Alternating Current – Learning Outcomes Describe alternating current in terms of voltage and current with time. Use an oscilloscope to show a.c. Discuss the use of a.c. in the national grid. HL: Solve problems about peak and rms values of voltage and current.

Alternating Current While in direct current (d.c.), current always flows in the same direction (positive to negative), alternating current (a.c.) continuously changes directions. The current does not change direction abruptly – it is a gradual process of acceleration, fitting models of waves and SHM. Thus when we graph a.c. it appears as a sine wave.

Alternating Current – Graphing For Ohmic conductors, 𝑉∝𝐼, so voltage-time and current-time graphs look the same (though may have different scales) We use lower case v and i to denote instantaneous values for voltage and current.

Oscilloscopes Oscilloscopes are instruments that show the variation in electrical signals flowing through them over time.

National Grid As mentioned in earlier, generators generate an emf by electromagnetic induction. Due to the particular way turbines work (rotating coil in a magnetic field), the emf generated is a.c. The voltage varies from +325 V to -325 V on the mains, changing direction every 1/100 seconds (thus a full cycle takes 1/50 seconds and the frequency is 50 Hz.

Root-Mean-Square With d.c. we can perform calculations using a constant current and voltage. Since current and voltage vary in a.c., various formulas cannot be used normally. We use an “rms” value for a.c. voltage and current, which are given by: 𝐼 𝑟𝑚𝑠 = 𝐼 0 2 𝑉 𝑟𝑚𝑠 = V 0 2 These are effectively average values, so we can discuss average effects of a.c. over time.

Root-Mean-Square e.g. Domestic electricity is supplied at an rms voltage of 230 volts. Find the maximum value of the voltage in any one cycle. e.g. An alternating current flowing in a wire of resistance 10 Ω produces heat at a rate of 60 W. Find: the rms value of the current, the peak value of the voltage across the wire. 400 turn oil has a resistance of 200 Ω and is connected to an a.c. supply. Over a 1 ms time interval the flux through the coil increases by 5 × 10-4 Wb. Find the average induced emf over the 1 ms interval. Find also the average current if the average voltage is 300 V.