AP Physics 2 - Electric Circuits

How is Current Produced?

  • When a high potential is connected by a conductive material to a low potential

    • Flow of electrons is called electric current

    • I = q/t

      • Current = charge/time

      • C/s = Amperes (Amps)

  • When the potentials are equal, the current stops flowing

    • To increase the Electric potential energy of the electrons an electron pump must convert (do work) another form of energy into electrical potential energy

      • Electron Pump examples:

        • Voltaic or galvanic cell (dry cell)

        • Several cells connected (battery)

        • Photovoltaic cell (solar cell)

        • Generator

The Electric Battery

  • Volta discovered that electricity could be created if dissimilar metals were connected by a conductive solution called an electrolyte

  • A battery transforms chemical energy into electrical energy

  • Chemical reactions within the cell create a potential difference between the terminals by slowly dissolving them

    • Potential difference can be maintained even if a current is kept flowing, until one or the other terminal is completely dissolved

  • Several cells connected together make a battery, although now we refer to a single cell as a battery as well

Electric Current

  • In order for current to flow, there must be a path from one battery terminal, through the circuit, and back to the other battery terminal

  • A complete circuit is one where current can flow all the way around

  • By convention, current is defined as flowing from + to -

    • Electrons actually flow in the opposite direction, but not all currents consist of electrons

Two Types of Current

  • Direct Current - (D.C.)

    • Flow of electrons in only one direction

      • Dry Cells and Batteries

  • Alternating Current (A.C.)

    • Flow of electrons at first in one direction and then the other direction (Oscillate)

      • Generator

      • In your house

      • 60 Hz

Ohm’s Law

  • Georg Simon Ohm studied what affects current

  • I = V / R

Some Clarifications

  • Batteries maintain a (nearly) constant potential difference; the current varies

  • Resistance is a property of a material or device

  • Current is not a vector but it does have a direction

  • Current and charge do not get used up

    • Whatever charge goes in one end of a circuit comes out the other end

Current in Electric Circuits

  • An electric circuit is a closed loop

  • Basic Circuits consist of three things

    • Electron Pump (Battery)

    • Device that reduces potential (User)

    • Conducting Connections (Wires)

Direct Current Circuits

  • Circuit

    • Closed loop in which current can flow

    • Basic Simple Circuit

      • Voltage Source

      • Voltage uses

      • Conducting Connections (wire)

    • Diagrams for Electric Circuits

      • Describe in words

      • Use Photographs / Drawings

      • Use symbols (schematics)

Drawing Schematic Circuits

  • Draw symbol for battery and indicate “+” and “-” terminals

  • Draw wire from "+” to the voltage user and draw its symbol

  • If a point where there are two current paths, draw connection symbol and follow one path to the user. Return to the second path and draw to its user and continue until paths join

  • Follow path to the “-” terminal

  • Check work

  • Connect ammeter in series

  • Connect voltmeter in parallel

Series Circuit Rules

  • Current is constant at every point

    • Itot = I1 = I2 = I3…

  • Total resistance is the sum of the resistance at all points

    • Reff = R1 + R2 + R3…

  • Total voltage is the sum of the voltage at all points

    • Vtot = V1 + V2 + V3…

Parallel Circuit Rules

  • Voltage is constant at every point

    • Vtot = V1 = V2 = V3…

  • Inverse of the total resistance is the sum of the inverse of resistance at all parts

    • 1 / Reff = 1 / R1 + 1 / R2 + 1 / R3…

  • Total current is the sum of the current at all points

    • Itot = I1 + I2 + I3…

Kirchhoff Rules

  • Junction Rule

    • ΣI = 0

    • Current splits into two separate current’s that rejoin into the same original current

  • Loop Rule

    • ΣV = 0

    • Start at biggest battery

    • + to - is -V

    • - to + is +V

RC Circuits

  • Circuits that contain resistors and capacitors

  • Values of resistance and capacitance in an RC circuit determine the time it takes the capacitor to charge or discharge

    • Current and voltage decay exponentially

    • Time Constant = T

      • characteristic time for circuit

      • long constant = slow decay; short constant = rapid decay

      • T = RC