CollegePhysics18

# CollegePhysics18 - Chapter 18 ELECTRIC CURRENT AND CIRCUITS...

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691 Chapter 18 ELECTRIC CURRENT AND CIRCUITS Conceptual Questions 1. ± R R S 2. Voltmeters have very large internal resistances, so there is usually no danger of a large current. Ammeters on the other hand have very small internal resistances, so they may draw very large currents in a circuit if they are not connected in series with any other significant resistance. 3. The resistance of a light-bulb filament increases significantly as it heats up. The largest current therefore flows through the bulb when it is cold. Thus, it is most likely to burn out just after being switched on. 4. If he connects three 300 Ω resistors in parallel, the equivalent resistance will be the desired 100 Ω . 5. This statement is not exactly true. The current flowing through a branch in a circuit is inversely proportional to the resistance of the branch. Thus, more current follows the path of least resistance than follows any other path, but every path has some current. 6. An ideal ammeter has zero resistance so as to have no effect on the current it is supposed to measure when connected in series in a circuit. An ideal voltmeter has an infinite resistance so that it does not perturb the voltage it is supposed to measure when connected in parallel in a circuit. 7. As the temperature increases, the atoms in the metal begin to vibrate with greater amplitudes. The chance of an electron colliding with one of the atoms is therefore increased. This effectively reduces the mean free path of the electrons and increases the resistance of the metal. 8. Some of the energy is dissipated as heat by the resistors and some of it gets stored in the electric fields of the capacitors as they are charged up. 9. Electric stoves and clothes dryers require relatively large amounts of power to operate. Supplying them with 240 V instead of 120 V decreases the magnitude of required current to supply the power. This reduces the rate ( P = I 2 R ) at which energy is dissipated in the wiring. 10. Elements connected in series in a circuit have the identical current flowing through them, while elements in parallel have identical potential differences. Therefore, ammeters are connected in series and voltmeters in parallel. It is true that if the element under consideration is a resistor, a measurement of the current allows one to calculate the voltage via Ohm’s law and vice versa. So, for a resistor, an ammeter could be used to measure the voltage in a sense. This doesn’t work for other elements though. A fully charged capacitor for example has no current flowing through it and an ammeter connected in series would tell us the current but give us no information about the voltage. Similarly, a voltmeter connected in parallel to a capacitor would allow us to measure the voltage but give us no information about the current flowing through it.

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## This note was uploaded on 04/07/2009 for the course PHY PHY2140 taught by Professor Talagala during the Spring '09 term at Wayne State University.

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CollegePhysics18 - Chapter 18 ELECTRIC CURRENT AND CIRCUITS...

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