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Current, resistance, and electromotive force (emf): Chapter 25 (sec. 1-5)

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## Current, resistance, and electromotive force (emf): Chapter 25 (sec. 1-5)

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**Current, resistance, and electromotive force (emf): Chapter**25 (sec. 1-5) • Charges (electrons) moving in a conductor • Ohm’s Law & resistance to flow of charge • Energy and power in electrical circuits C 2012 J. Becker**Learning Goals - we will learn:**• The meaning of electric current, and how electric charges move in a conductor.• What is meant by the resistivity of a substance.• How to calculate the resistance of a conductor from its dimensions and its resistivity.• How an electromotive force (emf) makes it possible for current to flow in a circuit.• How to do calculations involving energy and power in circuits.**ELECTRON MOTION IN A CONDUCTOR WITH AND WITHOUT AN ELECTRIC**FIELD**ANALOGY OF A CHARGE MOVINGIN A CONDUCTOR**12 Volts 0 Volts**CONDUCTOR WITH CONVENTIONAL CURRENT MOVING FROM HIGH**ELECTRICAL POTENTIAL (VOLTS) TO LOW POTENTIAL**“CONVENTIONAL” CHARGESDRIFTING IN A CONDUCTOR**HIGHER POTENTIAL LOWER POTENTIAL LAMP**Which Box (A, B, or C) has the most resistance to the flow**of electric charge (current)? Each lamp has the same amount of resistance to the flow of charge. Current is the flow of charge past a point in the circuit per unit time interval. C 1998 McDermott, et al., Prentice Hall**Which network has the most resistance to the flow of charge?**Rank the networks according to decreasing resistance. C 1998 McDermott, et al., Prentice Hall**Rank the brightness of the bulbs (bright to dim).**2. A wire is added as shown below. • Does the brightness of bulb C increase, decrease, or remain the same? • Does the brightness of bulb A increase, decrease, or remain the same? • Does the current through the battery increase, decrease, or remain the same? C 1998 McDermott, et al., Prentice Hall**Resistance (R) is proportional to resistivity (r): R =**r L / AThe resistivity (r) depends on temperature and the physical properties of the material, so it has a different value for each material. Temperature dependence of resistance (and resistivity) is generally linear over limited temperature ranges and is characterized by the temperature coefficient of resistivity (a): R(T) = R0 [ 1 + a (T-T0)] r(T) = r0 [ 1 + a (T-T0)] where R0 and T0 are the resistance and temperature at a standard temperature, usually room temperature or 20o C. (Measured in Lab #5)**Current – voltage relations**• a resistor obeys Ohm’s Law: I = V/R with constant slope = 1/R (or DV = I R) b) A vacuum tube diode c) A semiconductor diode**Electric potential (DV) rises and drops in a circuit (from**previous slide)**CIRCUIT ENERGY and POWER**P = Vab I = I2 R = Vab2/ Re I = rate of conversion of non-electrical (chemical) energy to electrical energy within the sourceI2 r = rate of electrical energy dissipation in the internal resistance of the source (battery)e I - I2 r = the rate at which the source delivers electrical energy to the load (headlight) R**You can view a 4-minute youtube video of a smoke detector**and how it works, from classmate Marjo Mallari http://www.youtube.com/watch?v=oFUUQcpGR3k