PHYA22 Chapter 32.doc

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17 Apr 2012
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Chapter 32: Fundamentals of Circuits
Circuit Elements and Diagrams
Circuit Diagram: a logical picture of what is connected to what.
Light bulbs have two ends and current passes through the bulb; it can be
thought that a light bulb is a resistor that gives off light when a current is
present.
Kirchhoff’s Laws and the Basic Circuit
Kirchhoff’s Junction Law: the total current into the junction must equal the total
current leaving the junction (∑Iin = ∑Iout).
Energy Conservation: sum of the potential differences around any loop or
closed path is zero.
Kirchhoff’s Loop Law: Vloop = ∑(V)I = 0
oDraw a circuit diagram.
oAssign a direction to the current.
oTravel around the loop.
Vbat = +ε, for an ideal battery in the negative-to-positive
direction.
Vbat = -ε, for an ideal battery in the positive-to-negative
direction.
VR = -IR for a resistor.
oApply the loop law.
Complete Circuit: a circuit that forms a continuous path between the battery
terminals.
Load: a resistor.
The battery is called the source.
The potential energy that the charges gain in the battery is subsequently lost
as they “fall through the resistor.
Energy and Power
Pbat = rate of energy transfer = dU/dt = (dq/dt)ε = Iε, where ε is the emf of a
battery and Pbat has the units of J/s or W.
Power: the energy transferred per second from the battery’s store of chemicals
to the moving charges that make up the current.
Energy-Transfer Process: Echem U (potential energy) K (kinetic energy)
Ethermal
The battery’s chemical energy is transferred to the thermal energy of the
resistors, raising their temperature.
The potential difference across the resistor is exactly the emf supplied by the
battery, the Pbat and PR are numerically equal (Pbat = PR).
PR = power dissipated by the resistor = IVR = I2R = (VR)2/R
The largest resistance will dissipate most of the power.
Eth = PRt
Series Resistors
Series Resistors: resistors that are aligned end to end with no junctions
between them.
The current must be the same through each of these resistors.
Equivalent Resistance: Req = R1 + R2 + … + RN
Resistors in series all have the same current (I).
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