Ch 18: Electric Force & Electric Fields

The Origin of Electricity

• The electrical nature of matter comes from atomic structure

Nucleus – • made up of protons

and neutrons• Most of the mass of

the atom is contained in the nucleus

• mp=1.673x10-27kg

• mn=1.675x10-27kg

Electrons• Orbit the nucleus

in a “cloud”• Relatively mass-

less• me=9.11x10-31kg

The Origin of Electricity

• The electrical nature of matter comes from atomic structure

Protons – • Have a positive

charge• +e = +1.60x10-19C

Electrons• Have a negative

charge• -e = -1.60x10-19C

Neutrons – • No net electrical

charge

The Origin of Electricity

• The electrical nature of matter comes from atomic structure• The magnitude of the charge on a proton

= the magnitude of the charge on the electron. The magnitude is e=1.6x10-19 C.• e is an elementary charge (it is indivisible)• The SI unit for electrical charge is the

coulomb (C).

The Coulomb

• The SI unit for electrical charge is the coulomb (C).• One coulomb is the charge which would flow

through a 120-watt light bulb in one second.• Two charges of 1 C each separated by a

meter would repel each other with a force of about 1,000,000 tons!• Usually, we measure charge in nano-

Coulombs (x10-9 C)

Electrical Charge

• In nature, atoms are usually found with equal numbers of protons and electrons => electrically neutral• A net charge (q) can be built up on an

object by adding or removing electrons• q=Ne , where N is an integer

Example 1

• How many electrons are there in one coulomb of negative charge?• q=Ne• N=q/e = 1.00C/1.60x10-19C• N=6.25x1018 electrons

Charged objects & electrical force

• Electrons can be transferred from one object to another• The object that gains electrons

acquires a negative charge• The object that loses electrons has a

positive charge

Charged objects & electrical force

•When a plastic rod is rubbed with fur, some of the electrons from the fur are transferred to the rod.• No electrons or protons are created or

destroyed• The net charge of the fur/rod system

remains constant (Law of conservation of electrical charge)

Charged objects & electrical force

• Two electrically charged objects exert a force on each other.• This electrostatic force can alter the

motion of an object. (Newton’s 2nd Law)

Example 2

• Object A and Object B are each electrically neutral. 2 million electrons are removed from A and placed on B. What is the resulting charge on A and B?• q=Ne• q=(2,000,000)(1.60x10-19C) = 3.2x10-13 C• A = +3.2x10-13 C• B = - 3.2x10-13 C

Example 3

• Object A has a charge of -1.6x10-13C, and Object B is electrically neutral. 2 million electrons are removed from A and placed on B. What is the resulting charge on A and B?• q=Ne• q=(2,000,000)(1.60x10-19C) = 3.2x10-13 C• A = -1.6x10-13 C – (-3.2x10-13C) = +1.6x10-13C• B = 0C + (- 3.2x10-13 C) = - 3.2x10-13 C

Conductors and Insulators

• Charge can exist on an object, and also move through an object.• Substances that readily conduct electric charge

are called electrical conductors. (ex: metals)•Materials that conduct electric charge poorly

are called electrical insulators. (example: rubber, plastic, wood)

Remember chemistry???

• Recall that a metallic bond has a “sea of electrons”• The valence electrons are not “attached” to the

parent atom – they can wander freely throughout the material• The ready movement of valence electrons is

what makes a good conductor.

Remember chemistry???

• In an insulator, there are very few electrons free to move throughout the material.• Insulators tend to have a more full valence shell

and higher ionization energies.• The valence electrons are being held tightly by

the nucleus of its parent atom. • Without the free electrons, there is very little

flow of charge electrical insulator

Charging by contact

•When a negatively charged rod is touched to a metal sphere, some of the excess electrons from the rod are transferred to the sphere.• Once the rod is removed, the electrons repel

each other and spread out as far away from each other as possible.

Charging by contact

• Identical conducting spheres equalize their charge upon touching.

+5q -3q

+5q -3q

+q+q

Total charge is +5q -3q = 2qequally distributed over both spheres

Charging by induction

• A negatively charged rod that is brought close to (but not touching) a metal sphere induces the electrons in the sphere to move away from the negative rod causing a separation of charge.

Charging by induction

• Grounding the sphere causes some of the electrons to leave the sphere and redistribute themselves over the earth (remember, they want to be as far away from each other as possible. The earth is a good conductor.)

Charging by induction

• If the grounding wire is removed, then the negative rod is removed, the sphere is left with a positive net charge.

Charging by induction

• If the sphere were made of an insulating material (like plastic), you would not be able to induce a net charge on it.• However, you would still be able to separate

the positive and negative charges slightly and induce a slight positive charge at the surface.

Example 4

• A rod made from insulating material carries a net charge (which may be positive or negative), whereas a copper sphere is electrically neutral. The rod is held close to the sphere but does not touch it. Which statement is true?• A) the forces are always attractive• B) the forces are always repulsive• C) the forces are attractive when the rod is negative and

repulsive when positive• D) the forces are repulsive when the rod is negative and

attractive when positive• E) there are no forces

Assignment

•Focus p. 563 #1,2, 4•Problems #1-6