It’s Electrifying! E-Mag
Electrostatics • Stored electricity, or electricity at rest
Static Electricity • A build up of electric charges in an object or in an area of an object. • Rubbing your feet on the carpet so that you can “shock” someone Phet simulation: http://phet.colorado.edu/simulations/sims.php?sim=Balloons_and_Static_Electricity 4.5 min, video: http://www.cbsnews.com/stories/2004/08/27/earlyshow/living/ConsumerWatch/main638947.shtml
How Do Refueling Fires Happen? • Static charge picked up when re-entering vehicle • Touching nozzle without discharging static electricity • A spark ignites fumes
More Safety Guidelines • DO NOT get back into your car when refueling • DO NOT top off your tank • If a flash fire occurs, back away, contact attendant, and most important: LEAVE NOZZLE IN VEHICLE!
Electrical Forces • unlike charges attract • like charges repel
Rub a balloon on your hair, the balloon attracts you hair. Is the amount of charge in the balloon and your hair • A: more than before rubbing? • B: the same as before? • C: less than before?
Conductors • allow charges to flow through them easily • electrons are not tightly anchored to the nucleus
Electrical Conductor Allow charges to easily flow through them Not this kind of conductor!
Insulators Plastic or rubber • Charges cannot easily flow through them. • Ex: plastic, styrofoam Metal, wire
Charging by Friction • Transfer of electrons by friction between two objects • rubbing a balloon so that it will “stick” to a wall Travolta simulation: http://phet.colorado.edu/simulations/sims.php?sim=John_Travoltage
Charging by Direct Contact • Conduction… transfer of electrons when one object touches another. • touching someone and “shocking” them
Charging by Induction • Transfer of electrons when objects are brought near each other, but do NOT touch.
Balloon on wall Induction: Wall is neutral and an insulator + move slightly towards balloon - move slightly away from balloon Wall is neutral but has small surface charge, sufficient to hold balloon
Van De Graaff Generator • Robert Jemison Van de Graaff invented the Van de Graaff generator in 1931. • The device has the ability to produce extremely high voltages -- as high as 20 million volts.
Electric Potential or Potential Difference • Commonly called Voltage Difference • Electric Potential is measured in volts (V)
Electric Current (I) • The flow of electric charges. • Measured in amperes (A) • There must be a potential difference in order for current to flow
Bird on a Power Line SAFE: both feet are on the same voltage line thus no potential difference! NOT SAFE: If one leg is on the ground or another wire and the other one is on the power line, then there is a potential difference between the bird’s two legs.
Voltage Sources • Generators and batteries provide sources of voltage. • Dry cells (Duracell) have a paste electrolyte. • Wet cells (car batteries) have a liquid electrolyte.
Generators • Convert mechanical energy into electrical energy • ex: household generators or car alternators
Batteries • Energy is released during a chemical reaction. • Dry cells (Duracell) have a paste electrolyte. • Wet cells (car batteries) have a liquid electrolyte.
Resistance • Opposition to the flow of electric charges. It is increased if length is increased or width of wire is decreased. • Measured in ohms ()
Relationship between voltage (V), current (I) and resistance (R) • V = IR Ohm’s Law sim: http://phet.colorado.edu/simulations/sims.php?sim=Ohms_Law
Electric Current • The flow of electrons
Fact or Fiction? • When you turn on a light switch, electrons move rapidly from the socket to the lamp to light the bulb.
Direct Current (DC) • Charges always flow in one direction. Ex: battery • electrons always flow from the neg. terminal to the pos. terminal AC/DC simulations http://phet.colorado.edu/simulations/sims.php?sim=Circuit_Construction_Kit_ACDC
Alternating Current (AC) • Current flows in one direction, and then in the other direction…back and forth. • Most US circuits operate at a frequency of 60 Hz.
Electric Power • Rate at which electric energy is converted into another form such as mechanical, heat or light energy.
Calculating Power • Electric power (PE) is equal to current (I) times voltage (V) • measured in watts (W) • PE = I V
Energy Converted • Electricity used (energy converted) is equal to power (kW) times time (hours) • Eelectric = Pt
What causes electric shock in the human body- current or voltage? • Current passing through the body depends on the voltage applied and the electrical resistance of the body. (ranges from 100 Ω if soaked in salt water to about 500,000 Ω if the skin is very dry)
Effects of Current on the body • 0.001 A – barely felt • 0.005 A – painful • 0.010 A – muscle spasm • 0.015 A – loss of muscle control • 0.1 A (100 mA) can be fatal if current goes through the heart
Electric Circuit • A closed path through which electrons can flow. • Ex: when you turn a light switch ON, you create a CLOSED circuit, when you turn it off you are opening the circuit Phet circuit http://phet.colorado.edu/simulations/sims.php?sim=circuit_construction_kit_dc_only
Types of Circuits • Series: current has only one path to take • Parallel: current can follow different paths throughout the circuit.
The basics of solving circuit problems… Parallel Circuits I adds up V is constant 1 = 1 + 1 + 1 + ... Req R1 R 2 R 3 SERIES CIRCUIT I is constant V adds up R adds up
Circuit Breakers • Guards against overheating. It has a piece of wire that bends at high temps, causing a switch to “flip” which “opens” the circuit
Transformers Not these: These:
Transformers • Used to step up (increase) or step down (decrease) voltage through electromagnetic induction
Electromagnetic Induction • The process of inducing voltage by changing the magnetic field around a conductor
Power Transmission • Power can be transmitted very long distances at high voltages and low currents, it begins at 120,000 V at power plants and is stepped down until it reaches the 120 V used in your home