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Download Chapter 17 Electric Current and Resistance
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Chapter 17 Electric Current and Resistance What is electric “current”? 17.1 Batteries and Direct Current Electric current is the flow of electric charge. A battery is a source of electric energy—it converts chemical energy into electric energy. 17.1 Batteries and Direct Current “Redox” Reaction Cathode – Oxidation Electrons lost Anode – Reduction Electrons gained 17.1 Batteries and Direct Current “Redox” Reaction Electrolyte Contains free ions Salt Bridge Allows flow of ions Allows continued use Typical “Alkaline” Batteries Potassium hydroxide serves as the electrolyte (an alkaline, not acid) 17.1 Batteries and Direct Current Potassium hydroxide (do not eat) 17.1 Batteries and Direct Current In a complete circuit, electrons flow from the negative electrode to the positive one. The positive electrode is called the anode; the negative electrode is the cathode. A battery provides a constant source of voltage—it maintains a constant potential difference between its terminals. 17.1 Batteries and Direct Current The potential difference between the battery terminals when the battery is not connected to anything is called the electromotive force, emf. 17.1 Batteries and Direct Current The actual terminal voltage of the battery is always less than the emf, due to internal resistance. Usually the difference is very small. 17.1 Batteries and Direct Current When batteries are connected in series, the total voltage is the sum of the individual voltages. 17.1 Batteries and Direct Current When batteries of equal voltage are connected in parallel, the total voltage does not change; each battery supplies part of the total current. 17.1 Batteries and Direct Current Battery Clean-Up • Anode / Cathode Mix-Up – Ben Franklin first described the movement of current under an assumption that the electron carried positive charge. – Today, we still refer to this as “current” or “conventional current” – The actual direction that electrons move is opposite of this and called “electron flow” 17.2 Current and Drift Velocity Battery Clean-Up • Different Voltages – In series, the voltages add, but the current will be the lowest of the batteries – In parallel, the current adds, but the voltage will be the lowest of the batteries • Probably will lead to something heating up and breaking Battery Clean-Up • Battery Naming (A, AA, AAA, B, C, D) – 1.5 Volts is a standard value used for portable electronics. – 1.5 V is a good multiple • Need 3V? Use two in series! – Letter refers to size • Started with A and went up • Advent of miniature electronics demanded establishment of AA and AAA 17.2 Current and Drift Velocity Electrons do not flow like water in a pipe. In the absence of voltage, they move randomly at high speeds, due to their temperature. When a voltage is applied, a very small drift velocity is added to the thermal motion, typically around 1 mm/s; this is enough to yield the observed current. 17.2 Current and Drift Velocity Current is the time rate of flow of charge. SI unit of current: the ampere, A How long would it take a circuit with 2.0 A of current to deliver 3.6x10-3 C of charge? 17.3 Resistance and Ohm’s Law If there is a potential difference across a conductor, how much current flows? The ratio between the voltage and the current is called the resistance. SI unit of resistance: the ohm, Ω 17.3 Resistance and Ohm’s Law It is helpful to think of resistance as… resistance to current moving No resistance? FULL SPEED AHEAD! So resistance is bad… right? (Hint: no) 17.3 Resistance and Ohm’s Law Resistance captures energy from electrons and lets us do stuff with it! How? Official Physics Terminology 17.3 Resistance and Ohm’s Law An ohmic material is one whose resistance is constant. 17.3 Resistance and Ohm’s Law Ohm’s law is valid only for “ohmic” materials: The resistance of a particular object depends on its length, cross-sectional area, material, and temperature. What is the resistance in a circuit with 0.2 A of current and a difference in potential electric difference of 12 J/C? 17.3 Resistance and Ohm’s Law As expected, the resistance is proportional to the length and inversely proportional to the cross-sectional area (why?): The constant ρ is called the resistivity, and is characteristic of the material. 17.3 Resistance and Ohm’s Law In this table, you can easily see the differences between the resistivities of conductors, semiconductors, and insulators. The speaker wires for the rear speakers in this room are approximately 8 m in length and have a cross-sectional area of 10-6 m2. Identify the resistance of these wires. 17.3 Resistance and Ohm’s Law For many materials, the temperature dependence of the resistivity is approximately linear, as long as the temperature change is not too large. The constant α is called the temperature coefficient of resistivity. Some values of α are listed in the table on the previous page. Find the resistivity of platinum when the temperature is increased to 35ºC. 17.3 Resistance and Ohm’s Law Some materials exhibit a curious phenomenon: at a very low temperature called the critical temperature, their resistivity drops abruptly to zero. These are called superconductors; they have a number of unique properties. They are impractical for everyday home use, however, as they must be cooled to cryogenic temperatures. 17.4 Electric Power Power, as usual, is the rate at which work is done. For work done by electricity: Rewriting, For ohmic materials, we can write: 17.4 Electric Power So, where does this power go? It is changed to heat in resistive materials. 17.4 Electric Power Electric appliances are rated in watts, assuming standard household voltage. 17.4 Electric Power 17.4 Electric Power The electric company typically bills us for kilowatt-hours (kWh), a unit of energy. We can reduce our energy usage by buying efficient appliances. Chapter 17 Quiz Thursday Review of Chapter 17 A battery produces emf; positive terminal is the anode, negative is the cathode. emf is measured in volts; it is the number of joules the battery supplies per coulomb of charge. An electric current can exist only in a complete circuit. Resistance: Review of Chapter 17 Ohm’s law is obeyed if the resistance is constant: The resistance of an object depends on its length, cross-sectional area, and resistivity. Review of Chapter 17 Power is the rate at which work is done.