Energy in Electrical Systems 5.3

Capacitors A. Electrical device that stores energy in an electric field using a pair of parallel plates B. Potential difference source removes electrons from one plate and deposits them on the other plate C. Charge separation and the buildup of electric field and voltage continue until potential difference across the capacitor equals that of the power supply D. Two conductors separated by an insulator

II. Capacitance A. Proportionality constant between charge and potential difference (C) B. Q = C ΔV C. Charge = capacitance x potential difference between plates D. Farad (F) = coulomb per volt (C/V) E. Most capacitors—microfarads (μF) 10-6F or picofarads (pF) 10-12F

III. Potential Energy in Capacitors A. PE = ½ C ΔV2 B. Potential Energy Stored in a Capacitor = ½ (capacitance) (potential difference)2 C. 1J = 1F · V2

IV. Magnetic Fields and Induced EMF A. Electric field creates a magnetic field B. Concentric circles around a wire C. Two wires—opposite fields attract, alike fields repel D. Coil or solenoid—series of wire loops to make a helix (see p. 267)

E. Electromagnetic Induction 1. Electric field can cause current to flow and therefore produce a magnetic field. 2. If a loop of wire moves through a magnetic field, current is produced in the wire.

F. Electromotive Force 1. Induced voltage in a wire when a magnetic field moves past the wire or if a wire moves past a magnetic field 2. Increase by: a. Using a larger magnet b. Using a coil with a larger diameter c. Using a coil with more turns of the wire d. Moving the magnet faster

3. Lenz’s Law—The changing magnetic field caused by the moving magnet induces a current, and the direction of the induced current is such that its own magnetic field opposes the changes responsible for producing it.

V. Inductors A. Electrical device that stores energy in a magnetic field B. Solenoid or coil of wire VI. Inductance A. Proportionality constant (L) between the EMF and rate of change of current B. EMF = -L · ΔI / Δt C. Henry (H) = 1 volt per ampere per second (1V·s/A)

VII. Potential Energy in Conductors A. PE = ½ LI2 B. Potential energy stored in an inductor – ½ (Inductance) (current)2 C. 1 J = 1 H · A2 Controlling Energy in Electrical Systems A. Filter circuits—smooth out voltage changes or current changes B. Electric motors are frequently equipped with capacitors or inductors to help the motors start.