4.1 - Charge and Current

Key info and definitions 
 Electric Circuit Components 
 
 
 Junction of conductors: Allows current to split; resistance is typically low but increases with temperature. 
 Conductors crossing (no connection): Allows current to flow across a circuit without connecting to the crossing wire; resistance is typically low. 
 Switch: Turns current on and off; resistance is low when closed and very high (due to an air gap) when open. 
 Cell: Provides a source of energy or e.m.f. (electromotive force); resistance is low or negligible. 
 Battery: A combination of two or more cells; internal resistance increases as more cells are added in series. 
 Terminals: Provides a connection point for a source of energy; resistance is low. 
 Lamp: Transfers electrical energy into light; resistance increases as the current increases. 
 Fixed resistor: Controls the amount of current; resistance is fixed and determined by its material (often semiconductors like silicon). 
 Variable resistor: Controls current flow manually; resistance changes based on the slider or dial setting. 
 Fuse: A safety device that melts if current is too high; resistance is typically low and depends on the wire's dimensions and material. 
 Heater: Transfers electrical energy into thermal energy; resistance is typically high. 
 Ammeter: Measures the electrical current in a circuit; resistance is very low. 
 Voltmeter: Measures the potential difference (e.m.f.) across a component; resistance is very high. 
 Thermistor: Responds to environmental temperature; resistance changes in response to the temperature of the surroundings. 
 Diode: Restricts current to one direction; resistance is low in forward bias and very high in reverse bias. 
 Light-emitting diode (LED): Allows current in one direction and emits light; resistance is low in forward bias and high in reverse bias. 
 Light-dependent resistor (LDR): Changes current based on light levels; resistance decreases as light intensity increases. 
 
 Electric Current and Charge 
 Current is the rate of flow of electrical charge. 
 
 ... where Q is charge and t is time. 
 Atoms become charged when they gain or lose electrons. 
 Conventional current travels in the  direction of flow of positive charge - i.e. the opposite of electron flow. 
 Kirchoff's Laws 
 
 The sum of electrical current into a junction is equal to the sum of electrical current out of a junction. Ensures conservation of charge, as flow of charge into a junction point = flow of charge out of the junction point. 
 In a closed loop of a circuit, the sum of potential differences is equal to the sum of emfs. Ensures conservation of energy, as energy into the circuit = energy out of the circuit. 
 
 Electron Drift Velocity 
 When conducting electricity, electrons move slowly through the wire at a drift velocity v. 
 
 ... where I is current, n is number density of the wire's conducting material, A is the cross sectional area of the wire, v is the electron drift velocity, and e is the elementary charge, 1.6e-19C. 
 The larger the value of n, the greater the conductivity of the metal.