Lab: AC Circuits Integrated Science II

Applications of AC Circuits AC (Alternating Current) vs. DC (Direct Current) Examples: Radio

Applications of AC Circuits Examples: Alternator (AC generator)

Applications of AC Circuits Examples: Transformer Used to step voltages up or down -Exist in MOST devices!

AC Circuit Elements Resistors Capacitors Two conductors (plates) separated by a gap Inductors (Solenoids) Coils of wire AC Power Supply (Function Generator)

AC Circuit Elements Resistors Same elements as used for DC circuits Ohm’s law still valid for AC currents Voltage difference across a resistor with AC current flowing through it: SI unit of resistance: Ohm (Ω)

AC Circuit Elements Capacitors (Energy Storage Devices!) Two conductors (plates) separated by a gap, One plate has +Q and the opposite has -Q Definition of capacitance: The ability of a body to store electric charge Capacitance is a constant that only depends on plate geometry (shape, spacing,…) SI unit of capacitance: Farad (F)

Activity/Example: Parallel Plate Capacitor Capacitance of a parallel plate capacitor: Area of plate’s face = A Separation distance between the plates = d ε0 = 8.854 x 10-12 F/m = permittivity of free space Use this formula to calculate the capacitance of a metal plate capacitor Consider a plate measuring 6 in. by 6 in. Calculate capacitance for d=0.2 mm and 2 mm Which is a better storage device?

AC Circuit Elements Capacitors Store energy in the electric field generated between the plates from the separation of (+) and (-) charges Voltage difference across a capacitor in an AC circuit, means that the charges have potential energy. The stored energy turns out to be U = ½ CV2

AC Circuit Elements Inductors (Solenoids) - Energy is stored in a magnetic field due to the current, and this can prevent current from changing rapidly in some circuits. Coils of wire -Current passing through generates magnetic field Quantified by inductance SI unit of inductance: Henry (H)

Example: Inductance of a Solenoid Consider a cylindrical solenoid that is 10.0 cm long, with a radius of 0.50 cm. Calculate the inductance of this coil, if it is also known that there are 200 turns of copper wire in the solenoid. Use the formula: with μ0 = 4π x 10-7 H/m

Example: Magnetic Field in a Solenoid Suppose the solenoid from the last example is connected to a DC power supply that passes 1.00 mA of current through its wires. What is the value of the magnetic field at the center of the solenoid? Use the formula: where the density of turns (turns per unit length) for the coil is given by

Transformers and Mutual Inductance Transformers on power poles step down the voltage before it goes into your house!

AC Power Supply Generates AC voltage wave (often sine wave)