Electromagnetism

Introduction Section 0 Lecture 1 Slide 1

Lecture 33 Slide 1

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 33

Electromagnetism

Electromagnetism

Introduction Section 0 Lecture 1 Slide 2

Lecture 33 Slide 2

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet

*Homework Handout

PHYSICS OF TECHNOLOGY - PHYS 1800 ASSIGNMENT SHEET

Spring 2009 Date Day Lecture Chapter Homework Due Feb 16 17 18 19 20

M Tu W H F*

Presidents Day Angular Momentum (Virtual Monday) Review Test 2 Static Fluids, Pressure

No Class 8 5-8 5-8 9

-

Feb 23 25 27

M W F*

Flotation Fluids in Motion Temperature and Heat

9 9 10

6

Mar 2 4 6

M W F*

First Law of Thermodynamics Heat flow and Greenhouse Effect Climate Change

10 10 -

7

Mar 9-13 M-F Spring Break No Classes Mar 16 18 20

M W F*

Heat Engines Power and Refrigeration Electric Charge

11 11 12

8

Mar 23 25 26 27

M W H F*

Electric Fields and Electric Potential Review Test 3 Electric Circuits

12 13 9-12 13

-

Mar 30 Apr 1 3

M W F

Magnetic Force Review Electromagnets Motors and Generators

14 9-12 14

9

Apr 6 8 10

M W F*

Making Waves Sound Waves E-M Waves, Light and Color

15 15 16

10

Apr 13 15 17

M W F*

Mirrors and Reflections Refraction and Lenses Telescopes and Microscopes

17 17 17

11

Apr 20 22 24

M W F

Review Seeing Atoms The really BIG & the really small

1-17 18 (not on test) 21 (not on test)

No test week 12

May 1 F Final Exam: 09:30-11:20am * = Homework Handout

Electromagnetism

Introduction Section 0 Lecture 1 Slide 3

Lecture 33 Slide 3

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 33

Electromagnetism

Magnetism and Currents

Electromagnetism

Introduction Section 0 Lecture 1 Slide 4

Lecture 33 Slide 4

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Magnetic Effects of Electric Currents

• Oersted discovered that a compass needle was deflected by a current-carrying wire.– With the wire oriented along a north-south line, the compass

needle deflects away from this line when there is current flowing in the wire.

Electromagnetism

Introduction Section 0 Lecture 1 Slide 5

Lecture 33 Slide 5

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• The magnetic field produced by the current is perpendicular to the direction of the current.

• The magnetic field lines produced by a straight, current-carrying wire form circles centered on the wire.– The right-hand rule gives the direction of the field lines: with the

thumb in the direction of the current, the fingers curl in the direction of the field lines produced by that current.

– The effect gets weaker as the compass is moved away from the wire.

Magnetic Effects of Electric Currents-Right Hand Rule

Electromagnetism

Introduction Section 0 Lecture 1 Slide 6

Lecture 33 Slide 6

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• Two parallel current-carrying wires exert an attractive force on each other when the two currents are in the same direction.– The force is proportional to the two currents (I1 and I2) and

inversely proportional to the distance r between the two wires:

– One ampere (A) is the amount of current flowing in each of two parallel wires separated by a distance of 1 meter that produces a force per unit length on each wire of 2 x 10-7 N/m.

Magnetic Effects of Electric Currents-Right Hand Rule

Electromagnetism

Introduction Section 0 Lecture 1 Slide 7

Lecture 33 Slide 7

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Two long parallel wires carry currents of 5 A and 10 A in opposite directions as shown. That is the magnitude of

the force per unit length exerted by one wire on the other?

a) 2.0 x 10-6 N/m b) 5.0 x 10-6 N/mc) 2.0 x 10-4 N/md) 50 N/me) 1000 N/m

N/m 100.2

m 05.0

A 10A 5N/A 1012

2

4

27

21

r

IIk

l

F

Magnetic Effects of Electric Currents-Right Hand Rule

Electromagnetism

Introduction Section 0 Lecture 1 Slide 8

Lecture 33 Slide 8

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Two long parallel wires carry currents of 5 A and 10 A in opposite directions as shown.

What are the directions of the forces on each wire?

a) The wires exert an attractive force on each other.

b) The wires exert a force repelling each other.

c) Each wire exerts a force on the other in the direction of the other wire’s current (the red arrows shown).

d) Each wire exerts a force on the other in the direction opposite to the other one’s current.

e) The wires exert no force on each other.

The wires repel each other.

