Electricity and Magnetism

Slides:



Advertisements
Similar presentations
Current, resistance and electromotive force
Advertisements

CHAPTER 4 CONDUCTION IN SEMICONDUCTORS
Have you ever held a wire that has current flowing through it? If so what did you notice about it? The wire gets hot. The increase in temperature causes.
Current Density and Drift Velocity Current And Resistance Perfect conductors carry charge instantaneously from here to there Perfect insulators carry.
Chapter 27 Current And Resistance Electric Current Electric current is the rate of flow of charge through some region of space The SI unit of current.
CH 20-1.
Current and Resistance FCI.  Define the current.  Understand the microscopic description of current.  Discuss the rat at which the power.
PHY 2054: Physics II. Calculate the Electric Field at P Calculate the el. potential at P.
UNIT 9 Electrostatics and Currents 1. Thursday March 22 nd 2 Electrostatics and Currents.
Chapter 17 Current and Resistance. Electric Current Let us look at the charges flowing perpendicularly to a surface of area A The electric current is.
15/20/2015 General Physics (PHY 2140) Lecture 6  Electrostatics and electrodynamics Capacitance and capacitors capacitors with dielectrics Electric current.
Physics for Scientists and Engineers II, Summer Semester Lecture 8: June 8 th 2009 Physics for Scientists and Engineers II.
1 Chapter 27 Current and Resistance. 2 Electric Current Electric current is the rate of flow of charge through some region of space The SI unit of current.
Chapter 25 Current, Resistance, Electromotive Force
Current and Resistance (Cont.)
Current, Resistance, and Electromotive Force
Voltage, Current, and Resistance
Current and Resistance. Current In our previous discussion all of the charges that were encountered were stationary, not moving. If the charges have a.
Chapter 26 Current and Resistance
DC Circuits P10-.
Electric Current Electric current is the rate of flow of charge through some region of space The SI unit of current is the ampere (A) 1 A = 1 C / s The.
-Electric Current -Resistance -Factors that affect resistance -Microscopic View of Current AP Physics C Mrs. Coyle.
Chapter 24 Electric Current. The electric current I is the rate of flow of charge through some region of space The SI unit of current is Ampere (A): 1.
Current and Direct Current Circuits
Current and Resistance. The Starting Point: Elements, Atoms and Charge Electrons and protons have, in addition to their mass, a quantity called charge.
Chapter 27 Current Resistance And Resistor. Review The current is defined and its unit is ampere (A), a base unit in the SI system I A The.
P212c26: 1 Charge carrier motion in a conductor in two parts Constant Acceleration Randomizing Collisions (momentum, energy) =>Resulting Motion Average.
Chapter 17 Current and Resistance. Electric Current Let us look at the charges flowing perpendicularly to a surface of area A The electric current is.
Electric Current and Resistance Unit 16. Electric Current  The current is the rate at which the charge flows through a surface Look at the charges flowing.
Chapter 27 Current and Resistance. Intro Up until now, our study of electricity has been focused Electrostatics (charges at equilibrium conditions). We.
10/8/2008 “+” REMEMBER, THE ELECTRONS ARE ACTUALLY MOVING THE OTHER WAY! - -
Current � and � Resistance Electric Current Resistance and Ohm’s Law A Model for Electrical Conduction Resistance and Temperature Superconductor Electrical.
Current Electricity Parallel CircuitSeries Circuit.
Electric Current AP Physics C Montwood High School R.Casao.
ELEC 3105 Basic EM and Power Engineering Conductivity / Resistivity Current Flow Resistance Capacitance Boundary conditions.
Current and Resistance FCI.  Define the current.  Understand the microscopic description of current.  Discuss the rat at which the power.
1 TOPIC 6 Electric current and resistance. 2 Conductors have free electrons, which Are in continuous rapid motion – thermal and quantum effects Undergo.
Current & Resistance - Current and current density - Ohm’s Law - Resistivity - Resistance.
Chapter 27 Current and Resistance. Electric Current The electric current I is the rate of flow of charge through some region of space The SI unit of current.
Chapter 17 Current and Resistance. Electric Current Whenever electric charges of like signs move, an electric current is said to exist The current is.
Chapter 27 Current Resistance And Resistor. Electric Current, the definition Assume charges are moving perpendicular to a surface of area A If ΔQ is the.
Current and Resistance
Current and Resistance FCI.  Define the current.  Understand the microscopic description of current.  Discuss the rat at which the power.
A positive test charge is used by convention to identify the properties of an electric field. The vector arrow points in the direction of the force that.
Chapter 16 Capacitors Dielectrics Chapter 17 Current Resistance.
Electric Current Chapter 7-2. Electric Circuit F A closed path through which electrons can flow F Electrons flow because of a difference in potential.
Chapter 26 Lecture 21: Current: I. Types of Capacitors – Variable Variable capacitors consist of two interwoven sets of metallic plates One plate is fixed.
ELECTRON THEORY OF METALS 1.Introduction: The electron theory has been developed in three stages: Stage 1.:- The Classical Free Electron Theory : Drude.
Chapter 27: Current and Resistance Fig 27-CO, p Electric Current 27.2 Resistance and Ohm’s Law 27.4 Resistance and Temperature 27.6 Electrical.
Dr. Jie ZouPHY Chapter 27 Current and Resistance.
2/2009 EXAMINATION #2 WEDNESDAY MARCH 4, 2009.
Chapter 27 Current Resistance And Resistor. Review The current is defined and its unit is ampere (A), a base unit in the SI system I A The.
Chapter 27 Current And Resistance. Electric Current Electric current is the rate of flow of charge through some region of space The SI unit of current.
Electric Current Chapter 27 Electric Current Current Density Resistivity – Conductivity - Resistance Ohm’s Law (microscopic and macroscopic) Power Dissipated.
-Electric Current -Resistance -Factors that affect resistance -Microscopic View of Current AP Physics C Mrs. Coyle.
TOPIC 6 Electric current and resistance
ELEC 3105 Basic EM and Power Engineering
Current Electricity Parallel Circuit Series Circuit.
J Current And Resistance Current Density and Drift Velocity Perfect conductors carry charge instantaneously from here to there Perfect insulators.
Our Story So Far  .
A Little Capacitance and Current and Resistance
Chapter 24 Electric Current.
Current and Resistance
Ch 17 Electrical Energy and Current
Microscopic Model of Conduction
Effects of moving charges
Current and Resistance
Chapter 9: Current and resistance
Circuits.
Physics 1 Electric current Ing. Jaroslav Jíra, CSc.
Presentation transcript:

