Electric charge - Uplift Education

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Interactive lectures – should
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All physics to date has led to one primary conclusion:
• There are four fundamental forces:
1)
2)
3)
4)
Gravitational
Electromagnetic
Strong nuclear
Weak nuclear
grand unified theory
GUT
all based on the
Electromagnetic Theory
~250 yrs or so since we first learned what electricity is
“Electricity” – from the Greek word electron (elektron) meaning “amber”. The ancients knew that if you rub an
amber rod with a piece of cloth, it attracts small pieces of
leaves or dust.
“amber effect”– the object becomes electrically charged
Electricity & Magnetism
• static electricity (Electrostatics)
– Why do I get a shock when I walk across the
rug and touch the door knob?
– Why do socks stick to my pants in the dryer?
– Why does my hair stick to my comb, and I
hear a crackling sound ?
– Why does a piece of plastic refuse to leave
my hand when I peel it off a package?
– What is lightning?
What is that all about?
It’s the CHARGE
No one has ever seen electric charge;
it has no weight, color, smell, flavor, length, or width.
Charge is an intrinsic property of matter
electron has it, proton has it, neutron doesn’t have it
– and that’s all
 Electric charge is defined by the
effect (force) it produces.
– positive charge
– negative charge
Benjamin Franklin
(1706 - 1790, American
statesman, philosopher
and scientist)
Electricity has origin within the atom itself.
10-15 m
Name
Electron
Symbol
e
Charge
-e
Mass
9.11x10-31 kg
Proton
Neutron
p
n
e
none
1.67x10-27 kg
1.67x10-27 kg
10-10 m
mnucleon ≈ 2000 x melectron
ratom ≈ 100000 x rnucleus
Atom is electrically neutral = has no net charge,
since it contains equal numbers of protons and electrons.
Electric forces
• charges exert electric forces on other charges
–two positive charges repel each other
–two negative charges repel each other
–a positive and negative charge attract each other
+ +
The repulsive electric force between 2 protons is
1,000,000,000,000,000,000,000,000,000,000,000,000
times stronger than the attractive gravitational force!
+
Attractive force between protons and electrons cause them to
form atoms.
Electrical force is behind all of how atoms ond…chemistry…
• charge is measured in Coulombs [C]
French physicist
Charles A. de Coulomb
1736 - 1806
• Every electron has charge -1.6 x 10-19 C,
and every proton 1.6 x 10-19 C
1C represents the charge of 6.25 billion billion (6.25x1018) electrons !
Yet 1C is the amount of charge passing through a 100-W light
bulb in just over a second. A lot of electrons!
The smallest amount of the free positive
charge is the charge on the proton.
The smallest amount of the free negative
charge is the charge on the electron.
quarks have 1/3, but
they come in triplets
let e = 1.6 x 10-19 C
Charge of the single proton is
qproton = e .
Charge of the single electron is
qelectron = - e
•Charge is quantized: cannot divide up charge into
smaller units than that of electron (or proton) i.e. all objects
have a charge that is a whole-number multiple of charge of
the smallest amount (a single e).
•The net charge is the algebraic sum of the
individual charges (+ 5 - 3 = 2).
Everyday objects - electronically neutral –
balance of charge – no net charge.
Objects can be charged – there can be net
charge on an object. How?
The only type of charge that can move around is the negative
charge, or electrons. The positive charge stays in the nuclei. So,
we can put a NET CHARGE on different objects in two ways
Add electrons and make
the object negatively
charged.
Remove electrons and make
the object positively
charged.
Some materials have atoms that have outer electrons (farthest
from nucleus) loosely bound. They can be attracted and can
actually move into an outer orbit of another type of atom. The
atom that has lost an electron has a net charge +e (positive
ion). An atom that gains an extra electron has a net charge of
– e (negative ion).
This type of charge transfer often occurs when two different
materials (different types of atoms) come into contact.

• Which object gains the electrons depends on their
electron affinity:
Conclusion:
 electrons can be transferred from one object to another
 During that process, the net charge produced is zero.
The charges are separated, but the sum is zero.
The amount of charge in the universe remains constant
(we think!!) It is CONSERVED!
 Another Law of Conservation:
Charge is always conserved: charge cannot be
created or destroyed, but can be transferred from
one object to another.
When objects are charged by rubbing, they don’t stay charged for
ever. They eventually return to neutral state – very often the
charge will “leak off” onto water (polar) molecules in the air.
Sometimes they will be neutralized by charged ions in the air
(formed, for example, by collisions with charged particles known
as cosmic rays).
