Physics 18: 1-6 Electric Current and Resistivity
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 18: Sections 1 to 6
- 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.
- Current definition:
- Temperature dependence of Resistivity:
- Electric power
Read the following weblecture before chat: Direct Current
Use the simulation below to explore how changibng resistivity, length, and cross-secttion 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
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- The chapter quiz is not yet due.
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