Physics 18: 1-6 Electric Current and Resistivity

Homework

Reading Preparation
Key Equations
WebLecture
Study Activity
Chat Preparation Activities
Chapter Quiz
Lab Work

Reading Preparation

Text Reading: Giancoli, Physics - Principles with Applications, Chapter 18: Sections 1 to 6

Study Points

18.1 Batteries. Batteries are a way of storing charge in solutions that have ions, charged molecules, trapped in different chambers. Connecting the chambers with a wire allows the electrons to flow.
18.2 Current. Current is the amount of charge moving through time. I = δQ/δt. Not only can electrons move down conduction wires, but ions can move through fluids, which has significant implications for geology (flow of current in metals creates planetary magnetic fields) and power sources (flow of ions in acids and bases allows us to create batteries).
18.3 Charges flow for the same reason massive objects role downhill in a gravitational field: because there is a difference in the potential energy of the field at two points in the field. Just as friction can retard the rate of descent for an object sliding on a rough surface, interactions between the flowing ions and obstacles created by other atoms in the material provide interference to the flow. Ohm's Law and Resistance. Ohm's Law relates resistance to the flow of charge to the difference in potential and the amount of charge that changes. I = V/R.
18.4 Resistivity. In any given situation, resistance depends on the type of material, the distance the electrical charges move along the length of the material, and the cross-section area through which the charge can pass: R = ρ * L/A.
18.5 Power. We've already used power = work done/time period for mechanical work. We can also use this definition for energy transformed form potential to kinetic energy (or vice-versa) a charge moving in an electric field, noting that a work done as charge Q moves through potential difference V is QV: Power = QV/t. Since Q/t is current I, P = VI, and a number of possible equivalencies can be set up.
18.6 Household circuits. Household circuits are set up to prevent fires by including a fuse in the circuit which will trip or break the circuit to prevent overheating of wires.

Key Equations

Current definition: $I = \frac{Δ Q}{Δ t}$
Resistance: $V = IR$
Resistivity: $R = ρ \frac{l}{A}$
Temperature dependence of Resistivity: $ρ_{T} = ρ_{0} [1 + α (T - T_{0})]$
Electric power $\begin{array}{l} P = IV = I (IR) = I^{2} R \\ P = IV = (\frac{V}{R}) V = \frac{V^{2}}{R} \end{array}$ $P = IV = (\frac{Q}{t}) V$

Web Lecture

Read the following weblecture before chat: Direct Current

Study Activity

Use the simulation below to explore how changing resistivity, length, and cross-section area changes the total resistance in a conducting wire.

What happens to Resistance R as you vary resistivity ρ? When is Resistance R the greatest? The least?
What happens to Resistance R as you vary length L? When is Resistance R the greatest? The least?
What happens to Resistance R as you vary area A? When is Resistance R the greatest? The least?
How can you maximize resistance? What is the largest value of R you can achieve?

Physics simulation Java Applets are the product of the PHET Interactive Simulations project at the University of Colorado, Boulder.

Chat Preparation Activities

Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

Chapter Quiz

The chapter quiz is not yet due.

Lab Work

If you want lab credit for this course, you must complete at least 18 labs; you may complete more if you are preparing for the AP exam.. One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!

Lab Instructions: Complete Circuits II

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Physics

Chapter 18: 1-6