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Slides Physics of Everyday • These Are the Slides That I Intended to Life Show in Class on Tue PHY205H1S Note on Posted Slides Physics of Everyday • These are the slides that I intended to Life show in class on Tue. Mar. 18, 2014. Class 17: Electric • They contain important ideas and Current questions from your reading. • Due to time constraints, I was probably not • Flow of Charge • Speed and Source of Electrons in a able to show all the slides during class. • Electric Current Circuit • They are all posted here for completeness. • Voltage Sources • Electrical • Electric Power Resistance • Light bulbs • Ohm’s Law • Electric Circuits • DC and AC [image from http://torontoist.com/2013/02/toronto-invents-the-incandescent-light-bulb/ ] Flow of Charge Electricity/Water Analogy: • When the ends of an electrical conductor are at • Water flows from different electric potentials—when there is a • Electric charge flows higher pressure to potential difference—charge flows from one end from higher potential to lower pressure, if it to the other. lower potential, if it can. Electrical Conductor can. Motion of charge Higher Lower High Electric Low Electric Potential Potential Potential Potential © 2010 Pearson Education, Inc. © 2010 Pearson Education, Inc. Continuing the Analogy… Electric Current • To maintain a continuing flow of charge in a • When charged particles move, transferring electric conductor, a battery must be provided to maintain charge from one place to another, this is called electric the potential difference (=voltage). current • To maintain a continuing flow of water in a pipe, a • A “conductor” is a material having free charged pump must be provided to maintain the pressure particles that easily flow through it when an electric force acts on them difference • In a conducting solid metal, negatively charged electrons are free to move around while the positive nuclei remain fixed in the atomic lattice • In certain types of solid semiconductors, positively charged electron holes are free to move around • In ionic liquids, positive or negative ions can carry electric charge from one place to another Battery © 2010 Pearson Education, Inc. 1 Electric Current CHECK YOUR NEIGHBOR Electric Current is a Number A capacitor begins with two neutral metal plates. A battery is attached and charges up the capacitor, so that 6 • Electric current is the rate that charge moves seconds later there is +3 Coulombs of charge on the positive from one point to another plate, and -3 Coulombs of charge on the negative plate. What was the average current running • The common symbol for current is I through the battery as the capacitor was Δ푞 charging? 퐼 = Δ푡 A. Zero • The unit of current is Coulombs per second (C/s) B. 0.33 Amps • 1 C/s = 1 Ampere = 1 Amp = 1 A C. 0.5 Amps D. 1 Amp E. 2 Amps [image from http://people.oregonstate.edu/~joneski/electric.htm ] Voltage is a Number How Does a Battery Work? • Electric potential difference is called “voltage” • Voltage between two points causes electric • Energy is stored in a battery in chemical bonds current in a conductor flow from higher potential • When current flows through a battery, work is to lower potential. done by chemical disintegration of zinc or lead in • No current flows when the voltage is zero acid • The unit of voltage is Volts • This energy is carried • 1 Volt = 1 V = 1 Joule per Coulomb by the electric charges • We say that when voltage is set up across a as they travel through conducting object, it can cause a current to flow the circuit, and can be through the conducting object. used to power a light- bulb, for example [image from http://people.oregonstate.edu/~joneski/electric.htm ] Electric Current CHECK YOUR NEIGHBOR Electric Resistance What does a good DC battery supply you Current in a circuit depends on: with, such as this brand-new Duracell? • voltage. • electrical resistance. A. A steady supply of current to put into a Resistors are circuit elements that use up circuit. energy; usually they create heat B. A steady rate of energy flow into a circuit. In a circuit diagram, the symbol for a C. A constant voltage to apply across a resistor is: circuit. 