Power I, VI, V I max  V max  I, VI, V I max AC Circuits use a similar definition of power as DC circuits. The one detail that need to be included is that the voltage and current may not be in phase. The power delivered to or generated by an element will be at a maximum when the voltage and current are in phase and a minimum when they are out of phase by . To determine the average power we can average the power over a single period.

The light bulb has a resistance R, and the emf drives the circuit with a frequency . The light bulb glows most brightly at 1. very low frequencies. 2. very high frequencies. 3. the frequency  = 1/ √ LC.

For the RLC series circuit shown, which statements is/are true: (i) Potential energy oscillates between C and L. (ii) The source does no net work: Energy lost in R is compensated by energy stored in C and L. (iii) The current through C is 90° out of phase with the one through L. (iv) The current through C is 180° out of phase with the one through L. (v) All energy is dissipated in R. 1. all of them 2. none of them 3. (v) 4. (ii) 5. (i), (iv), and (v) 6. (i) and (v) 7. none of the above Not energy sources Same current through R, L and C, current must be in phase!

Transformers: A transformer is used to convert the very high voltages and very low currents used in power transmission into the currents and voltages required by the end user. A transformer consists of N 1 turns of wire around one edge of an iron core and N 2 turns of wire around the opposite edge of the iron core. The iron core enhances the field from the coils and serves as a guide for the magnetic field. Primary Secondary Using Faraday’s Law If we assume the flux is the same through the primary and the secondary. For an ideal transformer the power developed in the primary coil is equal to the power delivered to the secondary coil. When N 1 > N 2 we have a step-down transformer – voltage decreases from 1 to 2. When N 1 < N 2 we have a step-up transformer – voltage increases from 1 to 2. A transformer requires an ac input.

When the switch is closed, the potential difference across R is 1. VN 2 / N VN 1 / N V. 4. zero. 5. insufficient information dc source!!