Forces

• By the early 19th century, physicists had classified the apparent myriad of forces in nature to just 3 kinds:– Gravitational force– Electric force– Magnetic force

Forces

• By the end of the 19th century, they had narrowed the list to just 2 forces:– Gravitational force– Electromagnetic force

Forces

• The 20th century first added two new forces to this list that are observed only inside the atomic nucleus:– Gravitational force– Electromagnetic force– Weak force– Strong force

Forces

• And then found theoretical links that narrowed the list back to 2 kinds of forces:– Gravitational force– Strong/Electroweak force

21.1 Electric forces

• Electric forces are created between all electric charges. • Because there are two kinds of charge (positive and negative)

the electrical force between charges can attract or repel.

Elementary Charges

• Protons carry the smallest positive charge.• Protons and uncharged neutrons generally

reside in an atom’s nucleus.• Protons are held in the nucleus by the strong

force.

21.1 Electric Charge

• All ordinary matter contains both positive and negative charge.

• You do not usually notice the charge because most matter contains the exact same number of positive and negative charges.

• An object is electrically neutral when it has equal amounts of both types of charge.

21.1 Electric Charge

• Objects can lose or gain electric charges.

• The net charge is also sometimes called excess charge because a charged object has an excess of either positive or negative charges.

• A tiny imbalance in either positive or negative charge on an object is the cause of static electricity.

21.1 Electric Charge

• Electric charge is a property of tiny particles in atoms.

• The unit of electric charge is the coulomb (C).

• A quantity of charge should always be identified with a positive or a negative sign.

Electrical Charge

• An object with an excess of electrons is negatively charged.

• An object with too few electrons (too many protons) is positively charged.

• An object with the same number of electrons and protons is neutral.

Units of Charge

• The SI unit of charge is the Coulomb.

1 Coulomb = the charge of 6.24 x 1018 electrons

21.1 Electric current

• In conductive liquids (salt water) both positive and negative charges carry current.

• In solid metal conductors, only the electrons can move, so current is carried by the flow of negative electrons.

The direction of current was historically defined as the direction that positive charges move.

Both positive and negative charges can carry current.

21.1 Electric current

• Current is the movement of electric charge through a substance.

Current (amps)

Charge that flows

(coulombs)

Time (sec)

I = q t

Two basics type of materials

• Conductors– Materials, such as metals, that allow the free

movement of charges

• Insulators– Materials, such as rubber and glass, that don’t

allow the free movement of charges

21.1 Conductors and insulators

• All materials contain electrons.

• The electrons are what carry the current in a conductor.

• The electrons in insulators are not free to move—they are tightly bound inside atoms.

21.1 Conductors and insulators

A semiconductor has a few free electrons and atoms with bound electrons that act as insulators.

21.1 Conductors and insulators

• When two neutral objects are rubbed together, charge is transferred from one to the other and the objects become oppositely charged.

• This is called charging by friction.

• Objects charged by this method will attract each other.

Superconductors

• Superconductors are materials that lose all resistance to charge movement at temperatures near absolute zero (0 K or about -273oC).

• Recently, “high temperature” (above 100 K) superconductors have been discovered.

Polarization

• Bringing a charged object near (but not touching) a neutral object polarizes (temporarily separates) the charge of the neutral object.– Like charges in the neutral object are repelled

by the charged object.– Unlike charges in the neutral object are

attracted by the neutral object.• The neutral object returns to normal when

the charged object is removed.

21.2 Coulomb's Law

• The force between two charges gets stronger as the charges move closer together.

• The force also gets stronger if the amount of charge becomes larger.

21.2 Coulomb's Law

• The force between two charges is directed along the line connecting their centers.

• Electric forces always occur in pairs according to Newton’s third law, like all forces.

21.1 Coulomb's Law

• The force between charges is directly proportional to the magnitude, or amount, of each charge.

• Doubling one charge doubles the force.

• Doubling both charges quadruples the force.

21.1 Coulomb's Law

• The force between charges is inversely proportional to the square of the distance between them.

• Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4).

• This relationship is called an inverse square law because force and distance follow an inverse square relationship.

21.2 Fields and forces

• Mass creates a gravitational field that exerts forces on other masses.

21.2 Fields and forces

• Gravitational forces are far weaker than electric forces.

21.2 Drawing the electric field

21.2 Fields and forces

• The concept of a field is used to describe any quantity that has a value for all points in space.

• You can think of the field as the way forces are transmitted between objects.

• Charge creates an electric field that creates forces on other charges.

Electric Field• The Electric Force is like the Gravitational

Force• Action at a Distance

• The electric force can be thought of as being mediated by an electric field.

Electric Field• We say that when a charged object is put at

a point in space,• The charged object sets up an Electric

Field throughout the space surrounding the charged object

• It is this field that then exerts a force on another charged object

Electric Field Lines

• Possible to map out the electric field in a region of space

• An imaginary line that at any given point has its tangent being in the direction of the electric field at that point

• The spacing, density, of lines is related to the magnitude of the electric field at that point

Electric Field Lines

• At any given point, there can be only one field line

• The electric field has a unique direction at any given point

• Electric Field Lines• Begin on Positive Charges• End on Negative Charges

21.2 Electric shielding

• Electric fields are created all around us by electric appliances, lightning, and even static electricity.

• These stray electric fields can interfere with the operation of computers and other sensitive electronics.

• Many electrical devices and wires that connect them are enclosed in conducting metal shells to take advantage of the shielding effect.

21.3 Capacitors

• A capacitor is a storage device for electric charge.

Capacitors can be connected in series or parallel in circuits, just like resistors.

21.3 Capacitors

• A capacitor can be charged by connecting it to a battery or any other source of current.

• A capacitor can be discharged by connecting it to any closed circuit that allows current to flow.

21.3 Capacitors

The current flowing into or out of a particular capacitor depends on four things:

1. The amount of charge already in the capacitor.

2. The voltage applied to the capacitor by the circuit.

3. Any circuit resistance that limits the current flowing in the circuit.

4. The capacitance of the capacitor.