E-Mag
description
Transcript of E-Mag
E-Mag
It’s Electrifying!
ElectrostaticsStored
electricity, or electricity at rest
Static ElectricityA 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 GuidelinesDO NOT get back into your car when
refuelingDO NOT top off your tankIf a flash fire occurs, back away,
contact attendant, and most important:
LEAVE NOZZLE IN VEHICLE!
Electrical Forces
unlike charges attract
like charges repel
A: more than before rubbing?B: the same as before?C: less than before?
Rub a balloon on your hair, the balloon attracts you hair. Is the amount of charge in the balloon and your hair
Conductorsallow charges to flow through them easily electrons are not tightly anchored to the nucleus
Electrical Conductor
Not this kind of conductor!
Allow charges to easily flow through them
Insulators
Charges cannot easily flow through them.
Ex: plastic, styrofoam
Metal, wire
Plastic or rubber
Charging by FrictionTransfer 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 InductionTransfer of
electrons when objects are brought near each other, but do NOT touch.
How does Lightning start and end?
Induction: Balloon on wallWall 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 DifferenceElectric 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 LineSAFE: 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.
Potential Difference and the flow of current:
Voltage SourcesGenerators 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 energyex: 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 ()
Electrical Resistance
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: batteryelectrons 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.
AC/DC
Electrical Relationships
Water in a Hose
DC in a Wire
Electrical Units
Pressure Voltage (V) Volts (V)
Rate of Flow
Current (I) Amperes, Amps (A)
Friction Resistance (R)
Ohms (Ω)
Electric Power
Rate at which electric energy is converted into another form such as mechanical, heat or light energy.
Electric power (PE) is equal to current (I) times voltage (V) measured in watts (W)
PE = I V
Calculating Power
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 body0.001 A – barely felt0.005 A – painful0.010 A – muscle spasm0.015 A – loss of muscle control0.1 A (100 mA) can be fatal if current
goes through the heart
Electric CircuitA 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 CircuitsSeries: current has only one
path to take
Parallel: current can follow different paths throughout the circuit.
Series Circuits
Parallel 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 CIRCUITI is constantV adds upR 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:
TransformersUsed 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 TransmissionPower 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
Power to Your Home
110 V vs. 220 V plug
Electric MeterEach month the number of
kilowatt hours (kWh) is read and you are billed according to your usage
Magnetic PolesProduce magnetic
forces.
Every magnet has a North and South pole.
Opposite poles attract each other. Like poles repel.
Magnetic Fieldsspace around a
magnet where a magnetic force is exerted acts like an electric
field
Magnetic Declination
Magnetic DomainsThe domain is
determined by the alignment of the atoms in a magnet
A strong magnet has perfectly aligned atoms
The Earth’s Magnetic FieldA compass points
to the earth’s magnetic field, however the geographic and magnetic fields are not perfectly aligned