1. Electrostatics – Forces, Fields, and Energy
Unit 8

2. Electric field – a field of force surrounding a charged particle
Electric force – an attractive or repulsive electrostatic force described by Coulomb's law
Coulomb's law – an equation describing the electrostatic force between electric charges
Electromagnetic force – force associated with electricity and magnetism
Magnitude – an amount, a quantity, or a size
Conductors – material through which heat and electrical charges can be transferred
Insulators – material that is a poor conductor of heat and electrical charges
Law of conservation of energy – the fundamental principle of physics that the total energy of an isolated system is constant, despite internal changes

3. Opposite charges attract
What is our model of matter?
Electrical Charge – Charge is a fundamental property of matter, like mass.
Objects are charged either positive, negative, or neutral.
Charges will move due to these “Electrical Forces”
The unit for electrical charge: The “coulomb”. It represents how strong a charge is.
Electrical Forces are very strong if 1 negative couloumb were 1 meter away from 1 positive coulomb the force would be 9 billion Newtons!
These forces are so strong because they have to be! They are the forces that hold together molecules!
Like charges repel
Opposite charges attract _ + + +
ATOMIC MODEL OF MATTER
Electrical charges work like magnetic poles.
Attraction and repulsion of electrical forces causes electrons to move!
Protons – Positive in the nucleus. POSITIVE charges are represented in RED.
ELECTRON – Negative, orbits the nucleus. NEGATIVE charges are represented in BLUE
We always speak in terms of the electrons, NOT the protons moving. Protons CAN’T move because they are in the nucleus. +
When electrons move we get :
NEUTRON – Neutral, in the nucleus.
ELECTRICITY!

4. Net Charge – How to calculate it.
Conductors vs. Insulators
Net Charge – How to calculate it.
If it had one more electron, our sum would have been zero. (11-11 = 0) and the atom would have been neutral. (Net charge = 0)
Conductors
Insulators
Allow electricity to flow
Resist the flow of electricity +
Sodium Ion
Metal
Water
Plastic
Air
Wood
Glass
Sand
Dirt
Styrofoam
Conductors of electricity also conduct heat, aka thermal energy.
More electrons than protons and our atom has a negative ‘net charge’.
Have weak attraction to valence electrons
Have strong attraction to valence electrons
1. Count number of protons Subtract number of electrons The resulting sum is the net charge 1
We would now say, “This atom has a positive charge”, because it has one more proton than neutron.

6. He imagined electricity as an invisible sort of fluid
Ancient Greeks - discovered static electricity when they rubbed animal fur and amber together
Our word electron comes from the Greek word for amber which was elektron
Benjamin Franklin – “coined” the terms positive and negative in his investigation of electric charge
He imagined electricity as an invisible sort of fluid
Amber
Coulomb – Measured the force of attraction/repulsion between two charges
Faraday – Faraday proved that the electricity induced from the magnet, voltaic electricity produced by a battery, and static electricity are all the same.
Millikan – Measured the fundamental unit of charge to be e = x C.

13. Electric Fields
Source charge = the charge that is producing the magnetic field that we want to measure the strength of.
Variable for the magnitude (strength) of the electric field.
Units are Newtons per Coulomb (N/C).
This is the amount of force per charge
Test charge = the charge that is acted on by the source charge. We will test the electric field at its location.
E=F/q
Example Problem =
Calculate the magnitude of the electric field produced by the positive charge at the negative charge.
Postive charge = 1 C
Negative charge = -2 C
Distance between the two charges = .003m
Use coulomb’s law to find (F) the amount of force between these two charges.
Figure out what is the test charge and what is the source charge. Since we are calculating the magnitude of the electric field PRODUCED by the POSITIVE CHARGE, it is the source charge. This leaves the other charge to be the test charge. The test charge will go on the bottom of our E=F/q equation.
Plug the numbers into the E=F/q equation and solve.
The amount of charge (in coulombs) of the test charge.
Usually given to you in the problem.
The force between the test charge and the source charge. It could be a repulsive or an attractive force.
Might be given to you in the problem.
Might have to be found using Coulomb’s Law.
Electric Field Lines – The arrows above represent electrical field lines. The arrows always point towards negative charges and away from positive charges.

14. Important Electrostatic Instruments
Electrophorus
Important Electrostatic Instruments

15. Calculating the PE of an Electric Field – Just like Coulomb’s Law, but without squaring the distance
Source charge = the charge that is producing the magnetic field that we want to measure the strength of.
Coulomb’s Constant
Test charge = the charge that is acted on by the source charge. We will test the electric field at its location.
Variable for PE of an electric field
Value of charge #1
Example Problem =
Calculate the magnitude of the electric field produced by the positive charge at the negative charge.
Postive charge = 1 C
Negative charge = -2 C
Distance between the two charges = .003m
Use coulomb’s law to find (F) the amount of force between these two charges.
Figure out what is the test charge and what is the source charge. Since we are calculating the magnitude of the electric field PRODUCED by the POSITIVE CHARGE, it is the source charge. This leaves the other charge to be the test charge. The test charge will go on the bottom of our E=F/q equation.
Plug the numbers into the E=F/q equation and solve.
U=k(q1q2)/r
The distance between charges
Value of charge #2