only by-products of the fuel cell’s reaction. They are 50-85% efficient.
Types of Primary Dry Cells
| Type | Uses | Pros / Cons |
|---|---|---|
| Zinc-carbon | Flashlights, portable stereos, CD players, remote controls, toys etc. | Not efficient at low temperatures |
| Alkaline | Flashlights, portable stereos, CD players, remote controls, toys etc. | Last longer than zinc carbon, but more expensive |
| zinc-air | Calculators, hearing aids, watches and smaller remote controls, car keys. | Highest energy per unit mass, but discharge rapidly |
Types of Secondary Dry Cells
| Type | Uses | Pros / Cons |
|---|---|---|
| Nickel-cadmium | Electric shavers, laptops, power tools, portable TV’s | Rechargeable hundreds of times |
| Nickel-metal hydride | Cameras, laptops, cell phones, hand tools, toys | Less toxic than NiCad – 40% more energy density than NiCad, rechargeable, no memory effect, lose charge when stored |
Types of Secondary Wet Cells
| Type | Uses | Pros / Cons |
|---|---|---|
| Lead acid | Cars, motorbikes, snowmobiles, golf carts | Dependable, but heavy and has a corrosive liquid |
Section 6: Generators and Motors
Magnetism
Magnetism is the ability of an object to push or pull another object that has magnetic property. Magnets also work with metals like iron and nickel.
A magnet has two poles, North (N) and South (S). Like poles repel, while unlike poles attract. Magnets always have a N and S pole, even if you break a magnet into two, it will form 2 magnets, each with a N pole and a S pole.
The Earth acts as a magnet with a North and South pole.
Atoms also act like magnets. In most non-magnetic materials, the N and S poles are random so they cancel each other. In materials where the N ans S poles of the atoms are aligned in the same direction, they form a permanent magnet. Iron, Nickel and Cobalt are attracted to magnets because their atoms can align to match those in the magnet thereby acting as weak magnets. When you place pieces of these metals over a magnet they form lines which correspond to the forces around a magnet called magnet field. The closer the lines are, the stronger the magnetic force in that area.
Electromagnets
An electromagnet is an electric circuit that produces a magnetic force. The moving electrons in the circuit generate a magnetic field.
The simplest electromagnet is a straight wire. The magnetic field circles around the wire when there is current flowing through the wire.
When you use the wire to make a loop, you increase the magnetic force. Many loops together result into a coil, which has a much stronger magnetic force.
If you place an iron rod in the coil, it becomes magnetized and makes the eletromagnet even stronger.
A generator is a device that generates an electric current by spinning an electric coil between the poles of a magnet. Energy is used to rotate the axle then as the coil moves through the magnetic field, the margentic forces push on its electrons and generate an electric current. Whenever the coil passes the pole of a magnet the direction of the current changes. This kind of current that regularly changes direction is called alternating current (AC). In US, generators produce alternating current that changes direction 120 times every second.
A DC generator is much the same as a DC motor, and is often called a dynamo. The spinning armature produces the electricity (if electricity is passed through a DC generator, it will spin like a motor). The armature is connected to a split ring commutator which enables this type of generator to send current through a circuit in only one direction. The DC generator’s pulsating electricity is produced in one direction - referred to as direct current - and coincides with the spinning of the generator.
Current electricity is first conducted to a transmission substation. The substation has many towers with power lines leaving them. Step-up transformers in the substation increase the voltage of the current electricity. This allows the electric charge to be transmitted over long distances more efficiently.
Electric Motors: Electric motors convert electric energy to mechanical energy. An electric motor is constructed in exactly the same way as a generator. Instead of producing electricity, it uses electrical energy to make a wire coil spins between the poles of a magnet. Faraday constructed the first motor. By coiling (copper) wire around a (iron) metal core a strong electromagnet can be made. When attached to an electrical source it will produce a strong magnetic field. To keep this electromagnet spinning in a magnetic field, the direction that the current is traveling through the coil must be switched. This is accomplished by with a gap, which allows the polarity of the electromagnet change just before it aligns with the permanent magnet.
DC motors use a commutator (a split ring that breaks the flow of electricity for a moment and then reverses the flow in the coil, when the contact is broken, so is the magnetic field) and brushes (contact points with the commutator) to reverse the flow of electricity through the magnetic field. The armature (the rotating shaft with the coil wrapped around it) continues to spin because of momentum, allowing the brushes to come into contact once again with the commutator.
AC motors have a rotating core, or rotor, made up of a ring of non-magnetic conducting wires connected at the ends and held in a laminated steel cylinder. Surrounding the rotor is a stationary component called a stator. The stator is a two-pole electromagnet. When the motor is turned on, the attraction and repulsion between the poles of the stator and the rotor cause the rotor to spin.
Section 7: Electricity in the Home
The high voltages that are used in transmitting current electricity over distances are dangerous. Therefore, before current electricity enters your home, the voltage is decreased at several stages.
