electricity and magnetism€¦ · 14/03/2016 · question: a beam of electrons is directed into...

ELECTRICITY AND

MAGNETISM

ELECTROMAGNETISM

3-16-16

ELECTRICITY AND MAGNETISM ( or

ELECTROMAGNETISM

• Electromagnetism is a branch of physical

science that deals with electricity,

magnetism and the physical relations

between electricity and magnetism

• It involves the study of the electromagnetic

force between electrically charged objects

The Four Fundamental Forces

1. Force of Gravity

2. Electromagnetic Force:

Electromagnetism is the force that acts between electrically charged particles.

The electromagnetic force usually shows electromagnetic fields, such as

electric fields, magnetic fields, and light.

Includes the electrostatic force acting between charged particles at rest

Includes combined effect of electric and magnetic forces acting between

charged particles moving relative to each other.

describes a number of everyday experience such as

friction,

rainbows,

lightning,

and all human-made devices using electric current, such as television,

lasers, and computers,

properties of the chemical elements, including all chemical bonding.

3. Strong Nuclear Forces

4. Weak Nuclear Forces

ELECTRIC CHARGES AND ELECTRIC FORCES

OBJECTIVES

• Calculate the charge on an object

• Describe the defenses between

conductors and Insulators

• Explain the difference between conduction

and induction

• Explain how an electroscope works

• Discuss the attractive and repulsive force

between charged objects

The Origin of Electric ChargeBuilding Blocks of Matter is Atom

Each atom consists of a dense core

(nucleus) of positively charged protons

and uncharged (neutral) neutrons.

These electrons orbit the nucleus in

distinct energy levels

kg10673.1 27pm

kg10675.1 27nm

kg1011.9 31em

C1060.1 19ecoulombs

The fundamental unit of charge is the

Coulomb [C]

The Origin of Electricity

Most atoms are neutral -- that is, they

have an equal number of positive protons

and negative electron (charges), giving a

net charge of 0.

If an atom loses or gain one or more

electrons, they form a positively or

negatively charged ions.

By adding or removing electrons from

matter it will acquire a net electric charge

(q)

Mathematically:

Neq

Net Electric Charge (q) = (e) times the number of electrons

added or removed (N).


CHARGE Charge is a fundamental measurement (base physical quantity) in

physics, much as length, time, and mass are fundamental

measurements.

The fundamental unit of charge is the Coulomb [C], which is a very

large amount of charge.

Charge of an electron is called an elementary charge; it is equal to

1.6*10-19 coulomb

Problem

Coulomb is a very large amount of charge. How many electrons are there

in one coulomb of negative charge?


Answer 1 A Lot of Electrons

Neq

18

19-1025.6

C101.60

C 00.1

e

qN

CHARGE

Charge is a fundamental measurement in physics, much as length,

time, and mass are fundamental measurements.

The fundamental unit of charge is the Coulomb [C], which is a very

large amount of charge.

Charge of an electron is called an elementary charge; it is equal to

1.6*10-19 coulomb

Problem

Coulomb is a very large amount of charge. How many electrons are there

in one coulomb of negative charge?

Conductors and Insulators

Not only can electric charge exist on an object, but it can also move

through an object.

Electrical Conductors: Are substances that readily allow electric charges to

move freely through them.

Examples of good conductors include metals such as gold, copper,

silver, and aluminum.

Electrical Insulators: Are materials that conduct electric charge poorly

Good insulators include materials such as glass, plastic, and rubber.

Semiconductors has an electrical conductivity that increases with temperature

and is intermediate between that of a metal and an insulator. Examples are silicon

and germanium

Superconductor: are materials that conduct electricity with zero resistance below

a certain temperature.

CLASS EXPERIMENTS/ACTIVITIES

CHARGING INSULATORS BY RUBBING/CONTACT

The law of conservation of charge.

It is possible to transfer electric charge from one object to another.

Materials can be charged by contact, or conduction e.g rubbing a

pen/ebonite rod or balloon against your hair/fur lead to transfer of electron

from atoms in your hair to the pen or balloon.

The body (your hair/fur) that loses electrons has an excess of positive

charge, while the body (balloon/pen/ebonite rod) that gains electrons has

an excess of negative charge.

The total charge of the hair/balloon system remains constant (zero)

Just like momentum and energy, charge is conserved: Total Charge Initial

= Total Charge Final

THE LAW OF CONSERVATION OF ELECTRIC CHARGE state that during

any process, the net electric charge of an isolated system remains constant

(is conserved).

Charged Objects and the

Electric Force1. Like charges repel each

other

2. Unlike charges attract

each other.

3. Charged objects (+ or - )

can charge and attract

neutral objects.

