# Electricity

• Electricity
• Current
• Current is the flow of electric charge. It will only flow around a closed circuit if there is a voltage.
• The unit of current is an amp
• In a single, closed loop the current has the same value everywhere
• Voltage is the driving force that pushes the current around the circuit
• The unit is the volt
• Resistance is anything that slows down the flow
• It has the unit ohm
• The greater the resistance upon a component, the smaller the current that flows
• The equation for current is: Charge flow = current x time or Q=It
• Resistance
• The equation for this is: Voltage = current x resistance or V=IR
• An ammeter measures the current flowing through a wire. It must always be placed in series with whatever your investiagting
• Current is measured in amps
• The voltmeter measures the potential difference across a wire. It must always be placed in parallel with what you're investigating
• Potential difference is measured in volts
• I-V characteristics
• Ohmic conductors
• The resistance of an ohmic conductor doesn't change with current. When at a constant temperature, the current is directly proportional to the voltage across it
• When an electrical charge flows through a filament lamp, it transfers some energy to the thermal store of the filament lamp, which is designed to heat up. Resistance increases with temperature so as current increases so does resistance
• For diodes, the resistance depends on the direction of current. They let current pass at a low resistance one way but at a high resistance the other way
• Graphs for ohmic conductors
• Ohmic conductor (e.g a resistor). The current through an ohmic conductor is directly proportional to voltage so you get a straight line
• Filament lamp: As the current increases, the temperature of the filament increases, so the resistance increases. This means less current can flow per unit of pd, so the graph curves
• Diode: Current will only flow in one direction. A diode has a very high resistance in the reverse direction
• Circuit devices
• LDR (light dependent resistor)
• An LDR is a resistor that is dependent on the intensity of light
• In bright light there is less resistance
• They have lots of applications including burglar alarms
• Thermistor
• A thermistor is a temperature dependent resistor
• In hot conditions the resistance drops
• Series circuits
• The total voltage is all the voltages added together
• Current is shared throughout
• Parallel
• The voltage is the same thorughout
• Electric in the home
• Neutral wire - blue. It completes the circuit and carries away current. It is at 0v
• Live wire- brown. Provides the AC supply
• Earth wire -yellow and green. For safety to carry away electricity in the event of a fault
• National Grid
• It is what distributes the electricity around the country
• Becuase production has to meet demand, power stations normally run well below the power output
• Transformers are used to distribute power along the national grid