Chapter 20: Electric Current, Resistance & Ohm s Law

Transcription

1 Chapter 0: Electric Current, Resistance & Brent Royuk Phys-11 Concordia University The Minds of Our Own Challenge Light a bulb with a battery and a wire. Could you do it? Introduction 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 Electric Current Current Flow Consider a simple circuit diagram What direction does the current flow? Electron flow vs. conventional current Curse you Ben Franklin! 3 4 Electric Current Definition: I = ΔQ Δt Unit: The ampere (A) amps 1 A = 6.5 x electrons/s The Water Pump Analogy 5 6 1

2 Drift Velocity Even without a potential, electrons are in constant motion Electric field --> force --> drift velocity How many conduction electrons are in a wire? So drift velocities are often very slow, like walking speeds. So why don t we have to wait for the light when we hit the switch? What moves fast? Marbles in a tube analogy 7 Two laws for resistive circuits: I α ΔV I α 1/R Put them together and you get V = IR 8 Definition of resistance: R = V/I Resistance Unit: The ohm (Ω) doesn t apply to all materials E.g. semi-conductors, lightbulb filaments (Known as Ohmic & Non-Ohmic materials) Resistivity Resistivity is a measure of how well a material conducts electricity. Resistance also depends on the material dimensions of the conductor. R = ρl A Further define conductivity as 9 σ = 1 ρ 11 Table of Resistivities Substance ρ (Ω m) Substance ρ (Ω m) Quartz 7.5 x Iron 9.71 x 10-8 Rubber 1 to 100 x Tungsten 5.6 x 10-8 Glass 1 to 1000 x Aluminum.65 x 10-8 Silicon 0.10 to 60 Gold.0 x 10-8 Germanium to 0.5 Copper 1.68 x 10-8 Temperature Dependence For most materials, resistivity increases with temperature. Approximate relationship: ρ = ρ o (1 + αδt) Semiconductors actually lower their resistivity as they heat up. Thermistors Lead x 10-8 Silver 1.59 x 10-8 How about a real copper wire? 1 13

3 Superconductors Type 1 Electric Power Pb, Hg, Sn, Cr, Al For Pb, Tc = 7. K Type A charge moving through a circuit expends energy 1986: Superconducting oxocuprates Tc > 100K This is due to collisions between electrons and resistor molecules. Voltage drops across a resistor U = VQ, so divide by time. P = VI A standard flashlight bulb is rated at 5. V, 850 ma. What is its wattage? Combining with, we get P = VI = V R = I R Electric Pie Chart Calculator Joule Heating 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 e.g. energy loss in power lines P R Ploss = I R = supplied R = Psupplied V V P = I R, so should heaters have high resistance or low resistance? Energy Bills AC vs. DC What is a kilowatt-hour (kwh)? How much does it cost to light a 100-W bulb for a month? V = Vo sin πft

4 Home Electricity Three-wire system: difference between two hot is 40 V, most appliances connect between one hot and a ground wire for a difference of 10 V Appliances are wired in parallel: Why? Home Electricity Demo with hot wire, voltmeter 1 Human Shock Risk Two problems to prevent: 1. Too much current can flow in a circuit, causing Joule heating of the circuit, possibly starting fires. Three Conditions for Danger Enough Voltage to Cause Current Enough Charge/Current to Cause Damage Electrical Path Through Body Solution: Fuses & circuit breakers. Electricity can electrocute people. Solution: Three prong plugs, polarized plugs, fuses & circuit breakers 3 4 Limiting Current in Circuits Fuses and Circuit Breakers Problem: disconnected wire can make appliances hot Demo grounding plug: polarized plug vs. dedicated grounding wire. Path through body is important; hand vs. chest Why isn t a car battery dangerous? 5 adapters have grounding lugs 6 4

5 Frequency Dependance The lower the current value, the more sensitive the body is at that frequency. Effects of Electric Current on the Human Body Current (approximate) Effect 00.5 ma none 0.5 ma Threshold of feeling 10 ma Pain; muscular contractions. 100 ma Increased muscular effect, some injury; above 16 ma is the'let-go' current above which a person cannot release held objects ma Respiratory paralysis 100 ma3 A Ventricular fibrillation; fatal unless resuscitation occurs immediately. above 3 A Cardiac arrest; heart can be restarted if shock is very brief; severe burns 7 8 The Resistor Code color number multipier tolerance black 0 1 brown 1 10^1 red 10^ orange 3 10^3 yellow 4 10^4 green 5 10^5 blue 6 10^6 violet 7 10^7 gray 8 10^8 white 9 10^9 gold 10^-1 5% silver 10^- 10% no color 0% 31 The Resistor Code B B ROY of Great Britain had a Very Good Wife Bad Betty runs over your garden but Violet Gray won't Big boys race our young girls, but violet generally wins Black Beetles Running On Your Garden Bring Very Good Weather Big Bears Run Over Your Gladiola Bed Vexing Garden Worms (go see now) Beer Bottles, Reminders Of Your Guests, Become Voluminous Glass Waste Behold Brother, Rain On Your Grapes Brings Very Good Wine Bad Booze Rots Our Young Guts, But Vodka Goes Well 3 5