# ELECTROSTATICS REVIEW ELECTRIC FIELDS ELECTRIC POTENTIAL ENERGY Charges

• Slides: 15

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 attract An 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 electrons k = 9. 00 * 109 Nm 2/C 2

Electric Fields An electric field is a force field that surrounds an electric charge or group of charges has both magnitude and direction Electric field lines go: AWAY FROM (+) charges INTO (-) charges (+) charges = mountains (-) charges = valleys

Electric Fields If a charge is placed in an electric field, it moves! (+) Charges move towards (-) charges and away from other (+) charges (-) Charges move towards (+) charges and away from other (-) charges

Remember: If you put a charge in an electric field, it feels a FORCE (and moves!) 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? = 6800 N/C away from the fly

Review: Potential Energy 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.

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 field The energy a charge has due to its location in an electric field is called electrical potential energy

Electric Potential IMPORTANT: Convenient when working with electricity to consider the electrical potential energy per charge Electrical Potential Energy AKA: total electrical potential energy divided by the amount of charge IS NOT THE SAME AS electric potential = electric potential energy charge 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 small �so the electrical potential energy is relatively small. Very little energy flows through the person. Electric Potential * Charge = Electrical PE

Deriving formulas 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

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 store electrical energy Made of two closely spaced metal parallel plates When connected to a battery, the plates become equally and oppositely charged The energy stored in a capacitor comes from the work done to charge it The energy is in the form of the electric field between its plates Electric fields are storehouses of energy

The field that exists between two charged parallel plates is uniform and depends upon the potential difference between the plates and the plate separation 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 the door? E= V Δd