PHYSICAL SCIENCE Electricity Part 3: Circuits

13.3 Circuits Objectives Use schematic diagrams to represent circuits. Distinguish between series and parallel circuits. Calculate electric power using voltage and current. Explain how fuses and circuit breakers are used to prevent circuit overloads.

Defining Circuits An electric circuit is an electrical device connected so that it provides one or more complete paths for the movement of charges. In a circuit involving a battery and light bulb, electrons move through the wires and bulb from the negative terminal to the positive terminal. The battery then adds energy to the charges as they move within the battery from the positive terminal back to the negative one.

Defining Circuits The electrons follow a closed-loop path. This complete path is called a closed circuit, if the path becomes incomplete, it becomes an open circuit. The voltage source is always a part of the conducting path of a closed circuit. A switch is a device used to open or close a circuit.

Circuits – Open and Closed

Schematic Diagrams A schematic diagram is a graphic representation of an electric circuit or apparatus, with standard symbols for the electrical devices. Schematic diagrams tend to use universal standard symbols. Each element used in a piece of electrical equipment is represented by a symbol that reflects the element’s construction or function.

Schematic Diagram Symbols

Series and Parallel Circuits A series circuit provides a single conducting path. Charges cannot build up or disappear at a point in a circuit. If one element of a series circuit is removed, the circuit will not work.

Series and Parallel Circuits In a parallel circuit, components in the circuit are connected across common points, providing two or more separate conducting paths. The voltage across each device is the same, although the current can vary. The sum of the current in all devices equals the total current.

Series and Parallel Circuits If one bulb of a parallel circuit were removed, charges would still move through the other loop or loops. The break of the flow in one path does not interrupt the flow of electric charge in other paths.

Electric Power and Electrical Energy Electrical energy is the energy associated with electrical charges, whether moving or at rest. The rate at which electrical work is done is called electrical power. Electrical power (P) is the product of total current (I) in and voltage (V) across a circuit. Formula: power = current x voltage, or P=VI.

Electric Power and Electrical Energy The SI unit for power is the watt (W). The power lost (dissipated) by a resistor can be calculated by the formula… P = I 2 R = V 2 /R Electric companies track consumption of energy in units of kilowatt-hour (kWh). One kilowatt-hour is the energy delivered in one hour at the rate of 1kW. In SI units, 1 kWh = 3.6 x 10 6 J.

Fuses and Circuit Breakers Electrical wires could carry an overload – more than a safe level of current. If the insulation on wires is worn, and two wires touch, this creates an alternative pathway for current called a short-circuit. Grounding appliances reduces the risk of electric shock from a short circuit.

Fuses and Circuit Breakers A fuse is an electrical device containing a metal strip that melts when current in the circuit becomes too great. The melting of the thin metal strip opens the circuit to prevent overload.

Fuses and Circuit Breakers A circuit breaker uses a bimetallic strip that responds to current overload by opening the circuit. Circuit breakers act like a switch and can be reset by turning the switch back on.