Electricity

• An electrical circuit is a closed loop that contains a power source, such as a cell, and something for the electrons to flow through, like a wire
• Circuits are generally represented with circuit diagrams
• The simplest type of circuit consists of a cell, wire, and possibly a filament lamp
• Symbols for different components in circuits are used in circuit diagrams
• A cell symbol is a simple version of a battery, as batteries are made up of two or more cells
• A filament lamp symbol represents a small light bulb in a circuit
• A switch can be added to a circuit to either allow or disrupt the flow of current
• Key terms in circuits are potential difference, current, and resistance
• Current (denoted by the letter i) is a measure of the flow of electrons around the circuit, measured in amperes (amps)
• Potential difference (denoted by V) is the force driving the flow of electrons, provided by the cell or battery, measured in volts
• Resistance (denoted by R) opposes the flow of electrons, measured in ohms
• Current flows from the positive terminal to the negative terminal in a circuit, following the convention of conventional current
• The equation V = IR is important in the electricity topic, where potential difference or voltage equals current times resistance
• In a circuit with a current of 4 amps and a resistance of 2 ohms, the voltage of the cell must be 8 volts (4 x 2)
• For a 24-volt battery producing a current of 8 amps, the resistance can be found using the equation R = V / I, giving 3 ohms (24 / 8)
• The relationship between voltage, current, and resistance: as long as resistance is constant, increasing voltage leads to a proportional increase in current, and vice versa
• In a graph of current against potential difference, as voltage increases (e.g., using a bigger battery), the current in the circuit also increases proportionally
• Straight lines in circuits with only wires or resistors due to constant resistance; different resistors result in lines of varying steepness
• Filament lamps: as current flows through the filament, the wire heats up, emitting light and increasing resistance, resulting in a less steep curve as current increases
• Diodes: devices allowing current flow in one direction only, showing current when potential difference is positive and having high resistance in the reverse direction to prevent current flow
• To draw circuit diagrams, we need to know the different components and their symbols
• To provide electric power to a circuit, we need either a cell or a battery
• To control the flow of electricity, we can add a switch that can be closed to allow electricity to flow or open to turn off the circuit
• Filament lamps are small bulbs used in circuits
• Fuses break if too much current flows through the circuit
• Diodes only allow current to flow in one direction
• A special type of diode is the light emitting diode (LED) which emits light when current flows through in the forward direction
• Ammeters measure current and are connected in series
• Voltmeters measure potential difference and are added in parallel
• Resistors can be fixed to provide a certain number of ohms worth of resistance or can be variable
• Basic resistors can be fixed or variable
• Light dependent resistors (LDRs) have resistance dependent on the intensity of light
• In bright light environments, LDRs have very low resistance allowing lots of current to flow
• In darkness, LDRs have high resistance, limiting current flow
• LDRs are used in automatic night lights and burglar alarms
• Thermistors have resistance dependent on temperature
• Higher temperatures cause the resistance to fall
• Lower temperatures cause the resistance to increase
• Thermistors are used as temperature receptors in car engines and electronic thermostats
• charge is a measure of how much current flowed into a period of time