25 Overview

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25 Overview

• electric current
• Direct current (DC) circuits
• Ohms Law
• Kirchoffs Rules
• RC circuits

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Circuit Analogy
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Current Charge Flow

• I Q/t (coulomb/second ampere, A)
• affected by material and field
• carriers electrons or holes

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current parameters

• q charge/carrier
• n carriers/volume
• A cross-section
• vd drift velocity

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Electrical Resistance
ohm, W volt/amp
Example Effect of 100ohm, 1000ohm resistors with
6.0 volts potential difference
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Ohms Law

• Ohmic V/I constant
• Non-Ohmic V/I ? constant

Ohmic
Non-Ohmic
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Example Non-Ohmic Diode
Sizeable current flows in only one direction
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Example Diode Application
Half-Wave Rectifier
Full-Wave Rectifier
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Resistivityspecific resistance

• Intrinsic (does not depend on shape)
• Resistance is Extrinsic (depends on shape)
• Symbol r (rho).
• SI Unit ohm-meter (W-m)

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Factors Affecting Resistivity

• Temperature
• Defects
• Impurities

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some resistivities in Wm _at_ 20C

• silver 16x10-9
• copper 17x10-9
• aluminum 28x10-9
• nichrome 1000x10-9
• silicon 640
• insulators 108 to 1016
• /

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Resistance Resistivity
where L length of wire A cross-sectional
area of wire
Example A wire cross-sectional area A 10-6 m2
is L 15m long. Is made from a material with
200x10-9 ohm-meter. Its resistance is
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Power Loss in a Resistor
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Terminal Voltage Real Battery
internal battery resistance
load resistor
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Connection Types
Series
Parallel
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Equivalent Resistances
Series
Ex. R1 4, R2 12 Req 4 12 16 ohms
Parallel
Ex. R1 4, R2 12 Req (4-1 12-1)-1 3
ohms
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0
Ex. Calc equivalent resistance, current,
voltages, powers dissipated
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0
Ex. Calc equivalent resistance, current,
voltages, powers dissipated
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Calc equivalent resistance
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• Kirchoffs Rules
• Loop Rule.
• Junction Rule.

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I I1 I2 (junction)
a
b
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Calculate Current in each resistor
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RC Circuits
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Charging/Discharging Cs

• Resistance affects time required
• Capacitance value also affects time needed
• time constant RC.
• Benchmarks values related to e 2.718
• discharge 1/e 0.3678, Q _at_ t RC
• charge 1 1/e 0.6321, Q _at_ t RC

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Summary

• electric current is flow of charge/time
• V IR R rL/A
• P I2R
• circuits obey charge and energy cons.
• RC exponential time constant

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Calc I 2A, I2 0.5A, I1 1.5A