•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

Charging and Discharging a CapacitorCharging and Discharging a Capacitor

Tactics: Using Kirchhoff’s loop lawTactics: Using Kirchhoff’s loop law

EXAMPLE 32.1 A single-resistor circuitEXAMPLE 32.1 A single-resistor circuit

EXAMPLE 32.1 A single-resistor circuitEXAMPLE 32.1 A single-resistor circuit

Energy and PowerEnergy and PowerThe 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 lightEXAMPLE 32.4 The power of light

EXAMPLE 32.4 The power of lightEXAMPLE 32.4 The power of light

Series ResistorsSeries 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 flashlightEXAMPLE 32.7 Lighting up a flashlight

EXAMPLE 32.7 Lighting up a flashlightEXAMPLE 32.7 Lighting up a flashlight

Parallel ResistorsParallel 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 ResistorsSeries and Parallel Resistors

EXAMPLE 32.10 A combination of resistorsEXAMPLE 32.10 A combination of resistors

QUESTION:

EXAMPLE 32.10 A combination of resistorsEXAMPLE 32.10 A combination of resistors

RC CircuitsRC 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

ApplicationsApplications

EXAMPLE 32.14 Exponential decay in an RC EXAMPLE 32.14 Exponential decay in an RC circuitcircuit

QUESTION:

EXAMPLE 32.14 Exponential decay in an RC EXAMPLE 32.14 Exponential decay in an RC circuitcircuit

General Physics 2 Circuits 24

Junction Rule

General Physics 2 Circuits 25

Resistance, Voltage

• Determine (a) the equivalent resistance of the circuit and (b) the voltage across each resistor.

General Physics 2Circuits

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.

€

P = VI = I2R =V 2

R

General Physics 2 Circuits 27

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 Current & Resistance 28

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