Complete the activity on charging and discharging capacitors located under Activities on the website sites.google.com/site/sienaphys140spring2011/activities/char ging-and-discharging-a-capacitor sites.google.com/site/sienaphys140spring2011/activities/char ging-and-discharging-a-capacitor Charging and Discharging a Capacitor

Tactics: Using Kirchhoff’s loop law

EXAMPLE 32.1 A single-resistor circuit

Energy and Power The power supplied by a battery is The units of power are J/s, or W. The power dissipated by a resistor is Or, in terms of the potential drop across the resistor

EXAMPLE 32.4 The power of light

Series Resistors Resistors that are aligned end to end, with no junctions between them, are called series resistors or, sometimes, resistors “in series.” The current I is the same through all resistors placed in series. If we have N resistors in series, their equivalent resistance is The behavior of the circuit will be unchanged if the N series resistors are replaced by the single resistor R eq.

EXAMPLE 32.7 Lighting up a flashlight

Parallel Resistors Resistors connected at both ends are called parallel resistors or, sometimes, resistors “in parallel.” The left ends of all the resistors connected in parallel are held at the same potential V 1, and the right ends are all held at the same potential V 2. The potential differences ΔV are the same across all resistors placed in parallel. If we have N resistors in parallel, their equivalent resistance is The behavior of the circuit will be unchanged if the N parallel resistors are replaced by the single resistor R eq.

Series and Parallel Resistors

EXAMPLE A combination of resistors QUESTION:

EXAMPLE A combination of resistors

RC Circuits Consider a charged capacitor, an open switch, and a resistor all hooked in series. This is an RC Circuit. The capacitor has charge Q 0 and potential difference ΔV C = Q 0 /C. There is no current, so the potential difference across the resistor is zero. At t = 0 the switch closes and the capacitor begins to discharge through the resistor. The capacitor charge as a function of time is where the time constant τ is

Applications

EXAMPLE Exponential decay in an RC circuit QUESTION:

EXAMPLE Exponential decay in an RC circuit

General Physics 2Circuits24 Junction Rule

General Physics 2Circuits25 Resistance, Voltage Determine (a) the equivalent resistance of the circuit and (b) the voltage across each resistor.

General Physics 2 Circuits 26 Rank in order of brightness Rank bulbs 1 through 6 in order of descending brightness. –Brightness is proportional to power Now assume the filament in B6 breaks. Again rank the bulbs in order of descending brightness.

General Physics 2Circuits27 Practice Problems Determine the equivalent resistance and the current through R 1 for the circuits shown. Assume R 1 = 10 , R 2 = 20 , and R 1 = 30 , and the battery is 12 V.

General Physics 2Current & Resistance28 Activities Exploration of Physics –E&M –Resistive circuits –Do each of the 5 circuits – set all to 50 ohms –Calculate I, V, and P for each resistor and then check answers in the program