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 billion electrons k = 9.00 * 109 Nm2/C2

Electric Fields An electric field Electric field lines go: 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

If a charge is placed in an electric field, it moves! 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

Example = 6800 N/C away from the fly 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 If the charge is released, it will accelerate in a direction away from the sphere, and its electrical potential energy will transform into kinetic energy.

IMPORTANT: IS NOT THE SAME AS Electrical Potential Energy Electric Potential (V) 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 If we push two charges instead of one, we do twice as much work. The two charges in the same location will have twice the electrical potential energy as one; a group of ten charges will have ten times the potential energy; and so on.

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!

Electric Potential * Charge = Electrical PE 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 In contrast, it would be unadvisable to intentionally become the short-circuit for the household 110 V because, although the voltage is much lower, the transfer of energy is appreciable. Less energy per charge, but many many more charges!

Deriving formulas V * q = W V = W q 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 = 3.5 * 10-15 J V = W q W = q*V = 1.6 * 10-19 c * 22,000 V 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 = ? W = q*V = 1.6 * 10-19 c * 22,000 V = 3.5 * 10-15 J V = W q

Capacitors 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 and the door? E = V Δd