Given enough time, the particles in the air will remove the excess
charge from the object leaving it neutrally charged. This explains
why on dry days we tend to have more trouble with static
electricity build-up than on humid (moist) days. On moist days
there are more water molecules in the air to steal charge more
rapidly. On dry days there are fewer particles in the air to steal
charges so we accumulate charge until we touch something and
get discharged (shocked).
Electrical conductors, insulators,
semiconductors and superconductors
- distinction based on their ability to conduct electric charge.
Any material that allow charges to move about more or less freely
is called conductor. So, if you transfer some electrons to the
metal rod, that excess of charge will distribute itself all around rod.
Tap water, human body and metals are generally good
conductors.
That’s all very nice, but why is that so?
What makes conductors conduct?
• Atoms have equal numbers of positive and negative charges,
so that a chunk of stuff usually has no net charge  the
plusses and minuses cancel each other.
• However, in metal atoms the valence electrons – the electrons
in the outermost orbits - are loosely bound, so when you put a
bunch of metal atoms together (to form a metal) an amazing
thing happens  valence electrons from each atom get
confused and forget which atom they belong to.
• They now belong to the metal as the whole. As the result,
positive ions which are tightly bound and can only oscillate
around their equilibrium positions, form a positive background.
All the homeless electrons - “Free electrons”
wander around freely keeping ions
from falling apart – metallic bond!!
Electrons in insulators are tightly bound to atomic nuclei and so
cannot be easily made to drift from one atom to the next. Only if a
very strong electric field is applied, the breakthrough (molecules
become ionized resulting in a flow of freed electrons) could result
in destruction of the material.
The markings caused by electrical
breakdown in this material – look
similar to the lightening bolts
produced when air undergoes
electrical breakdown.
Materials like amber, pure water, plastic, glass, rubber, wood…
are called insulators. They do not let electricity flow through
them. Electrons are tightly bound to nuclei, so it is hard to make
them flow. Hence, poor conductors of current and of heat.
Semiconductors
• Materials that can be made to behave sometimes as
insulators, sometimes as conductors.
Eg. Silicon, germanium. In pure crystalline form, are insulators.
But if replace even one atom in 10 million with an impurity
atom (ie a different type of atom that has a different # of
electrons in their outer shell), it becomes an excellent
conductor.
• Transistors: thin layers of semiconducting materials joined
together.
Used to control flow of currents, detect and amplify radio
signals, act as digital switches…An integrated circuit contains
many transistors.
The movement of electrons in semiconductors is impossible to
describe without the aid of quantum mechanics.
As the conductivity of semiconductors can be adjusted by adding
certain types of atomic impurities in varying concentrations, you
can control how much resistance the product will have.
ADVANTAGE – A HUGE ONE
Superconductors
• Have zero resistance, infinite conductivity
• Not common! Have to cool to very very low temperatures.
• Current passes without losing energy, no heat loss.
• Discovered in 1911 in metals near absolute zero (recall this
is 0oK, -273oC)
• Discovered in 1987 in non-metallic compound (ceramic) at
“high” temperature around 100 K, (-173oC)
• Under intense research! Many useful applications eg
transmission of power without loss, magnetically-levitated
trains…
Conductors and Insulators – and how
to charge them
REMEMBER:
Electrons are free to
move in a conductor
Electrons stay with their
atom in an insulator
Most things are in between
perfect conductor/ insulator
ELECRTOSTATIC CHARGING
1. Charging by Friction:
The transfer of charge is due the rubbing - friction
between two previously neutral materials.
When you move your comb through your hair, the friction
between the comb and hair can pull some of the electrons
out of your hair and onto the comb. As a result your comb
ends up with a net negative charge and attracts your hair
which is now positive.
Rubbing: rubber rod with fur or cloth, glass rod with silk, hair
with balloon, shuffling across a carpeted floor.
2. Charging by Conduction (Contact):
2.1
Conductors:
When a charge is placed on a conductor, the
mutual repulsion of the individual charges
causes them to move as far away from each
other as possible. Thus, a charge deposited on a
conductor quickly spreads out over its surface.
2. 2
Insulators:
When a charge is placed on an insulator, it remains
where it is deposited and surrounding molecules
become polarized.
An external (negative) charge distorts the shape of
an atom by forcing its negatively-charged electron
clouds to shift away from the charge and the
positively charged nuclei to shift toward the charge.
Such a distorted atom is said to be polarized.
Metal
sphere
Insulated
stand
Glass
sphere
Insulated
stand
Question:
Consider a negatively charged rod touching a conductor
versus touching an insulator.
