Electricity. Chapter 20

Transcription

1 Electricity Chapter 20

2 Types of electric charge Protons + charge Electrons - charge SI unit of electric charge is the coulomb (C)

3 Interactions between charges Like charges repel Opposite charges attract

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5 Static Charge Latin word Stasis which means Stays Objects are typically Neutral w/ the same # of protons and electrons They can become charged by gaining or losing electrons NOT PROTONS! They stay in the nucleus! The buildup of these charges is Static Electricity In Static Electricity the charges build up and STAY; they don t flow as they do in electric currents

6 Transferring Static Charge Friction transferred from rubbing i.e. get shocked after walking on the carpet Conduction transferred by direct contact w/ another object hair standing on end w/ Van de Graff machine Induction the force field of a highly negatively charged object pushes the electrons away from nearby objects causing them to become + charged, they then are attracted to each other. i.e. statically charged balloon attracts small pieces of torn up paper

7 Static Discharge Objects don t hold a static charge forever objects tend toward equilibrium they want to be neutral When electrons move toward this equilibrium static discharge occurs Humidity water vapor in the air pulls electrons off negatively charged objects, preventing static charges to build up (remember that water is a polar molecule) Sparks & Lightning - objects reaching static equilibrium

8 Wimshurst Generator

9 Lightning Rods

10 Electrical Current Water flowing thru a pipe depends on more than the angle of the pipe. It also depends on the length of the pipe, diameter of the pipe and if the pipe is clogged or open. Electrical Current is how much electric charge flows! Electrical Current is measured in Amperes Represented by an I (for intensity) Amount of Electrical Current (amps) depends on more than just Voltage, it depends on the Resistance found in the circuit.

11 Conductors Allow the easy flow of electricity loosely bound electrons that are free to move from atom to atom metals like aluminum, gold, copper and silver

12 Insulators Insulators resist the flow of electrons hold more tightly to their valence electrons: plastic, rubber, glass

13 Electroscope Experiment

14 Electrical Resistance the opposition to the flow of electricity measured in Ohms symbol is the Greek letter Omega - Electricity will take the path of least resistance Remember the water pipe analogy? a. Longer wires have greater resistance than short wires b. Thin wires have more resistance than thick wire c. High conductors have less resistance than insulators The greater the resistance, the less current there is for a given voltage.

15 Ohm s Law V = IR Let s look at worksheet 20.2 Electric Current and Ohm s Law

16 Voltage = Power! Voltage causes current to flow through an electrical circuit Volt unit of measure to measure this electrical potential energy A Voltage Source (battery or generator) is required to maintain the electrical potential in a circuit.

17 Power P=IV Power = Current x Voltage Power is measured in watts

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19 Please list which letters and SI units represent the following: Electric Charges C Coulomb Electric Current I amps Resistance R ohms Voltage V volts

20 Circuit Circuit- a complete path through which charge can flow There are 2 types: Series Parallel

21 Circuit (Schematic) Diagrams All circuits need at least the following Power supply, wire, resistors, other items include switches, connectors, meters, etc. There is a set of standard symbols used to represent these items in a diagram of the circuit

22 Series Circuits Series Circuits provides only one path for the electrons to follow 1. A break in the circuit stops the flow of electricity to all other parts of the circuit 2. With multiple light bulbs (more resistance) the current reduces & the dimmer the lights become

23 Parallel Circuits Parallel circuits the different parts of the circuit are on separate branches A break (burned out light bulb) in the circuit doesn t stop the flow to the remaining devices Multiple light bulbs will remain the same brightness since the resistance is not decreasing as it does in a series circuit. Each pathway can be separately switched off w/out affecting the others Household circuits Wired in parallel, with a standard of 120 volts Voltmeters are wired in parallel

24 Parallel vs Series Circuits Demo

25 Parallel Circuits The more paths the LESS the resistance Water example again: added pipes coming from a large tank will allow more water to flow out than a single pipe. Therefore as resistance decreases, current increases; they are inversely proportional

26 Electric Energy E=Pt Energy = Power x time Energy is measured in kilowatt hours 1kwh= 3,600,000j

27 Electric safety In an overloaded circuit, too much current can cause circuits to explode and catch fire Fuse: blows when current gets too strong Circuit breaker: switch that opens when current gets too strong Ground: current goes through the ground instead of you during a short circuit

28 Have you ever thought about who changes the light bulbs in radio towers? 1. [OH-] =7.5 x M gda

29 1. Series or Parallel Circuits? Hello

30 Amps/Volts can kill If your body resistance is 100,000 ohms, then the current which would flow would be the 2 nd of each Voltage listed, but if you have just played a couple of sets of tennis, are sweaty and barefoot, then your resistance to ground would be lower, therefore the 1 st voltage of each example would be used:.001 A (1-100 V) tingle.005 A (5-500 V) small shock (maximum harmless current).030 A ( V) painful, lose muscle control.100 A (100-10,000 V) extreme pain, stop breathing 1.00 A ( ,000 V) nerve damage, heart stops, death possible 10.0 A severe burns, death is certain

31 If you need more help on Circuits, please see this!! Series and Parallel Circuits

32 Circuits! Electricity Lab

33 Chapter 21 MAGNETISM & ELECTROMAGNETISM

34 Magnetic Poles Magnetic Poles the ends of the magnet, area where the magnetic effect is the strongest. If a bar magnet is suspended by a thread or string, it will align itself so that one strong end points north and the other points south, hence the names for the North and South poles of the magnet. Like poles of separate magnets repel push away from each other Unlike poles attract each other magnet video

35 Magnets If you snap a magnet in half, the inside pieces become the opposite poles:

36 Magnetic Fields that region around a magnet that is affected by the magnet. Strongest at the poles, the Force forms lines that go out of the North Pole and wrap back around to enter in at the South Pole.

