11/1/19

1

Unit 7: ElectricityHewitt Chapters 22-23

Brent RoyukPhys-109

Concordia University

1

Electric Charge• Electrostatics vs. Electricity• Historical Development

– Elektrum and Magnesia

2

Electric Charge Basics• Likes repel, opposites attract• Charge can move• Two kinds of charge

– How do you know it’s only two?• Two materials: insulators and conductors• Neutral means balanced charge

– So a water stream is always attracted: polarization

3

11/1/19

2

Electric Charge Basics• Charge by contact or induction

– Charging by induction: which charge does it get?– What is an electrical ground?

4

Electric Charge• Franklin’s single fluid model, 1747

– surplus (glass) or deficiency (rubber/amber)– Unfortunate coin toss

• Actually, we know it’s electrons that move: surplus = –, deficiency = +. – And electron is negative, proton is positive. Bummer.

• Metric unit of charge: the Coulomb (a lot of charge)– 1 Coulomb = 6.25 x 1018 electrons– Usually more useful: charge of electron– e = 1.6 x 10-19 C

• Notice inverse relationship• Classical electromagnetism does not require the use of

electrons• Charge symbol, q, and q = ne (charge is quantized)

5

Coulomb’s Law• The inverse square relationship

– Just like gravity• The Law:

– k = 9.0 x 109 Nm2/C2

• So two coulombs of charge one meter apart would experience a force of…?

• What makes the force happen?

7

11/1/19

3

Electric Field Lines• Lines can be drawn to show the

direction of the electric field in regions of space.– Field lines never cross.– More lines/area means stronger field.

8

Voltage• Jacob’s Ladder• Interesting terminology: Voltage = Electric Potential

– Electric potential ≠ Electric potential energy • Units

– In MKS, energy/charge = Joule/Coulomb = 1 volt (V) • Remember: potential is energy per charge.• Voltage makes electrical currents flow.• What if current flows through your body?

9

Electric Safety• Three Conditions for Danger

1. Enough Voltage to Cause Current2. Enough Charge/Current to Cause Damage3. Electrical Path Through Body

10

11/1/19

4

Charge Application: Lightning• Formation is somewhat mysterious• Bottom of clouds generally become negative

– There are several current hypotheses for the charge separation mechanism.– Perhaps the charges separate due to freezing of water droplets: outer parts of

droplets become positive, shattering makes small positive fragments that are carried upward in the cloud.

• Most lightning is within clouds: not visible• Can see cloud to cloud and cloud to ground discharges

11

Charge Application: Lightning• Beginning of discharge: stepped leader. Met by streamers.

Then 3 or 4 return strokes (the visible part)• Lightning rods

– Intercepts the stepped leader (path to ground), protecting the structure

12

Charge Application: Lightning• Lightning bolt throws photographer in the air

The amazing image of the thunderbolt caught on cameraWhen the sky darkened and lightning began to flash, Kane Quinnell grabbed his new digital camera, hoping to snap some pictures of the approaching storm.Not having a tripod, he balanced the camera on his car, parked under the carport of his then Old Toongabbie home, and aimed the lens southward.It never occurred to Mr Quinnell that his new hobby - photographing storms - could be dangerous."In the north you could see a few stars and it wasn't raining," he recalled."The storm looked like it was five to 10 kilometres to the south. I thought it was perfectly safe to be outdoors, taking photos."After setting the camera for a four-second exposure he began shooting pictures, suspecting there was little chance of lightning flashing while the shutter was open."I hit the button ? and there was nothing. I hit the button again ? and nothing. On about the fourth attempt I hit the button again and I saw this lightning and heard the thunder."It was like a crack. The next thing I was about two metres in the air - it scared the hell out of me."Mr Quinnell estimated the lightning struck about 20 metres away. "I think it hit the house behind me."Unhurt, but buzzing with adrenaline, he rushed inside to check the photo on his computer."I was amazed. It was the first storm picture I had really taken.”http://www.dailymail.co.uk/news/article-407650/Lightning-bolt-throws-photographer-air.html

13

11/1/19

5

Charge Application: Lightning

14

Franklin’s KiteBEN FRANKLIN'S KITE WAS STRUCK BY LIGHTNING? Never happened!

