Physics 7C SS1, Lecture 9: Field Model & EM Waves

Physics 7C SS1, Lecture 9:Field Model & EM Waves Magnetic Field Electromagnetic Waves Polarization

Final Exam Review Sessions • Definitely some on Monday. Which other day would you like most? • Thursday • Friday • Saturday • Sunday

Magnetic Force F out of the screen v RHR2 (for positive charge): your thumb points in the direction of the moving charge, B is along your index finger, and F is the middle finger. Very Bad Finger B v B q F F = qvBsinq, where q is the angle between B and v

Magnetic Force • Suppose a large magneticfield points downward at every point in the room. What direction is the force on a positive particle traveling along the chalkboards, to your left? Into the board Out of the board Left (along particle path) Right (opposite path) Down Up No Force

Magnetic Force • Suppose a large magneticfield points downward at every point in the room. Which direction should the particle be traveling in to experience a force to the left? Into the board Out of the board Left Right Down Up No Force

Magnetic Force • Suppose a large magneticfield points downward at every point in the room. What direction is the force on a positive particle traveling upwards, toward the ceiling? Into the board Out of the board Left (along particle path) Right (opposite path) Down Up No Force

v B q F Field Model of Magnetism • A source moving charge creates a magnetic fields in a direction given by RHR1. • Another moving charge, placed in a magnetic field, experiences a magnetic force • Magnitude given by F=qvBsin • Direction of force given by RHR2 • Reverse direction for negative test charge

Inducing current • Imagine a region with a magnetic field away from you in some regions (into the screen) and zero in other regions, as shown below. Right wire is blue wire. Left wire is red wire. At t=0, loop is outside the field. Our goal: What happens as the loop enters the magnetic field? What happens while the loops moves within B. What happens as the loop exits the magnetic field? Connecting to chaning fields.

Applying RHR2:t=0, before entering the field Describe the force at the instant shown on positive charges in the blue wire: • Left • Right • Up • Down • Into Screen • Out of screen • Zero • Other Why?

Applying RHR2:As entering the field Describe the force at the instant shown on positive charges in the blue wire: • Left • Right • Up • Down • Into Screen • Out of screen • Zero • Other Why?

Applying RHR2:As entering the field Repeat for charges in the top, bottom, and red wire: • Left • Right • Up • Down • Into Screen • Out of screen • Zero • Other Why?

Applying RHR2:As entering the field Draw the current that results from the forces we just describes as loop enters field. Draw the magnetic field from the induced current. (focus on the inside the loop) Would this analysis change if I had asked for the forces on the electrons?

Applying RHR2:Within the field Describe the force at the instant shown on positive charges in the blue wire: • Left • Right • Up • Down • Into Screen • Out of screen • Zero • Other Why?

Applying RHR2:Leaving the field Describe the force at the instant shown on positive charges in the blue wire: • Left • Right • Up • Down • Into Screen • Out of screen • Zero • Other Why?

Applying RHR2:As leaving the field Draw the current that results from the forces we just describes as loop leaves field. Draw the magnetic field from the induced current. (focus on the inside the loop) Would this analysis change if I had asked for the forces on the electrons?

A new way to analyze situations with changing B • Magnetic Flux: the “amount of B-field through an area • Depends on… • B: strength of B-field • A: area bound by conductor • : orientation of loop with respect to B-field.  How should B-field be oriented for maximum magnetic field to pass through the loop? B

Applying Magnetic Flux • In which of the previous times was the amount of field passing through the loop changing? • Before entering field • While entering field • Within field • Leaving field When was there an induced current?

A changing B-field induces a current that creates another B-field • Induced current makes a field opposite to the change in amount of field through loop: 1) Entering field 3) Leaving field ti tf ti tf Iind Iind No field Ext field into page Ext field into page No field Induced field out of page Induced field out of page

Consequences of changing magnetic fields • Anything that changes the flux through a conductor causes a current to flow in the conductor. • Before: cause of current flow is a voltage difference (like from a battery). • New model: changing magnetic flux induces voltage differences (which cause induced currents and induced magnetic fields)

Switching Gears: Rethinking Light • What “waves” in light? • What propagates?

Image from http://www.monos.leidenuniv.nl/smo/index.html?basics/light.htm

A vertical wave traveling through a vertical fence passes unimpeded. The second fence also lets the wave pass. If we place the second fence with horizontal slats, the vertical vibrations cannot pass through the fence. Image from http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/light/u12l1e.html

Image from http://www.lbl.gov/MicroWorlds/teachers/polarization.pdf

What happens in each of the following cases? (use RHR2) (a) Wire carrying current is placed in a B-field N X S (b) A wire (without current) is moved up and down as shown N S

What happens in each of the following cases? (use RHR2) (c) A loop of wire (no current) is turned within a magnetic field N S (b) A loop of wire carrying current is placed in a B-field N S I

What happens in each of the following cases? (use B) (c) A loop of wire (no current) is turned within a magnetic field N S

Physics 7C SS1, Lecture 9: Field Model & EM Waves