# Circuits Resistors, Capacitors and Inductors 1 Resistors Capacitors Inductors.

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Circuits Resistors, Capacitors and Inductors 1 Resistors Capacitors Inductors

Circuits Resistors, Capacitors and Inductors 2

Circuits Resistors, Capacitors and Inductors 3 Resistors slow down the electrons I

Circuits Resistors, Capacitors and Inductors 4 Resistors slow down the electrons + - + -

Circuits Resistors, Capacitors and Inductors 5 In most instances, the voltage across a piece of material is proportional to the current that flows through it This is known as Ohm’s law and is usually written as The constant of proportionality is called resistance, R, and has units of Ohms (Ω) In a circuit diagram we draw resistance like this R

Circuits Resistors, Capacitors and Inductors 6 For many materials, the resistance is proportional to the length of the resistor and inversely proportional to the cross-sectional area Table 28.2 in the text gives resistivity for some materials The constant of proportionality is called resistivity, ρ, and is different for different materials As length increases, the electrons are blocked for a longer time, slowing them down even more. Thus, the current is smaller and the resistance is greater As area increases, more electrons pass through it. Thus, the current is greater and the resistance is smaller

Circuits Resistors, Capacitors and Inductors 7 Power is defined as the change in energy per unit time In the case of electric potential energy, we can write The power may be written in different forms using Ohm’s law

Circuits Resistors, Capacitors and Inductors 8 E ΔsΔs +σ

Circuits Resistors, Capacitors and Inductors 9 V=0 V=4πkσd -σ d E +σ

Circuits Resistors, Capacitors and Inductors 10 -σ E0E0 EdEd V=4πkσd 0 /κ d V=0 d0d0 +σ

Circuits Resistors, Capacitors and Inductors 11 Capacitors store the electrons + - + -

Circuits Resistors, Capacitors and Inductors 12 A capacitor is two conductors of equal but opposite charge separated by a dielectric. A dielectric is a substance made up of stationary dipoles. The capacitance of a capacitor is given by the equation. Q is magnitude of the charge of one of the conductors. V is the magnitude of the difference in potential between them. The capacitance depends only on the geometry (shape and size of the capacitor, not on Q or V because they are proportional to each other.

Circuits Resistors, Capacitors and Inductors 13 For any capacitor, the electric potential energy is given by But where is that energy found? For a parallel plate capacitor… This is the electrostatic field energy density. The energy lies in the electric field!

Circuits Resistors, Capacitors and Inductors 14 Inductors create magnetic fields + -