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Transcript
What is Electricity? • Electricity is generated from the motion of tiny charged atomic particles called electrons and protons! • Protons = + • Electrons = electron neutron proton • Electric Charge and Electrical Forces: • Electrons have a negative electrical charge. • Protons have a positive electrical charge. • These charges interact to create an electrical force. – Like charges produce repulsive forces – so they repel each other (e.g. electron and electron or proton and proton repel each other). – Unlike charges produce attractive forces – so they attract each other (e.g. electron and proton attract each other). • (A) A neutral atom has no net charge because the numbers of electrons and protons are balanced. • (B) Removing an electron produces a net positive charge; the charged atom is called a positive ion (cation). • (C) The addition of an electron produces a net negative charge and a negative ion (anion). Electrostatic Charge • The charge on an ion is called an electrostatic charge. • An object becomes electrostatically charged by: – Friction: Rubbing one object on another object causes electrons to transfer from one object to the other, – Conduction: a charged object contacts another object which results in the transfer of electrons, – Induction: a charged object comes close to another object which causes the electrons to be rearranged in the second object. Name the form of electrostatic Charging! • Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before and (B) after combing. • Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair. Name the form of electrostatic Charging! • The comb has become charged by friction, acquiring an excess of electrons. The paper (A) normally has a random distribution of (+) and (-) charges. • (B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges. This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb. What is static electricity? • When two objects rub against each other electrons transfer and build up on an object causing it to have a different charge from its surroundings. • Like the shoes rubbing against the carpet. Electrons are transferred from the carpet to the shoes. What causes you to be shocked when you rub your feet across carpet? An electrical discharge is the passing of an electric current through the air from a negatively charged object to a positively charge object. This is what causes lightning! Electrical Conductors and Insulators: • Electrical conductors are materials that can move electrons easily. – Good conductors include metals. Copper is the best electrical conductor. • Electrical nonconductors (insulators) are materials that do not move electrons easily. – Examples are wood, rubber etc. • Semiconductors are materials that sometimes behave as conductors and sometimes behave as insulators. - Examples are silicon, arsenic, germanium. Current Electricity An easy way to think of electric current is to picture cars going through a Turnpike or Parkway Toll. The cars could represent electrons or charge, and the toll booth could represent the cross sectional area of the wire at a certain point. If you counted the number of cars or electrons, that passed through the toll booth or a certain cross sectional area of the wire, and divided that number by the time it took for those cars or charges to pass, you would get the current! What is the difference between static electricity and current electricity? • Static electricity is stationary or collects on the surface of an object, whereas current electricity is flowing very rapidly through a conductor. • The flow of electricity in current electricity has electrical pressure or voltage. Electric charges flow from an area of high voltage to an area of low voltage. Water pressure and voltage behave in similar ways. There are some similarities between the flow of water in a pipe and the flow of electric current through a circuit. • The pressure of the water flowing through the pipes on the last slide compare to the voltage (electric potential) flowing through the wires of the circuit. The unit used to measure voltage is volts (V). • The flow of charges in a circuit is called current. Current (I) is measured in Amperes (A). Charge Passing Through A Given Area Electric Current = ------------------------------Time The Ampere The Ampere is a measure of how much electrical current is flowing and is measured in units of amps. Q I = ---t The Ampere The current varies depending on the force behind the current and the resistance to flow. What is electrical resistance? Resistance (R) is the opposition to the flow of an electric current, causing the electrical energy to be converted to thermal energy or light. The metal which makes up a light bulb filament or stovetop eye has a high electrical resistance. This causes light and heat to be given off. The unit for measuring resistance is the ohm (Ω). Resistance and Temperature Resistance and Thickness Electric Circuits – Electric current means a flow of charge in the same way that a water current flows. – It is the charge that flows, and the current is defined as the flow of the charge. An electrical circuit contains some device that acts as a source of energy as it gives charges a higher potential against an electrical field. The charges do work as they flow through the circuit to a lower potential. The charges flow through connecting wires to make a continuous path. A switch is a means of interrupting or completing the circuit. The source of the electrical potential is the voltage source. A simple electric circuit has a voltage source (such as