Lab AC Circuits Integrated Science II Applications of

Lab: AC Circuits Integrated Science II

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

Applications of AC Circuits • Examples: Ø Alternator (AC generator)

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

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

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

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

Activity/Example: Parallel Plate Capacitor • Capacitance of a parallel plate capacitor: Ø Area of plate’s face = A Separation distance between the plates = d ε 0 = 8. 854 x 10 -12 F/m = permittivity of free space Ø Use this formula to calculate the capacitance of a metal plate capacitor Ø Consider a plate measuring 6 in. by 6 in. Ø Calculate capacitance for d=0. 2 mm and 2 mm Ø 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 Ø 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 CV 2

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. Ø Coils of wire -Current passing through generates magnetic field Ø 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 μ 0 = 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 m. A 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 Ø Generates AC voltage wave (often sine wave)