Chapter 25a

# Chapter 25a - Electronics The modern mastery of electricity...

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Electronics The modern mastery of electricity and magnetism has given us countless technological benefits that enrich our lives. Communications Entertainment Navigation Time keeping Computing

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The devices that make up this technology are themselves made up of electronic components . These can be roughly divided into two types. Active components like transistors (a subject for a more advanced course) and so called passive components like: DC power supplies (e.g. batteries) Capacitors (Capacitance) Resistors (Resistance) Inductors (Inductance) + symbol chapter 25, 26, 27 25, 27, 31 26, 27, 31 30, 31
Direct current (DC) power supply (first look) + variable V + + + A DC power supply has two terminals one marked positive (+) and the other marked negative (–). 1.5 V 9 V It is a source of electrical energy that works to keep its positive terminal at its rated electric potential (V) greater than its negative terminal. This drives charge around the circuit to which the power supply is attached.

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We could label the (–) terminal with potential V and the (+) terminal V + , both measured with respect to the potential being zero at infinity (as we’ve done before), but the electric potential available for moving charge around a closed circuit is just the potential difference between the two terminals. Previously we labeled potential difference as Δ V where, VV V + Δ= But in circuits it is is conventional to simply label this potential difference as just V (it still means V + –V ). V V + V When we connect the power supply to the circuit this potential (difference) causes movement of charge in the circuit.
Capacitance As our first circuit element we consider a capacitor. The prototypical capacitor consists of two, parallel metal plates, each of inner face area A , with a separation between the inner faces d and a wire electrically connected to each plate. Inner face area A (same for both plates) d In the simplest circuit, the wires are connected to each terminal of a power supply. A quantity of negative charge – q is driven from the (–) terminal of the supply onto one plate while the same quantity of negative charge – q is simultaneously driven off of the other plate into the (+) terminal of the supply. This leaves behind a charge of +q on the plate connected to the (+) terminal.

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Inner face area A (same for both plates) d The net result, some time after connecting the capacitor to the power supply, is that a charge of +q resides on one plate while an equal but opposite charge –q resides on the other plate .
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Chapter 25a - Electronics The modern mastery of electricity...

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