What is the difference between how the electrons are
arranged on the conductor and insulator?
• charges can be transferred from/to conductors or nonconductors but they can only move through conductors.
Would spread out evenly on a good conductor,
because the transferred e’s repel each other.
But on insulator, or poor conductor, would be
more localized at where the rod touched.
3. Charging by Induction
3.1
Conductors:
a. Neutral conductor with free electrons
b. free electrons in the metal are
repelled as far as possible from the
charged object.
c. The Earth is reservoir of any charge. It
can easily accept or give up electrons.
Connect conductor with a conducting wire
to the ground - many of free electrons in
metal are able to move even further from
charged object down the wire into the Earth.
d. Object is left positively charged
e. cut the wire, remove the rod and the
metal sphere has evenly distributed
positive charge.
Charge has been
separated, but metal
sphere is still neutral
Or you can touch
it with finger,
electrons flow
from your finger,
through you, to
the ground.
3.2
Insulators:
Positive surface
charge
Insulators:
When insulator is charged by induction, there
will be no change of charge on that object.
Instead of that charge is moved within the
molecule/atom (the net charge is kept zero)
Therefore we call it rather:
Charging by Polarization
A charge placed near an insulator polarizes its atoms.
While the insulator’s interior remains electrically neutral, a net charge
appears on the surface, and can produce force on other charges near
the insulator.
Even though sphere is neutral
there is attraction force acting between the rod and sphere.
Charge polarization
When bring a charged object near an insulator, electrons are not free to migrate
throughout material. Instead, they redistribute within the atoms/molecules
themselves: their “centers of charge” move
Here, usual atom,
with center of
electron cloud at
positive nucleus
When a – charge is
brought near the right,
electron cloud shifts to
the left. Centers of +
and – charges no longer
coincide.
Atom is electrically polarized
Surfaces of material look like this. A – charge
induced on right, and + on the right.
(Zero net charge on whole object)
EXAMPLE - QUESTION Charging by induction
Bring a charged object near a conducting surface, electrons will
move in conductor even though no physical contact: Due to
attraction or repulsion of electrons in conductor to the charged
object – since free to move, they will!
Once separated from each other with rod still close they’ll
remain charged. Charge is conserved, so charges on spheres
A and B are equal and opposite.
Note, the charged rod never touched them, and retains its original charge.
QUESTION:
A metal ring receives a positive charge by contact.
What happens to the mass of the ring?
Does it increase, stay the same, or decrease?
Will the object have deficiency or excess of electrons?
When the positively charged ball touches the ring, electrons inside
it are attracted to the ball. Some will leave the ring trying to
neutralize the ball. Only a tiny fraction leaves the ring. The mass
of the electrons is so small compared to the atoms, so although
the mass of the ring decreases, measuring it would not be
possible. (By the way, both will be positively charged, but the ball
will be less then before)
EXAMPLES AND CONSEQUECIES:
Example:
Van de Graaff
The sphere gives the girl a large
negative charge. Each strand of
hair is trying to:
1)
2)
3)
4)
5)
Get away from the charged sphere.
Get away from the ground.
Get near the ceiling.
Get away from the other strands of hair.
Get near the wall outlet.
Like charges attached to the hair strands repel,
causing them to get away from each other.
What is his secret?
Seeing the effects of charge:
the electroscope
• the electroscope is a simple
device for observing the
presence of electric charge
• it consists of a small piece of
metal foil (gold if possible)
suspended from a rod with a
metal ball at its top
++
++
• If a negatively charged rod is placed near the ball,
the electrons move away because of the repulsion.
The two sides of the metal foil then separate.
Charging by Induction
• Bring a charged rod in near
Positive
charged rod
results in
positive leaves.
Attracting uncharged objects
+
+
+
+
uncharged
metal sphere
• A negatively charged
rod will push the
electrons to the far
side leaving the near
side positive.
• The force is attractive
because the positive
charges are closer to
the rod than the negative
charges
Charge polarization is why a charged object can attract
a neutral one :
•DEMO: Rub balloon on your hair – it will then stick to the wall !
Why?
Balloon becomes charged by friction when rub on
hair, picking up electrons. It then polarize molecules
on the surface, induces + charge layer on the wall’s
surface closest to it , and next negative furthest away.
So balloon is attracted to + charges and repelled by –
charges in wall, but the – charges are further away so
repulsive force is weaker and attraction wins.
• Charge a comb by rubbing it
through your hair, and then see it
attracts bits of paper and fluff…
You can bend water with charge!
