5.1 Electric Forces & Fields

Chapter 18

The Origins of Electricity

• In the mid 18th century Ben Franklin created the idea of positive and negative electric charge.

• It wasn’t until 150 years later the electron was discovered.

• Franklin described an electric “fluid” that would flow depending on electric pressure.

Electric Charge

• In 1909 Robert Millikan discovered charge was “quantized.”

• This means there is a smallest amount.• Thing about it like this, In order to have a

car, all the pieces must be there. If you try to take the engine out, it’s no longer a car.

• Electric charge is much the same.• The electron has a set charge, take some

away and it’s no longer an electron.

The Millikan Experiment

• Click here to recreate the Millikan oil-drop experiment

Charged Particles

• Protons (+e): Mass = 1.673 x 10-27 kg, Charge = 1.60 x 10-19 C

• Neutron: Mass = 1.675 x 10-27 kg, Charge = 0

• Electron (-e): Mass = 9.11 x 10-31 kg, Charge = -1.60 x 10-19 C

e = 1.60 x 10-19 C

Neutral Objects

• If the number of electrons equals the number of protons the object is said to be electrically neutral.

• In general q (charge) = Ne, where N is an integer.

• Since protons are much more difficult to remove, most objects are charged by removing or adding electrons.

Charged Objects

• When two dissimilar materials are rubbed together, electrons usually go from one to the other.

• Look on the triboelectric scale to see which way they go, positive or negative

• Also, charge is conserved.

• The net charge of an isolated system is constant.

Like and Unlike charges

• Like charges repel each other

• Unlike (opposite) charges attract

Conductors & Insulators

• Materials that have loose valence electrons are conductors.

• Materials with tightly held valence electrons are insulators.

• Can you think of some?

Charging by Induction (Conductors)

Click on the picture to open an applet

Example Question

• Two separated, identical conducting spheres are charged with 4 C and -12 C, respectively. If the spheres are allowed to touch and then separated again, what will be the charge on each sphere?

Answer:• The net charge is -8 C. So each sphere

will have -4 C of charge.

Polarization (Insulators)

Electroscope

Van De Graff Generator

Coulomb’s Law

• F = 1/(4o) q1q2 / r2

– F = Force (N)

– o = 8.85 x 10-12 (electric permittivity of a vacuum)

– q = charge (Coulombs)– r = distance between charges

• 1/(4o) = k

Point Charges

• When more than two charges are acting on each other we sum the forces.

• Treat each pair independently, then add the forces.

q1 q2 q3

r1 r2

Point Charges in 2D

• When more than two charges are acting on each other in 2D, sum the forces for x and y dimensions.

• Again, treat each pair independently, then add the forces.

q1 q2

q3

r1r 2

Electric Field

• Just like mass creates gravitational fields, charges create electric fields

• With gravity the field strength is measure as Newton per kilogram

• What do you think Electric fields are measured in?

Newton's per Coulomb

Measuring the Electric Field

• If the unit is Newton's per Coulomb, what is the equation?

E = F / qo = k q/ r2

• Simple enough, right.

• E = Electric Field

• F = Force

• qo = charge producing field

Summing electric Fields

• It is the surrounding charges that create an electric field at a given point in space.

• Look at Example 8 – Figure 18.18

• Two charged objects contribute as follows to the net electric at point P: Ea = 3.00 N/C directed to the right, and Eb = 2.00 N/C directed downward. What is the net electric field at P?

Solution

• We use the electric field vectors to determine the resultant.

• Add the vectors with the Pythagorean theorem.

• And find the angle with arctan.

• E = 3.61 N/C @ 33.7o

EA

EB

E

+

++

Electric Field Rules

• Fields start at positive and end at negative, or start or end at infinity.

• This is by convention. The field is said to predict the movement of a positive charge.

• The density of lines should represent the strength of the field.

• A positive charge will have a velocity tangent to a field line.

• Field lines do not actually exist since the are an infinite number of paths a test charge can take.

Picturing the Electric Field

Click on the picture to open an applet

Electric Fields Inside Conductors

• Excess electric charge moves to the surface of a conductor

• At equilibrium the electric field inside a conductor is zero– This comes from the fact that free

electrons will not move inside the conductor.

• So the electric field lines don’t penetrate the conductor

• The electric field outside a conductor is perpendicular to the surface

Click here for more info

Triboelectric Scale• Human hands (usually too

moist, though) (Very positive)• Rabbit Fur• Glass• Human hair• Nylon• Wool• Fur• Lead• Silk• Aluminum• Paper• Cotton• Steel (Neutral)

• Wood• Amber• Hard rubber• Nickel, Copper• Brass, Silver• Gold, Platinum• Polyester• Styrene (Styrofoam)• Saran Wrap• Polyurethane• Polyethylene (like Scotch

Tape)• Polypropylene• Vinyl (PVC)• Silicon• Teflon (Very negative )