Electrostatics Test Review February 28, 2011

Warning Our next unit is Magnetism. Magnetism is covered by chapter 14 in “5 Steps to a 5” There will be NO retest offered for the next unit’s test. TONIGHT’S HW: Read and Take detailed notes on chapter 12 found on page 178 in “5 Steps to a 5”

Point of Clarification “Magnitude” used by itself DOES NOT EXIST. It is not possible to “Find the magnitude.” You ALWAYS have to find the magnitude of something. Ex: Determine the magnitude and direction of the net force on charge Q1.

Basic Definitions Electric current: a movement or flow of electrically charged particles, typically measured in amperes. Field: A property of a region of space that can affect objects found in that region of space. Electric field: an influence produced by an electric charge on other charges in its vicinity. Electric potential: the capacity of an electric field to do work on an electric charge, typically measured in volts.

Electric Field Electric field: an influence produced by an electric charge on other charges in its vicinity. Every point in an electric field has a certain value called, surprisingly enough, the “electric field value,” or E,

Electric Potential When you hold an object up over your head, that object has gravitational potential energy. If you were to let it go, it would fall to the ground. Similarly, a charged particle in an electric field can have electrical potential energy. For example, if you held a proton in your right hand and an electron in your left hand, those two particles would want to get to each other. Keeping them apart is like holding that object over your head; once you let the particles go, they’ll travel toward each other just like the object would fall to the ground.

Electric Potential: Bottomline Potential energy provided by an electric field per unit charge. The zero of electric potential is assumed to be far, far away. This means that if you have two charged particles and you move them farther and farther from each another, ultimately, once they’re infinitely far away from each other, they won’t be able to feel each other’s presence.

Electrical Potential Energy U= qV It is extremely important to note that electric potential and electric field are NOT the same thing.

Electrical Potential Energy

Electric Potential vs Potential Energy This is a rather simple conservation of energy problem, but it’s dressed up to look like a really complicated electricity problem.

Equipotential Lines Equipotential Lines: Lines that illustrate every point at which a charged particle would experience a given potential. Moving a charge from one equipotential line to another takes energy.

Equipotential Lines Well, potential energy is equal to qV; here, q is 1.6 × 10−19 C, the charge of a proton. The potential energy at point A is (1.6 × 10−19 C)(50 V) = 8.0 × 10−18 J; the potential energy at point B is (1.6 × 10−19 C)(60 V) = 9.6 × 10−18 J. Thus, the proton’s potential is 1.6 × 10−18 J higher at point B, so it takes 1.6 × 10-18 J of work to move the proton there. Um, didn’t the problem say that points A and B were 30 cm apart? Yes, but that’s irrelevant. Since we can see the equipotential lines, we know the potential energy of the proton at each point; the distance separating the lines is irrelevant.

Equipotential Lines