Unit 3Electricity and Simple Circuits

Electrical forces arise from particles in atoms. The protons in the nucleus attract the electrons and hold them in orbit. Electrons are attracted to protons, but electrons repel other electrons.

Structure of the atom

Atoms consist of a nucleus and electron orbital shells

Particles in the nucleus Neutrons – neutral charge Protons – positive charge

Particles in the shells Electron – negative charge

What is a charge?

The fundamental electrical property to which the mutual attractions or repulsions between electrons or protons is attributed is called charge.

• By convention, electrons are negatively charged and protons positively charged.

• Neutrons have no charge, and are neither attracted nor repelled by charged particles.

Important facts about atoms

1. Every atom has a positively charged nucleus surrounded by negatively charged electrons.

2. All electrons are identical.3. The nucleus is composed of protons and

neutrons. All protons are identical; similarly, all neutrons are identical.

4. Atoms usually have as many electrons as protons, so the atom has zero net charge.

Note: A proton has nearly 2000 times the mass of an electron, but its positive charge is equal in magnitude to the negative charge of the electron.

The fundamental rule of all electrical phenomena

Like charges repelOpposite charges attract

Neutral atom

Electrons and protons have electric charge.

In a neutral atom, there are as many electrons as protons, so there is no net charge.

When is an object electrically charged?

An object that has unequal numbers of electrons and protons is electrically charged.

If an electron is removed from an atom, the atom is no longer neutral. It has one more positive charge than negative charge. There is an imbalance of charges.

A charged atom is called an ion.A positive ion has a net positive charge; it has

lost one or more electrons. A negative ion has a net negative charge; it

has gained one or more extra electrons.

How electrons are transferred

Electrons are being transferred by friction when one material rubs against another.

Electrons can also be transferred from one material to another by simply touching.

Show Balloon and Static Electricity pHet

Coulomb’s Law

The electrical force between any two objects obeys a similar inverse-square relationship with distance.

The relationship among electrical force, charges, and distance—Coulomb’s law—was discovered by the French physicist Charles Coulomb in the eighteenth century.

Coulomb’s Law

For charged objects, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them.

Where: d is the distance between the charged particles. q1 represents the quantity of charge of one particle.

q2 is the quantity of charge of the other particle.

k is the proportionality constant.

The SI unit of charge is the coulomb, abbreviated C. A charge of 1 C is the charge of 6.24 × 1018 electrons. A coulomb represents the amount of charge that passes through a common 100-W light bulb in about one second.

Conductor and Insulator

Materials through which electric charge can flow are called conductors.

Outer electrons of the atoms in a metal are not anchored to the nuclei of particular atoms, but are free to roam in the material.

Metals are good conductors for the motion of electric charges because their electrons are “loose.”

Conductor and Insulator

Insulators are materials that tightly bound their electrons to the nucleus and are not free to wander.

Materials such as rubber or glass.

These materials are poor conductors of electricity.

Conductor and Insulator

A substance is classified as a conductor or an insulator based on how tightly the atoms of the substance hold their electrons.

The conductivity of a metal can be more than a million trillion times greater than the conductivity of an insulator such as glass.  In power lines, charge flows much more easily through hundreds of kilometers of metal wire than through the few centimeters of insulating material that separates the wire from the supporting tower.

Conductor and Insulator

Electrons move easily in good conductors and poorly in good insulators.

Induction

If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface.

Charging by induction can be illustrated using two insulated metal spheres.Uncharged insulated metal spheres touching each other, in effect, form a single noncharged conductor.

Induction

When a negatively charged rod is held near one sphere, electrons in the metal are repelled by the rod.

Excess negative charge has moved to the other sphere, leaving the first sphere with an excess positive charge.

The charge on the spheres has been redistributed, or induced.

Induction

When the spheres are separated and the rod removed, the spheres are charged equally and oppositely.

They have been charged by induction, which is the charging of an object without direct contact.

What is an electric field?

It is a region around a charged particle or object within which a force would be exerted on other charged particles or objects.

Force field

The space around a concentration of electric charge is different from how it would be if the charge were not there. If you walk by the charged dome of an electrostatic machine—a Van de Graaff generator, for example—you can sense the charge. Hair on your body stands out—just a tiny bit if you’re more than a meter away, and more if you’re closer. The space is said to contain a force field.

Van de Graaff Generator

An electric field has both magnitude and direction. The magnitude can be measured by its effect on charges located in the field.Imagine a small positive “test charge” placed in an electric field.

Where the force is greatest on the test charge, the field is strongest.

Where the force on the test charge is weak, the field is small.

Direction of field

The direction of an electric field at any point, by convention, is the direction of the electrical force on a small positive test charge.

● If the charge that sets up the field is positive, the field points away from that charge. ● If the charge that sets up the field is negative, the field points toward that charge.

Positive chargeNegative charge Neutral charge

a. In a vector representation of an electric field, the length of the vectors indicates the magnitude of the field.

b. In a lines-of-force representation, the distance between field lines indicates magnitudes.

a. The field lines around a single positive charge extend to infinity.

Electric Field Lines

a. The field lines around a single positive charge extend to infinity.

b. For a pair of equal but opposite charges, the field lines emanate from the positive charge and terminate on the negative charge.

Electric Field Lines

a. The field lines around a single positive charge extend to infinity.

b. For a pair of equal but opposite charges, the field lines emanate from the positive charge and terminate on the negative charge.

c. Field lines are evenly spaced between two oppositely charged capacitor plates.

Electric Field Lines

Show electric field demo.

Electric Field equation

E = electric field strengthQ = charged = distancek = 9.0 x 109 Nm2 / C2

Examples

The electric field strength in a region is 2,200 N/C. What is the force on an object with a charge of 0.0040 C?

What equation are we going to use?

E = F / q

E = 2200 N/Cq = 0.0040 CF = ?

Rearrange the equation and we get F = EqF = (2200)(0.0040) = 8.8 N

Example

If two charges (q1= 2.3mC & q2=1.0mC) are placed 0.50m apart what force is experienced by q1? By q2?

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Example

What is the field strength 2.0m away from a -0.060C charge? Is the field directed towards or away from the charge?

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Since the charge is negative, the direction would be towards the charge.