a generator or battery) that maintains the electrical potential, some device (such as a lamp or motor ) where work is done by the potential, and continuous pathways for the current to follow. Voltage Sources - Electricity Relationships • Generator: creates electrical energy from mechanical energy • Batteries (cells): creates electrical energy from chemical energy • Motor: conforms electrical energy into mechanical energy Battery (cells) • A battery produces electricity by transforming chemical energy into electrical energy Anatomy of a Battery Carbon Electrode Zinc Electrode Sulfuric Acid What are batteries? Batteries are composed of a chemical substance which can generate voltage which can be used in a circuit. There are two kinds of batteries: dry cell and wet cell batteries. Below is an example of a dry cell. The zinc container of the dry cell contains a moist chemical paste surrounding a carbon rod suspended in the middle. Wet cell batteries are most commonly associated with automobile batteries. A wet cell contains two connected plates made of different metals or metal compounds in a conducting solution. Most car batteries have a series of six cells, each containing lead and lead oxide in a sulfuric acid solution. What are electric circuits? Circuits typically contain a voltage source, a wire conductor, and one or more devices which use the electrical energy. What is a series circuit? A series circuit is one which provides a single pathway for the current to flow. If the circuit breaks, all devices using the circuit will fail. What is a parallel circuit? A parallel circuit has multiple pathways for the current to flow. If the circuit is broken the current may pass through other pathways and other devices will continue to work. What is the difference between an open circuit and a closed circuit? A closed circuit is one in which the pathway of the electrical current is complete and unbroken. An open circuit is one in which the pathway of the electrical current is broken. A switch is a device in the circuit in which the circuit can be closed (turned on) or open (turned off). Circuit Diagrams Scientists usually draw electric circuits using symbols; Circuit Diagrams Circuit Diagrams Circuit Diagrams What is Magnetism • The magnet is surrounded by an invisible force field • All magnets have a North and South POLE • Opposite poles attracted each other • All ferrous (containing iron) materials can be magnetized • The earth is the biggest magnet around • Magnetic Fields: – A magnet that is moved in space near a second magnet experiences a magnetic field. • A magnetic field can be represented by field lines. – The strength of the magnetic field is greater where the lines are closer together and weaker where they are farther apart. These lines are a map of the magnetic field around a bar magnet. The needle of a magnetic compass will follow the lines, with the north end showing the direction of the field. • The core is probably composed of iron and nickel, which flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field. • The Earth’s magnetic field is thought to originate with moving charges. The earth's magnetic field. Note that the magnetic north pole and the geographic North Pole are not in the same place. Note also that the magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth. You know that it must be a magnetic south pole since the north end of a magnetic compass is attracted to it and opposite poles attract. What are magnetic domains? Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms. • Force Fields: – The condition of space around an object is changed by the presence of an electrical charge. – The electrical charge produces a force field, that is called an electrical field since it is produced by electrical charge. Hans Christen Oersted Flux Lines Electricity and Magnetism • a wire carrying an electric current always has a magnetic field around it – the magnetic field in a straight wire is not very strong – if the wire is wound into a coil, the magnetic field becomes much stronger as the individual magnetic fields overlap • the greater the number of coils, the stronger the magnetic field Oersted’s Experiment All magnetic fields originate from moving electric charges. A magnetic field appears only when relative motion is present between an electric charge and an observer. Electric and magnetic fields are different aspects of a single electromagnetic field. When a current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field like the magnetic field of a bar magnet. The solenoid is known as electromagnet. Michael Faraday Electromagnets • electromagnet – temporary magnet made by wrapping a current-carrying wire around an iron core – the center of an electromagnet is called the core • it is often made of iron • as long as current is flowing, an electromagnet has a magnetic field • when current is turned off, there is no longer a magnetic field Joseph Henry’s Bigger and “Badder” Electromagnet Strength of Electromagnets • there are two ways to make an electromagnet stronger – increasing the number of coils – increasing the amount of current • changing the direction of an electric current causes the poles of an electromagnet to reverse – this feature is important in the production of electric motors An electromagnet consists of an iron core placed inside a wire coil. The magnetic field strength of a wire coil carrying an electric current increases in direct proportion to the number of turns of the coil. Electromagnets in Your World! • electromagnets have many important uses – ex. radios, telephones, computers, electric motors • electromagnets are useful because they can be turned on and off • An electromagnet can be used to move large quantities of metal. When the current is on the magnet will pick up the metal. When you want to drop it you turn off the power and the electromagnet is disabled and the metal drops. In September of 1831, Michael Faraday made the discovery of Electromagnetic Induction. Faraday attached two wires to a disc and rotated the disc between the opposing poles of a horseshoe magnet creating an electric current. If you place a magnet and a conductor (copper wire), in a room together there will be no electric current generated. This is because motion, from our equation for electricity, is missing! An electric current is not generated unless the magnetic field is moving relative to the copper wire, or the copper wire is moving relative to the magnetic field. Magnetic Force on a Current A magnetic field exerts a sideways push on an electric current with the maximum push occurring when the current is perpendicular to the magnetic field. Currents exert magnetic forces on each other. The forces are attractive when parallel currents are in the same direction and are repulsive when the parallel currents are in opposite directions. Electric Motors An electric motor uses the sideways push of a magnetic field to turn a current-carrying wire loop. Electric motors use a commutator to change the direction of the current in the loop. Alternating current electric motors do not use commutators. So simple electric generators found in power plants contain, magnets and copper wire that when put into motion relative to one another create the electric current that is sent out to homes. The major problem in electricity generation is where does the motion come from that keeps the copper wire and magnets moving relative to one another. In this case, wind power applies a force to the blades that turns them. The spinning blades, spin an armature that turns the copper wire relative to the magnetic field. As long as the blades spin, electricity will be generated! Types of Sources used to make Electricity • • • • • • Thermal Geothermal Nuclear Hydroelectric Solar Wind Pictures of each Source -Solar Power – uses the sun energy to either boil water or directly converts solar energy to electrical energy -Ocean Thermal Energy Conversion – uses temperature differences between different depths of ocean water to drive a heat engine. Working fluid is ammonia which is gas at room temperature. -Biomass Energy: Municipal Solid Waste – burning wastes to drive heat engines -Geothermal Energy – based on naturally occurring heat in the Earth in the Earth due to radioactive decay -Tidal Energy – uses the gravitational pull of the moon on our oceans to drive turbines Proportion of World’s energy consumption - 1997 Proportion of the world’s Electricity generation - 1997 Saskatchewan’s Electricity http://www.pressprogre ss.ca/5_things_saskatch ewan_premier_brad_w all_doesnt_seem_to_ge t_about_climate_chang e How do we get Electricity? • Energy from one of the sources is converted by machines at the power plant to Electricity and then put onto the Electric Power Grid • Electric Power Grid – Power Plants – Transmission Lines – Substations – Power Lines – Transformers – Electrical Wiring and Circuit Box Pictures of each piece of the Power Grid! Electrical Energy Electricity by definition is electric current that is used as a power source! This electric current is generated in a power plant, and then sent out over a power grid to your homes, and ultimately to your power outlets. What happens at Home? At home, electric current that was generated by generators in the power plant is used to power electric appliances. The electric current, running through the copper wire causes the armature to spin which is how most motors generate motion. This meter measures the amount of electric work done in the circuits, usually over a time period of a month. The work is measured in kWhr. Electric Power The power of an electric current is the rate at which it does work and is equal to the product of the current and the voltage of a circuit: The unit of electric power is the watt.The commercial unit of electric energy is the kilowatthour (kWh). Electric Power Typical Power Ratings Appliance Stove Clothes Dryer Heater Dishwasher Photocopier Iron Vacuum Cleaner Coffee Maker Refrigerator Portable Sander Fan Personal Computer TV Receiver Fax Transmitter/Receiver Charger for Electric Toothbrush Power (W) 12,000 5,000 2,000 1,600 1,400 1,000 750 700 400 200 150 150 120 65 1 How is household wiring arranged? Most household wiring is logically designed with a combination of parallel circuits. Electrical energy enters the home usually at a breaker box or fuse box and distributes the electricity through multiple circuits. A breaker box or fuse box is a safety feature which will open How is Electrical Power calculated? Electrical Power is the product of the current (I) and the voltage (v) The unit for electrical power is the same as that for mechanical power– the watt (W) Example Problem: How much power is used in a circuit which is 110 volts and has a current of 1.36 amps? P=IV Power = (1.36 amps) (110 V) = 150 W Electric current generation - whether from fossil fuels, nuclear, renewable fuels, or other sources is usually based on the: Careers in the Field of Electricity http://learn.org/articles/What_are_Some_Common_Careers_in_the _Electricity_Industry.html Videos/Animations • http://ippex.pppl.gov/interactive /electricity/static.html • http://videos.howstuffworks.co m/hsw/19720-electricity-andmagnetism-creating-electricityvideo.htm