Unit 10: Electrodynamics notes

Transcription

1 Name Period Unit 10: Electrodynamics notes Electrostatics is the study of charges at. Electrodynamics is the study of charges in. Lightning is a gigantic electric. This means to lose charge. Fill in the diagram below after your teacher explains: A is a metallic enclosure that prevents the entry or escape of electromagnetic. o Since cell phones use electromagnetic fields to communicate, being inside a metallic enclosure like this is usually for reception. o A car can act like a Faraday cage resulting in it being a place to be during a lightning storm. (As long as you aren t touching the metal frame.) is a faint glow due to a slow leakage of charge at the points of charged objects. This is also called a corona. is credited for inventing "May not the knowledge of this power of points be of use to mankind, in preserving houses, churches, ships, etc., from the stroke of lightning, by directing us to fix, on the highest parts of those edifices, upright rods of iron made sharp as a needle...would not these pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief!" Ben Franklin

2 Lightning rods are to carry the current from a lightning strike to the ground safely, but their primary purpose is to charge off the building so as to make the building less attractive to lightning. To do this they are best built with points. The first device that could acquire electric charge and it until a scientist wanted to use it for an experiment or demonstration was a Leyden jar. o These were the first. o The basic make-up is two separated by an. o These are used anywhere you need to store and quickly release electrical energy, like in the of a camera. o In the above capacitor, would flow from to if the two metal plates of the capacitor were connected by a conductor. An electric in a conductor is the rate of flow of through a cross section of the conductor. I = Q / Δt I = measure in units (A). Q = measured in units (C). Δt = measured in units (s). Current refers to the movement of not energy. o In conductors, negative move. o In nerve cells, K and Na ions flow. o Conventional current refers to charge motion so this is the direction we ll use even though we know it s really electrons moving in the wires.

3 o What direction will the (conventional) current flow in the circuit below? The are 2 types of current: (1) Current (DC) o Charges move in only one direction o Batteries provide DC. (2) Current (AC) o Charges alternate between forward and reverse direction. o Generators provide AC. o In the USA, most outlets are AC at 120 Volts and oscillate at 60 Hz meaning they switch directions 60 times per. is the difference in potential between two locations. o Variable = o Units = o If there s no voltage there s no. o Voltage is sometimes called potential difference. Potential is the amount of per charge. Batteries, generators, etc. make potential since they provide energy to the circuit. = a voltage rise Light bulbs, toasters, etc. make potential since they use the electrical energy. = a voltage drop

4 Incandescent Light Bulbs: Bulbs light up when a is set up in its filament. In order for that to happen both the side and the bottom of the bulb must be part of the circuit. Cross-section view: When a light switch is turned on, an field is set up in the entire circuit that gets the electrons moving. This field moves at the speed of (300,000,000 m/s). The electrons though, on average, move much (about m/s). This average velocity of electrons is known as velocity. o The huge difference in speeds is mainly due to between the electrons and the fixed particles of the metal. These collisions result in internal energy being produced meaning the wire in temperature ( heats up ). o This opposition to the movement of charges is called. Variable = Unit = Resistance of a wire depends on: (1) of the wire. The longer the wire, the the resistance. o This is a relationship. (2) of the wire. The thicker the wire, the the resistance. o This is an relationship. (3) of the wire. The hotter the wire, the the resistance. o This is a relationship. (4) What the wire is made of. A copper wire has resistance than a wire made of wood. At extremely low temperatures (below C), some metals have shown zero resistance. These are called.

5 What is Ohm s Law as found in our lab? Electric is the time rate of changing electrical energy. (P = E / Δt ) o For electric circuits, the equation is used. Power is times. o Variable = Unit = Schematic Symbols: Electric Shock Voltage causes, but does the most damage. The body's resistance depends on its conditions. o Soaked with salt water = 100 Ω o Very dry skin = 500,000 Ω As you in R, you the current. (V is constant) 3 rd prong on a plug o Is a. o It releases any charge that may build up on an appliance s surface. Short Circuits: Charges can be thought of as wanting to take the path of least resistance. When a load is bypassed, the drops significantly, increasing. (ΔV =IR) Fuses and breakers are used, in series, to break the circuit when a certain I is reached.

6 Magnetism Magnetism results from forces between electric charges in. o Magnetite (Fe2O3, natural magnets) (= lodestones) o Found more than 2000 years ago in Magnesia Greece. Subject to Earth s magnetic field for very long time periods. Atoms are grouped in microscopic regions called, which are oriented in all possible directions. Each domain is billions of aligned atoms. o Nonmagnetic substances = domains cancel out o Magnetic substances = domains line up, or can randomize the domains, making a magnet nonmagnetic again. Poles: o North and South poles (arbitrary like + & -) o Like poles and unlike poles. Magnetic Field Lines: o to outside the magnet o to inside the magnet

7 Terrestrial Magnetism o Earth can be considered to have one giant bar magnet running N to S. o Earth's geographic South pole is close to the magnetic North pole and its geographic North pole is close to the magnetic South pole. (think compass needles) Hans Christian Oersted, 1820 o Observed that a needle moves when placed near a carrying wire. o Concluded that a exists in the region near a carrying conductor.

8 Andre Ampere o Found that a field encircles an electric charge in motion. o A is a coil of current carrying wires. If wrapped around a ferromagnetic substance = an 2 ways to strengthen an electromagnet: (1) More of wire = stronger the magnetic field (2) Higher = stronger magnetic field Michael Faraday, 1831 o Found that moving a in or out of a wire coil could produce a in that wire. This provides an alternative to using batteries as sources of current. Generators: o Convert energy into energy

9 Motors o Convert energy into energy o Reverse of generator (current is supplied) Transformers o Utilizes EM induction to change the of electricity o Allows electricity to be transmitted over long distances at high voltages, but safely used at a lower voltage o windings yield lower voltages and vice versa