Physics 1402: Lecture 8 Today’s Agenda Announcements: –Lectures posted on: www.phys.uconn.edu/~rcote/ www.phys.uconn.edu/~rcote/ –HW assignments, solutions etc. Homework #3:Homework #3: –On Masterphysics today: due next Friday –Go to masteringphysics.com
Today’s Topic : Review Chapter 24: –Definition of Capacitance – Example Calculations (1) Parallel Plate Capacitor (2) Cylindrical Capacitor (3) Isolated Sphere –Energy stored in capacitors –Dielectrics –Capacitors in Circuits
Definitions & Examples d A - - - - - + + a b L a b ab
Capacitors in Parallel V a b Q2Q2 Q1Q1 V a b Q C = C 1 + C 2 Capacitors in Series ab +Q-Q ab +Q-Q
Energy of a Capacitor The total work W to charge a capacitor to Q is : In terms of the voltage V: The energy density u in the field is : Units: J/m 3 The energy is stored in the Electric field itself. Dielectrics INCREASE the capacitance of a capacitor: dielectric constant of the material
Overview Charges in motion –mechanical motion –electric current How charges move in a conductor Definition of electric current Text Reference: Chapter 25
Charges in Motion Up to now we have considered –fixed charges on isolated bodies –motion under simple forces (e.g. a single charge moving in a constant electric field) We have also considered conductors –charges are free to move –we also said that E=0 inside a conductor If E=0 and there is any friction (resistance) present no charge will move!
Charges in motion We know from experience that charges do move inside conductors - this is the definition of a conductor Is there a contradiction? no Up to now we have considered isolated conductors in equilibrium. –Charge has nowhere to go except shift around on the body. –Charges shift until they cancel the E field, then come to rest. Now we consider circuits in which charges can circulate if driven by a force such as a battery.
Current Definition l Consider charges moving down a conductor in which there is an electric field. If I take a cross section of the wire, over some amount of time t I will count a certain number of charges (or total amount of charge) Q moving by. l We define current as the ratio of these quantities, I avg = Q / t or I = Q/ t l Units for I, Coulombs/Second (C/s) or Amperes (A) E + + + + + + Note: This definition assumes the current in the direction of the positive particles, NOT in the direction of the electrons!
How charges move in a conducting material l Electric force causes gradual drift of bouncing electrons down the wire in the direction of -E. l Drift speed of the electrons is VERY slow compared to the speed of their bouncing motion, roughly 1 m / h ! (see example later) Good conductors are those with LOTS of mobile electrons. E
How charges move in a conducting material Q is the number of carriers in some volume times the charge on each carrier (q). l Let n be the carrier density, n = # carriers / volume. The relevant volume is A * (v d t). Why ??? So, Q = n A v d t q And I avg = Q/ t = n A v d q l More on this later … E
Drift speed in a copper wire Because each copper atom contributes one free electron to the current, we have (n = #carriers/volume) Volume of copper (1 mol): The copper wire in a typical residential building has a cross-section area of 3.31e-6 m 2. If it carries a current of 10.0 A, what is the drift speed of the electrons? (Assume that each copper atom contributes one free electron to the current.) The density of copper is 8.95 g/cm 3, its molar mass 63.5 g/mol.
Drift speed in a copper wire, ctd. We find that the drift speed is with charge / electron q Thus
Resistance Resistance is defined to be the ratio of the applied voltage to the current passing through. Is this a good definition? i.e. does the resistance belong only to the resistor? Recall the case of capacitance: (C=Q/V) depended on the geometry, not on Q or V individually Does R depend on V or I ? V I I R UNIT: OHM =
Ohm's Law Vary applied voltage V. Measure current I Does ratio ( V / I ) remain constant?? V I slope = R = constant V I I R
Resistivity L A E j Property of bulk matter related to resistance of a sample is the resistivity defined as: where E = electric field and j = current density in conductor = I/A. For uniform case: n 0 : carrier density (carriers/volume) material constant q : charge per carrier v : carrier speed : viscosity material constant
Resistivity L A E j eg, for a copper wire, ~ 10 -8 -m, 1mm radius, 1 m long, then R .01 So, in fact, we can compute the resistance if we know a bit about the device, and YES, the property belongs only to the device !
Make sense? L A E j Increase the Length, flow of electrons impeded Increase the cross sectional Area, flow facilitated The structure of this relation is identical to heat flow through materials … think of a window for an intuitive example How thick? How big? What’s it made of? or
Alternative Version of Ohm’s Law L A E j A related empirical observation is that: This is an alternative version of Ohm’s Law. It can also be written as, with We can show this is also Ohm’s Law using the relations, and
Lecture 8, ACT 1 Two cylindrical resistors, R 1 and R 2, are made of identical material. R 2 has twice the length of R 1 but half the radius of R 1. –These resistors are then connected to a battery V as shown: V I1I1 I2I2 –What is the relation between I 1, the current flowing in R 1, and I 2, the current flowing in R 2 ? (a) I 1 < I 2 (b) I 1 = I 2 (c) I 1 > I 2
Current Idea l Current is the flow of charged particles through a path, at circuit. l Along a simple path current is conserved, cannot create or destroy the charged particles l Closely analogous to fluid flow through a pipe. Charged particles = particles of fluid Circuit = pipes Resistance = friction of fluid against pipe walls, with itself. E
Lecture 8, ACT 2 R I 1 2 3 4 + - x 1 234 + - 1 234 + - 1 234 + - Consider a circuit consisting of a single loop containing a battery and a resistor. Which of the graphs represents the current I around the loop?
A more detailed model I avg = Q/ t = n A v d q l Difficult to know v d directly. l Can calculate it. E
A more detailed model I avg = Q/ t = n A v d q l The force on a charged particle is, l If we start from v=0 (on average) after a collision then we reach a speed, or l Substituting gives, (note j = I/A) : average collision-free time E
A more detailed model l This formula is still true for most materials even for the most detailed quantum mechanical treatment. l In quantum mechanics the electron can be described as a wave. Because of this the electron will not scatter off of atoms that are perfectly in place in a crystal. l Electrons will scatter off of 1. Vibrating atoms (proportional to temperature) 2. Other electrons (proportional to temperature squared) 3. Defects in the crystal (independent of temperature) E
Lecture 8, ACT 3 I am operating a circuit with a power supply and a resistor. I crank up the power supply to increase the current. Which of the following properties increases, A) nB) qC) ED) E
Conductivity versus Temperature In lab you measure the resistance of a light bulb filament versus temperature. You find R T. This is generally (but not always) true for metals around room temperature. For insulators R 1/T. At very low temperatures atom vibrations stop. Then what does R vs T look like?? This was a major area of research 100 years ago – and still is today.