Electric Forces
The Electrostatic Force
Like charges repel.
Unlike charges attract.
What if there existed a force like gravity but billions of times stronger?
Atomic Model
# Every atom is composed of a
positively charged nucleus su
rrounded by negatively charged
electrons.
Model of a helium atom
# The electrons of all atoms are
identical. Each has the same
quantity of negative charge and
the same mass.
# Protons and neutrons compose
the nucleus. (The common form of
hydrogen that has no neutron is
the only exception.) Protons are
about 1800 times more massive
than electrons but carry an
amount of positive charge equal
to the negative charge of
electrons. Neutrons have slightly
more mass than protons and have
no net charge.
# Atoms usually have as many
electrons as protons, so the atom
has zero net charge.
However the electrons in matter
can travel-but atoms themselves
remain electrically neutral
• The positively
charged
particles in
ordinary matter
are protons,
and the
negatively
charged
particles are
electrons.
1.How does the charge of an electron
differ from the charge of a proton?
2 If you scuff electrons onto your feet
while walking across a rug, are you
negatively or positively charged?
A thin stream of water bends toward a negatively
charged rod. When a positively charged rod is placed
near the stream, it will bend in the
1. opposite direction.
2. same direction.
3. … but it won’t bend at all.
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Charging By Induction
Coulomb’s law
• It states that for two charged objects
that are much smaller than the distance
between them, the force between the
two objects varies directly as the
product of their charges and inversely
as the square of the separation
distance.
The unit of charge is the coulomb,
abbreviated C. It turns out that a
charge of 1C is the charge
associated with 6.25 million trillion
electrons.1 C= 6.25 x1018 electrons
Coulomb Constant
• The proportionality constant k in Coulomb's
law is similar to G in Newton's law of
gravitation. Instead of being a very small
number like
• G =(6.67 ×
10−11 N-m2/Kg2 ),
Electric Force is 1024
times as big as
gravitational force
• the electrical proportionality constant k is a
very large number. It is approximately or, in
scientific notation,
• k = 9 × 109 N-m2 /C2 .
Check yourself
• 1. The proton that is the nucleus of the
hydrogen atom attracts the electron that
orbits it. Does the electron attract the proton
with the same amount of force, less force, or
more force?
• 2. If a proton is repelled by a charged particle
with a given force, by how much will the force
decrease if the proton is moved three times
farther away from the particle? Five times
farther away?
• 3. What is the sign of charge of the particle in
this case?
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Conductors and Insulators
The Electric Force and Field
• It is easy to establish an electric current in
metals because one or more of the electrons
in the outer shell of its atoms are not
anchored to the nuclei of particular atoms.
Instead they are free to wander in the
material. Such materials are called good
conductors. Metals are good conductors of
electric current for the same reason they are
good heat conductors. Electrons in their outer
atomic shell are loose. Insulators are not
free to wander about among other atoms in
the material.
Electric Field
http://www.colorado.edu/ph
ysics/2000/applets/nforcefie
ld.html
• An electric field has both magnitude
(strength) and direction. The
magnitude of the field at any point is
simply the force per unit of charge. If
a body with charge q experiences a
force F at some point in space, then
the electric field E at that point is
• Note that the lines
emanate from the
positive particle and
terminate on the
negative particle.
Electric Shielding
• Electric Field Inside any conductor is
zero
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Electric Potential
• a) The spring has
more mechanical PE
when compressed.
• (b) The charged
particle similarly has
more electrical PE
when pushed closer to
the charged sphere. In
both cases the
increased PE is the
result of work input.
+
-
The Volt
• it is convenient when working with
charged particles in an electric field to
consider the electric potential energy
per unit of charge. We simply divide the
amount of potential energy in any case
by the amount of charge.
(N-m/C
or Joules/C)
The Battery
Thus a 12-volt
battery gives 12
joules of energy to
every 1 coulomb of
charge passing
through the battery
Check yourself
1. If there were twice as many coulombs in the test charge
near the charged sphere in Figure 22.24, would the electric
potential energy of the test charge with respect to the
charged sphere be the same or would it be twice as great?
