Download Goal: To understand what Electric Fields are

Goal: To understand
Electrostatics
Objectives:
1) Understanding what charges are.
2) Knowing how to produce a charge.
3) How to calculate an electric field
4) Electric Force
5) Electric Potential
6) Other random stuff
What is charge?
• For the most part, charge is a measure of
how many protons or electrons you have
somewhere.
• Charge is measured in units of Coulombs
(C).
• An elementary charge from a proton or
electron has magnitude of 1.602 * 10-19 C.
• Like charges repel. Opposite attract.
• Charges can move.
How do you get charge?
• 1) Rubbing (static electricity)
• 2) Induction (charge obtained from a
changing magnetic field)
• 3) Conduction (moving charge along a
wire)
Electric Field
• Suppose you wanted to know where the
water would flow when it rains.
• How would you do that?
Fields
• Fields are just a listing of possible
potential at any given point.
• For rain you look at the Gravitational Field
– which is just a fancy way of saying the
topography.
• Water will want to flow downwards.
• We can do the same with electric fields.
Electric “Field”
• The Electric Field is just a measure of the
electric topography.
• Since protons repel each other you can think of
the protons as hills.
• The electrons would be pits or valleys.
• The elevation of some point near some charges
would depend on the distribution of charges
(much like your elevation depends on where you
are compared to the hills and valleys).
• Units are in N / C.
Calculating the Electric Field
• First lets do it for just one charge.
• For one charge the equation is pretty
straightforward:
• E = -k * charge / (distance * distance)
• k is a constant, and is 9 * 109 N m2 / C2
• There is another equation for the electric
field we will use later.
Force translation
• the Electric field is a topography of electric
charges around you.
• At any point the electric field is just a sum
of the topography from each charge.
• For each charge E = -qk / r2
• How would this translate to a force?
Ball downhill
• If you have a gravitational topography a ball will
want to roll downhill.
• That is it will roll from a high elevation to a low
one or a high field to a low one.
• The same is true of electric fields.
• A positive charge will want to move to a lower
electric field.
• A negative charge will do the opposite and will
want to move up to a higher valued electric field
(moving uphill).
Now for the math
• The force on a charge is:
• F=q*E
• Where q is the charge the force is being
applied to and E is the electric field that
charge q is located at.
• Much like for gravity that F = m * g on the
surface of the earth.
If we add in E
• If we have 2 charges called q1 and q2 then the
force is:
• F = q1 * E, but E = -q2 k / r2
• So, F = -q1 * q2 * k / r2
• (k is the same constant we had before)
• Notice this is almost the same as the
gravitational force – where:
• F = -G * M1 * M2 / r2
Which force is stronger?
• During the next 5 min break think about
which of the following is a stronger force?
• Electric force: F = -q1 * q2 * k / r2
• Or gravity: F = -G * M1 * M2 / r2
Electric Potential Energy
• Electric Potential energy is similar to
gravitational potential. It is the potential
energy between any two charges.
• Energy is a force times a distance, so if
you multiply the Electric Force times a
distance (I am oversimplifying a little here)
you get the Electric Potential.
• U = k q1 q2 / r (and is in units of Joules)
• Note how similar it looks to the equation
for the force.
Direction?
• Other than calculating distances between
charges will the directions matter?
• Why or why not?
• NO! Potential energies are SCALER
values!
Insulators
• Insulators (like insulation for the house) is
a material that won’t let charges move
through it.
• Wood would be an insulator…
• What is a real world use for an insulator?
Conductor
• Materials that help the flow of charges are
conductors.
• Ideally, you want to make a wire out of a
good conductive material.
What about in between
• There are materials that can act as both
conductors and inductors.
• These are called semi-conductors.
• These form vital electronic parts such as
transistors (which revolutionized the
electronics industry)
Super strength conductors
• The only draw back to conductors is that
they heat up (and that means you are
loosing power).
• If you send power across a long power
line, you loose energy, and that looses you
MONEY!
• But what if you could build a conductor
that didn’t give any resistance to the flow
of charge?
• You would have a superconductor
Superconductors
• Typically superconductors are made of
materials that when cooled to VERY low
temperatures (-300 to – 400 F) they allow
charges to flow freely.
• There is a lot of uses for this, but it is not
very practical unless you can build them to
operate at temperatures that are much
closer to room temperature.
Saving charges
• Collecting charges requires a device called a
capacitor.
• This is usually a pair of sheets separated by a
small distance on which you store charge.
• The negative electrons collect on one sheet.
• One the opposing sheet the electrons are
repelled which can then flow through the rest of
the circuit – leaving only a positive charge.
Flood!
• However, there is a limit.
• If you try to collect too much charge, you
get a discharge.
• The electrons flow over like water flowing
over a filled to capacity dam.
Dielectrics
• As you read in the book (hopefully) – you can
shield charges.
• One way is by filling the center of the capacitor
with a substance.
• This substance is a dielectric.
• The type of material will determine what fraction
of the charge the other side actually sees.
• Water, for example, allows you to collect 80
times more charge than air.
Conclusion
• 1) We learned how to find the Electric
Field and electric force
• 2) We have found how to do the electric
potential.
• We have seen applications for
electrostatics such as conductors and
inductors.
• We have seen how to store charge.