Presentation on theme: "Homework Test Corrections p. 526: 9, 10, 13. Quiz What is an electric circuit? What are the fundamental units of an Ampere? How do I find the voltage."— Presentation transcript:
Quiz What is an electric circuit? What are the fundamental units of an Ampere? How do I find the voltage required to push a current through a given resistance? What is the relationship? How do I find the power generated when a current is moved through a potential difference? What is a potentiometer? What is a kilowatt-hour? What is a kilowatt-hour a measure of? (_ _ _ _ _ _ )
A flow of charged particles in a closed system. (Which can be very large…) Electricity
Electron Energy Kinetic Energy Schematic Current (I): Definition Potential Difference Battery Internal Resistance Circuit kWH and Usage Ohms / Mhos V: Definition Franklin RT = R1+ R2 + … RT = 1/R1+1/R2 +1/R3 + … R = (T)L/A P = IV Parallel V = IR P = I 2 R Graphs: VI(Linear) IV(Linear) V=IR Light bulb R(t) Work Resistance (R): Definition Series Positive and Negative Current SuperConductor Ohm’s Law Tools: Voltmeter Ammeter VOM DMM Rheostat Potentiometer Battery Material Resistivity / Reference Tables R=R(T)
Voltage, Current & Circuits Voltage is the potential difference that drives the movement of charged particles. (Named after Volta, who rediscovered the battery) Current is the flow of charged particles. An electric circuit is composed of three elements: 1) conductor 2) potential difference (V), supplied by battery, solar cell, thunderstorm, … 3) closed loop
Voltage, Current & Circuits Voltage is the potential difference that drives the movement of charged particles. Voltage is analogous to height in a gravitational field: gh Multiply by m mgh – and you now have gravitational potential energy (Joules). Electrical potential energy is W = qV = Joules. The unit of V is Joules / coulomb
Current Electric current is the rate of flow of charge I = Δq/ Δt Ampere = Coulomb per second (passing by a point in a circuit) Ampere made discoveries regarding the interrelationships between electricity and magnetism in the early 1800’s
Current – which way does it go? Two types of current: Conventional current: Current flows from the positive terminal to the negative terminal Electron flow: Follow the electrons! Which are going the wrong way (by definition) Why are these not the same? –What famous American guessed wrong? When was the electron discovered?
Current – which way does it go? Two types of current: Conventional current: Current flows from the positive terminal to the negative terminal Electron flow: Follow the electrons! Which are going the wrong way (by definition) Why are these not the same? –What famous American guessed wrong? When was the electron discovered? –1898 (JJ Thompson) with his cathode ray tube. Long after current was defined.
How do I visualize what is going on in an electric circuit?
What is driving the current?What is driving the flow?
Ancient Video http://www.stmary.ws/highschool/physics/home/ notes/electricity/circuits/default.htm http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/ CurrentBasics.htm http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/ OhmsLawEquation.htm 30 seconds with Mr. G.
Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.
Linear equation with zero intercept: y = mx V = IR The slope is the resistance, R
Resistance –the ratio of V/I Units: Ohms –Its inverse is conductance, unit is mhos Georg Ohm promulgated his famous law in 1827 –He was a high school Physics teacher!
Which is the dependent and which the independent variable? The slope is the inverse of the resistance, 1/R
Ohm’s Law V = IR Why do I care? Fix wiring problems in my home* Fix wiring problems in my car Fix my kid’s toys Figure out why my circuit breaker / fuse blows Do my job at work (the one I have now, not the one I had for the last 27 years) *Disclaimer: Consult your parents or a qualified electrician before trying this yourself.
Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I. When does it not apply? Circuit elements that change temperature Examples? Circuit elements with large capacitance or strong magnetic fields (that are changing) Semiconductors: materials that are natural insulators that are made to be somewhat conductive Diode – like a one-way path
Example A 10V battery is placed across a 5 Ohm resistor. What is the current? A 10A current flows through a 10 Ohm resistor. What is the voltage? A 10V battery drives a 1A current through a resistor. What is the resistance?
Power Power is the rate of doing work (Watts) Power = Work / time Power = Volt * q / t but q/t = I Electrical Power = Voltage * Current = VI Electrical Energy = Power * Time = VIt
Resistance and Ohm’s Law Resistance is the impedance to the flow of charge Resistance is defined as the ratio of potential difference to current R = V/I Unit is the Ohm, Ω A device is said to obey Ohm’s Law of its resistance is independent of the Voltage
Find Resistance of a material Depends on four factors –R ∞ Temp –R ∞ RHO (ρ) resitivity –R ∞ Length –R ∞ A -1 (cross-sectional Area)
Kirchoff’s Junction Rule Conservation of charge
Kirchoff’s Loop Rule Conservation of Energy The sum of the changes in potential around any closed path (loop) of a circuit must be zero.
Series Circuit Current : One path for the flow of charge I t =I 1 =I 2 =I 3 Resistance must increase because length increases R t -=R 1 +R 2 +R 3 Voltage must add to Zero V t -V 1 -V 2 -V 3 =0 V t =V 1 +V 2 +V 3
VIRP Table V t = V 1 = V 2 = I t = I 1 = I 2 = R t =R 1 =R 2 = P t = P 1 = P 2 =
Practice Series Circuits /www.stmary.ws/physics/home/animations3/ electricity/ElectricPowerChallenge.html.net /ed1_files/circuits1.html http://www.stmary.ws/highschool/physics/ho me/notes/electricity/circuits/default.htm