Would the electric potential of the test charge be the same
or would it be twice as great?
2. What does it mean to say that your car has a 12-volt
battery?
• The standard metric unit on electric potential
difference is the volt, abbreviated V and
named in honor of Alessandra Volta. One Volt
is equivalent to one Joule per Coulomb. If the
electric potential difference between two
locations is 1 volt, then one Coulomb of
charge will gain 1 joule of potential energy
when moved between those two locations. If
the electricpotential difference between two
locations is 3 volts, then one coulomb of
charge will gain 3 joules of potential energy
when moved between those two locations.
VanDeGraaff
10 C
100 C
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Flow of Charge
• Consider two spheres at a different
potential. Charge flows from one end to
the other if a conductor is connected
+
-
Flow of Charge
(N-m/C
+
-
or Joules/C)
Analogous to mass
dropping in a
gravitational field. It
loses PE as it drops.
Check Your Understanding
• The battery simply supplies the energy to
do work upon the charge to move it from
the negative terminal to the positive
terminal. By providing energy to the charge,
the battery is capable of maintaining an
electric potential difference across the two
ends of the external circuit.
The quantity electric potential is defined
as the amount of _____
• a. electric potential energy
• b. force acting upon a charge
• c. potential energy per unit charge.
• d. force per charge
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Current
• The rate of electrical flow is measured in
amperes. One ampere is a rate of flow equal
to 1 coulomb of charge per second. (Recall
that 1 coulomb, the standard unit of charge, is
the electric charge of 6.25 million trillion
electrons.) In a wire that carries 5 amperes,
for example, 5 coulombs of charge pass any
cross section in the wire each second. So
that's a lot of electrons! In a wire that carries
10 amperes, twice as many electrons pass
any cross section each second.
• I=Q/t (C/s) or Ampere
Check Your Understanding
• A 2 mm long cross section of wire is
isolated and 20 C of charge are
determined to pass through it in 40 s.
• I=____ Amperes
Check Your Understanding
A current is said to exist whenever
_____.
• A. a wire is charged.
• B. a battery is present
• C. electric charges are unbalanced
• D. electric charges move in a loop
• In a hydraulic circuit, a narrow pipe
(green) offers resistance to water flow.
(right) In an electric circuit, a lamp or
other device (shown by the zig zag
symbol for resistance) offers resistance
to electron flow.
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The Ohm
• Electrical resistance is measured in
units called ohms. The Greek letter
omega, Ω, is commonly used as the
symbol for the ohm. An ohm is that
amount of resistance when a volt
results in a current of an amp i.e.
Ohm’s Law: Analogous to a=F/m
• Ohm discovered that the current (rate of flow
of charge) in a circuit is directly proportional
to the voltage established across the circuit,
and is inversely proportional to the
resistance of the circuit.
• Ohm=Volt/amp
• Check yourself
• 1. How much current will flow through a
lamp that has a resistance of 60 ohms
when 12 V are impressed across it?
• 2. What is the resistance of an electric
frying pan that draws 12 A when
connected to a 120-V circuit?
• The bird can stand
harmlessly on one
wire of high
potential, but it had
better not reach
over and grab a
neighboring wire!
Why not?
Electric Shock
• What causes electric shock in the
human body current or voltage?
• Check yourself
• 1. At a resistance of 100,000 Ω, what
will be the current in your body if you
touch the terminals of a 12-volt battery?
• 2. If your skin is very moist so that your
resistance is only 1000 Ω and you touch
the terminals of a 12-volt battery, how
much current do you receive?
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What causes electric shock,
current or voltage?
• The round prong connects
the body of the appliance
directly to ground (the
earth). Any charge that
builds up on an appliance
is therefore conducted to
the groundﾑ preventing
accidental shock.
