Sinai University -Faculty of Dentistry
Course name: General Physics II,
Dentistry
Text references
1- Physics for biology and premedical students,Burns
2- College Physics, Serway
3- Lecture notes
4- Internet sites
ellawindy@su.edu.eg
Department site:
www.engineering.su.edu.eg
www.dendistry.su.edu.eg
Assessment
%15
5%
10%
20%
50%
50 %
15
5
10
20
50
50
marks
marks
marks
marks
class work
Pre. term exam 1
Mid term exam 2
Lab. Works
marks
Final yea exam
100%
100
marks total
Electricity Course objectives
At the end of this course, students will be able
to:
1- Define and apply coulomb force law.
2- Illustrate electric field, electric potential, and
Ohm law.
3- Analyze simple electric circuits.
3- Explain the conduction mechanisms.
4- Use the gained information to understand the
operational functions of electrical instruments.
5- To link the course to future applications
Sinai University
Faculty of dentistry
Subject
Ch 1 Electrostatics
Prof Ahmed El-Lawindy
Physics II
Code: SGS 122
Lectures time table
Weeks
Date
W1+W2
5/3/2011-12/3/2011
Ch 2 Potential, current and their sources W4
26/3/2011
Quiz 1 (15 min)
W5
2/4/2011
Ch 3 Ionic conduction
W5- W6
2/4/2011-9/4/2011
Ch 4 Magnetic fields and magnetic forces W7-W8
16/4/2011- 23/4/2011
Midterm Exam (30 min)
W9
30/4/2011
Ch 5 Magnetic induction
W9
30/4/2011
Quiz 2 (15 min)
W10
Ch 6 X-rays
W10-W11
Quiz 3 (15 min)
Ch 7 Radioactivity
W12+ +W14
7/5/2011 -14/5/2011
21/5/2011 -5/6/2011
W12-W13-W14 21/5/2011 -5/6/2011
Chapter 1
Electrostatics
Introduction
Up to now, we have for the most part dealt
with topics which are macroscopic,
Force, pressure, work etc…
For the next few chapters, we will be
concerned with the microscopic world, which
is largely hidden from our senses and our
common sense.
Gravity, electric fields, magnetic fields, etc..
Introduction
The basic problem which electricity presents to our
intuition is: how can the electric force act at a distance,
without direct contact?
When you get closer to a charged sphere, your hair will
rise, defying gravity.
What actually happens is
a separation of “electric
charges" from the boy's
hair such that the charges
on hair nearer to the
charged sphere are of
opposite
type
of
the
charge on it. This is an
attractive force between
what is called positive and
negative charges.
Coulomb force
Coulomb force
23.3 Coulomb’s Law
t=kq
Q
Q
Q/2
Q/2
=FxR
Faq
Q/2
Q/4
Q/4
Q/8
Q/8
Q/16
Q/4
Q/8
Q/16
Coulomb force Law
At the heart of this simple phenomenon is one of the most
universally applicable ideas in physics: action at a distance is
caused by charges, which are the sources of forces.
or, in scalar form,
F = q1 q2 /( 4 p e0 r 2). it is a vector quantity
=K q1 q2 / r 2
K=1/4peo= 8.99 x l09 N.m2/C2,
eo= 8.85419x10-12 C V-1 m-1
eo is the electric permittivity of space
Electric charges detection
Neutral bodies are charged by different ways.
The most common ways are charging by
1- conduction,
2- induction
3- rubbing
++++++++++++
++++++++++++
1-Charging by conduction
+
2-Charging by induction
3-Charging by rubbing
# Glass rubbed with silk +ve
# Hard rubber rubbed with fur –ve
Charge is Quantized
Electric charge was thought to be a
continuous fluid
q = ne
n=0,+1,+2,+ 3, …..
Millikan’s Experiment
Milliken's apparatus
V drift velocity, constant velocity, D, is the distance traveled by an electron in a time t
Derivation
Step 1 Free Fall
Mg= bv
Step 2 Motion in an Electric Field, E
QE-Mg=- bu
Then:
QE-bv=bu
QE=bu-bv=b(u-v)
QV/d=b(D/t1-D/t2)
Q=ne= bdD(1/t1-1/t2)
Sample Problem 1
A penny, being electrically neutral, contains equal amounts of
positive and negative charge. What is the magnitude of these
equal charges?
Solution
The charge q is given by NZe, in which N is the number of atoms in a penny and Ze is
the magnitude of the positive and the negative charges carried by each atom.
The number N of atoms in a penny, assumed for simplicity to be made of copper, is
NAm/M, in which NA is the Avogadro constant. The mass m of the coin is 3.11 g, and the
mass M of 1 mol of copper (called its molar mass) is 63.5 g. We find
N= NAm = (6.02 x 1023 atoms/mol x (3.11 g)
M
63.5g/mol
= 2.95 x 1022 atoms.
Q= NZe=2.95 x 1022 atomsx26x2 atoms-1x1.6x10-19 C
= 245440 C
It is a huge quantity of charge, but remember that these charges neutralize each other
so that we don’t feel it.
The force of gravity and
the Coulomb force
Fx = - G m1 m2 Rx / r3 Or F = - G m1 m2 / r2.
("Newton's Constant", equal to 6.673 x 10 - 11 N m2 / kg2)
Fx = q1 q2 R x / 4 p e r 3,or, in scalar form, F = q1 q2 / 4 p e r 2.
The force of gravity and
the Coulomb force
We can now see how to compute the value of g, the "constant" acceleration
due to gravity.
The force due to a spherical source is equivalent to a point source at its center.
Approximating the earth by a sphere, we set m2 equal to the earth's mass (mE) and r
to its average radius (rE). The acceleration g is then G mE / rE2, or approximately 9.8
m / s2.
m1g= G m1mE / rE2
g= G mE / rE2
=9.8 m / s2.
Sample Problem 2
The average distance r between the electron and the proton in the
hydrogen atom is 5.3x10-11 m. (a) What is the magnitude of the average
electrostatic force that acts between these two particles? (b) What is the
magnitude of the average gravitational force that acts between these
particles?
Solution (a) From Eq. (2-1) we have, for the electrostatic force,
Fe =
1
q 1 q2
4peo
r2
= (8.99 X 109N.m2/C2)(1.60X l0-19C)
(5.3 X 10-11 m)2
= 8.2 X 10-8 N.
While this force may seem small (it is about equal to the weight of a speck of dust), it produces an immense effect,
namely, the acceleration of the electron within the atom.
(b) For the gravitational force, Eq. (3-1), we have
Fg= G m1m2/ r2
= (6.67 x 10-11 N.m/kg2)(9.ll x l0-31 kg) ( 1.67 X 10-27kg)
(5.3 X 10-11 m)2
= 3.6 X l0-47 N.
Fe/ Fg= 8.2 X 10-8 /3.6 X l0-47=~1039
Sample Problem 3
The nucleus of an iron atom has a radius of about 4x10-15 m and contains
26 protons. What repulsive electrostatic force acts between two protons in
such a nucleus if a distance of one radius separates them?
Solution From Eq. (2-1), we have
F =
1
4peo
qp qp
r2
(8.99 x l09N.m2/C2)(l.60x l0-19 C)2
(4 X l0-15)2
=14 N.
=
This enormous force, more than 1 kg and acting on a single proton,
must be more than balanced by the attractive nuclear force that
binds the nucleus together. This force, whose range is so short that its
effects cannot be felt very far outside the nucleus, is known as the
"strong nuclear force" and is very well named.
Problems
Solve the following problems
1-3-5
Hand it tomorrow