Outline Chapter 6 Electricity and Magnetism Positive and Negative Charge Positive and Negative Charge

Transcription

1 Outline Chapter 6 Electricity and Magnetism 6-1. Positive and Negative Charge 6-2. What is Charge? 6-3. Coulomb s Law 6-4. Force on an Uncharged Object 6-5. Matter in Bulk 6-6. Conductors and Insulators 6-7. Superconductivity 6-8. The Ampere 6-9. Potential Difference Ohm's Law Electric Power Magnets Magnetic Field Oersted's Experiment Electromagnets Magnetic Force on a Current Electric Motors Electromagnetic Induction Transformers 6-1. Positive and Negative Charge Opposites attract-same charges repel Let s try it! 6-1. Positive and Negative Charge Electrons are stripped from one component and transferred to the other to cause both to be oppositely charged. 1

2 Fig. 6.2 Ben Franklin discovered electric charge What is Charge? Atoms are composed of protons (+), electrons (-) and neutrons. The nucleus contains the protons and neutrons and the electrons surround the nucleus What is Charge? Protons are much larger than electrons but have an equal and opposite charge. The coulomb (C) is the unit of electric charge. The basic quantity of electric charge (e) is 1.6 x C. 2

3 6-3. Coulomb s Law Q 1 Q 2 F = K R 2 Charles Coulomb ( ) 6-4. Force on an Uncharged Particle Initially the paper is uncharged, but the comb polarizes the charges in the paper Matter in Bulk Coulomb's law resembles the law of gravity; however, gravitational forces are always attractive, whereas electric forces may be attractive or repulsive. Coulomb s Law Q 1 Q 2 F = K R 2 Law of Gravity M 1 M 2 F = G R 2 Gravitational forces dominate on a cosmic scale; electric forces dominate on an atomic scale. 3

4 6-6. Conductors and Insulators A conductor is a substance through which electric charge flows readily. An insulator is a substance that strongly resists the flow of electric charge. Semiconductors are substances whose electrical conductivity is between that of conductors and insulators. Semiconductors Transistors are switches that conduct electricity only when a second source of electricity is energized. John Bardeen Nobel Prizes in 1956 and 1972 for developing the transistor and superconductivity. A C B What a Transistor looks like. A current will not flow from A to B unless C is energized. Fig Enlargement of 5 mm square computer chip. 4

5 6-7. Superconductivity Superconductivity refers to the loss of all electrical resistance by certain materials at very low temperatures. Substances that are superconducting at 150K are now known which is warmer than liquid nitrogen (77K). Maglev Train in Japan 6-8. The Ampere The Ampere is a measure of how much electrical current is flowing and is measured in units of amps. Q I = ---- t 6-8. The Ampere The current varies depending on the force behind the current and the resistance to flow. 5

6 6-9. Potential Difference Potential difference, or voltage, is the electrical potential energy per coulomb of charge. Alessandro Volta ( ) J V = ---- C Fig Ohm s Law André Marie Ampére ( ) V I = ---- R Georg Ohm ( ) Resistance is a measure of opposition to the flow of charge and is measured in ohms (Ω) 6

7 Fig Fig Electric Power P= IV 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). 7

8 6-11. Electric Power Typical Power Ratings Appliance Power (W) Stove 12,000 Clothes Dryer 5,000 Heater 2,000 Dishwasher 1,600 Photocopier 1,400 Iron 1,000 Vacuum Cleaner 750 Coffee Maker 700 Refrigerator 400 Portable Sander 200 Fan 150 Personal Computer 150 TV Receiver 120 Fax Transmitter/Receiver 65 Charger for Electric Toothbrush Magnets Every magnet has a north pole and a south pole Magnet Field How to make a magnet: Heat in magnetic field. Magnetic force lines. 8

9 6-14. Oersted s Experiment Hans Christian Oersted ( ) Hans Christian Oersted discovered in 1820 that an electric current near a compass causes the compass needle to be deflected. Oersted's experiment showed that every electric current has a magnetic field surrounding it Oersted s Experiment According to the right-hand rule, the electron current in a wire and the magnetic field it generates are perpendicular to each other 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. 9

10 6-15. Electromagnets 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 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 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. 10

11 Fig The experimental Japanese Maglev train uses magnetic forces for both support and propulsion. How a TV works 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. 11

12 6-18. Electromagnetic Induction The effect of producing an induced current is known as electromagnetic induction. The direction of the induced current can be reversed by reversing the motion of the wire or reversing the field direction. The strength of the current depends on the strength of the magnetic field and the speed of the wire's motion. Fig Michael Faraday ( ) built the first electric motor and discovered magnetic induction. The stationary windings of a large electric motor. magnetic forces underlie the operation of such motors. Fig Sharks navigate with the help of the earth s magnetic field. They detect the field using electromagnetic induction. 12

13 Alternating and Direct Current Alternating current (ac) is current that flows in a back-andforth manner; household current changes direction 120 times each second (60 Hz). Direct current (dc) flows in one direction. The ac generator (or alternator) produces an ac current and can be modified to produce dc current by 1. Use of a commutator. 2. Use of a rectifier which permits current to pass through it in only one direction Transformers A transformer is a device composed of two unconnected coils, usually wrapped around a soft iron core, that can increase or decrease the voltage of ac current Transformers A transformer is used to step the voltage down and the current up (P=IV) so that we can use it. Low power is desired for the transport of electricity long distances to avoid loss of energy to heat loss. A moving coil activated by voice vibrations is used as a microphone. The coil induces a current in the magnet that can be amplified or recorded. 13

14 6-19. Transformers A taperecorder records signals from a microphone on magnetic tape which then can be run across a magnet and played back. Lecture Quiz 6 1. What happens when two positive charges come together? 2. What is Ohm s Law? 3. Why are transformers used on telephone poles? 4. What is the difference between an electric motor and an electric generator? 5. Why is AC current best to use at home? 14