Electrostatics

Unit 11

Electric Charge

Symbol: qUnit: Coulomb (C)

Two kinds of Charge:PositiveNegative

Law of Electrostatics: Like charges REPEL- they don’t want to be anywhere near each other.Unlike charges ATTRACT- they want to be as close as possible.

Chemistry Review:

Fundamental Amount of Charge for 1 electron or 1 proton:

e = ± 1.6 x 10-19 C

q = n (e)

n = # of particlese = charge per particleq = total charge of object

Transfer of Charge

Electroscope: a device that detects the presence of electric charge.

Only ELECTRONS can be transferred from one object to another because protons and neutrons are fixed in the nucleus.1. Conductors: material that transfer electrons easily

ex: most metals2. Insulators: materials that don’t easily transfer electrons

ex: glass, plastic, rubber3. Semiconductors: in between the properties of conductors and insulator

ex: metalloids4. Superconductors: perfect conductors at or below a certain temperature.

Methods of ChargingCONDUCTION

Conduction: charging by CONTACT.• rubbing two objects together, the electrons from one are transferred to the

other• one object becomes negatively charged and one object becomes

positively charged.

Ex: Rubbing a balloon on your hair. • The balloon gains electrons from your hair-• the balloon becomes negatively charged, your hair becomes positively charged• Opposite charges attract!

Methods of ChargingINDUCTION

Grounded: when an object is connected to the earth by a wire, pipe, finger, etc so that electrons can transfer to or from the earth.Why connect to the Earth? The earth is so large that we say it can ACCEPT or DONATE an unlimited amount of electrons.

Steps:1. Start with a negatively charged rod and a neutral

metal sphere.2. A negatively charged rod brought near a neutral

metal sphere, the negative charges are repelled and redistribute themselves.**Part of the sphere closest to the rod has more

POSITIVE charges.3. Ground the sphere by connecting to the earth, the

electrons will flow into the earth leaving a NET POSITIVE CHARGE on the sphere.

4. Pull the rod away and the positive charges redistribute themselves.

INDUCTION

*** NO CONTACT WITH THE CHARGED ROD!

Methods of ChargingPOLARIZATION

Polarization: the redistribution of charge within a group of particles that produces an apparent charge on the surface of an object.

When Polarized- net charge = 0 * No charge has actually been transferred. * Charge is only REDISTRIBUTED.

Ex: A polar molecule- one side of the molecule is more positive, one side is more negative. I.e. Water Molecule

++++

+H2O

Coulomb’s Law

Electric Force- the force between any two charges (or charged objects)Attractive between opposite chargesRepulsive between like charges

Fe kq1q2

d2

Where:k = 9 x 109 Nm2/C2 Coulomb’s Constantq1 & q2 = two chargesd = distance between charges (always in METERS)

Relationship between Fe and q1q2 is DIRECT. • Double one charge and Fe will be doubled.• Double both charges (x4) and Fe will be quadrupled.

Relationship between Fe and distance is an Inverse Square Law.• Double the distance and Fe will be 1/4 as large.• Triple the distance and Fe will be 1/9 as large.

Electric Force

Fe always acts along a straight line between two charged objects.

Always draw the force ON the object you’re talking about.

Fe is a FIELD FORCE. Def: A force that acts on objects at a distance;

* NO CONTACT.Another Example: Gravitational Force (Fg)

Fe kq1q2

d2

Fg Gm1m2

d2 Differences: a. Fg is only attractive; Fe can be

attractive or repulsive

b. Fg is a relatively weak force; Fe is very strong.

+

-

q1

q2

F2,1

Superposition- Net Electric Force

If there are more than 2 charges, the net electric force on any charge is found by getting the vector sum of all electric forces. **Draw all forces directly on the charge that you are finding the net force on.

1-Dimension: forces in same direction are ADDED forces in the opposite direction are SUBTRACTED- (right - left)*if you draw it correctly, do not use negatives in coulomb’s law formula!*

+ + -

A B CFc,a

Fc,b

+ - -

A B CFc,a

Fc,b

Net force on Charge C:

Fc,a + Fc,b

Fc,b - Fc,a

Superposition- Net Electric Force

2-Dimensions:Add up all the x’s, add up all the y’s and get the RESULTANT.

+

--

A

B C

Fb,a

Fb,c

Net force on Charge B:

Fnet x 2 y 2

tan 1 y

x

Fnet Fb,c2 Fb,a

2

Electric Fields

Def: a region in space around any charged object in which another charged object will experience an electric force.

E Feq0

Units: N/C q0 is a small positive test charge placed near a second object with a charge

Fe is the magnitude of the electric force

E is a VECTOR quantity- depends on direction** the direction of the E field shows the direction of the electric force that would be exerted on a small positive test charge.

+ ++

+ ++ Near a Positive Object- the test charge would

be repelled so the field is away from a positive.

E

- --

-- +

Near a Negative Object- the test charge would be attracted so the field is toward a negative.

E

E Fields

The direction of the Electric Field depends on the charge of the object producing the field.

The magnitude of the electric field (strength) depends on: charge and distance

* The greater the charge of the object, the stronger the field will be around that object.* The greater the distance from the object, the weaker the field.

E kq

d2 q is the charge of the object producing the field

If q is + the field is always AWAY FROM THE OBJECT.If q is (-) the field is always TOWARDS THE OBJECT.

E fields exist anywhere near a charged object whether there is a test charge or not.

Electric Field Lines

Imaginary lines that represent the size and direction of an electric field.-The # of lines is proportional to the size (strength) of the field.

Rules for Drawing E Field Lines:

1. Lines begin on positives and end on negatives.

2. The number of lines leaving a + charge or approaching a (-) charge must be proportional to the magnitude of the charge.

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

E Field Lines

Electric Dipole: two point charges of equal magnitude but opposite charge

The closer the lines, stronger the strength of the field.

Because size of charges is the same, the # of lines leaving the + charge is equal to the # of lines ending on the (-) charge.

E fields between 2 like charges won’t go from one to the other. Between the charges, lines will repel.

E Field Lines

Between Charges of Unequal Magnitude

+2 Charge & -1 Charge

* the # of lines leaving the +2 charge is twice as much as the # ending on the -1 charge.

* Half of the lines that leave the +2 charge will end on the -1 charge. The other half will terminate at infinity.

Electric Field Inside a Conductor

When a positive charges totaling Q are placed on a sphere, they all go to the outside and distribute themselves in such a way to get as far from each other as possible.

Inside the sphere (r < R), the electric field is zero.Outside the sphere (r > R), the electric field behaves as if the sphere is a point charge centered at the center of the sphere, that is,

E kq

d2