Physics 30S. A changing magnetic field creates an electric field A changing electric field creates...

Electromagnetic Induction

Physics 30S

Electromagnetism

What is Electromagnetism?

A changing magnetic field creates an electric field

A changing electric field creates a magnetic field

Basis for electricity generation, transmission, most uses and applications

Maxwell’s Equations

Complicated, but in essence:

Electric fields and magnetic fields are one phenomena: an electromagnetic field

United concepts of electricity and magnetism into 4 equations

James Clerk MaxwellJune 13 ,1831 – November 5, 1879

Michael Faraday

Faraday was a great researcher

Intuitive knowledge about electricity and magnetism

Performed many experiments which paved the way for an understanding of electromagnetism Primitive motor

September 22, 1791 – August 25, 1867

Hans Christian Oersted

Oersted made one of the first electricity/magnetism insights all because of a messy desk

Current carrying wire caused compass needles to divert

Oersted’s basic principle of electromagnetism: moving electric charges produce a magnetic field

August 14,1777 – March 9, 1851

Field around a Current Carrying Wire

Current through a wire creates a circular magnetic field; weakens with distance

Right Hand Rule 1: Point thumb in the direction of the

current Make a fist Fingers show direction of magnetic field

Magnitude of field:B is the magnetic field (T)I is the current (A)r is the distance from the wire (m)

Right Hand Rule 1

Point your thumb in the direction of current flow

Fingers wrap to show direction of the magnetic field

Visualizing the Magnetic Field around a Wire

Point your thumb with the current

Curl your fingers to determine the direction of the field

What is ?

Permeability of free space4π x 10-7 Tm/APhysical constantPermeability relates the ability of the

medium to induce a magnetic field Inductance

Notation

Current Coming out of the screen

Going into the screen

Meant to visualize an arrow

Example1 Right Hand Rule 1Add arrows to show the direction of

the magnetic field

Solution

Example 1: Calculating Field

a) What is the strength of the magnetic field 15 cm away from a wire carrying 75 A?B = 1.0 x 10-4 T

b) At what distance is the strength of the magnetic field 1.5 x 10-5 T?r = 1.0 m

Example 2: Calculating Field

a) How much current is a wire carrying if the magnetic field is measured to be 3.03 x 10-3 T at 0.500 meters away?I = 7580 A

b) What is the strength of the magnetic field 1.50 m from the wire?B = 1.01 x10-3 T

Homework

3. Magnetic Fields Homework Handout

Force on a Current Carrying Wire Current carrying wires experience a force

in an external magnetic field Right Hand Rule 2:

Make an L shape with your hand Thumb points in the direction of the current Fingers point in the direction of the magnetic

field Palm shows the direction of the force

Magnitude of the Force:B is the magnetic field (T)I is the current (A)l is the length of the wire (m)Θ is the angle between the magnetic field and the wire (not in the text)

Example 1 Right Hand Rule 2

What direction is the force on the wire?

Solution: Into the screen

Example 1: Calculating Force

A 25cm wire in a motor carrying 15 A of current is in a magnetic field of 0.2T. What will be the force on the wire, assuming that the wire and magnetic field are perpendicular?

FB = 0.08 N

Example 2: Calculating Force

What length of conductor, running at right angles to a 0.033 T magnetic field and carrying a current of 20.0 A, will experience a force of 0.10N?

I = 0.15 m

What about Charged Particles? Magnetic fields exert a force on moving

electrical charges, including charged particles What might the formula be?

Example 1: Moving Particles

An electron is shot through a cathode ray tube TV at 5.0 x 105 m/s, perpendicular to the direction of the field. If the force acting on the particle is 2.0 x 10-15 N and the length of the tube is 10 cm, what is the strength of the magnetic field?

B = 0.025 T

Example 2: Moving Particles

An alpha particle (He2+) is shot through a magnetic field at 3.33 x 106 m/s, perpendicular to the direction of the field. If the magnetic field strength is 1.5 x 10-4T, what is the magnitude of the force acting on the particle?

F = 1.6 x10-16 N

Homework - Textbook

Pg.569 Force on a Wire: #7-9

Pg. 574 Force on a Moving Particle: #10-11

Defining Magnetic Field

Magnetic field hasn’t been defined qualitatively

Magnetic field is a force per unit current element

Electromagnetism is needed

Solenoid Picture

A solenoid is our first electromagnet Magnet caused by electricity

Solenoids

A solenoid is a coiled wire Contains many loops

Magnetic field of each loop sums to make the magnetic field of the solenoid http://webphysics.davidson.edu/Applets/BField/sol

enoid.html Capable of producing strong magnetic fields

Right Hand Rule 3: Coil fingers with the direction of current Thumb points in the direction of magnetic north

Magnetic field created is similar to a bar magnet

Right Hand Rule 3: SolenoidsRight Hand Rule 3:

Coil fingers with the direction of current Thumb points in the direction of

magnetic north

Diagram of a Solenoid’s Field

Where are the North and South Poles?

Homework

Calculating Magnetic Forces Exercises Handout

Lab Experiment

Lab Manual 24.1

Electric and Magnetic Fields

Research an Electromagnet

Pick from one among the list.Research about how it worksSpecifically, where is

electromagnetism involved and how does it make the device function?

2 minute presentation at the end of class explaining the device to the class

Include multi-media if possible!Questions?

The List

SpeakersCathode Ray Tubes/TelevisionAlarmsElectromagnets for Lifting SteelGeneratorsElectric motorsMaglev trains

Technologies/Electromagnets Primitive motor

Make the Motor Generator

Reverse motor Electromagnet

How do you think this works? Speakers

http://electronics.howstuffworks.com/speaker5.htm

Cathode Ray Tube Alarm/bell Maglev trains

http://player.discoveryeducation.com/index.cfm?guidAssetId=6581C80B-C521-4467-9A8A-E56533E3FC83&blnFromSearch=1&productcode=US

Summary of Hand Rules

Right Hand Rule 1: Point thumb in the direction of the current Make a fist Fingers show direction of magnetic field

Right Hand Rule 2: Make an L shape with your hand Thumb points in the direction of the current Fingers point in the direction of the magnetic field Palm shows the direction of the force

Right Hand Rule 3 (Solenoids): Coil fingers with the direction of current Thumb points in the direction of the magnetic field

The Plan!

Day 1 -2: What is Electromagnetism, Maxwell, Oersted, Field around a wire, Up to slide 16

Day 2: Force around a wire, Force on moving particles Up to slide 24

Day 3: Definition of Magnetism, Solenoids, In-class work Up to slide 26 Solenoid example

Day 4: Lab Experiment Day 5: Electromagnet applications research, summary of right

hand rules (Friday) – Gr. 11 up to here Up to Slide 36

Day 6: Review Day 7 Test