Circuits Chapter 17.3 Notes

What are circuits? An electric circuit is a set of electrical components connected such that they provide one or more complete paths for the movement of charges In a circuit, there is a closed-loop path for electrons to follow Because charges are moving, there is current in the circuit The conducting path produced when a load, such as a string of light bulbs, is connected across a source of voltage is called a closed circuit When there is no complete path (no source of voltage, no charge flow, and no current) the circuit is called an open circuit A switch can be added to a circuit to either open or close the circuit

Schematic Diagrams A schematic diagram depicts the construction of an electric circuit using symbols Each element used in a piece of electrical equipment is represented by a symbol that reflects the element’s construction or function Any circuit can be drawn by using a combination of these symbols

Series Circuits Electrical devices can be connected as a series circuit so that the voltage is divided among the devices When appliances or other devices are connected in a series circuit, they form a single pathway for charges to flow The amount of charge that enters one device in a given time interval equals the amount of charge that exits that device in the same amount of time Because there is only one path for a charge to follow, the current in each device is the same Devices may have different resistances, and therefore different voltages

Series Circuits If one element along the path in a series circuit is removed, the circuit will not work An open switch, burned-out light bulb, a cut wire, or any other interruption will cause the entire circuit to fail

Parallel Circuits Devices can also be connected in parallel circuits so that the voltage is the same across each device Although the voltage across each device is the same when connected in parallel, the current does not have to be the same The sum of the currents in all of the devices equals the total current in the circuit If one bulb in a parallel circuit is removed, charges would still flow to the other devices in the circuit because there are multiple pathways for current to flow

Electric Power When a charge moves in a circuit, the charge loses energy Some of this energy is transformed into useful work, such as the turning of a motor, and some energy is lost as heat The rate at which electrical energy is changed to other forms of energy is called electric power Electric power is calculated by multiplying the total current, I, by the voltage, V, in a circuit P=IV

Unit for Power The SI unite for power is the watt Most light bulbs are labeled in terms of watts (W) In terms of energy, 1 W is equal to 1 J/s; 1 W is also equal to 1 A x 1 V Another common unit of power is the kilowatt (kW); one kilowatt is equal to 1,000 W Power companies charge for energy, not power The unit of energy power companies use to track consumption of energy is the kilowatt-hour (kW·h)

Fuses and Circuit Breakers When too many devices are connected across an electrical outlet, the resistance is lowered and the wires carry more than the safe level of current—the circuit is said to be overloaded Overloaded, or short circuits, can be dangerous and cause fires To prevent overloading in circuits, fuses are connected in series along the supply path: ribbon of wire that melts when current is too large and the circuit is opened Many homes have circuit breakers: uses a magnet or bimetallic strip that acts as a switch when the circuit is overloaded; opens the circuit but the switches can be reset