The water molecule
has a positive end and
a negative end.
charged rod
When a negative rod is
brought near the stream
of water, all the positive
ends of the water molecules turn to the right
and are attracted to the
negative rod.
What happens if the rod is
charged positively?
stream of water
As we said Like charges repel, and opposite
charges attract.
This is the fundamental cause of almost ALL
electromagnetic behavior.
But how much?
How Strong is the Electric Force between
two charges?
ELECTROSTATIC – ELECTRIC - COULOMB FORCE
The force between two point charges is proportional to
the product of the amount of the charge on each one,
and inversely proportional to the square of the distance
between them.
q1q2
F k 2
r
k  8.99  109 N  m 2 / C 2
Force is a vector, therefore it
must always have a direction.
SHE accumulates a charge q1 of 2.0 x 10-5 C
(sliding out of the seat of a car). HE has
accumulated a charge q2 of -8.0 x 10-5 C
while waiting in the wind.
What is the force between them a) when she opens the door 6.0 m
from him and b) when their separation is reduced by a factor of 0.5?
2.0  105 C
a) q qThey
equal forces on each other only in opposite direction
 0.40exert
N
F k
1 2
r2
b) r’ = 0.5 r
q1 q 2
F  k 2  0.40N
r
(“-“ = attractive force)
q1 q 2
F '  k 2  1.6 N  4 F
r'
At very small separation - spark
How many electrons is 2.0 x 10-5 C ?
2.0 10 5 C
 1014 electrons
1.6 10 19 C
When you comb your hair with a plastic comb,
some electrons from your hair can jump onto it
making it negatively charged.
Your body contains more than 1028 electrons.
Suppose that you could borrow all the electrons from a friend’s
body and put them into your pocket. The mass of electrons would
be about 10 grams (a small sweet). With no electrons your friend
would have a huge positive charge. You, on the other hand, would
have a huge negative charge in your pocket.
If you stood 10 m from your friend the attractive force would be
equal to the force that 1023 tons would exert sitting on your
shoulders – more 100,000 times greater than the gravitational
force between the earth and the Sun. Luckily only smaller charge
imbalances occur, so huge electrical forces like the one described
simply do not occur.
Three point charges : q1= +8.00 mC; q2= -5.00 mC and q3= +5.00 mC.
(a) Determine the net force (magnitude and direction) exerted on q1 by
the other two charges.
(b) If q1 had a mass of 1.50 g and it were free to move, what would be
its acceleration?
Force diagram
1.30 m
230
q1
q2
F3
230
1.30 m
F k
2
q3
qq
1 2
r2
 0.213N
F3  k
qq
q1
1 3
r2
F2
 0.213N
Force
diagram
F
F3
q1
F2
x-components will cancel,
because of the symmetry
F  Fy  F2 sin 230  F3 sin 230
F  0.213sin 230  0.213 sin 230
ma = F
.
2
a  0166
m
/
s
15
. 103
F = 0.166 N
a = 111 m/s2
in y - direction
electric force is very-very strong force, and resulting
acceleration can be huge
A positive and negative charge with equal magnitude are
connected by a rigid rod, and placed near a large negative
charge. In which direction is the net force on the two
connected charges?
1) Left
2) Zero
3) Right
Positive charge is attracted (force to left)
Negative charge is repelled (force to right)
Positive charge is closer so force to left is larger.
-
+
-
• Calculate force on +2mC charge
due to other two charges
– Calculate force from
+7mC charge
– Calculate force from
–3.5mC charge
– Add (VECTORS!)
Three Charges
F7
Q=+2.0mC
F3
(9 109 )(2 106 )(7 106 )
F7 
N
25
F7  5 103 N
(9 109 )(2 106 )(3.5 106 )
F3 
N
25
3
F3  2.5 10 N
4m
kq1q2
F 2
r
Q=+7.0mC
6m
Q=-3.5 mC
Adding Vectors F7+F3
•Decompose into x and y components.
F7x = F7 cos q = F7(3/5) = 3x10-3 N
F7y = F7 sin q = F7(4/5) = 4x10-3 N
F3x = F3 cos q = F3(3/5) = 1.5x10-3 N
F3
Fx = 3  10-3 N + 1.5  10-3 N
Fy = 4  10-3 N – 2.0  10-3 N
Fx = 4.510-3 N
6m
Fy = 2.010-3 N
Q=+7.0mC
F  Fx2  Fy2  4.9  10 3 N
F
Q=+2.0mC
4m
F3y = F3 sin q = F3(4/5) = -2x10-3 N
F7
Q=-3.5 mC
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