# Electricity _ Magnetism by wanghonghx

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```									Electricity
If you rub a piece of amber

With the fur of a rabbit

It will attract bits of stuff (paper, leaves, etc)
• Equal charges repel.
• Opposite charges attract.

The force between two charges:
• Proportional to the magnitude
of the charges
• Decreases as the square of the
distance
(a) electrons are
transferred to the metal
sphere by rubbing the
negatively charged rod
(b) when the rod is
removed, electrons
distribute themselves
uniformly over the
surface
Five Styrofoam balls are suspended from insulating threads.
Several experiments are performed on the balls and the
I. Ball A attracts B and repels C.
II. Ball D attracts B and has no effect on E.
III. A negatively charged rod attracts both A and E.
What are the charges, if any, on each ball?

A B C       D E
a.   +   -   +   0   +
b.   +   -   +   +   0
c.   +   -   +   0   0
d.   -   +   -   0   0
e.   +   0   -   +   0
The first statement indicates that A attracts B and
repels C. This indicates that both A and C are
charged, but B could be charged by induction.
The second statement indicates that D attracts B, but
has no effect on E. This means that D and E must
not be charged because if either were charged and
the other not, then they would by attracted to each
other by induction.
A negative rod attracts A and E. We know then that A
is positively charged. This then tells us from
statement one that B is negatively charged. C is also
positively charged.
Therefore, choice (C) is the correct answer.
COULOMB’S LAW

The magnitude of the electrostatic force exerted by one point charge
on another point charge is directly proportional to the magnitude of the
charges and inversely proportional to the square of the distance between
them.
q1 q2
F k                                1
r2                   k
4 o
 8.99  109 N  m 2 C 2

  8.85 1012 C2  N  m2 
mp  1.6731027 kg
mn  1.675 1027 kg
me  9.1110 31 kg
19
e  1.60 10          C

An atom contains a small, positively charged nucleus,
about which negatively charged electrons move. The
orbits shown are only symbolic. In reality electrons,
occupy a region of space that encircles the nucleus.
In nature, atoms are normally
found with equal numbers of protons
and electrons, so they are electrically
neutral.

from matter it will acquire a net
electric charge with magnitude equal
to e times the number of electrons

q  Ne
An electrical conductor is a material in which
electrons move easily and are readily conducted.

metals:
silver, copper, gold, aluminum, tungsten, iron, etc.
An electrical insulator is a
material in which electrons are
poorly conducted.

insulators:
mica, rubber, teflon, wood, paper

semi-conductors:
silicon, germanium, carbon
(semi-metal)
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:

F              SI Units of Electric Field: newton per coulomb (N/C)
E
qo
Electric fields from different sources

The Electric Fields from Separate
Charges May Cancel
THE PARALLEL PLATE CAPACITOR

Parallel plate
capacitor
WAB  EPE A  EPE B
WAB EPE A EPE B
    
qo   qo   qo
The potential energy per unit charge
is called the electric potential.

EPE
V
qo
SI Unit of Electric Potential:
joule/coulomb = volt (V)
Work, Potential Energy, and Electric Potential

The work done by the electric force as a test +2.0x10-6 C charge moves
from point A to point B is + 5.0x10-5J.

(a) Find the difference in EPE between these
points.

(b) Determine the potential difference between
these points.

WAB  EPE A  EPE B
5
EPE B  EPE A  WAB  5.0  10 J
5
WAB 5.0 10 J
VB  VA                  25 V
qo   2.0 10 C
-6
Batteries

Within a battery, a chemical reaction occurs that transfers
electrons from one terminal to another terminal.
The electric current is the amount of charge per unit time that passes
through a surface that is perpendicular to the motion of the charges.

q
I
t
One coulomb per second equals one ampere (A).
If the charges move around the circuit in the same
direction at all times, the current is said to be direct
current (dc).

If the charges move first one way and then the
opposite way, the current is
said to be alternating current (ac).
A Pocket Calculator

The current in a 3.0 V battery of a pocket calculator is 0.17 mA. In one hour
of operation, (a) how much charge flows in the circuit and (b) how much energy
does the battery deliver to the calculator circuit?

(a)        q  I  t    0.17 103 A   3600 s   0.61 C

Energy
(b)     Energy  Charge               0.61 C  3.0 V   1.8 J
Charge
The resistance (R) is defined as the
ratio of the voltage V applied across
a piece of material to the current I through
the material.

OHM’S LAW

The ratio V/I is a constant, where V is the
voltage applied across a piece of material
and I is the current through the material:

V
 R  constant             or        V  IR
I
SI Unit of Resistance:
volt/ampere (V/A) = ohm (Ω)
A Flashlight

The filament in a light bulb is a resistor in the form
of a thin piece of wire. The wire becomes hot enough
to emit light because of the current in it. The flashlight
uses two 1.5-V batteries to provide a current of
0.40 A in the filament. Determine the resistance of
the glowing filament.

V 3.0 V
R         7.5 
I 0.40 A
Solar
Array
ELECTRIC POWER

When there is current in a circuit as a result of a voltage, the electric
power delivered to the circuit is:

P  IV
SI Unit of Power: watt (W)

Many electrical devices are essentially resistors:

P  I  IR   I 2 R

V    V2
P   V 
R    R
The Power and Energy Used in a
Flashlight

In the flashlight, the current is 0.40A and the voltage
is 3.0 V. Find (a) the power delivered to the bulb and
(b) the energy dissipated in the bulb in 5.5 minutes
of operation.

P  IV   0.40 A3.0 V  1.2 W

E  Pt  1.2 W330 s   4.0 102 J
100 watt bulb
What is the resistance
and how much
current flows through
a 100 W bulb?
Note: The wattage on
a bulb is its power
output and assumes
that you will use it in
the US where the
voltage in 110 V.
60 W bulb
Redo the calculations
for a 60 W bulb.

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