13.3 Alternating Current (AC) Electromagnetic induction requires a changing magnetic field to produce an electric current. If you were to push a permanent magnet through a coil, current flows for only as long as you move the magnet in one direction. It is not possible to move the magnet in the same direction indefinitely. At some point the magnet will need to be pulled in the opposite direction. As soon as you reverse the direction of the magnet being pulled through the coil, the electric current produced in the coil will also reverse direction. Alternating current is current that periodically reverses direction.

13.3 What is Alternating Current? Alternating current is the back and forth motion of charges. Both voltage and current continuously increase to a positive maximum and decrease to a negative minimum. This process repeats itself at a frequency of 60 Hz. This means that the current is flowing in a positive direction, reverses, then flows in a negative direction 60 cycles every second. Note the graphs go through zero twice per cycle as the current changes direction. This implies that the circuit shuts off for an instant, yet we do not notice this effect because the cycles occur too quickly for our eyes to detect.

13.3 Edison vs. Tesla Today’s electrical power grids do not rely on direct (DC) current because it is limited to how far it can be transferred without significant energy losses, due to friction, in the form of thermal energy. Direct current was the standard used in the first electrical power grids. In 1882, Thomas Edison built the Pearl Street power station in Manhattan, but he was only able to provide energy to 193 homes. Nikola Tesla, an electrical engineer who at one point worked for Edison, developed a competing system using alternating current. The AC system could transfer energy from a power plant more efficiently than the DC system. Edison and Tesla were bitter rivals who both fought to have their respective systems accepted, but in the end, it was Tesla and his AC system that became the favoured choice.

13.3 Household Circuits Your home is designed for a voltage of 240 V. Most household appliances run on 120 V, yet larger appliances, such as an electric stove or clothes dryer run on 240 V. A three-wire system is used, with red and black being ‘hot’ and white being neutral. Electricians wire homes mainly using two wires; a white and a ‘hot’ for 120 V. For appliances that require 240 V, the red and black are used. For safety purposes, a fourth wire called a ground is also used to direct stray currents into the ground, preventing them from reaching you when you touch a circuit.

13.3 Safety Systems Many safety systems exist in your home to prevent harm to you, damage to your appliances, or electrical fires. Fuses are devices that placed in series with one of the branches in your home to prevent excess current. If the current exceeds the maximum rated value, a fine wire inside the fuse melts, opening the circuit to prevent any more current. Circuit breakers also prevent excess current in a wire for extended periods of time, potentially causing a fire. A bimetallic strip heats up and bends, causing the circuit to trip, turning it off. This may occur if too many loads are connected to a branch and require more current.

13.3 Safety Systems Ground Fault Circuit Interrupters (GFCIs) are installed in bathroom outlets to detect any small difference in current going into the a circuit compared to going out. This prevents electrocution if a wet hand touches the outlet. GFCIs are designed to trip if a current difference of A is detected, as opposed to 15 A to trip a circuit breaker. Arc Fault Circuit Interrupters (AFCIs) prevent sparking or arcing, which could start a fire. If insulation around a wire becomes frayed, the bare wire could move close to another metal part causing arcing to occur. AFCIs prevent current flow if an arc is detected.

13.3 Homework Questions # 1-3, 6 p.598