37 Attract & Repel Magnets attract because force comes out of North Pole and goes into the South Pole Attraction Repulsion Magnets repel because the forces are pushing away from each other

38 Making Magnets Since Magnetism and electricity are so closely related, it is relatively easy to make magnets Temporary magnets materials that become magnetized while in contact w/ strong magnets ie a paperclip is able to pick up more paper clips when stuck to a strong magnet Permanent magnets materials that maintain their magnetism when the magnet is removed from it.

39 Earth s core is Iron Earth is a giant magnet Earth s magnetic north pole is not the same as Earth s axis north pole. It is about 1250 km (776 miles) away from the true north pole Magnetic Earth

40 Electric Current & Magnetic Fields An electric current produces a magnetic field An electric current through a coil of wire around a nail produces a magnet Solenoid- coil of current-carrying wire (more coils = more magnetic force) (less coils = less magnetic force) Electromagnet- placing a ferromagnetic material in the coil of a solenoid

41 How does a stereo speaker work?

42 Generating Electric Current Induction of electric current - making a current flow in a wire 1. Moving a coil of wire up and down in a magnetic field or 2. Moving a magnetic field up and down through a coil of wire

43 Alternating Current The flow of an induced current may be constant or may change direction Alternating current AC as a coil is moved up & down on a magnet repeatedly the current would reverse direction each time A current that changes direction The electricity in our homes is AC AC generators- simply a backwards motor a. requires a mechanical source to spin the axle b. which in turn spins the loop/armature which will induce a current. c. Attached to each end of the coil loop are Slip Rings which spin & d. transfers the electricity to the brushes & the rest of the circuit

44 Direct Current Direct current DC the current resulting in electrons flowing from high potential to lower potential a. Therefore it moves in one direction only b. The electricity stored in batteries is DC DC Generator Similar to an AC generator but has a single Commutator instead of two slip rings

45 Batteries -Electrochemical cells 1. Converts chemical energy into electrical energy 2. Consists of two different metals the electrodes 3. Electrodes immersed in a chemical bath that conducts electricity called the electrolyte 4. The part of the electrodes above the electrolyte is the terminal and used to connect the battery to the circuit. There is a chemical reaction between the electrodes and the electrolyte resulting in a buildup of electrons on one of the terminals (it becomes the - terminal) The other terminal gives up its electrons and becomes the + terminal. This difference sets up the electrical potential of the system = Volts When cells are connected in series the voltages of the cells are added together

46 Transformers 1. Remember resistance occurs anytime current is sent thru a wire. 2. Power companies have found that very high voltages can travel more efficiently thru the wires 3. Once electricity is generated, it is transformed (in a step up transformer) to a very high voltage ( up to 750,000 volts) then sent along the transmission lines 4. Voltage is then reduced at a substation at a step-down transformer to a lower voltage ( between 2,000 & 5,000 volts) 5. Electricity is then sent throughout the neighborhood and as it comes into the home it is step-downed one more time to the 110 volts required for our household appliances and tools

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48 Changing Voltage with a Transformer 1. Transformer piece of iron with two wires coiled around it. The coils do not come into contact w/ each other 2. Transformers work only with AC currents, not DC currents and is accomplished by Induction 3. The loops are labeled primary coil/winding, electricity coming into the transformer; the secondary coil is the loop/winding coming out.

49 Changing Voltage with a Transformer Step-up or step-down voltage is directly proportional to the number of coils present

50 Electricity and Transformers #coils secondary/ #coils primary = v out/v in Ex. A transformer has 50 coils in its primary coil and 200 in its secondary coil. The input voltage is 25 volts, what is the output voltage? What type of transformer was this?

51 Using Electric Power Electric power Remember that Power is the rate at which work is done and the unit of power is the Watt. Formula is: Power = Voltage x Current Formula is: Watts = Volts x Amps Or Amps = Watts / Volts Or Volts = Watts / Amps P = Volts x Amps P = 6 x.5 P = 3 Watts P = Volts x Amps 500 = 120 x A 500/120 = A 4.17 Amps

52 Calculating Electrical Energy Cost Paying for energy we are charged by the electric company for the power we use. It is calculated and billed to us by the kilowatt hour. 1. The formula used is Energy = Power x Time = (VoltsxCurrent)xTime 2. The formula used is Kilowatt hours = Kilowatts x Hours