UNIT 3: ELECTRICITY
Concerns charges (we don’t really know what they are, but we can describe their behaviour very well).
Static Electricity:
charges do not move
Current Electricity: negative charges move around
Conductors and Insulators
Substances can be classified by their ability to let charges move:
Insulators:
charges cannot easily move through these substances
e.g.: rubber, glass, plastic
Conductors:
charges can move easily through these
e.g.: metals, graphite, earth (allow electrons to move only)
salt solutions (allow both positive and negative ions to more)
Semiconductors:
charges can move through these, but not very well
e.g.: people, silicon
Protons (positive charges) do NOT move in most substances.
This is because protons are locked into the nucleus while electrons orbit around it.
Static electricity can only exist in insulators, or in conductors that are not connected to any other
conductors.
Charges
All matter is made of atoms: protons, neutrons, electrons.
protons:
+ charge, locked in to the nucleus
electrons:
- charge, in electron shells, easy to remove
Therefore, there are two types of charge: + and All objects have both types of charge
Charged Objects
Objects can be charged or neutral:
neutral objects
positive objects
negative objects
# of p+ = # of e# of p+ > # of e# of p+ < # of e-
So, even neutral objects are full of charges.
All objects, even if they are charged, contain both positive and negative charges.
positive
neutral
negative unnatural
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The Static Electricity Lab
What did we learn in the lab about charged objects?
1) Objects can be made charged by rubbing objects made from two different substances together.
2) Objects with like charges repel each other.
3) Objects with opposite charges attract each other.
4) Charged objects attract uncharged objects.
Can we explain these observations?
1) We know from the chemistry unit that electrons are easy to remove from or add to atoms.
We also know from the chemistry unit that different substances have different strengths of attraction
for electrons.
Therefore, based on what we learned in chemistry, when two objects are rubbed together, the object
made from the substance with the weaker attraction for electrons loses electrons to the object made
from the substance with the stronger attraction for electrons.
Charging By Friction
Points to remember:
a) only electrons move
b) electrons move from the weaker object to the stronger object
c) the weaker object becomes positive
d) the stronger object becomes negative
e) rubbing provides the energy needed to remove the electrons from the weaker object
Strengths of Attraction for Electrons
The following table can be used to determine which of two objects will have the stronger attraction for
electrons.
Weakest Attraction
If two substances are rubbed, the one with the
Human Skin
strongest attraction takes electrons (and gets
Rabbit Fur
negative) from the weaker (which gets positive).
Acetate
Glass
Quartz
Human Hair
Nylon
Wool
Cat Fur
Silk
Aluminum
Paper
Cotton
Amber
Hard Rubber
Copper
Silver
Gold
Polyester
Styrofoam
Plastic Wrap
PVC plastic
Silicon
Teflon
Example 1:
human hair is rubbed with silk
the hair becomes positive,
the silk becomes negative
Example 2:
a plastic pen is rubbed with silk
the silk becomes positive
the plastic becomes negative
Strongest Attraction
2) Like charges repel: we can’t explain this (at our level), it is just the way the universe works.
3) Opposite charges attract: we can’t explain this (at our level), it is just the way the universe works.
4) We will be able to explain why charged objects attract neutral ones, once we learn a few more things.
Charging and Discharging Objects
Charging:
creating an imbalance in the number of p+ and e- in an object
occurs by e- moving from one object to another
only electrons move (PDM: protons don’t move!)
e- move between objects, they never exist by themselves
Discharging: restoring the balance in the number of p+ and e- in an object
occurs by e- moving from one object to another
Charging
1) Friction:
2) Contact:
2 positive objects rub together
both start out neutral
one becomes negative, the other positive
e- move from weak object to strong object
2 objects touch (or get very, very close)
at least one object is charged
objects start out with different charges
they both end up with the same charge
e- move from the most negative to most positive object
before
after
Discharging
1) Grounding:
make electrical contact between a charged object and the earth
The earth is very, very large (6x1024 kilograms)
The earth has so many p+ and e- that it can lose or gain billions with out becoming significantly charged.
Therefore, the earth is:
a good source for electrons to neutralize a positive object
a good sink for extra electrons to neutralize a negative object
Therefore, to discharge a charged object, connect it with a conductor to the earth.
Plumbing usually provides a good conductive connection to the earth.
