Pre AP Physics

Electrostatics and Electric Fields

Starting the electricity portion of physics

Will first look at electrostatics (electricity at rest)

We have looked at force of gravity which attracts us to the Earth & we call it force of weight.

There is a force that is acting on you that is billions and billions times stronger – so strong it could crush you to the size of about the thickness of a piece of paper, but we aren’t crushed because there is a repelling force that is equally as strong.

The 2 forces act to balance each other so that there is no noticeable effect, even though they act on us all the time. They are electrical forces which come from particles in atoms.

Remember from Chem – basic structure of the atom

Nucleus contains protons (+) & neutrons (0)

Electrons (-) travel (orbit) around the nucleus

Protons in nucleus attract negative electrons to help hold in orbit

Protons & electrons both have a charge (property that describes attracting and repelling behavior of a particle)

Remember: like charges repel, opposite charges attract

Have you ever shocked someone when you touched them on a non-humid day?

Charges were involved

How do objects become charged?

(keep remembering chem class last year J) The valence (outermost) electrons are able to transfer to other atoms & if an electron decides to leave for another atom, you end up with an electric charge

any object with unequal numbers of e- & p+ is electrically charged

if an atom has more e- than p+ = negative electric charge

if an atom has less e- than p+ = positive electric charge

example: if rub balloon on hair (have student do this J)

hair’s e- are transferred to the balloon, which leaves balloon negatively charged

and hair positively charged

amount of negative charge on balloon = amount of positive charge on hair due to conservation of charge (no creating or destroying net electric charge – just a transfer of the charges (electrons))

Another example: dragging your feet on the carpet so you can shock someone

You gain electrons and become more negatively charged & carpet is left positively charged

Types of Materials involved in electricity/electrostatics

Conductors:

Materials in which electrons can move freely (have more freedom to roam than in other materials) which allows charge to move through the material easily

Also good conductors of heat

Most metals are good conductors

Insulators:

Materials in which electrons are tightly bound and have little to no freedom to move which causes charge to not flow easily

Also NOT good conductors of heat

Examples: rubber, glass, plastic, foam

Semiconductors:

Materials that fall between conductors and insulators

In pure state, act as insulators

If introduce impurity, can increase ability to conduct significantly

Examples: germanium, silicon à used in electronic devices, layered to form transistors

Superconductors:

At certain temperatures are perfect conductors which allow the electrons to flow indefinitely

For some metals this occurs at absolute zero (0K or -273 oC)

Examples: tin, aluminum, metallic alloys

Ways to charge objects (give objects a charge)

by friction

by contact

by induction

by polarization

Charging by friction:

Electrons are transferred by friction when one material rubs against another

Examples: stroke cat’s fur and hear crackle of sparks

Comb your hair on dry day and hear/see sparks of electricity

Feel shock when touch doorknob

Fleece blankets

Playground slides

Charging by contact:

Electrons are directly transferred from one object to another through touching one object to the second

A charged rod contacts a neutral object

If neutral object is a good conductor, the extra charge will spread out throughout the surface of the object since like charges repel each other

If neutral object is a poor conductor, may have to touch the rod in several places on the object in order to get a more uniform distribution of charge

Charging by induction:

Electrons move and charge is produced, but no contact occurs

Inducing a charge in two conducting spheres:

Inducing a charge in a single object through a grounding wire:

bring negatively charged object near neutral object

electrons are repelled and able to leave through conducting wire

if remove wire while charged object is still near, 2nd object keeps net + charge

If remove charged object before remove wire, electrons will return to the object and will be neutral again

“grounding” something allows the object to have a conducting path to an infinite reservoir for electric charge – the Earth

This is why lightning rods are on buildings

Lightning occurs because there is an electrical discharge between oppositely charged parts of the clouds

Negatively charged bottom of the clouds induce a positive charge on the ground/surface of the earth

Charging by polarization:

Similar to charging by induction in conductors, but polarization occurs only in an insulator where the electrons are not as free to move

Instead of free electrons moving, the molecules within the object rearrange so that all positive ends are pointing one direction and negative ends are pointing opposite direction

Once object is polarized, still has no net charge (is neutral), but can attract/repel objects due to the realignment of charge within the object

Ex. Charged comb picks up pieces of paper, charged balloon by stream of water can attract water

Coulomb’s Law

Remember Newton’s Law of Universal Gravitiation?

Fgravitiational = Gm1m2

Similarly, the electrical force between two charges obeys a similar relationship as discovered by French physicist Charles Coulomb

F = kQ1Q2

k = Coulomb’s constant: 9 x 109 Nm2/C2

Q1 = quantity of charge of 1 particle (size of electric charge)

Q2 = quantity of charge of 2nd particle (size of electric charge)

r = distance between the two particles

F inversely related to r2

F directly proportional to the charges

Units for Coulomb’s Law:

Q = Coulombs (C)

r = meters (m)

k = Nm2/C2

The “size” of charge for an electron and proton are equal in magnitude but opposite in charge (1.6 x 10-19 C) ß need to know!

Gravitational force and electrical force are similar in that they are both inverse square laws (F inversely related to square of distance)

The two forces differ in that electrical force is MUCH stronger than gravitational force (see problem #1 on WS) and gravitational force is only an attractive force, while electrical force can be attractive or repulsive

[Problems #1-5]

Equilibrium:

Occurs where the net electric force on a charge is zero

Have to find the position where a third charge can be placed so that the electric force from charge 1 is equal and opposite to the electric force from charge 2

[Problems #6-9]

Electric Field:

Michael Faraday, a British scientist came up with the idea of a field to help look at situations involving a force acting over a distance

An electric field fills the space around every electric charge

The force that one electric charge exerts on another electric charge is actually the interaction between one charge and the other’s electric field à there is no direct contact between the two charges

Electric fields are vectors à have both magnitude and direction

The magnitude (electric field strength) is measured by its effects on charge located within the field

Can measure the magnitude by placing a small positive test charge in an electric field

- Has to be small enough charge that does not alter original charge

- Where force on test charge is greatest, field is strongest

- Where force on test charge is weakest, field is weakest

- Measured in N/C (Newtons per Coulomb)

The direction of the electric field is the direction of the electric force exerted on the small positive test charge by the charged particle producing the electric field

If particle producing the field is positive à electric field points away from the particle

If particle producing the field is negative à electric field points toward the particle

To find electric field strength: E = kQ

Electric field strength depends on two things:

1. Charge creating the field

2. Distance from object to a specific point in space

Electric field lines:

Useful way to describe an electric field

Also called lines of force

When electric field is strong à lines are closer together

When electric field is weak à lines are farther apart

Rules for drawing electric field lines:

1. Lines must begin on positive charge and end on negative charge

2. Number of lines drawn leaving or entering a charge is proportional to magnitude of the charge

3. No two field lines from the same field can cross each other

Electric dipole is formed when two equal charges of opposite sign create an electric field

high density of lines in between 2 charges à strong electric field

Direction of electric field at any point is tangent to the field line at that point (shown above)

Field lines for two equal positive charges

Field lines for two unequal opposite charges

look at proportion of electric field lines as compared to magnitude of charge

Field lines between two parallel plates carrying equal but opposite charge

Conductor with one end more pointed

The charge gathers more densely at the sharp point and the electric field is stronger there