Tactics: Using Kirchhoff’s loop law

EXAMPLE 32.1 A single-resistor circuit

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

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 Req.

EXAMPLE 32.7 Lighting up a flashlight

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 V1, and the right ends are all    held at the same potential V2. 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 Req.

Series and Parallel Resistors

EXAMPLE 32.10 A combination of resistors QUESTION:

EXAMPLE 32.10 A combination of resistors

Junction Rule General Physics 2 Circuits

Resistance, Voltage Determine (a) the equivalent resistance of the circuit and (b) the voltage across each resistor. Redraw so the 820 is in parallel with 680 = R for these two is 372 ohm. Now add Rs = 372+470 = 842 ohm General Physics 2 Circuits

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. students can set up circuits to help solve problem Assume R is same for all: brightness goes as current – B5 = B6, B2 = B3, B4, B1 I in B1; I/2 in B2 and B3 because current splits for parallel and R is same for each arm; I in B4 is twice that in B5 and B6. So I in B4 is 2I/3 and I is I/3 in B5 and B6 General Physics 2 Circuits

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 Q0 and potential difference    ΔVC  = Q0/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 32.14 Exponential decay in an RC circuit QUESTION:

EXAMPLE 32.14 Exponential decay in an RC circuit

RC Circuits

RC Circuits  = RC time constant time to reach 63% of maximum voltage

Resistor and capacitor in series Applications RC Circuits Resistor and capacitor in series Applications windshield wipers pacemakers camera flash timing of traffic lights

Think-Pair-Share A heart pacemaker is designed to operate at 72 beats/min using a 7.5-F capacitor in a simple RC circuit. What value of resistance should be used if the pacemaker is to fire (capacitor discharge) when the voltage reaches 63% of its maximum?

Practice Problems Determine the equivalent resistance and the current through R1 for the circuits shown. Assume R1 = 10, R2 = 20 , and R1 = 30 , and the battery is 12 V. General Physics 2 Circuits

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