2 Electric Current CHECK YOUR NEIGHBOR Resistance is a Number A pump is pushing water through a narrow wiggly pipe, • When you apply a voltage across an object, the called a “line”. amount of current depends on the object’s If you increase the length of the line, without changing the resistance. pressure of the pump, how will this affect the rate that • Resistance of an object is measured in Ohms water flows through it? • 1 Ohm = 1 Ω = 1 Volt per Amp A. The flow rate will decrease • The higher the resistance of an object, the less B. The flow rate will increase current will flow through it for a given voltage. C. The flow rate will stay the same [image from http://people.oregonstate.edu/~joneski/electric.htm ] Electric Current CHECK YOUR NEIGHBOR Electric Resistance A pump is pushing water through a narrow wiggly pipe, • Factors affecting electrical resistance of an called a “line”. object: If you increase the diameter of the line, without changing the pressure of the pump, how will this affect the rate that 1. Material water flows through it? – Rubber is much more resistive than copper 2. Cross-sectional area A. The flow rate will decrease – Thin wires have more resistance than thick B. The flow rate will increase wires C. The flow rate will stay the same 3. Length – An object twice as long has twice as much resistance. Electrical Resistance Ohm’s Law • For some materials, the resistance can • The current in a circuit varies in direct proportion increase dramatically with temperature to the potential difference, or voltage, and inversely with the resistance • For example, the Current voltage filament in a light bulb resistance increases its 푉 resistance after it has 퐼 = been on for a short 푅 time, as it warms up. Examples: • For a constant resistance, current will be twice as much for • The current is high at twice the voltage. first, then decreases. • For twice the resistance and twice the voltage, current will be unchanged. 3 Electric Current Continuing the Analogy… CHECK YOUR NEIGHBOR • If you increase the pressure on the pump, water will A battery is set up to push current through a resistor. flow faster through the line (analogous to increasing The battery has a voltage of 100 V. The resistor has a voltage) resistance of 500 Ω. • If you keep the pressure the same, but make the line When you close the switch, what is the current flowing longer or narrower, water will flow slower through the around the circuit? line (analogous to increasing resistance) A. 0.2 A B. 5 A C. 100 A D. 500 A E. 50,000 A You are a resistor!! Direct and Alternating Current • The human body has a resistance, which depends on the • Direct current (dc) exact path through your body, and whether your hands are – Flows in one direction only. wet or dry. – Current always flows from • If your hands are moist, the the positive terminal toward resistance between them can be the negative terminal. as low as 500 Ω • Alternating Current (ac) • So if there is a voltage of 100 V – Current in the circuit flows between your hands, this will set first in one direction and then up a current of 0.2 A, straight in the opposite direction, through your heart! alternating to and fro many • Electrical current traveling through times per second. your body can damage tissue, and – This is accomplished by interfere with the beating of your alternating the polarity of heart (which is electrical) voltage at the generator or other voltage source. [image from http://www.retronaut.com/2012/10/30-ways-to-die-by-electrocution ] © 2010 Pearson Education, Inc. Direct and Alternating Current Direct and Alternating Current • Commercial electricity in North America is • Converting from ac to dc alternating current (ac): – Household current is ac, but current in laptop or – 60 cycles per second cell phone is dc. – Voltage is 120 V – The AC to DC converter uses a diode, a tiny • Power transmission is more efficient at electronic device that acts as a one-way valve to allow electron flow in one direction only. higher voltages. – Europe and China adopted 220 V as their standard. AC in – Canada and the U.S. continued with 120 V because so much equipment was already installed. © 2010 Pearson Education, Inc. © 2010 Pearson Education, Inc. DC out 4 Electric Current Speed and Source of Electrons in a Metal CHECK YOUR NEIGHBOR Wire When we flip the light switch on a wall and the circuit, an electric field is established inside the conductor. When you turn on a light switch, • The electrons continue their random motions while simultaneously being A. An electric field is set up in along the wire, which nudged by the electric field. immediately begins pushing electrons in the light. • Current is established through the B. Electric current immediately begins flowing in the wires at nearly the speed of light. switch, which pushes electrons who bump into • It is not the electrons that move at neighbouring electrons, all the way up to the light, like a this speed. sequence of dominoes. • It is the electric field that can travel C. The switch sends a stream of electrons toward the light, through a circuit at nearly the speed which, when it reaches the light, makes the light go on. of light. © 2010 Pearson Education, Inc. Speed and Source of Electrons in a Circuit Speed and Source of Electrons in a Circuit • If the voltage source is dc, like the battery, electric field Misconceptions about electric current: lines are maintained in one direction in the conductor.
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