Many people believe that Ben Franklin's kite was hit by a lightning bolt, and this was how he proved that lightning was electrical. A number of books and even some encyclopedias say the same thing. They are wrong. When lightning strikes a kite, the spreading electric currents in the ground can kill anyone standing nearby, to say nothing of the person holding the string! So what did Franklin actually do? He showed that a kite would collect a tiny bit of electric charge out of the sky during a thunderstorm. Electric leakage through the air caused his kite and string to become electrified and so the hairs on the twine stood outwards. Twine is slightly conductive, so the imbalanced charge spread to all parts of the kite string. Franklin used the twine to electrify a metal key, and tiny sparks could then be drawn from the key. (He used a metal object because sparks cannot be directly drawn from the twine, it's not conductive enough.) This suggested that some stormclouds carry strong electrical net-charge. It IMPLIED that lightning was just a large electric spark.

The common belief that Franklin easily survived a lightning strike is not just wrong, it is dangerous: it may convince kids that it's OK to duplicate the kite experiment as long as they "protect" themselves by holding a silk ribbon. Make no mistake, Franklin's experiment was extremely dangerous, and if lightning had actually hit his kite, he certainly would have been killed.

Taken from "ELECTRICITY" MISCONCEPTIONS IN TEXTBOOKS - - - William J. Beatyat http://www.eskimo.com/~billb/miscon/elect.html16

Charge Application: Laser Copiers• Selenium is a good insulator, unless light hits it. So charge

up a Se plate, expose it with light in the shape of letters, let toner be attracted to the Se, transfer onto paper and bake it on.

18

11/1/19

6

Batteries• Batteries supply charge to produce a current

– How? Electrochemistry stuff: oxy/redux• cathode and anode• dry cell vs. battery

– Electric current = moving charges• dc vs. ac

• How does this relate to electrostatics?– Electroscope and D-cell?– Voltage of charge strips

• So when is high voltage dangerous?– Energy = VQ

• Are car batteries dangerous?– They hold a lethal amount of energy, but the voltage is too low to produce a

dangerous current.

20

The “Minds of Our Own” Challenge• Light a bulb with a battery and a wire.

Could you do it?

21

Electrical Circuits• Electricity

– Electricity is moving charge (almost always electrons)

– More voltage creates more current.– Voltage is the “pump” of a circuit

22

11/1/19

7

Electrical Circuits• Current Flow

– Consider a simple circuit diagram• What direction does the current flow?

– Electron flow vs. conventional current• Curse you Ben Franklin!

• Charge Speed– Charges actually move rather slowly in your average

wire.– Walking/Crawling speeds

• So why don’t we have to wait for the light when we hit the switch?– What moves fast?– “Marbles in a pipe” analogy

24

Electric Current• Unit: The ampere (A)

– “amps”

• 1 A = 6.25 x 1018 electrons/s• What about AC current?

25

Ohm’s Law• Two laws for resistive circuits:

– I α V– I α 1/R

• Put them together and you get V = IR– Ohm’s Law

• Definition of resistance: R = V/I– Resistance Unit: The ohm (Ω)

• Ohm’s Law doesn’t apply to all materials– E.g. semi-conductors, lightbulb filaments– Ohmic vs. non-ohmic materials

26

11/1/19

8

Resistivity• Resistivity (ρ) is a measure of how

well a material conducts electricity.– Resistance also depends on the physical

dimensions of the conductor.