Magnetic Effects of Electric Currents-Right Hand Rule

Electromagnetism

Introduction Section 0 Lecture 1 Slide 9

Lecture 33 Slide 9

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Two long parallel wires carry currents of 5 A and 10 A in opposite directions as shown.

What is the total force exerted on a 30-cm length of the 10-A wire?

F

l

2 k I1I2

r2.010 4 N/m

F F

l

l 2.010 4 N/m 0.30 m

610 5 N

a) 2.0 x 10-6 N b) 3.0 x 10-6 Nc) 2.0 x 10-5 N d) 6.0 x 10-5 Ne) 2.0 x 10-4 N

Magnetic Effects of Electric Currents-Right Hand Rule

Electromagnetism

Introduction Section 0 Lecture 1 Slide 10

Lecture 33 Slide 10

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 33

Electromagnetism

Magnetic Forces

Electromagnetism

Introduction Section 0 Lecture 1 Slide 11

Lecture 33 Slide 11

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• Magnetic forces are exerted by magnets on other magnets, by magnets on current-carrying wires, and by current-carrying wires on each other.– The force exerted by one wire on the other is attractive

when the currents are flowing in the same direction and repulsive when the currents are flowing in opposite directions.

– The magnetic force exerted on a moving charge of an electric current is perpendicular to both the velocity of the charges and to the magnetic field.

– This force is proportional to the quantity of the charge and the velocity of the moving charge and to the strength of the magnetic field:

Magnetic Forces

Electromagnetism

Introduction Section 0 Lecture 1 Slide 12

Lecture 33 Slide 12

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Two long parallel wires carry currents of 5 A and 10 A in opposite directions as shown.

What is the strength of the magnetic field produced by the 5-A wire at the position of the 10-A wire?

a) 2.4 x 10-6 Tb) 2.0 x 10-5 Tc) 1.2 x 10-5 Td) 1.2 x 10-4 Te) 2.4 x 10-4 T

T 102

mN/A 00002.0

m 30.0A 10

N 106

current.10A theis and

current,5A the todue is

10A wire, theoflength theis m 30.0 where

N 106

5

5

5

Il

FB

I

B

l

IlBF

Magnetic Forces

Electromagnetism

Introduction Section 0 Lecture 1 Slide 13

Lecture 33 Slide 13

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• For this relationship to be valid, the velocity must be perpendicular to the field.

• This actually defines the magnetic field as the force per unit charge and unit of velocity:

units: 1 tesla (T) = 1 N/Am

• If the index finger of the right hand points in the direction of the velocity of the charge, and the middle finger in the direction of the magnetic field, then the thumb indicates the direction of the magnetic force acting on a positive charge.

Magnetic Forces

Electromagnetism

Introduction Section 0 Lecture 1 Slide 14

Lecture 33 Slide 14

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• The force on a moving positively charged particle is perpendicular to the particle’s motion and to the magnetic field, just as the force on a current is perpendicular to the current and to the field.

– The force on a negative charge is in the opposite direction of the force on a positive charge: q -q.

• Because the force is perpendicular to the velocity of the particle, the force does no work on the particle.

– It cannot increase the particle’s kinetic energy; it only serves to change the direction of the particle’s motion.

– It provides a centripetal acceleration.– If the charge is moving perpendicular to a

uniform magnetic field, the particle will follow a circular path.

Magnetic Forces

Electromagnetism

Introduction Section 0 Lecture 1 Slide 15

Lecture 33 Slide 15

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Two long parallel wires carry currents of 5 A and 10 A in opposite directions as shown.

What is the direction of the magnetic field produced by the 5-A wire at the position of the 10-A wire?

a) Perpendicular to the plane of the page and into the page

b) Perpendicular to the plane of the page and out of the page

c) Upwardd) Downwarde) Inward toward the other wiref) Outward away from the other

wire

Perpendicular to plane of page and into page

Magnetic Forces

Electromagnetism

Introduction Section 0 Lecture 1 Slide 16

Lecture 33 Slide 16

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

A straight wire with a length of 15 cm carries a current of 4 A. The wire is oriented perpendicularly to a magnetic field of 0.5 T. What is the size of the

magnetic force exerted on the wire?

a) 0.3 Nb) 0.48 Nc) 0.6 Nd) 1.0 Ne) 2.0 N

The direction of this force will be perpendicular to both the current in the wire and to the magnetic field, as described by the right-hand rule.

