Circuits
Schematic diagrams depict the construction of a circuit Uses symbols to represent specific circuit elements Documents how elements are connected so that anyone reading diagram can understand the circuit s construction and maintenance Symbols are fairly standard, but variations do exist
Connected elements of an electrical device form a circuit Path through which charges can be conducted Components that dissipate energy: loads Simple circuit: source of potential difference and electrical energy, wires, load(s) Wires assumed to dissipate negligible amount of energy
If charges can flow from one terminal of potential difference source to other without interruption: closed circuit Potential difference exists and current flows
Without complete path, there is no potential difference No flow of charge No current Open circuit Load does not operate Switches used to selectively open and close circuits Fuses and circuit breakers open circuits automatically in dangerous situations
Without a load, resistance in circuit is very low Current increases due to lowered resistance High current flow can overheat wires and cause fires Short circuits route charge around loads and create increased currents
All circuits require source of potential difference and electrical energy Batteries and generators are common sources of potential difference Any device that increases potential energy of charges in a circuit is a source of emf emf: energy per unit charge supplied by a source of electric current Charge pump: forces charges to move in a certain direction
For conventional current, terminal voltage is less than emf emf source has internal resistance to charge flow Acts as both an emf source and a resistor Potential difference across terminals actually less than emf Potential difference listed for emf sources should be interpreted as the terminal voltage and not the emf value of the source
When charges leave emf source, encounters with loads and resistors dissipate energy Potential decrease across loads must equal potential increase across battery Conservation of energy must hold in circuits
Multiple loads can have different arrangements in circuits Series, parallel Series circuits: when charges move through single path Must go through each load in order Law of conservation of charge says that same charge that moves through one load must move through other loads
In series circuits, current through each load is the same Same charge must enter and exit loads in same time interval to prevent accumulation or disappearance of charge at any given point Total current in series circuit depends on how many resistors are present and the magnitude each resistor possesses Total current found by summing individual resistances to determine total resistance of circuit
The equivalent resistance of a series circuit is always greater than any individual resistance Use Ohm s Law to find current in circuit: ΔV = IR eq Charges dissipate energy through each load Potential across each load in a series circuit varies Use Ohm s Law to find potential difference across each load: ΔV load = I circuit R load Series circuits require all elements to function for current to flow Burnt filament, broken wire, open switch
Alternate pathways for charges to travel from and to potential difference source Loads or resistors in parallel have same potential differences across them Charges carrying same amount of energy from potential difference source to each load Potential difference equal to terminal voltage of potential difference source
Current through each load can vary Different numbers of charges move through different loads Loads with less resistance experience more current Sum of current in parallel circuits equals total current I total = I resistors Use terminal voltage and individual resistance value to find current flow through load
Total equivalent resistance for parallel circuit calculated using reciprocal relationship 1/R eq = 1/R 1 + 1/R 2 + 1/R 3 Equivalent resistance for parallel circuit always less than the smallest individual resistance Adding resistors in parallel lowers overall resistance overloading parallel circuit can increase current to dangerous level Parallel circuits do not require all elements function alternate path(s) for charges
Series and parallel circuits often combined in overall electrical scheme When determining equivalent resistance for complex circuit, simplify circuit into groups of series and parallel resistors and find equivalent resistance of each group Recombine equivalent groups into simplified circuit and analyze circuit as a whole Work backward to determine current and potential difference across parts of circuit