Electric Circuits
Series Circuits: Water Analogy
• Series Circuits
As more
resistors (bulbs)
are added,
resistance gets
bigger, current
gets smaller
(dimmer)
RT=R1+R2 +R3
1.What happens to current in other lamps if one lamp in a
series circuit burns out?
2. What happens to the light intensity of each lamp in a series
circuit when more lamps are added to the circuit?
Parrallel Circuits
I = V/R
Parallel Circuits: Water Analogy
As R gets bigger, total resistance gets
smaller. Total Resistance is smaller
than the smallest resistor.
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Demo: Electric Circuits
A Problem
Two 10 ohm bulbs are connected in series to
a 20 V battery. What is the total resistance
and what current flows?
Two 10 ohm bulbs are connected in parallel to
a 20 V battery. What is the total resistance
and what current flows?
• 1.Each device connects the same two points
A and B of the circuit. The voltage is
therefore the same across each device.
• 2.The total current in the circuit divides
among the parallel branches. Since the
voltage across each branch is the same, the
amount of current in each branch is inversely
proportional to the resistance of the branchﾑ
Ohm's law applies separately to each
branch.
• 3.The total current in the circuit equals the
sum of the currents in its parallel branches.
• 4.As the number of parallel branches is
increased, the overall resistance of the circuit
is decreased. Overall resistance is lowered
with each added path between any two points
of the circuit. This means the overall
resistance of the circuit is less than the
resistance of any one of the branches.
• Lights and wall outlets are connected
in parallel, so all are impressed with
the same voltage, usually about 110ﾐ
120 volts. As more devices are
plugged in and turned on, more
pathways for current result in lowering
of the combined resistance of each
circuit. Therefore, a greater amount of
current occurs in the circuits. The sum
of these currents equals the line
current, which may be more than is
safe. The circuit is then said to be
overloaded. The heat generated by an
overloaded circuit may start a fire.
• Check yourself
• 1. What happens to the current in other
lamps if one of the lamps in a parallel
circuit burns out?
• 2. What happens to the light intensity of
each lamp in a parallel circuit when
more lamps are added in parallel to the
circuit?
Review Questions
So what is ______.
a Coulomb: a Volt: Current: Amp: Ohm’s Law
A 6 volt battery is connected to a 2 ohm lamp.
What is the current thru the lamp? Voltage
across the lamp?
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Review Questions
A 6 volt battery is connected to three 2 ohm
lamps.
What is the current thru each lamp? Voltage
across each lamp?
Review Questions
Review Questions
A 6 volt battery is connected to two 2 ohm lamps as
shown below with switch open. Repeat with switch closed.
•Voltage across the lamp?
•What is the current thru each lamp?
•Total current put out by battery?
Direct Current and
Alternating Current
A 1 ohm lamp is connected across the 12 V batter.
If another 60 ohm lamp is connected in parallel then the
total resistance in the circuit:
In a parallel circuit the
•Is more than 60 ohms
total resistance is
•Is less than 1 ohm
smaller than the smallest
resistance
•remains the same
Electric Power
10
• When it is observed that a
light bulb is rated at 60
watts, then there are 60
joules of energy delivered to
the light bulb every second.
A 120-watt light bulbs draws
120 joules of energy every
second
The power and voltage on the light bulb read 100 W 120 V.
How many amperes will flow through the bulb?
The Kilowatt-hour
• A kilowatt is a unit of power and an hour is a unit of
time. So a kilowatt ･ hour is a unit of Power ･ time. If
Power = Energy / time, then Power ･ time =
Energy. So a unit of power ･ time is a unit of energy.
The kilowatt ･ hour is a unit of energy. When an
electrical utility company charges a household for
the electricity which they used, they are charging
them for electrical energy. The utility company is
responsible for assuring that the electric potential
difference across the two main wires of the house is
110 to 120 volts. And maintaining this difference in
potential requires energy.
• Check yourself
• 1. If a 120-V line to a socket is limited to
15 A by a safety fuse, will it operate a
1200-W hair dryer?
• 2. At 10 cents/kWh, what does it cost to
operate the 1200-W hair dryer for 1 hr?
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