27

Table of ResistivitiesSubstance ρ (Ω m) Substance ρ (Ω m)

Quartz 7.5 x 1017 Iron 9.71 x 10-8

Rubber 1 to 100 x 1013 Tungsten 5.6 x 10-8

Glass 1 to 1000 x 1011 Aluminum 2.65 x 10-8

Silicon 0.10 to 60 Gold 2.20 x 10-8

Germanium 0.001 to 0.5 Copper 1.68 x 10-8

Lead 22 x 10-8 Silver 1.59 x 10-8

28

Series vs. Parallel

29

11/1/19

9

Electric Power• A charge moving through a circuit expends energy

– This is due to collisions between electrons and resistor molecules.– Voltage drops across a resistor

• P = VI– A standard flashlight bulb is rated at 5.2 V, 850 mA. What is its wattage?

• Energy that heats a resistor is sometimes called the Joule heat– That’s how electric heaters, hair dryers, etc. work– Sometimes this is bad, as in energy loss in power line

30

Application: Light Bulbs• Hot filament glows. Must be very hot to be white.

– Filament must not be surrounded by oxygen so it doesn’t burn.– Inert gas prevents evaporation.

• Edison’s search for a filament material: Tried over 6000 materials. – Early filaments: Carbon fiber– Modern filaments: Tungsten alloy

31

Application: Fluorescent Bulbs

32

11/1/19

10

Transformers• Transformers only work with AC Current.• They change the voltage from one AC circuit to another. • Step-Up vs. Step-Down

33

The Power Grid• Edison vs. Westinghouse

34

The Power Grid

35

11/1/19

11

Energy Bills• What is a kilowatt-hour (kWh)?• How much does it cost to light a 100-W bulb for a month?

36

Home Electricity• Three-wire system: difference between two

hot is 240 V, most appliances connect between one hot and a ground wire for a difference of 120 V– Demo with hot wire, voltmeter

• Appliances are wired in parallel: Why?

37

Home Electricity

38

11/1/19

12

Electric SafetyTwo problems to prevent:1. Too much current can flow in a circuit, causing

Joule heating of the circuit, possibly starting fires.• Solution: Fuses & circuit breakers

2. Electricity can electrocute people.• Solution: Three prong plugs, polarized plugs, fuses &

circuit breakers

39

Limiting Current in Circuits• Fuses and Circuit Breakers

40

Electric Safety Redux• Three Conditions for Danger

1. Enough Voltage to Cause Current2. Enough Charge/Current to Cause Damage3. Electrical Path Through Body

41

11/1/19

13

Electric Shock Safety• Problem: disconnected wire can make appliances “hot”• Demo grounding plug: polarized plug vs. dedicated grounding wire.

– adapters have grounding lugs• Path through body is important; hand vs. chest • So if you’re “grounded” are you safe?

42

Electric Safety• Effects of Electric Current on the Human Body

• Current (approximate) Effect

• 0–0.5 mA none• 0.5–2 mA Threshold of feeling• 2–10 mA Pain; muscular contractions.• 10–20 mA Increased muscular effect, some

injury; above 16 mA is the'let-go' current above which a person cannot release held objects.

• 20–100 mA Respiratory paralysis• 100 mA–3 A Ventricular fibrillation; fatal unless

resuscitation occurs immediately.• above 3 A Cardiac arrest; heart can be

restarted if shock is very brief; severe burns

44

Electric Safety•Electrical resistance of a dry body: ~100,000 Ω•Electrical resistance of a wet body: ~1500 Ω

An electrical model of the human body isa leather bag filled with salt water. The resistanceof the skin (leather) can be high when dry, but dropsby orders of magnitude when it get wet. Ions in theskin go into solution. Below the skin the resistanceis very low (blood, organs etc). If the skin isbreached, a voltage well below 100 volts can producelethal currents.

Some people survive brushes with very high voltages (like 100kV) because they draw an arc inches before they actually touch theconductor. With luck the muscular contractions pushthe victim away from the danger rather than into it.

45