Two of the three major elements of a circuit, current and resistance, give us the rate of flow of electrons and how hard they have to work to get through the circuit, respectively. While electrons are always moving, the net flow is zero unless an electric field is present, in which case a drift velocity is present. It is from this drift velocity that we can calculate a current. Similarly, we can calculate the current from the current density. Taking this current and a resistance of the circuit, we can combine them with the total voltage to obtain Ohms Law. From Ohms Law, we can relate the electric field to the current density, and we can derive a few equations for calculating the power required to move the electrons through their circuit. With the basics under our belts, its time to look at circuits themselves.
Current is the flow rate of electric charge. Conventional current flows in the direction that a positive charge moves. If negative electrons are the charge carriers, conventional current flows opposite the direction of electron flow.
In a conductor, electrons are in constant thermal motion. The net electron flow, however, is zero because the motion is random. When an electric field is applied, a small net flow in a direction opposite the electric field is observed. The average velocity of these electrons due to the electric field is known as electron drift velocity.
Resistance is the ratio of the potential drop across an object to the current flowing through the object.
Objects which have a fixed resistance are known as ohmic materials and follow Ohm’s Law (R=V/I).
The current density through a surface is the current per area, and is a vector quantity (J).
The resistance of a wire depends on the geometry of the wire as well as the resistivity of the wire, a material property relating to the ability of the material to resist the flow of electrons.
Current & Resistance
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.
This book includes a comprehensive review of the key AP Physics C concepts and targeted strategies for acing every section of the exam. Additionally, the book includes two full length practice tests with full answer explanations.
The book offers a complete review of your AP course, strategies to give you the edge on test day, and plenty of practice with AP-style test questions. It includes 2 full length practice exams modeled on the real test, 3 separate plans to fit your study stle, review material updated to the most recent tests, and all the terms and concepts you need to know.