Electrostatics Review Electric Fields - PowerPoint Presentation

Slide1

Electrostatics Review

Electric Fields

Electric Potential (Energy)

Charges

The fundamental rule at the base of all electrical phenomena is that

like charges repel and opposite charges attractAn object that has unequal numbers of electrons and protons is electrically charged.

Coulomb’s Law

For charged particles or objects that are small compared with the distance between them, the

force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. 1 C (coulomb) is the charge of 6.24 billion billion electronsk = 9.00 * 109

Nm

2

/C2

Electric Fields

An

electric field is a force field that surrounds an electric charge or group of chargeshas both magnitude and directionElectric field lines go:

AWAY FROM (+) charges

INTO (-) charges

(+) charges = mountains(-) charges = valleys

If a charge is placed in an electric field,

it moves!

Electric Fields

(+) Charges move towards (-) charges and away from other (+) charges

(-) Charges move towards (+) charges and away from other (-) charges

Example

A fly accumulates 3.0*10

-10

C of positive charge as it flies through the air.

What

is the magnitude and direction of the electric field at a location 2.0 cm away from the fly?

Remember:

If you put a charge in an electric field, it feels a

FORCE (and moves!)

= 6800 N/C away from the fly

An

object has potential energy by virtue of its

location in a force field. When you raise it through some distance, you are doing work on the object.

You

are also increasing its

gravitational potential energy.

The

greater the distance it is raised, the greater is the increase in its gravitational potential energy.

Review: Potential Energy

Electrical Potential Energy

A charged object can have potential energy by virtue of its location in an electric field

Work is required to push a charged particle against the E fieldThe energy a charge has due to its location in an electric field is called electrical potential energy

Electric Potential

Convenient when working with electricity to consider the

electrical potential energy per charge

AKA: total electrical potential energy divided by the amount of charge

electric potential = electric potential energy

charge

IMPORTANT:

Electrical Potential Energy

IS NOT THE SAME AS

Electric Potential (V)

Electric Potential is measured in volts (V) and is commonly called voltage

1 volt = 1 joule/Coulomb

Balloon becomes negatively charged to

several thousand

volts

charge on a balloon rubbed on hair is typically much less than a millionth of a

coulomb

the amount of energy associated with the charged balloon is very, very small—about a thousandth of a

joule

Although

the voltage of the charged balloon is high, the electrical potential energy is low because of the

small amount of charge!

Van de

Graaff

The generator is normally charged to thousands of volts, so why do I not get hurt when I touch it?The amount of charge is relatively smallso the electrical potential energy is relatively small. Very little energy flows through the person.

Electric Potential

*

Charge = Electrical PE

Electric Potential * charge = Electrical PE

Electric Potential = Electrical PE

charge Remember: the word “energy” can be replaced with “work” because to store energy in or give energy to an object, work must be done

V *

q

= W V = W

q

Deriving formulas

Example

An electron in Tammie’s old black and white TV is accelerate toward the screen across a potential difference of 22,000 V. How much energy does the electron lose when it strikes the TV screen?

q

= 1.6 * 10

-19

c

V = 22,000 V

W = ?

V = W

q

W =

q

*V = 1.6 * 10

-19

c

* 22,000 V

= 3.5 * 10

-15

J

Capacitors

Capacitors

store electrical energyMade of two closely spaced metal parallel platesWhen connected to a battery, the plates become equally and oppositely chargedThe energy stored in a capacitor comes from the work done to charge itThe energy is in the form of the electric field between its platesElectric fields are storehouses of energy

Example

Bob shuffles his feet across the living room rug, building up a charge on his body. A spark will jump when there is a potential difference of 9000 V between the door and the palm of Bob’s hand. This happens when his hand is 0.003

m from the door. At this point, what is the electric field between Bob’s hand and the door?

The field that exists between two charged parallel plates is uniform and depends upon the potential difference between the plates and the plate separation

E = V

Δd