Symbol for Grounding:
When a positive object is grounded, electrons enter it from the earth
When a negative object is grounded, electrons leave it into the earth
Insulator Breakdown: Sparks and Lightning
When an insulator, such as air, separates two very strongly charged objects, the
strong charges can literally pull the molecules of the insulator apart into positive
and negative ions. This makes the insulator become a conductor. The channel of
ions in the insulator allows charges to move between the two objects. As this
happens, energy is released. This creates light, heat, and sound.
We call this a spark or lightning.
2) Discharging by Humidity
Water molecules are very good at transporting charges.
If the air is humid, the water molecules in the air can:
- carry electrons away from negatively charged objects and give them to the earth
- carry electrons from the earth to positively charged objects
Behaviour of Excess Charges in Conductors and Insulators
Insulators
Excess charges cannot move and stay were they were first put.
This is why the balloon in the lab could only stick to the wall on the side where it was charged.
Conductors
Excess charges can move, and since electrons are repelled from each other, they push as
far away from each other as they can.
They always end up around the surface of the object.
Another method for discharging.
3) Discharging from a Point
Smooth surfaces: excess charges can only push parallel to the surface
(because they are all at the surface) and, so, they stay in place.
Pointy surfaces: the charges at the tip of the point can be pushed off the tip
by the charges behind them.
Therefore, any charged conductor that has sharp points will gradually lose
its excess charges off the tip of the point.
Another method for charging.
3) Charging by Induction
If a charged object is brought close to a neutral conductor, the charged object will cause electrons in the
neutral object to move from one side to the other.
This results in the neutral object having a positive side and a negative side, even though it is still neutral
overall. The side nearest the charged object is oppositely charged compared to that object.
This is why charged objects can attract neutral ones.
They make the near side of a neutral object oppositely charged
Permanent Charging By Induction
step 1) ground a neutral conductive object
step 2) bring a charged object near the grounded object.
the charged object will cause electrons to either
- leave the object into the earth
- get pulled out of the earth into the object
step 3) break the grounding connection
this prevents the excess charges from returning to the earth
step 4) the conductive object is now permanently charged
the charge is opposite the charge of the object that was
brought near
the e- that came from the earth
are now trapped since the
ground connection is broken.
Detecting Static Electric Charge
We can use inductive charging to detect the presence of static charges on objects.
The simplest example of this is when you walk past a strongly charged object. Your electrons are pushed
to one side of you body as you get near the strong charge, and this makes your hair stand on end.
The Electroscope
An electroscope is a pair of metal foil leaves connected to a large metal ball by a
metal rod. The leaves are inside a glass flask to keep them from being affected
by the movement of air.
Normally, all of the metal parts are neutral, and the leaves hang vertically below
the metal rod.
When a charged object is brought near the top of the electroscope, electrons are
pulled up into the ball (if the object is positive) or down in to the leaves (if the
object is negative).
In either case, spreading of the leaves is an indication of a charged object.
An Electroscope
No charged
object is
nearby
Leaves
together
Positively charged object
brought close to top
Negatively charged object
brought close to top
e- attracted up into the top
leaves are both positively charged
e- pushed down into the leaves
leaves are both negatively charged
Leaves move apart.
Leaves move apart.
A neutral electroscope can detect the presence of charged objects.
They cannot be used to determine whether the charged object is positive or negative.
Determining the Charge of a Charged Object
- use a charged electroscope
e.g.: an electroscope with a permanent positive charge
the leaves stay spread apart, because they are positively charged
If a negative rod is brought near a positively charged electroscope:
- electrons from the top are pushed down into the leaves
- the leaves become less positive
- the leaves move closer together
If a positive rod is brought near a positively charged electroscope:
- electrons from the leaves are drawn up into the top
- the leaves become more positive
- the leaves move farther apart
We also could use a negatively charged electroscope. In that case, the leaves would move together in
response to a positive object and apart in response to a negative object.
We can tell the sign (“+” or “-”) of the charge of an object using a charged electroscope.
The direction that the leaves move tells us the sign of the charge of the object.
Charging an Electroscope
1) Charge by contact:
touch it with a charged object
(especially one that is coated with a conductive substance)
The electroscope will have the same charge as the object that touches it.
Electrons are moved from the more negative to less negative object.
2) Charge by induction:
bring a charged object near a grounded electroscope.
remove the grounding
remove the other charged object.
The electroscope will have the opposite charge of the object that is brought near.
e- will be drawn into the electroscope from the earth or pushed into the earth from the electroscope.