SCS 139 Applied Physics II

Lecturer: Assoc.Prof.Dr.Thawatchai Onjun

Text. Books: 1. Halliday, D., et al, Fundamentals of Physics, 9th Edition,

John Wiley & Sons, Inc.

2. Giambattista A., College Physics, 2nd Edition, McGraw-Hill

Education

3. Tippens P. E., Physics, 7th Edition, McGraw-Hill Education

4. Young and Freeman, University Physics, 10th Edition,

Addison-Wesley

Class Evaluation

Quiz 10%

Homework 10%

Mid-term Exam. 40%

Final Exam 40%

Class Schedule

Week Subject Reference [1]

1 Electric Charge Ch.21

2 Electric Fields Ch.22-23

3 Electric potential Ch.24

4 Capacitance Ch.25

5 Electric Current and Circuits Ch.26-27

6 Magnetic Forces and Fields Ch.28-29

7 Induction and Inductance Ch.30

8-9 Midterm Exam

10 Alternative Current Ch.31

11 Maxwell’s Equations Ch.32

12 Images Ch.34

13 Interference Ch.35

14 Diffraction Ch.36

15 Photons and Matter Waves Ch.38-39

16 All about Atoms Ch.40

17-18 Final Exam

4

Electric Charge

There are two kinds of electric charge: positive and negative.

A body is electrically neutral if the sum of all the charges in a

body is zero.

Charge is a conserved quantity.

+

-

A hydrogen atom

A positive charge (proton)

A negative charge (electron)

Discovery of Electric Charge

• In 600 BC, ancient Greek

found that charge could be

accumulated by rubbing fur on

various substances, such as

amber.

• In 18th, Michael Faraday

studied about properties of

positive charges and

Benjamin Franklin explored

about positive and negative

charges.

Experimental Observation of Electric Charge

• Electric charge is an intrinsic property of particles that objects are made of.

Charges with the same electrical sign repel each other,

and charges with opposite electrical signs attract each other

7

The elementary unit of charge is e = 1.60210-19 C.

The charge on the electron is 1e.

The charge on the proton is +1e.

The charge on the neutron is 0e.

Keep in mind! Experiments show that likes charges will repel

each other and unlike charges will attract each other and that

the force decreases with increasing distance between

charges.

This body is electrically neutral. + + +

+

+

unit of charge

Example

8

A metallic sphere has a charge of +4.0 nC. A negatively

charged rod has a charge of 6.0 nC. When the rod touches

the sphere, 8.2109 electrons are transferred. What are the

charges of the sphere and the rod now?

Electric Charge & Materials

• Electrons in objects move and transfer while nuclei are immobile.

• Materials can be classified by the ability of their electrons to move.

• Four groups of materials can be found: – Insulator: are materials through which charge cannot

move freely. – Conductor: are materials through which charge can

move rather freely. – Semiconductor: are intermediate materials between

insulator and conductor. – Superconductor: are materials that are perfect

conductors.

Four groups of materials

Insulator Conductor

Semiconductor Superconductor

Charge is quantized

• Any positive or negative charge q can be

written as

in which e, the elementary charge, has the

value of

12

Coulomb’s Law

The magnitude of the force

between two point charges is:

where q1 and q2 are the charges, r is the separation between

the two charges and k = 8.99109 Nm2/C2.

2212

0

0

/NmC 1085.8 and 4

1 where

k

and 0 is called the permittivity of free space.

13

r

q1 q2 F21 F12

r

q1 q2 F21 F12

The electric force is directed between the centers of the two

point charges.

The electric force is an example of a long-range or field

force, just like the force of gravity.

Attractive force

between q1 and q2.

Repulsive force

between q1 and q2.

Example

14

What is the net force on the charge q1 due to the other two

charges? q1 = +1.2 C, q2 = 0.60 C, and q3 = +0.20 C.

The net force on q1 is Fnet = F21 + F31

F31

F21

15

The magnitudes of the forces are:

Example continued:

16

Example continued:

The components of the net force are:

17

Example continued:

The magnitude of the net force is:

The direction of the net force is:

Example • Two positively charged particles fixed in place on an

x-axis. The charges are q1 = 1.6x10-19 C and q2 =

3.2x10-19 C, and the particle separation is R = 0.02

m. What are the magnitude and direction of the

electrostatic force F12 on particle 1 from particle 2?

Net Force:

Example • Similar to the previous Example, except that

particle 3 now lies on the x axis between particles 1

and 2. Particle 3 has charge q3 = -3.2x10-19 C and

is at a distance 0.75R from particle 1. What is the

net electrostatic force F1,net on particle 1 due to

particles 2 and 3? Net Force:

Example • Similar to the previous Example, except that particle

4 now included. It has charge q4 = -3.2x10-19 C, is

at a distance 0.75R from particle 1, and lies on a

line that makes an angle = 60 with the x axis.

What is the net electrostatic force F1,net on particle 1

due to particles 2 and 4?

Net Force:

Example

• Four particles form a square as shown in

the figure below. The charges are q1 = q4

= Q and q2 = q3 = q. What is Q/q if the net

electrostatic force on particles 1 is zero?

The x-component of the force experienced

by q1 = Q is

which (upon requiring F1x = 0) leads to

.

If y direction is considered, the same result is obtained

Example

22

What is the ratio of the electric force and gravitational force

between a proton and an electron separated by 5.310-11 m

(the radius of a Hydrogen atom)?

The ratio is:

Charges in Equilibrium

• A charge can be in equilibrium on electrostatic

force if the net electrostatic force is equal to zero:

0 netF

24

Learning Summary

We have covered an introduction of E&M. Three topics

have been discussed.

• Electric Charges

• Properties of Conductors/Insulators

• Coulomb’s Law

• Charges in Equilibrium