Introduction to Electromagnetism - 13

Administration Electrical Safety I Electrical Safety II Electrical Safety III  Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level Click to edit Master text styles Second level Third level Fourth level Fifth level Instructor Franco Gaspari PHY 1040U (Physics for the biosciences) Introduction to Electromagnetism and Optics Lecture 13 March 02, 2007 Ideal versus real battery An ideal source of emf maintains a constant potential difference between its terminals, independent of the current, I, through it or the resistance, R, across it. The formula for an ideal source of emf V Thus the equation for a source with internal resistance is Example of loop rule, resistance rule and emf rule (single loop). Resistors in series The current is the same through each resistor since any charge that flows through R1 must flow through R2 (KCL). Potential drop from a to b IR1 Potential drop from b to c IR2 In general, for resistors in series X-mas lights Resistors in parallel At the junction at a the current splits in 2, i.e., I I1 I2 (KCL). Since the potential drop is the same, we have Household circuits are always in parallel. Examples (series, parallel, Kirchhoff) Example 28.4 page 866 The Ammeter and the Voltmeter R for ammeters is very small. R for voltmeters is very large. I1 I2 R1 R2 Why The current passes directly through the ammeter. Since the resistance of the ammeter is in series, then We place the voltmeter between the 2 terminals of a resistor to measure the potential drop. However, with the voltmeter connected, in reality we measure 1. A battery has an emf of 15.0 V. The terminal voltage of the battery is 11.6 V when it is delivering 20.0 W of power to an external load resistor R. (a) What is the value of R (b) What is the internal resistance of the battery An emf is the potential of the battery without considering the internal resistance. The terminal voltage is the actual potential at the output of the battery terminals (that is, including the internal resistance) From the definition of Power, we get Where V is the terminal voltage, then M 6. (a) Find the equivalent resistance between points a and b in the Figure. (b) A potential difference of 34.0 V is applied between points a and b. Calculate the current in each resistor. We are now left with 3 resistors in series, then The current across the 3 resistors in series is the same, that is I 1 I 2 The current I splits into I1 and I2 when it meets the 2 resistors in parallel. The potential drop across the resistors in parallel is Fig 28-32a, p.882 Fig 28-32b, p.882 Figure 28.32 (b) This shock can be avoided by connecting the drill case to ground through a third ground wire. In this situation, the drill case remains at ground potential and no current exists in the person. t Then, by rearranging the terms in the equation, solving for Rshoes Figure 28.32 (b) This shock can be avoided by connecting the drill case to ground through a third ground wire. In this situation, the drill case remains at ground potential and no current exists in the person.