NOTE: The only way to tell if an object is

charged is by repulsion. A positively charge

object can only be repelled by another positive

charge, and a negatively charged object can only be

repelled by another negative charge. Both can be

attracted to neutral and opposite charges.

Charging by Contact:

insulator and conductor

Charging by contact.

Charging by Contact or Conduction: Conductors

Question: If a conductor carrying a net charge of 8 elementary

charges is brought into contact with an identical conductor with

no net charge, what will be the charge on each conductor after

they are separated?

Question: What is the net charge (in coulombs) on each

conductor after they are separated?

Conductors can also be charged by contact. If a charged conductor is

brought into conduct with an identical neutral conductor, the net charge

will be shared across the two conductors.

Charging by Contact or Conduction: Conductors

Question: If a conductor carrying a net charge of 8 elementary

charges is brought into contact with an identical conductor with

no net charge, what will be the charge on each conductor after

they are separated?

Answer: Each conductor will have a charge of 4 elementary

charges.

Question: What is the net charge (in coulombs) on each

conductor after they are separated?

Answer:

Conductors can also be charged by contact. If a charged conductor is

brought into conduct with an identical neutral conductor, the net charge

will be shared across the two conductors.

The Electroscope A simple tool used to detect small electric charges known as an electroscope

functions on the basis of conduction.

The electroscope consists of a conducting rod attached to two thin conducting

leaves at one end, and isolated from surrounding charges by an insulating

stopper placed in a flask.

If a charged object is placed in contact with the conducting rod, part of the charge

is transferred to the rod. Because the rod and leaves form a conducting path, and

like charges repel each other, the charges are distributed equally along the entire

rod and leaf apparatus. The leaves, having like charges, repel each other, with

larger charges providing greater leaf separation!

Charging by Induction

Charging by induction involves charging an object without coming into contact

with another charged object that induces charge on it.

When an object is connected to the Earth by a conducting path, known as

grounding, the Earth acts like an infinite source for providing or accepting excess

electrons.

To charge a conductor by induction, we first bring it close to another charged

object. When the conductor is close to the charged object, any free electrons on

the conductor will move toward the charged object if the object is positively

charged (since opposite charges attract) or away from the charged object if the

object is negatively charged (since like charges repel).

Then the ground connection is removed and the net charges in the conductor

redistribute.

Charging a neutral insulator (plastic) by

Contact and by Induction

The negatively charged rod induces a slight positive surface charge

on the plastic.

Coulomb's Law

ke = 8.99×109 N m2 C−2)

q1 and q2 are the signed magnitudes of

the charges

r is the distance between the charges

F- Electrostatic Force of repulsion or

attraction between charges. (q)

C-Coulomb

Sir Augustin Coulomb's law

French physicist (1736 to 1806)

Coulomb’s Law

COULOMB’S LAW

The magnitude of the electrostatic force (F) exerted by one point charge

(q1) on another point charge (q2) is directly proportional to the magnitude

of the product of the charges (q) and inversely proportional to the square

of the distance (r) between them.

2

21

r

qqkF

229 CmN1099.841 ok

2212 mNC1085.8

Coulombs law is an inverse-

square relationship.

Notice how similar this formula is to the formula for the gravitational

force!

Both Newton's Law of Universal Gravitation and Coulomb's Law follow

the inverse-square relationship.

The further you get from the charges, the weaker the electrostatic

force.

If you were to double the distance from a charge, you would quarter

the electrostatic force on a charge and vise versa

Direction of Coulombs Force If the objects have opposite charges, they are being attracted, and

if they have like charges, they must be repelling each other.

We use common sense to figure out direction based on repulsion

or attraction.

Question: A beam of electrons is directed into the electric field

between two oppositely charged parallel plates, as shown in the

diagram below.

1.into the page

2.out of the page

3.toward the bottom of the page

4.toward the top of the page

Question: A beam of electrons is directed into the electric field

between two oppositely charged parallel plates, as shown in the

diagram below.

Answer: (4) toward the top of the page because the electron beam

is negative, and will be attracted by the positively charged upper

plate and repelled by the negatively charged lower plate.

Question: The diagram below shows a beam of electrons fired through the region between two

oppositely charged parallel plates in a cathode ray tube. After passing between the charged plates, the

electrons will most likely travel path A or B or C or D

Question/Example: Three protons are separated from a single

electron by a distance of 1*10-6 m. Find the electrostatic force between

them. Is this force attractive or repulsive?

Coulomb’s Law

Group Problem: A Model of the Hydrogen Atom

Question: In the Bohr model of the hydrogen atom, the electron is in orbit about the

nuclear proton at a radius of 5.29x10-11m.

