Download Electric

Sci-101
Introduction to Physics
Chap. 9 Static and Current
Electricity
Dr. Alexander Dzyubenko
http://www.csub.edu/~ adzyubenko
© A.B. Dzyubenko 2003, 2004
Electric Force and Charge
• A universal force, like
gravity
• Varies inversely as the
square of the distance, like
gravity
• Billions and billions time
stronger than gravity
repel
attract
• There are two types of
charges “+” and “-”,
unlike gravity
2
L8: Electricity
Atomic structure




Every atom is composed of a
positively charged nucleus
surrounded by negatively
charged electrons
The electrons of all atoms are
identical: same charge, same
mass, same everything
Protons and neutrons
compose the nucleus
 Neutrons have
Protons (+) are about 2000
slightly more mass
more massive than
than protons
electrons (-)
3
L8: Electricity
Atomic structure cont



Normally an atom has many
electrons and equal number
of protons => neutral as a
whole
Hydrogen atom: one proton,
one electron
When an atom loses one or
several electrons, it has a
positive net charge; called a
positive ion
-
 An
atom gains one
or several electrons,
it has a negative net
charge; called a
negative ion
4
L8: Electricity
Making neutral objects charged




Material objects are
made of atoms
Ordinarily have equal
numbers of electrons
and protons => neutral
as a whole
The electrons farthest
from the nucleus can be
stripped away
Easily? It depends on a
substance!
Electrons are
transferred from the
fur to the rod

5
L8: Electricity
Conservation of Charge
Whenever something is
charged, no electrons
are created or
destroyed
 The conservation of
charge is a significant
principle in physics

6
L8: Electricity
Coulomb’s Law
The force between two charges varies
directly as the product of their charges
 and inversely as the square of the
distance between them

7
L8: Electricity
Coulomb’s Law cont

The unit of charge, Coulomb (C)

Equals the amount of charge that passes
through a 100 W bulb in about 1 s

The proportionality constant, k, is similar
to the gravitational constant G

k = 9x109 Nm2/C2
8
L8: Electricity
Gravitational vs Electric Forces




The gravitational
force holds the satellite
in orbit
The electric force
holds the electron in
orbit
Qualitatively, very
similar
Quantitatively?
9
L8: Electricity
http://www.people.vcu.edu/~rgowdy/mod/077/xmp.htm
L8: Electricity
10
Charge polarization


Charge an inflated balloon by rubbing it
on your hair
Place it again a wall and it sticks …
Negatively charged balloon polarizes atoms in the wall
11
L8: Electricity
Electric Fields
Electric (like gravitational)
forces act between charged
objects that are not in contact
 There exists a force field that
influences distant charges
(masses)
 Electric field near
 Electric field, E, is a vector
a negative charge
 Field (or force) lines are used
 Direction? That of
to represent E

the force on a
positive charge
12
L8: Electricity
Electric field near
two charged metallic
plates. A capacitor.

Examples
Electric field
near a negative
charge.

Electric field of a
dipole, a pair of “+”
and “–” charges. Net
charge=0.

13
L8: Electricity
Electrical Potential Energy

A charged object has
electrical potential
energy by virtue of its
location in an electric
field
(a) gravitational PE
 (b) PE of charged particles

14
L8: Electricity
Electric Potential Energy, cont


Work done on the
spring goes to the
increase of its
mechanical PE
Work done on the
charge goes to its
electrical PE
15
L8: Electricity
Electric Potential
Electric potential energy
Electric potential 
amount of charge
Unit of energy: Joule (J)
 Unit of charge: Coulomb (C)
 Unit of Electric Potential:
Joule/Coulomb = Volt
1 V = (1 J)/(1 C)
 Terms electric potential and voltage are used

16
L8: Electricity
Electric Potential and PE



Whose energy is
larger?
Whose potential
is larger?
Two different charges at
same distance from the
charged dome
17
L8: Electricity
Voltage Sources
The ends of an electrical
circuit are at different
electric potentials
 (+) Charges in the
conductor flow from high
to low potentials
 the flow ceases when
 To attain a sustained
the difference in
flow of charge, we need a pressures ceases
battery or generator
 The difference in
 Electrical “pumps”
pressures is maintained

with the pump
L8: Electricity
18
Batteries
A common automobile battery provides
12 Volts
 We say there is a 12-volt voltage drop in
the circuit
 12 Joules of energy are supplied to each
1 Coulomb of charge that is made to
flow in the current

19
L8: Electricity
Electric current





Is the flow of charged particles
In metals, electrons are flowing
charged particles
These are called conduction
electrons
Positively charged nuclei
(protons+ neutrons) are locked
in fixed positions
The unit of electric current is
Ampere (A): it is a rate of flow of
1 Coulomb per second through a
wire
20
L8: Electricity
Electric current, cont
The current-carrying wire is not
electrically charged
 The charges flow through a circuit when
voltage is applied across a circuit

21
L8: Electricity
Direct and Alternating Currents




DC current: charges flow
in one direction
AC current may be
accomplished by
periodically switching the
sign at the terminals
Commercial US standard:
60 cycles per second or 60
Hertz (Hz)
European standard: 50 Hz
22
L8: Electricity
Electrical Resistance
The longer the pipeline,
the larger the resistance
The longer the wire, the
larger the resistance
23
L8: Electricity
Ohm’s Law







Current = Voltage / Resistance
For a given circuit (Resistance), current is
linearly proportional to voltage
Good conductors (like metals) obey Ohm’s
Law
Not all substances obey Ohm’s Law
Units for Resistance: Ohms, 
Amperes = Volts / Ohms
1 A = 1 V/ 1 
24
L8: Electricity
Quick Quiz
How much current flows through a
lamp that has a resistance of 60 
when 12 Volts is impressed across it?
 Current = Voltage / Resistance
 Amperes = Volts / Ohms
 Current = (12 Volts) / (60 ) = 5 A

25
L8: Electricity
Electric Shock







Which causes electric shock in
human body – current or voltage?
Current!
The resistance of the human body ranges from
about 100  (when soaked with salty water) to
500,000  (when the skin is very dry)
0.001 A can be felt
0.05 A is painful
0.01 A causes involuntary muscle contractions
0.07 A probably fatal if lasts > 1 s
26
L8: Electricity
Electric Circuits





A circuit, any path through
which electrons can flow
Usually contains more than
just one device
Devices connected to each
other
Connections in series
Connections in parallel
Conduction electrons
surge to and fro in the
filament: AC voltage =>
AC current

27
L8: Electricity
Connection in Series
Electric current has
just one path

L8: Electricity
Resistance of the
circuit = SUM of all
resistances

28
Connection in Parallel
Each device operates
independently but at
same voltage

The total current=
SUM of all currents

29
L8: Electricity
Appliances are connected in
parallel
Circuits
that carry
more that a safe
amount of current are
said to be overload
 A fuse
is connected in
series to prevent a
possible overload
30
L8: Electricity
Electric Power
The moving charges do work (electric
heaters and ovens, electric motors, …)
 The rate at which work is done is called
electric power
 Power = Current x Voltage
 1 Watt = 1 Ampere x 1 Volt
 Quiz: A 60 W lamp operates on a 120-V
line. Current?
 Answer: Current = 60 W / 120 V = 0.5 A

31
L8: Electricity