W 06 D 1 Current Current Density Resistance

W 06 D 1 Current, Current Density, Resistance and Ohm’s Law, Magnetic Field, Magnetic Force Today’s Reading Assignment: Current, Current Density, and Resistance and Ohm’s Law, Magnetic Fields and Forces Course Notes: Sections 6. 1 -6. 5, 8. 1 -8. 3 1

Announcements Week 6 Problem Solving and Math Review Tuesday from 9 -11 pm in 26 -152 PS 5 due Week 6 Tuesday at 9 pm in boxes outside 32 -082 or 26 -152 W 06 D 2 Reading Assignment Course Notes: Magnetic Forces, Currents & Dipoles; Sections 8. 3, 9. 1 -9. 2 Exam 2 Thursday March 21 7: 30 - 9: 30 pm 2

Outline Current and Current Density Resistance and Ohm’s Law Magnetic Field Magnetic Forces 3

Current: Flow Of Charge Average current Iav: Charge flowing across area A in time Instantaneous current: differential limit of Iav Units of Current: Coulomb/second = Ampere 4

How Big is an Ampere? • • Household Electronics ~1 A Battery Powered ~100 m. A (1 -10 A-Hr) Household Service 100 A Lightning Bolt 10 to 100 k. A • To hurt you • To throw you • To kill you 40 (5) m. A DC(AC) 60 (15) m. A DC(AC) 0. 5 (0. 1) A DC(AC) • Fuse/Circuit Breaker 15 -30 A 5

Direction of the Current Direction of current is direction of flow of pos. charge or, opposite direction of flow of negative charge 6

Why Does A Skydiver Fall At Constant Speed (eventually)? 7

Why Does A Skydiver Fall At Constant Speed (eventually)? 8

Why Does A Skydiver Fall At Constant Speed (eventually)? When you first jump you are accelerating downward at g As you build up speed, you encounter aerodynamic drag (proportional to speed squared) which decreases your acceleration Eventually your downward speed is large enough that the aero drag exactly balances downward force of gravity, and you no longer accelerate Then you move at constant speed, “terminal velocity” 9

Why Does Current Flow? If an electric field is set up in a conductor, charge will move (making a current in direction of E) Note that when current is flowing, the conductor is not an equipotential surface (and Einside ≠ 0)! 10

Microscopic Picture Drift velocity is the average velocity forced by applied electric field in the presence of collisions. Magnitude is typically 4 x 10 -5 m/sec, or 0. 04 mm/second! To go one meter at this speed takes about 10 hours! 11

Summary Current: Charge Displacement Drift speed 12

Current Density J Let n = number of charged objects per unit volume q = charge of object = drift velocity of object The current density is current per unit area Generalization for many charged moving objects 13

Current and Current Density J Current is the flow (flux) of current density through an open surface Special case: uniform and perpendicular to surface 14

Concept Question: Current Density A current I = 200 m. A flows in the wire below. What is the magnitude of the current density J? 5 cm 10 cm 1. 2. 3. 4. 5. 6. J = 40 m. A/cm J = 20 m. A/cm J = 1 m. A/cm 2 J = 2 m. A/cm 2 J = 4 m. A/cm 2 20 cm P 18 - 15

Concept Q. Answer: Current Density Answer: 6. J = 4 m. A/cm 2 5 cm 20 cm 10 cm The area that matters is the cross-sectional area that the current is punching through – the 50 cm 2 area shaded grey. So: J = I/A = 200 m. A/50 cm 2 = 4 m. A/cm 2 P 18 - 16

Conductivity and Resistivity Ability of current to flow depends on density of charges & rate of scattering Two quantities summarize this: σc: conductivity ρ r: resistivity 17

Microscopic Ohm’s Law and depend only on the microscopic properties of the material, not on its shape 18

Demonstrations: Temperature Effects on Resistance F 4 http: //tsgphysics. mit. edu/front/? page=demo. php&letnum=F%204&show=0 Conducting Glass F 1 http: //tsgphysics. mit. edu/front/? page=demo. php&letnum=F%201&show=0 Conductivity of Ionizing Water F 5 http: //tsgphysics. mit. edu/front/? page=demo. php&letnum=F%205&show=0 19

Why Does Current Flow? Instead of thinking of Electric Field, think of potential difference across the conductor 20

