Electrostatics – Forces, Fields, and Energy

Unit 8

• 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

• 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.

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Like charges repel

+ _

Opposite charges attract

ATOMIC MODEL OF MATTERElectrical charges work like magnetic poles.

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!

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ELECTRON – Negative, orbits the nucleus. NEGATIVE charges are represented in BLUE

Protons – Positive in the nucleus. POSITIVE charges are represented in RED.

NEUTRON – Neutral, in the nucleus.

Attraction and repulsion of electrical forces causes electrons to move!

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 :

ELECTRICITY!

Net Charge – How to calculate it.

1. Count number of protons 112. Subtract number of electrons -103. The resulting sum is the net charge 1

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+

++

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+ ++

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We would now say, “This atom has a positive charge”, because it has one more proton than neutron.

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)

More electrons than protons and our atom has a negative ‘net charge’.

Conductors vs. Insulators

Sodium Ion

Conductors of electricity also conduct heat, aka thermal energy.

MetalWater

Conductors

PlasticAirWoodGlassSandDirtStyrofoam

Insulators

Allow electricity to flow

Resist the flow of electricity

Have weak attraction to valence electrons

Have strong attraction to valence electrons

Amber

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

Coulomb – Measured the force of attraction/repulsion between two chargesFaraday – 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 = 1.602 x 10-19 C.

Electric FieldsSource charge = the charge that is producing the magnetic field that we want to measure the strength of.

Test charge = the charge that is acted on by the source charge. We will test the electric field at its location.

E=F/q

Variable for the magnitude (strength) of the electric field.

Units are Newtons per Coulomb (N/C).This is the amount of force per charge

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.

The amount of charge (in coulombs) of the test charge.

Usually given to you in the problem.

Example Problem =Calculate the magnitude of the electric

field produced by the positive charge at the negative charge.

Postive charge = 1 CNegative charge = -2 CDistance between the two charges

= .003m

1. Use coulomb’s law to find (F) the amount of force between these two charges.

2. 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.

3. Plug the numbers into the E=F/q equation and solve.

Electric Field Lines – The arrows above represent electrical field lines. The arrows always point towards negative charges and away from positive charges.

Electrophorus

Important Electrostatic Instruments

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.

Test charge = the charge that is acted on by the source charge. We will test the electric field at its location.

U=k(q1q2)/r

Variable for PE of an electric field

Value of charge #2

The distance between charges

Example Problem =Calculate the magnitude of the electric

field produced by the positive charge at the negative charge.

Postive charge = 1 CNegative charge = -2 CDistance between the two charges

= .003m

1. Use coulomb’s law to find (F) the amount of force between these two charges.

2. 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.

3. Plug the numbers into the E=F/q equation and solve.

Value of charge #1

Coulomb’s Constant