matter has a property we call electric charge (symbol q)
Electric Charges and Forces
Experiment shows there are two types of charge: we call them positive (+) and negative (-)
ATTRACT REPEL REPEL
Like charges repel, opposite cha
Motion of a charged particle in a constant B field
B into paper
FB qv B
FB is always perpendicular to v, so the particle goes in a circle 2 = =
= Because FB is always perpendicular to the motion, FB does no work on the particle i.e. it c
Chapter 29 Magnetic Fields due to currents
A compass needle always points in the direction of the local magnetic field. If we put a compass near a current-carrying wire, it will
A. B. C. D. E. Not be affected Point parallel to the wire Point perpendicular
Ch 29 (contd.) Magnetic Fields due to currents
Magnetic field around a long, straight currentcarrying wire
Interaction between two current carrying wires
i i The two wires may exert forces on each other through Magnetic Interactions. 1. Think of the Red W
Faradays law of Induction
Wireless transmission of power
This is a coil of wire with a hunk of iron in it A. If a steady current flows in the wire, the iron becomes a magnet B. If the iron is a magnet, a steady current is
Magnetic Induction (contd.)
Magnetic Induction can be used to power a Bike light
Lenzs Law: Opposition to motion
I must always do positive work to move the magnet. It takes energy to make a current
Inductors and Inductance
Inductor = (coil of wire)
Magnetic field of a straight wire with current
Coiling the wire: magnetic field lines go through the coil
Changing the current in a coil can induce an emf in a second coil. A related effect happens with a
Alternating Current (AC)
Please see updated course calendar
Generator: rotate a coil in a B field
Coil rotates with: frequency f (cycles per sec Hertz)
angular frequency = 2f (radians per sec) angle between the coil area and the B field changes with time:
Chapter 31: Simple AC circuits
Notation: Use lowercase for instantaneous values, Uppercase for peak values vR = = VR sin t iR = vR /R = IR sin t In an AC circuit, resistor voltage and current oscillate in phase.
VR I R R
Let there be light!
The relationships between electric and magnetic fields and their sources
What have we seen so far
Qenc E dA
Gauss Law for Electric Fields
What it says: How charges produce Electric field: Elect
Magnetism in Matter
Moving charges (currents) create B-fields.
Where are the moving charges?
Magnetism in matter arises from atomic current loops associated with orbiting and spinning electrons.
Diamagnetic materials atoms are not permanent magnetic dip
What produces a magnetic field?
Magnetic charges? B-fields are created by moving charges (currents).
B-fields exert a force on moving charges.
Where are the moving charges?
Atoms have moving charges. I
What is the current CT 29.18b through R3? flowing
A: 1.5 A B: 2 A C: 3 A D: 4 A E: 12 A
University of Colorado, Boulder (2008)
A Light bulb is a resistor
We buy a light bulb rated 120V 100W. What is its resistance? V2 (120)(120) 100
Chapter 21 (continued) Work on Worksheet 21a for the first 5 mins of class
Work with a partner Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now! Come to a consensus answ
Which are the best Conductors?
David's Whizzy Periodic Table Periodic Table of the Elements Electrical Conductivity
How does q2 know that q1 is present?
Charge q1 creates electric field E
E exerts a force on q2
Q = "source charge"
E-field at P due to point charge Q:
distance = r
E-field at point P
kQ E 2 r r
Work with a partner Come to a consensus
1 4 0
Work on Worksheet 22a (A thru F)
Electric Field Lines
Field lines start on + charges and
Chapter 23 Gauss Law
The first of Maxwells four equations
A new technique for calculating Efields
Whats in this box?
Whats in this box?
From the field lines in the picture, which of the following statements is most likely to be true ab
Making sense of Gauss Law
E field lines start on + charge, end on charge Think about a box with field lines going in and out of the surface
# of lines out # of lines in Q inside the box flux out flux in Q inside Net flux Q inside net flux = Q inside/ 0 (
Electrostatic Potential Energy and Electric Potential V
Electrostatic Potential Energy
R A 2 positive charges repel I must do positive work to bring q2 to point A a distance R away from q1 The amount of work I do is called the electrostatic potentia
The relationship between Electric Field [E] and Electric Potential [V] (Voltage)
More about conductors
Recap: Electric Potential [V] at a point in space is the work I have to do to bring unit + charge from to that point
Chapter 25: Capacitors
Capacitor: A pair of conductors, insulated from each other, used to store energy
For a charged capacitor, one conductor is + the other is (net charge is zero) The work used in separating charge is stored as electrostatic potential
Capacitors (contd.) Discussion of Exam 1
Capacitors in parallel
Have the same potential difference V across their plates
Net charge q = q1 + q2 + q3
(Work out on board)
Think about them like one giant capacitor with the area of 1 2
Current and Resistance
Wire loop in equilibrium conductor all points at same potential
Insert a battery no more equilibrium. Battery supplies energy to maintain a fixed potential difference between points A and B. Surface charge exists on the wires. Insid
Chapter 27: DC Circuits
Symbols for Electric Circuit Components.
Capacitor stores charge and potential energy, measured in Farads (F) Battery generates a constant electrical potential difference (DV) across it. Measured in Volts (V).
Resistor resists flow
Experiment 24: The Potentiometer
(1) To study the principle of the potentiometer.
(2) To measure EMFs of batteries.
(a) a slide wire, a DC power supply; an ammeter, a voltmeter
(b) a standard cell, a holder for batteries an