Electricity and Magnetism

1. Electricity and Magnetism Ch. 11 and 12

2. http://www.ecoprints.net/blog/environment/earth_night.jpg

3. Usage • Energy Usage • “Did you know that the typical U.S. family spends about $1,900 a year on home utility bills? • large portion of that energy is wasted • each year, electricity generated by fossil fuels for a single home puts more carbon dioxide into the air than two average cars” (US Dept. of Energy)

4. History of Electricity 1752: Benjamin Franklin’s kite experiment 1879: Thomas Edison and the light bulb Late 1880’s: small stations in cities 1930: larger towns and cities had, but only 10% rural farms 1935: Rural Electric Administration-President Franklin Roosevelt’s plan to bring electricity to all areas, especially rural farmers

5. Electricity • Presence of electric charges and their flow • Opposite charges attract and like repel • Types • Static Electricity • AC and DC

6. Static Electricity • No flow of electricity • Negative (electrons) or positive (ions) charges accumulate on surface of object • If charges of two objects are opposite they will attract each other; like repel

7. Coulomb’s Law • The electrical force that exists between 2 charged objects depends on their charges and the distance between them • F = k ((q1*q2) / d2) • F=force • k=electric constant • q=objects’ electrical charge • d=distance between objects

8. Electron Flow Electricity flows through wire when there is a voltage difference A battery or a source of electricity is needed to complete the circuit so the current is able to flow through it

9. Electrical Resistance • The amount material resists flow of electricity (charged particles) through it • Measured in ohm’s • V = I * R • V = voltage (volts) • I = current (amps) • R = resistance (ohms) • Toaster: 15-20 ohms resistance • Human: dry-500,000; soaked in salt water-100 • examples

10. Practice Problems What is the current of flow through the lamp with a resistance of 60 ohm and a voltage of 12 V? What is the resistance of an electric frying pan that draws a current of 12 A when connected to a 120 V circuit?

11. Electric Shock • Need voltage difference to create • Small amounts are used for muscle therapy • Large amounts can result in death • Electric shock video

12. Direct Current (DC) • Electricity flows in ONE direction • Electrical circuit: source of DC electricity with a wire connecting it to an electrical device(s) and then back to the source • Typical source is a battery • Example: flashlight • Runs smaller devices

13. Alternating Current (AC) • Electricity alternates direction back and forth (because the + and – charge alternates) • Typical source is a generator • Wire in homes is set up for AC • Voltage can be changed (high voltage of power lines can be reduced for use in the home) • Example: runs lights, computers, appliances…

14. Circuits Series: everything is connected in a line Parallel: devices are set up parallel to each other Current is charged NOT the wire

15. Series Circuit • Use: fuses in homes • If you increase devices connected the total resistance increases • Current is lowered in each device • Total energy or voltage is divided equally among the devices (so energy decreases for ea.)

16. Parallel • Uses: car fans and the radio…; lights • If you increase devices connected the total resistance decreases • Therefore current increases b/c voltage in each device is the same as what the source is producing

17. Calculating Power “Appliance-cost formula” Power (P-watts) = Current (I-amps) * Voltage (V-volts) W = ?

18. Calculating Power W = ? What is the current traveling through a 60 W light bulb at 120 V?

19. Overloading Circuits • Most outlets are 120 V • In parallel circuits: • As more devices are plugged in the current through each device increases • Too much of an increase can lead to overloading and cause a fire • General circuits are connected to a fuse box • If an overload occurs  fuse blows and circuit breaks • Voltage in power lines is HIGH so the corresponding current is LOW and the line does not OVERHEAT

20. Magnetism vs. Electricity Has north and south poles North and south poles never exist on own Cause objects to attract and repel Same strength Opp. Attract; like repel Has positive and negative charges Positives and negatives (electrons and protons) can exist on own

21. Magnetism • Magnetic fields: space around magnet • Closer the lines  stronger the field • Direction of field is from North  South • Field: produced by motion of ELECTRIC CHARGE • Magnetic field is produced when electrons (electrical charge) move and ALSO spin • Moving electrons have both electric & magnetic fields

22. Magnetic Fields Magnetic North Pole is in the South and the South Pole is in the North Compass: North attracts to Earth’s South mag. Pole and vice versa

23. Magnets Break a bar magnet into two pieces  each piece is magnetically ½ as strong

24. Electromagnetic Induction • Pass electric current through magnetic field • Results in a stronger magnet • Can produce large voltages (used in generators) • How a junkyard magnet works

25. Electromagnetic Induction • 1831: Henry and Faraday • Discovered electric current can be produced by moving a magnet in or out of a wire • As increase number of loops of wire  voltage increases • Faraday’s Law • Voltage = # of coil loops * rate mag. field changes • Increase # of loops  increase voltage

26. Electromagnetic Induction • Traffic Lights • Car drives over buried wires, activates light • Security System at Airport • Change in voltage by a metal object sounds an alarm • Credit Cards and Hotel room key card • Magnetic strip  induced voltage pulses identify the card

27. Unit 3 Test • Be able to: • Calculate using ohm’s law (v=I*R) • Calculate using the appliance cost/power equation (P=I*V) • Understanding what Coulomb and Faraday’s laws mean • Know the basics of electricity and magnetism (how they work-charges) • Basics about AC and DC • Basics of parallel and series circuits • **Study your notes if you take any—if any questions ask or look in ch. 11 or 12 in book