Electricity and Magnetism Electric Circuits

CH 25: Current and Resistance

We have been discussing electrostatics We have been discussing electrostatics. Stationary charges and the resulting forces, electric fields, electric potential and capacitance for these charged objects. We will now begin a discussion about charges that are moving. Charges can be forced to move from point A to point B through a certain region of space. The amount of charge that passes through that specified region within a certain period of time will define a flow rate for charge (similar to how we would describe the flow of a fluid). The flow rate of charge is more commonly called Current. Current can be mathematically defined as the amount of charge per unit time. DQ – amount of charge Dt – time interval IAv – Average current If we look at the instantaneous value of the current (a infinitesimally small time interval). I – instantaneous value of the current dQ – infinitesimally small quantity of charge dt – infinitesimally small time interval The current is measured in Amperes (or Amps).

We know that electrons typically have much greater freedom of movement, but when we discuss current it is important to remember that current is defined as the flow of positive charge. Moving charges will be traveling at some velocity, therefore there must be a way to relate the current to the speed of the charges. Volume charges are moving through We can examine the rate at which a certain amount of charge will move through a specified volume. The total amount of charge that is within that volume can be determined from the number density of charge carriers, the volume containing the charge carriers and the amount of charge on each charge carrier. Number density of charge carriers Amount of charge in the specified volume.

The amount of charge contained in the specified volume can be used to determine the rate at which charge flows through that region. Drift velocity – Average speed charges move through material The average current is proportional to the average speed at which the charges move. It is sometimes more convenient to discuss the current that passes through a specified area, called the Current Density. J – Current Density [A/m2] I – Current [A] A – Cross-Sectional Area [m2] n – Carrier Density [1/m3] q – Charge [C] vd – Drift Velocity [m/s] This microscopic view of current is used in certain situations, but most of the time is not necessary. Used by solid state, quantum and nuclear physicists to look at the motion of small groups of charges. Used by chemists to look at charges moving through a variety of mediums in a variety of shapes.

Charges do not move through materials in straight lines Charges do not move through materials in straight lines. They follow a random path that is defined through the interactions (e.g. collisions) of that charge with all other charges in the material, as well as any external influences. The net effect is for the charge to drift in the direction of current flow. The drift velocity for charges tends to be relatively slow as compared to the observable effects (vd for copper wire is approximately 0.2 mm/s). If you turn on a light by flipping a switch how long does it take for the light to turn on? The light turns on nearly instantaneously. This means that there must be some other effects that are responsible for the light turning on. What causes the charges to move through the wire in the first place? When the switch is flipped the charges begin moving in the wire. The charges must have undergone an acceleration. A force must be applied to the charges to cause an acceleration. Where does the force come from? The presence of an external electric field will exert a force on the charges. The external electric field travels through the wires from the positive plate to the negative plate of the power supply (battery), pushing positive charges towards the negative plate.

We should therefore be able to relate the electric field from the power supply to the current in the wires. Acceleration of a charge due to an electric field. t is the average time between collisions. s - Conductivity Conductivity is how easily charges move through a material. It is a property of the material. Ohm’s Law It is sometimes more convenient to look at the potential difference instead of the electric field. Let us look at how this expression changes.

1/s = r - Resistivity R - Resistance Resistivity is how hard it is to move charges through a material. Resistance is an impedance to the flow of charge. R – Resistance [W] r – Resistivity [Wm] l – Length of material [m] A – Cross-sectional area of material [m2] Ohm’s Law The “collisions” between charges is the primary reason that the drift speed is so slow. These collisions hinder the motion of the charges through the wire. This impedance to the flow of charge through a material is called Resistance. Resistance is essentially a decrease in the amount of current that can pass through a material.

Ohm’s law is only valid for materials where there exists a linear relationship between voltage and current. The depends primarily on the properties of the material. If you apply a voltage across an electrolyte (conducting liquid) the current is exponentially related to the voltage. Ohm’s law is not valid for this case. Example: A section of copper wire that is 2 m long with a radius of 0.5 mm has a potential difference of 12 V applied across the two ends. (rcu = 1.7x10-8 Wm) How much current is flowing through this section of wire? What is the current density contained in the wire? What is the magnitude of the electric field traveling through the wire? a) } This is an extremely large current and could not be supplied by most power supplies. 1 A is typically considered a moderately high current 10 A is considered a very high current b) A very small electric field is necessary to set up this large current within a very good conducting material. c)