(a) Find Fnet (Coulombs force)

(b) Determine the speed of the electron, assuming the orbit to be circular.

Hint: Use net force= centripetal force

Use constants given in the equation sheet or notes.

2

21

r

qqkF

Coulomb’s Law and Electrostatic Force

N1022.8m1029.5

C1060.1CmN1099.8 8

211

219229

2

21

r

qqkF

rmvmaF c

2

sm1018.2

kg109.11

m1029.5N1022.8 6

31-

118

mFrv


Group Question Three Charges on a Line

Determine the magnitude and direction of

(a) F12

(b) F13

(c) The Total or Net Force on q1.


N7.2m20.0

C100.4C100.3CmN1099.82

66229

2

21

12

r

qqkF

N4.8m15.0

C100.7C100.3CmN1099.82

66229

2

31

13

r

qqkF

5.7NN4.8N7.21312 FFF

STOP HERE ON 3-16-16

The Electric Field

The positive charge experiences a force which is the vector sum of the

forces exerted by the charges on the rod and the two spheres.

This test charge should have a small magnitude so it doesn’t affect

the other charge.

The Electric FieldExample A Test Charge

The positive test charge has a magnitude of

3.0x10-8C and experiences a force of 6.0x10-8N.

(a) Find the force per coulomb that the test charge

experiences.

(b) Predict the force that a charge of +12x10-8C

would experience if it replaced the test charge.

CN0.2C100.3

N100.68

8

oq

F(a)

(b) N1024C100.12CN0.2 88 F

The Electric Field

DEFINITION OF ELECRIC FIELD

The electric field that exists at a point is the electrostatic force experienced

by a small test charge placed at that point divided by the charge itself:

oq

FE

SI Units of Electric Field: newton per coulomb (N/C)

The Electric Field

It is the surrounding charges that create the electric field at a given point.

The Electric Field

Example 7 An Electric Field Leads to a Force

The charges on the two metal spheres and the ebonite rod create an electric

field at the spot indicated. The field has a magnitude of 2.0 N/C. Determine

the force on the charges in (a) and (b)

The Electric Field

N1036C100.18CN0.2 88 EqF o(a)

(b) N1048C100.24CN0.2 88 EqF o

The Electric Field

Electric fields from different sources

add as vectors.

The Electric FieldExample The Electric Field of a Point Charge

The isolated point charge of q=+15μC is

in a vacuum. The test charge is 0.20m

to the right and has a charge qo=+15μC.

Determine the electric field at point P.

oq

FE

2

21

r

qqkF

The Electric Field

N7.2m20.0

C1015C1080.0CmN1099.82

66229

2

r

qqkF

o

CN104.3C100.80

N 7.2 6

6-

oq

FE

The Electric Field

2r

qkE

The electric field does not depend on the test charge.

o

o

o qr

qqk

q

FE

12

Point charge q:

The Electric Field

Example 11 The Electric Fields from Separate Charges May Cancel

Two positive point charges, q1=+16μC and q2=+4.0μC are separated in a

vacuum by a distance of 3.0m. Find the spot on the line between the charges

where the net electric field is zero.

2r

qkE

The Electric Field

2

6

2

6

m0.3

C100.4C1016

dk

dk

21 EE 2r

qkE

22m0.30.4 dd

m 0.2d

Electric Field Lines

Electric field lines or lines of force provide a map of the electric field

in the space surrounding electric charges.

The Electric Field

THE PARALLEL PLATE CAPACITOR

Parallel plate

capacitor


Electric field lines are always directed away from positive charges and

toward negative charges.


Electric field lines always begin on a positive charge

and end on a negative charge and do not stop in

midspace.


The number of lines leaving a positive charge or entering a

negative charge is proportional to the magnitude of the charge.


Conceptual Example 13 Drawing Electric

Field Lines

There are three things wrong with part (a) of

the drawing. What are they?

The Electric Field Inside a Conductor: Shielding

At equilibrium under electrostatic conditions, any

excess charge resides on the surface of a conductor.

At equilibrium under electrostatic conditions, the

electric field is zero at any point within a conducting

material.

The conductor shields any charge within it from

electric fields created outside the conductor.


The electric field just outside the surface of a conductor is perpendicular to

the surface at equilibrium under electrostatic conditions.


Conceptual Example 14 A Conductor in

an Electric Field

A charge is suspended at the center of

a hollow, electrically neutral, spherical

conductor. Show that this charge induces

(a) a charge of –q on the interior surface and

(b) a charge of +q on the exterior surface of

the conductor.

electricity and magnetism€¦ · 14/03/2016 · question: a beam of electrons is directed into...

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