Electrostatics

Proton has a charge of

+1.60 x 10-19 C

Electron has a charge of

-1.60 x 10-19 C

Conductors in Electrostatic Equilibrium

A conductor that is in electrostatic equilibrium has no net motion of its charges. We find the conductor has: Electric field is zero everywhere inside the conductor. Excess charge on an isolated conductor resides entirely on the conductor's outer surface. Electric field just outside a charged conductor is perpendicular to the conductor's surface.

What happens to excess charge just seconds later if a small amount of the negative charge is placed near an electrically neutral insulating material?

All of it remains at the given point.

Capacitance depends on...

Capacitance depends on the size of the plates and the distance between them. Area increases as the capacitance increases (direct). Distance between the plates decreases the capacitance increases (inverse).

Capcitance

Capacitance means the charge stored per volt. Capacitance : Symbol : C Unit : Farad (F) = C/V 1*F = 1 x 10-6F (micro Farad) 1nF = 1 x 10-9F (nano Farad) 1pF = 1 x 10-12F (pico Farad)

Capacitor

Capacitor - stores charge and energy that can be reclaimed when needed. Used to tune the frequency of radio receivers Eliminate sparking in auto ignition systems Storage in electronic flashes (cameras) Two parallel metal (conductors) plates separated by a distance. An insulator (dielectric) is usually between the two plates. A capacitor is charged with a the battery. Once charged, you can replace the battery with a light bulb and close the switch and the light bulb will light until all the charge is transferred. When the capacitor is fully charged, it has the same electrical potential difference as the battery.

Charge Separation by Polarization

Charge Separation by Polarization Objects can have charges moved within it and yet keep a net charge of zero. Electric dipole is neutral overall, but has equal, yet opposite, amounts of charge at both ends. When a charged object comes in contact with an insulator, the molecules on the insulator surface will shift the electrons around. These molecules have become polarized. Once the charged object is removed, the surface molecules return to their normal configuration.

Electrical Forces and Charges

Charges : Positive and negative Like charges repel Opposite charges attract Neutral objects are attracted to charged objects.

properties of electrical force

Charges exert forces on other charges over a distance. This electrical force is greater than the force of gravity (being the weakest of all forces). This electrical force can cause an object to accelerate! (F = ma)

Charging by conduction

Charging by Conduction A charged object touches a neutral object and there is a flow of charge during the short period of time the two are in contact.

Charging by Friction

Charging by Friction Electrification Electrons are rubbed off and transferred. Amount of charging depends on the materials. You become electrically charged when jumping on the trampoline, or walking across the carpet.

Charging by induction

Charging by Induction A charged object is brought close to a neutral object. The two objects do not touch. The neutral object is grounded (the electrons have a path to escape). The ground is removed while the charged object is kept close to the neutral object. Then the charged object is removed and the neutral object is charged.

conductors

Conductors - electrical charges can move freely. The valence electrons are loosely bound. Examples include metals and graphite.

conservation of charge

Conservation of Charge : The net charge of an isolated system remains constant.

E- electric field

E = Fe / q Symbol : E Vector quantity, magnitude & direction Units : N/C

electric field lines

Electric Field Lines are used to represent the electric field produced by a point charge. Point away from positive charges and point towards negative charges. Number of lines drawn is proportional to the magnitude of the charge. No two field lines from the same field can cross each other. Lines are vectors representing the direction and magnitude of the force applied by the charge on the positive test charge.

Coulomb's Law

Electric Force is : Inversely proportional to the square of the distance between the charges. Directly proportional to the product of the magnitude of the charges. Attractive force if the charges are opposite and repulsive force if the charges are the same.

electric field

Electric field (like the gravitational field) acts through space, producing a force even when there is no physical contact between the charges. We test the direction and strength of the electric field with a small positive test charge, q0.

electric charge

Electrical Charge Symbol : q

electroscope

Electroscope - a device that can measure the quantity and type of charge.

electrostatics

Electrostatics - study of electrical charges that are at rest.

Energy Stored in a Capacitor

Energy stored in a capacitor : W capacitor = ½ CV^2

Fe: electric field force

Fe : electric force between two point charges. Units of Newtons, N Force is a vector quantity : magnitude and direction. +Fe Repulsive Force (+q1)(+q2) = +F (-q1)(-q2) = +F -Fe Attractive Force (+q1)(-q2) = -F (-q1)(+q2) = -F +Fe or -Fe does not tell direction!

