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Ohm’s Law

Ohm’s Law. Physics 102 Professor Lee Carkner Lecture 14. PAL #13 Capacitors. What is the charge stored on the capacitor? Jury-rig a replacement out of metal foil and Teflon coating (k = 2.1, thickness = 0.01 mm). C = ke 0 A/d A = Cd/ ke 0 = (5X10 -6 )(0.00001)/(2.1)(8.85X10 -12 )

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Ohm’s Law

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  1. Ohm’s Law Physics 102 Professor Lee Carkner Lecture 14

  2. PAL #13 Capacitors • What is the charge stored on the capacitor? • Jury-rig a replacement out of metal foil and Teflon coating (k = 2.1, thickness = 0.01 mm). • C = ke0A/d • A = Cd/ke0 = (5X10-6)(0.00001)/(2.1)(8.85X10-12) • How can such a device be portable?

  3. Circuit Theory • There are three key variables used in circuit theory: • DV provides energy and causes charges to move • Energy can be extracted from the current due to resistance (symbol: R)

  4. Current • The current is the flow rate of charge and is defined as: • The units are amperes (amps) or coulombs per second • The most common charge carrier is the electron

  5. Inside a Wire • What goes on inside a current carrying wire? • An applied potential difference makes them want to move in a certain direction (against the field) • They undergo many collisions and move in a random walk

  6. Drift Speed • We can find the drift speed in terms of the properties of the wire: • Where I is the current, n is the electron density, q is the charge on the electron and A is the cross sectional area of the wire

  7. Electron Motion

  8. Current Conundrums • The drift speed is very small (~mm per second), yet the effect of current is felt instantaneously • Electrons move randomly, yet current flows in only one direction • The direction of the current is opposite the motion of the electrons

  9. emf • A battery maintains a potential difference across its terminals which can do work by moving charge • The amount of work done by a battery is just the amount of charge moved times the emf W = DQE

  10. Resistivity • Why? • The materials resist the flow of current • Good conductors have low resistivity, good insulators have high resitivities

  11. Resistance • The total resistance of the material also depends on its size • The resistance can be written as: R = r (L/A) • The units of resistance are ohms (volts per ampere)

  12. Ohm’s Law • How much current do you get if you put a potential difference V across a wire with resistance R? I = V/R • This relationship is called Ohm’s Law V = IR • Ohm’s law is very important, memorize it! • However, the law only holds for certain types of materials (called ohmic)

  13. Simple Circuit

  14. Using Ohm’s Law • Ohm’s law quantifies the way circuits work • Can write in different ways: • V = IR • I = V/R

  15. Today’s PAL • A 1.5 volt battery produces 167 A of current when connected to a 1 meter long, 2 mm thick wire. What is the wire made of? • Discuss the validity of the following claim: • “The relationship R = V/I tells us that the resistance of a wire is directly proportional to the potential difference applied to it.”

  16. Temperature and Resistance • Electronic devices get hot! • Temperature also affects electronic properties • This increased random motion means collisions are more frequent and it is harder for current to flow

  17. Superconductivity • If we set up a current in a wire and then take away the battery the current fades to zero • If the resistance was zero the current would keep flowing even without a battery • Such materials are called superconductors • Resistance generally decreases with decreasing T

  18. Energy in Electric Circuits • As the charges flow (as current) they convert the potential energy to kinetic energy • We should be able to relate the potential difference, current and resistance to the energy produced

  19. Energy Dispersion Rate • Each charge that passes through the battery gains energy that it will later lose as heat • Each charge then gives up its energy so the total power (energy per second) depends on the rate of charge flow or current IDV = P

  20. Power • Using Ohm’s law (DV = IR) we can write: P = I2R and P = (DV)2/R • Current and power can then be computed

  21. Lightbulbs • A common circuit element is the lightbulb • Household lightbulbs are rated in watts • In the US, most power outlets produce 120 volts of potential difference • Those that do not use a transformer

  22. Joule Heating • The conversion of electrical energy into heat is called joule heating • Joule heating is seen in the natural world: • Can produce energy in the Earth’s atmosphere

  23. Next Time • Read 21.4-21.5 • Homework Ch 21, P: 24, 26, 42 • Final: • Section 1: Tuesday, Feb 25, 9-11 am • Section 2: Thursday, Feb 27, Noon-2pm

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