Ohm’s Law What is relationship between electric potential difference and current? 21

Ohm’s Law R has units of Ohms (W) = Volts/Amp 22

How Big is an Ohm? • Short Copper Wire milliohms (m ) • Notebook paper (thru) ~1 G • Typical resistors to 100 M • You (when dry) 100 k • You (when wet) 1 k • Internally (hand to foot) 500 Stick your wet fingers in an electrical socket: You’re dead! 23

Concept Question: Resistance When a current flows in a wire of length L and cross sectional area A, the resistance of the wire is 1. Proportional to A; inversely proportional to L. 2. Proportional to both A and L. 3. Proportional to L; inversely proportional to A. 4. Inversely proportional to both L and A P 18 - 24

Concept Question Answer: Resistance 3. Proportional to L; inversely proportional to A. The longer the wire the higher the resistance. The bigger the cross-sectional area of the wire, the more ways that current can flow through it, so the lower the resistance. So, if resistivity is , then P 18 - 25

Group Problem: Calculating Resistance Consider a hollow cylinder of length L and inner radius a and outer radius b. The material has resistivity . Suppose a potential difference is applied between the ends of the cylinder and produces a current flowing parallel to the axis. What is the resistance measured? 26

Magnetic Fields 27

Magnetic Field of the Earth North magnetic pole located in southern hemisphere http: //www. youtube. com/watch? v=At. DAOxa. J 4 Ms 28

Demonstrations: Magnetic Field Lines of a Bar Magnet G 2 http: //tsgphysics. mit. edu/front/? page=demo. php&letnum=G%202&show=0 Magnetic Field Lines of a Single Wire G 12 http: //tsgphysics. mit. edu/front/? page=demo. php&letnum=G%2012&show=0 29

How Big is a Tesla? • Earth’s Field 5 x 10 -5 T = 0. 5 Gauss • Brain (at scalp) ~1 f. T • Refrigerator Magnet 1 m. T • Inside MRI 3 T • Good NMR Magnet 18 T • Biggest in Lab 150 T (pulsed) • Biggest in Pulsars 108 T 30

Moving Charges Feel Magnetic Force Magnetic force perpendicular both to: Velocity v of charge and magnetic field B 31

Concept Question: Units Magnetic Field What are the correct SI units for the magnetic field? 1. 2. 3. 4. 5. C/N-m-s/C N-s/C-m C-m/N-s P 18 - 32

Concept Q. Answer: B Field Units Since This is called 1 Tesla (T) 33

Cross Product: Magnitude Computing magnitude of cross product A x B: area of parallelogram 34

Cross Product: Direction Right Hand Rule #1: 1) Curl fingers of right hand so that you are moving A into B through the smallest angle 2) Thumb will point in direction of C 35

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Cross Product: Direction Right Hand Rule #1: 1) Curl fingers of right hand so that you are moving A into B through the smallest angle 2) Thumb will point in direction of C 37

Cross Product: Signs Cross Product is Cyclic (left column) Anti-commutative: Switching vectors changes sign (right column) 38

Concept Question: Cross Product An electron is traveling to the right with speed v in a magnetic field that points up. What is the direction of the force on the electron 1. 2. 3. 4. 5. 6. up down left right into page out of page P 18 - 39

Concept Question Answer: Cross Product and Magnetic Force Answer: 5. points out of the page but the charge of the electron is negative so points into the page. P 18 - 40

Demonstration: Magnetic Deflection of TV Image G 6 http: //tsgphysics. mit. edu/front/? page=demo. php&letnum=G%206&show=0 41

Concept Question: Force Direction Is this picture (deflection direction) correct? 1. Yes 2. No 3. I don’t know 42

Concept Q. Answer: Force Direction Answer: 1. Yes Field from N to S, beam velocity right to left, cross product is up. But charges are negative so force is down, as pictured. 43

Putting it Together: Lorentz Force on charged particles in electric and magnetic fields Electric Force Magnetic Force 44

Velocity Selector Particle moves in a straight line when 45

What Kind of Motion in Uniform B Field? 46

Group Problem: Cyclotron Motion A charged particle with charge q is moving with speed v in a uniform magnetic field B as shown in the figure. (1) Draw a sketch of the orbit Find (2) R : radius of the circle (3) T : period of the motion (4) : cyclotron angular frequency 47