Magnetic Forces

Electromagnetism

Introduction Section 0 Lecture 1 Slide 17

Lecture 33 Slide 17

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 33

Electromagnetism

Current Loops

Electromagnetism

Introduction Section 0 Lecture 1 Slide 18

Lecture 33 Slide 18

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Magnetic Effects of Current Loops

• When a current-carrying wire is bent into a circular loop, the magnetic fields produced by different segments of the wire add to produce a strong field near the center of the loop.

Electromagnetism

Introduction Section 0 Lecture 1 Slide 19

Lecture 33 Slide 19

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• The magnetic field produced by a current loop is identical to one produced by a short bar magnet (a magnetic dipole).– In fact, in an external magnetic field, a current loop will

experience a torque just as a bar magnet would.

Magnetic Effects of Current Loops

Electromagnetism

Introduction Section 0 Lecture 1 Slide 20

Lecture 33 Slide 20

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• Consider a rectangular loop:– Each segment of the

rectangular loop is a straight wire.

– The force on each segment is given by F=IlB.

– Using the right-hand rule, you can verify that the loop will tend to rotate in the direction indicated.

– The forces on the two ends of the loop produce no torque about center of the loop, because their lines of action pass through the center of the loop.

– The forces on the other two sides combine to produce a torque that tends to line up the plane of the loop perpendicular to the magnetic field.

Magnetic Effects of Current Loops

Electromagnetism

Introduction Section 0 Lecture 1 Slide 21

Lecture 33 Slide 21

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

A current-carrying rectangular loop of wire is placed in an external magnetic field as shown. In what direction will this loop tend to rotate as a result of the magnetic

torque exerted on it?

a) Clockwiseb) Counterclockwise

The loop will rotate counterclockwise. The forces on the long arms are outward and because they do not share a common line of action, impart a counterclockwise torque on the loop.

End view

Electromagnetism

Introduction Section 0 Lecture 1 Slide 22

Lecture 33 Slide 22

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• Since the magnetic forces on the loop segments are proportional to the electric current flowing around the loop, the magnitude of the torque is also proportional to the current.

Thus, the torque on a current-carrying coil can be used for measuring electric current.An electric meter consists of a coil of wire, a permanent magnet, and a restoring spring to return the needle to zero when there is no current flowing through the coil.

Electromagnetism

Introduction Section 0 Lecture 1 Slide 23

Lecture 33 Slide 23

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• This torque is also the basis of operation for electric motors.– The current must reverse directions every half turn to keep the coil

turning.– This can be achieved by using alternating current, or by using a

reversing direction of dc current with a split ring commutator.

• One design for a simple dc motor consists of a wire-wound rotor mounted on an axle between the pole faces of a permanent magnet.• The split ring causes the current to reverse directions every half turn, thus keeping the coil turning the same direction.

Electromagnetism

Introduction Section 0 Lecture 1 Slide 24

Lecture 33 Slide 24

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

• The magnetic field produced by a coil of wire will be stronger than one produced by a single loop carrying the same current.– The magnetic field produced

by each loop all add together.

– The resulting field strength is proportional to the number of turns N that are wound on the coil.

– The torque on the coil,when placed in an external magnetic field, is also proportional to both the current and the number of turns in the coil.

Electromagnetism

Introduction Section 0 Lecture 1 Slide 25

Lecture 33 Slide 25

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Can we utilize the similarities between a current-carrying coil of wire and a magnet?

•By winding a coil around a steel needle or nail, the magnetic field produced is enhanced.

•The nail then behaves like a magnet that is stronger than most natural magnets.

•This is an electromagnet.

Electromagnetism

Introduction Section 0 Lecture 1 Slide 26

Lecture 33 Slide 26

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 33

Electromagnetism

Faraday’s Law

Electromagnetism

Introduction Section 0 Lecture 1 Slide 27

Lecture 33 Slide 27

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Faraday’s Law: Electromagnetic Induction

• We have seen that an electric current produces a magnetic field.

Can magnetic fields produce electric currents?

• Faraday tried, at first unsuccessfully, to detect a current in a coil as a result of a current in a nearby coil.– The primary coil was connected to a battery to produce a current.– The secondary coil was connected to a galvanometer, a device to

detect magnitude and direction of current.

Electromagnetism

Introduction Section 0 Lecture 1 Slide 28

Lecture 33 Slide 28

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

Next Lab/Demo: Electric Circuits

Magnetism Thursday 1:30-2:45

ESLC 46 Ch 13 and 14

Next Class: Friday 10:30-11:20

BUS 318 roomRead Ch 14