Transformers are again used to change the voltage of the current electricity. A step-up transformer increases voltage at the generating plant prior to distribution to the power grid over high voltage transmission lines. Step-down transformers decrease the voltage of the electric charge. By the time the electric current reaches your home, the voltage is 240V.
Electrical power enters a meter on the side of your house where electrical usage is recorded. Power is then routed into the service panel. The main circuit breaker shuts off all the power in the house at once, in case of an overload. The individual circuit breakers in the service panel control the branch circuits, located throughout the entire house. Each branch circuit is connected in parallel to wall plugs, lights and wall switches within a particular area of the house.
A regular wall socket in your home delivers 120V, and this is what most appliances need to operate. Sockets that deliver higher voltage are used for specially designed appliances, such as certain types of clothes dryers, stoves/ovens or air conditioners.
Power is defined as energy per unit time. Electric power describes the amount of electric energy that is converted into other forms of energy (heat, light, sound, or motion) every second. The formula that is used is: Power (watts) = Energy (joules) / Time (seconds) A kilowatt is 1000 watts.
The power rating of a device can be used to determine the amount of energy the device uses. Multiply the power rating by the time the device is operating.
(E) Energy in joules, (P) Power in watts, (J/s) (t) time in seconds.
E = P x t
P = E / t
t = E / P
Kilowatt Hours is used as a unit for energy. The energy calculation is the same, except that hours are substituted for seconds and kilowatts (kW) are substituted for watts. Electricity meters measure the energy used in kilowatt hours and then bills you for every kilowatt hour used.
Devices and Efficiency
Energy is neither created nor destroyed. It doesn't appear and then disappear, but is transformed from one form to another. Most of the energy transformed in a light bulb is wasted as heat. Known as the Law of Conservation of Energy, no device is able to be 100% efficient in transforming energy. Most often, the energy is lost, or dissipated as heat. The efficiency of a device is the ratio of the useful energy that comes out of a device to the total energy that went in. The more input energy converted to output energy, the more efficient the device is.
Efficiency ( % ) = useful energy output (J) x 100% / total input energy (J)
Home Electric Safety
- Cover electrical outlets with child-proof covers if they are within reach of small children
- Don't use devices that have a frayed or exposed power cord
- Always unplug an electrical device before disassembling it
- Don't put anything into an electrical outlet - except a proper plug for an electrical device
- Don't overload an electrical circuit, by trying to operate too many devices at once
- Don’t bypass safety precautions when you are in a hurry
- Pull on the plug, not the wire
- Never remove the third prong from a 3 prong plug
- Never handle electrical devices if you are wet or near water
Electric Safety Outdoors
- Never allow yourself to come into contact with anything that is touching live electrical wires.
- Never use ungrounded or frayed 2 prong electrical cords outdoors
- Do not operate electrical equipments outdoors in the rain
- Check before you dig – you could end up digging into electrical cables or wires for communication causing injury and disruption.
Section 7: Electricity and the Environment
Fuel oil, natural gas, and coal are used in large thermo-electric generating plants to produce roughly 25% of North America’s electrical energy needs. Coal is mined, crushed into a powder, blown into a combustion chamber and burned to release heat. This heat boils water and superheats the resulting steam to a high temperature and pressure, which then turns a turbine. The turbine shaft rotates large electromagnetic coils in the generator to produce electricity.
Hydro-electric plants use falling water (gravity), and pressure to generate electricity. Large dams raise the water above the power plant (which is usually built inside the dam), near the base. A channel, called a penstock, directs the water (at high pressure) to a turbine. The turbine then converts mechanical energy to electrical energy. Although these hydro-electric plants appear to be doing no harm to the environment, the reservoir they have to create behind the dam, destroys habitat and displaces whoever lived in the area prior to the reservoir being created.
Energy from Atomic Reactions: Bombarding uranium atoms with tiny particles, called neutrons cause the uranium to split into two smaller atoms. This is called nuclear fission. The process creates a huge amount of energy, which is used to generate electricity in a thermonuclear plant.
All thermonuclear and thermo-electric-generating plants release thermal energy into the environment. 43% of the water used in the cooling process enters the environment. Thermal pollution occurs when this heated water is not cooled before it re-enters the water system.
Cogeneration is the dual generation of electrical and thermal energy. The cogeneration systems usually are associated with industries, or commercial complexes.
Alternative Energy Sources
Wind - this energy is harnessed by large propeller-type blades, which turn a shaft - connected to a generator.
Sunlight - Solar cells (made from silicon) enable the energy from the sun to be transformed (photoelectric effect) into electricity.
Tides - moving water can power turbines, which then run generators. When the tide comes in, the water is trapped in large reservoirs and then allowed to flow out past turbines.
Geothermal - Heat from the Earth's core can also be used to generate electricity. This geothermal energy (hot water and steam) is channeled through pipes to drive turbines - connected to generators, which produce the electricity.