# TOC 1 Physics 212 and 222 Circuit Elements and Electric Power Resistors Capacitors Inductors.

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TOC 1 Physics 212 and 222 Circuit Elements and Electric Power Resistors Capacitors Inductors

TOC 2 Physics 212 and 222 Circuit Elements and Electric Power

TOC 3 Physics 212 and 222 Circuit Elements and Electric Power Resistors slow down the electrons I

TOC 4 Physics 212 and 222 Circuit Elements and Electric Power Resistors slow down the electrons + - + -

TOC 5 Physics 212 and 222 Circuit Elements and Electric Power 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

TOC 6 Physics 212 and 222 Circuit Elements and Electric Power 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

TOC 7 Physics 212 and 222 Circuit Elements and Electric Power 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

TOC 8 Physics 212 and 222 Circuit Elements and Electric Power E ΔsΔs +σ+σ

TOC 9 Physics 212 and 222 Circuit Elements and Electric Power V=0 V=4πkσd -σ-σ d E +σ+σ

TOC 10 Physics 212 and 222 Circuit Elements and Electric Power -σ E 0 E d V=4πkσd 0 /κ d V=0 d 0 +σ

TOC 11 Physics 212 and 222 Circuit Elements and Electric Power Capacitors store the electrons + - + -

TOC 12 Physics 212 and 222 Circuit Elements and Electric Power 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.

TOC 13 Physics 212 and 222 Circuit Elements and Electric Power 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!

TOC 14 Physics 212 and 222 Circuit Elements and Electric Power Inductors create magnetic fields + -

TOC 15 Physics 212 and 222 Circuit Elements and Electric Power Resistors Capacitors Inductors

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