Introduction to Electromagnetism - 14

Administration RC Circuits in Neurons An RC time constant determines the speed at which nerve impulses travel Click to edit Master title style 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 14 March 06, 2007 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 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. 2 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 RC Circuits (a) A capacitor in series with a resistor, switch, and battery. (b) Circuit diagram representing this system at time t 0, before the switch is closed. (c) Circuit diagram at time t 0, after the switch has been closed. Charging a Capacitor Discharging a Capacitor Analyze qualitatively the behavior of the current in the circuit below just after the switch is closed a long time after the switch is closed. Flashing Lamp A fluorescent lamp L is connected in parallel with C. There is a current through the lamp only when the potential difference across it reaches a breakdown voltage VL. When V VL the capacitor discharges through the lamp. We want 2 flashes per second. The time it takes for the voltage to reach VL is given by 0 Myelin sheath Axoplasm Semipermeable membrane Simplified Model of Myelinated Axon Myelin sheath Axoplasm Semipermeable membrane Myelin sheath Axoplasm Semipermeable membrane Myelin sheath Axoplasm Semipermeable membrane An estimate of how fast the electrical impulse travels is