 # Electric Forces and Electric Fields

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Electric Forces and Electric Fields

The primary particle that carries charge (and therefore can be lost or gained) in an atom is a/an:
Proton Neutron Electron

When a positively charged object comes close to an negatively charged object, the negative object will: Be attracted Be repelled Do nothing

When a positively charged object comes close to an neutral object, the neutral object will:
Be attracted Be repelled Do nothing

Anytime an object at rest starts to move, what must be present to cause it to move?
Difference in electric charge Electricity A force Matter

A Bit of History Ancient Greeks
Observed electric and magnetic phenomena as early as 700 BC Found that amber, when rubbed, became “electrified” and attracted pieces of straw or feathers

Properties of Electric Charges
Two types of charges exist They are called positive and negative Arbitrarily named by Benjamin Franklin Like charges repel and unlike charges attract one another

More Properties of Charge
Positive charge – protons Negative charge – electrons Gaining or losing electrons is how an object becomes charged; protons remain with the nucleus

A little review: What are some conservable quantities in physics? Mass
Momentum Energy Charge

Conservation of Charge
Electric charge is always conserved Charge is not created, only exchanged Objects become charged because negative charge is transferred from one object to another

More review: A force is …? Contact vs. Field forces?
Anything that produces acceleration or a change in motion. Contact vs. Field forces? Contact: require matter to be in contact (ex. friction) Field: matter not required (ex. gravitational, electrical, magnetic)

Fig. 15.1, p. 467

Properties of Charge, final
Charge is quantized All charge is a multiple of a fundamental unit of charge, symbolized by e Electrons have a charge of –e Protons have a charge of +e The SI unit of charge is the Coulomb (C) 1 e = 1.6 x C

Fig. 15.T1, p. 472

Conductors Conductors are materials in which the electric charges move freely Copper, aluminum and silver are good conductors

Insulators Insulators are materials in which electric charges do not move freely Glass and rubber are examples of insulators When insulators are charged by rubbing, only the rubbed area becomes charged There is no tendency for the charge to move into other regions of the material as opposed to conductors

Three Methods of Charging
Friction Conduction (or Contact) Induction

Charging by Friction The act of rubbing creates friction which removes or adds electrons to the objects involved in the friction.

Charging by Conduction
A charged object (the rod) is placed in contact with another object (the sphere) Some electrons on the rod can move to the sphere When the rod is removed, the sphere is left with a charge The object being charged is always left with a charge having the same sign as the object doing the charging

Charging by Induction A negatively charged rubber rod is brought near an uncharged sphere The charges in the sphere are redistributed Some of the electrons in the sphere are repelled from the electrons in the rod

Charging by Induction, final
The wire to ground is removed, the sphere is left with an excess of induced positive charge The positive charge on the sphere is evenly distributed due to the repulsion between the positive charges Charging by induction requires no contact with the object inducing the charge

Not enough information to tell
A charged rod is brought close to a neutral electroscope. When touched by the rod, the leaves both become positive and repel. The rod must have been … Positively charged Negatively charged Not enough information to tell

Not enough information to tell
A neutral electroscope is charged by induction. When touched by the rod, the leaves both become negative and repel. The rod must have been … Positively charged Negatively charged Not enough information to tell

Not enough information to tell
A postively charged rod is brought close to a neutral electroscope to charge it by induction. The top of the electroscope must be: Positively charged Negatively charged Not enough information to tell

Coulomb’s Law Mathematically, kc is called the Coulomb Constant
kc = 8.99 x 109 N m2/C2

Coulomb’s Law Typical charges can be in the µC range
Remember, Coulombs must be used in the equation Remember that force is a vector quantity It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is attractive or repulsive after the magnitude – 3 N attractive)

How is the magnitude of the charges proportional to the electric force between them?
Directly Inversely Exponentially

How is the square of the distance between two charges proportional to the electric force between them? Directly Inversely Exponentially

Coulomb’s Law It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is

Things that make you go, “Hmmmm…”
A) The electric force is significantly stronger than the gravitational force. However, although we are attracted to Earth by gravity, we do not usually feel the effects of the electric force. Explain why.

B) An ordinary nickel contains about 1024 electrons, all repelling one another.
Why don’t these electrons fly off the nickel?

C) When the distance between two negatively charged balloons is doubled, by what factor does the repulsive force between them change?

Electrical Force Compared to Gravitational Force
Both are inverse square laws The mathematical form of both laws is the same Electrical forces can be either attractive or repulsive Gravitational forces are always attractive

Gravitational Force Electric Force Same Unable to tell
If all other variables are held constant, is the electric force or the gravitational force greater for two oppositely charged objects? Gravitational Force Electric Force Same Unable to tell

Compare Gravitational and Electric Force
Calculate the gravitational force as well as the electric force for an electron and a proton which are located 1 cm from each other.

Electrical Field An electric field is said to exist in the region of space around a charged object When another charged object enters this electric field, the field exerts a force on the second charged object

Electric Fields 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.

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

Gravitational vs. Electric Fields
Gravitational forces are far weaker than electric forces.

Van de Graaff Generator
An electrostatic generator designed and built by Robert J. Van de Graaff in 1929 Charge is transferred to the dome by means of a rotating belt Eventually an electrostatic discharge takes place

Electrical Field An electric field is said to exist in the region of space around a charged object When another charged object enters this electric field, the field exerts a force on the second charged object

Electric Field, cont. A charged particle, with charge Q, produces an electric field in the region of space around it A small test charge, qo, placed in the field, will experience a force

Electric Field Lines Electric field is a vector.
There are electric field lines that help visualize this field and were introduced by Michael Faraday

Electric Field Lines Electric field lines are drawn to visualize electric field strength and direction. Introduced by Michael Faraday

Electric Field Line Rules
Electric Field Lines are drawn pointing in the way that a positive point charge would move when placed by the charged object. Field lines can never cross Electric fields exist even in the absence of a point charge.

Electric Field Line Rules
The direction of the electric field is in the same direction as the electric force on the point charge. The relative strength of the electric field is proportional to the number of field lines in a given location. Electric field lines accumulate as sharp points more than rounded objects.

Proton A positive test charge would be repelled by the field + +

Electron A positive test charge would be attracted by the field + -

Opposite charges attract

Like Charges repel

Electric Field Lines for a Dipole

Electric Field for Parallel Plate Capacitor

Summary of Electric Field Lines

Electric Potential Energy of a Charge
Wants to move when it has high PE Point b PE = max KE = min Point a PE = min KE = max

Electric Field Intensity Equation
E = F/q F is the force in Newtons acting on the test charge q in coulombs. Combined with Coulomb’s law, E = kQ/r2

Electric Fields Between Charges – Superposition Principle
Find “E” for each charge, add all together. ET=E1+E2+E3…