Coulomb's Law Formula

Fe = Kq,q / d^2

Grounding

Grounding - a path to allow excess electrons to flow to the earth.

insulators

Insulators - electrical charges can not move freely. The valence electrons are tightly bound. The outside of an insulator can be charged, but the charges are not transferred throughout the material. Examples include glass, rubber, silk, and plastic.

What happens to (a capacitor's) capacitance if you double the area AND distance between the two circular plates in a circuit?

It stays the same. Remember a capacitor just stores the charge it is provided!

What happens to excess charge just seconds later if a small amount of the negative charge is placed near a neutral hollow metal sphere?

It will distribute evenly over the outside of the sphere.

What happens to the force between two positively charges particles if the NET charge is +1 together and some distance apart and a +4 charged particle replaces one and the distance becomes three times greater between them?

It will still be four times the force, but divided by 9 since the distance is squared.

If you have a light in a series circuit and close the switch without a capacitor to store the charge, what happens to the light?

It would be bright, then eventually grow dimmer until it ran out.

Millikan's Oil Drop Experiment

Millikan could measure the charge of an electron (-1.6 x 10-19C). He determined that objects had a multiple of -1.6 x 10-19 C of charge. The electron is a basic unit of charge so charge is quantized. You could only change the charge of an object by adding or removing electrons.

What happens to the motion of a positively charged particle that is released from rest in a UNIFORM electric field?

Moves at a constant acceleration

Sharing Charges and Voltage

Objects tend to minimize their energy. If a charged object touches another object that is identical, the charges will distribute themselves between the two objects. Both objects will have the same charge (q) and the same Voltage (charges have equal distances). If two objects touch and are different in size, the charges will also spread between them. The large sphere has more charge than the small sphere, but they both have the same voltage. This is why the earth (a very large sphere) will allow excess electrons to flow to it.

irregularly-shaped conductors

On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature of the surface is smallest, that is, sharp points.

Which type of circuit will have the greatest total current delivered by a battery in general?

One in parallel only, because voltage is the same across all resistors.

What happens to brightness if you unscrew or remove a bulb?

One would dim and the other would be brighter depending on the combined circuit arrangement.

Three charges form a triangle in attraction. One positive charge (left) and one negative charge (right) have the same value in magnitude (just opposite charges) and form the bottom of the triangle. What will be the direction of the force on the positive charge at the top of the triangle? Sketch the picture if it helps.

Right, because opposites attract. It will repel the other positive charge.

electric charge unit

SI unit of electric charge is Coulomb, C

semi-conductors

Semiconductors - in their pure state are insulators. When specific atoms are added as impurities, the compound can conduct electric charge. Silicon and germanium.

super-conductors

Superconductors - perfect conductors when they are below a certain temperature (very, very cold).

Electrical Potential Difference (aka voltage)

Symbol : V Unit : Volts = J/C 1 Coulomb of charge passes through a potential difference of 1V gains 1 Joule of energy. Measure the Electrical Potential Difference (aka voltage) with a Volt Meter. It compares the PE of the charges in two different locations.

q0 (test charge)

Test charge symbol : q0 Units : C, coulombs

How would a circuit look if it contained a voltmeter and an ammeter to measure both the current and voltage across a resistor?

The ammeter should be placed before the resistor and the voltmeter after the resistor in its path.

Which bulbs tend be brighter in a combined (series and parallel) circuit?

The one in series. The parallel will have equal brightness, but dimmer than the series bulb.

4 methods of charging an object

There are 4 methods of charging an object: Friction Conduction Induction Separation by Polarization

How do you describe the motion of an electron with an electric potential of +8V?

There is not enough information to predict its motion.

Electrical Potential Energy

To move a positive test charge through a uniform electric field, you must do work. This work done gives the positive test charge potential energy. PE Electric = q0 E d

Two types of charges

Two types of charges : Positive (cations) : protons Negative (anions) : electrons

Electric Potential Difference (V)

V = PE electric/ q0 V = Ed A charge gains the electrical potential energy (difference) when it is moved a distance through an electric field.

Work and Voltage

W = qV Do work to move a charge through an electrical potential difference!

d (distance)

d : the distance between the two point charges Units of meters, m

What happens to the force between two positively charges particles if the NET charge is +1 together and some distance apart and a +4 charged particle replaces one?

it increases to four times the force

k : Coulombs constant

k : Coulombs constant 9.0 x 10 ^9 NC^2/M^2

q (charge)

q1 and q2 are two particles that are spherically symmetric and are considered point charges. The force of q1 on q2 is equal but opposite in direction as the force of q2 on q1. Units of coulombs, C Micro coulomb = C = 10-6C