Unit 3 Electricity & Magnetism Electric Fields Physics 5e. Students know charged particles are sources of electric fields and are subject to the forces of the electric fields from other charges.

Electric Charge Key Question: How do electric charges interact?

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.

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.

Electric Charge Static electricity video hill.com/sites/dl/free/ / / html hill.com/sites/dl/free/ / / html

Electric Charge The innermost electrons in an atom are attracted strongly to positively charged atomic nucleus. The outermost electrons (valence electrons) of many atoms are attracted loosely, therefore these electrons can be removed more easily.

Electric Charge 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.

Electric Charge Example: When you rub a comb against your hair, electrons transfer from the hair to the comb. –The comb then has excess electrons and is said to be negatively charged. –Your hair, has a deficiency of electrons and is said to be positively charged.

Practice Question Beth rubs an ebonite (polished rubber) rod with wool. She finds that the wool acquires a positive charge. Which conclusion can BEST be drawn from her observation?

Practice Question Beth rubs an ebonite (polished rubber) rod with wool. She finds that the wool acquires a positive charge. Which conclusion can BEST be drawn from her observation? Answer: C. Many negatively charged particles moved from the wool to the ebonite rod.

Practice Question If James were to pick up a negatively charged metal ball and touch it to a neutral metal disk, what would happen?

Practice Question If James were to pick up a negatively charged metal ball and touch it to a neutral metal disk, what would happen? Answer: D. The metal disk would become negatively charged.

Charge Polarization Since there are no free electrons to move throughout the material, there is a rearrangement of the positions of charges within the atoms and molecules themselves.

Charge Polarization This charge polarization does not last. Once the balloon is removed, the molecules go back to their usual positions.

Electric Charge Conservation of charge: –Whenever something is charged, no electrons are created or destroyed. –Electrons are simply transferred from one material to another.

Electric Charge Electric charge is a property of tiny particles in atoms. The unit of electric charge is the coulomb (C). 1 charge of coulomb is 6.24x10 18 electrons A quantity of charge should always be identified with a positive or a negative sign.

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.

Electric Forces

Charge can be transferred by conduction. –Electrical Conduction: is the movement of electrically charged particles through a transmission medium.

Practice Question The diagram below shows the electric force between the electrons and the nucleus of an atom. -Which conclusion can be drawn about the electric force?

Practice Question The diagram below shows the electric force between the electrons and the nucleus of an atom. -Which conclusion can be drawn about the electric force? Answer: B. The electric force between the electrons and the nucleus acts at a distance.

Coulomb’s Law Coulomb’s law relates the force between two single charges separated by a distance. F = K q 1 q 2 r 2 Force (N) Constant 9 x10 9 N. m 2 /C 2 Charges (C) Distance (m)

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

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.

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.

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 2 2 = (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.

Practice Question The electrostatic force between two charged objects is influenced by the size of the charges and the distance between the objects. Which expression correctly describes the magnitude of the force between two charged objects?

Practice Question The electrostatic force between two charged objects is influenced by the size of the charges and the distance between the objects. Which expression correctly describes the magnitude of the force between two charged objects? Answer: C. the electrostatic constant (k) multiplied by the product of the charges divided by the square of the distance between them

Fields & Forces Charge creates an electric field that creates forces on other charges.

Fields & Forces Mass creates a gravitational field that exerts forces on other masses.

Electric Field Lines

Show the direction of electric field Show the intensity of the electric field

Fields & Forces

Practice Question Which diagram shows opposing positive electric field lines?

Practice Question Which diagram shows opposing positive electric field lines? Answer:

Conductors All materials contain electrons. Conductors are materials that allow charged particles to pass through them easily. The electrons are what carry the current in a conductor. Examples: –Copper, silver, and other metals

Insulators The electrons in insulators are not free to move—they are tightly bound inside atoms. Examples: –Rubber, glass

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