Electricity

• Current is the rate of flow of charge through a circuit, measured in amps (A)
• For charge to flow:

  • The circuit must be closed
  • There must be a source of potential difference (battery/cell)
• In a single closed loop, the current has the same value at any point in a circuit (series)
• The greater the resistance across a component, the smaller the current that flows for a given potential difference across a component
• The resistance of ohmic conductors(e.g. a wire or resistor) doesn't change with the current. At a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it.
• For some components and resistors, the resistance does change (e.g. diode and filament lamp).

  1. When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of filament which is designed to heat up.
  2. Resistance increases with temperature, so as the current increases, the filament lamp heats up more and the resistance increases.
• Potential difference is a measure of energy, per unit charge, transferred between two points in a circuit
• Resistance is the oppositions to the flow of electric current in a circuit
• An LDR is a resistor that is dependant on light. The darker it is, the higher the resistance. They are used in automatic night lights, outdoor lighting and burglar detectors
• A thermistor is a temperature dependant resistor. The cooler it is, the higher the resistance. It can be used in electronic thermostats or car engine temperature sensors
• In a series circuit, the potential difference is shared between components so V(total) = V(1) + V(2) + ...
• In a series circuit the same current flows through all components I(1) = I(2) = .... . The size of current is determined by the total pd of the cells and total resistance (I = V / R)
• In a series circuit the total resistance of 2 components is the sum of their resistance R(total) = R(1) + R(2)
• In a parallel circuit, the potential difference is the same across all components V(1) = V(2) = .... . This means that identical bulbs connected in parallel will be at the same brightness.
• In a parallel circuit the total current flowing around the circuit is equal to the total of all the current through the separate components I(totat) = I(1) + I(2) + ... .
• A direct current (a battery) is a current that is always flowing in the same direction. It is created by direct voltage.
• In ac supplies the current is constantly changing direction. Alternating currents are produced by alternating voltages in which positive and negative keeps alternating. The UK mains supply is an ac supply at around 230 V
• Live wire is brown. The live wire provides the alternating potential difference from the mains supply at around 230V.
• The neutral wire is blue and completes the circuit. When the appliance is operating normally, current flows through the live and neutral wires. It is around 0V
• The earth wire is green and yellow. It is for protecting the wiring and safety. It stops the appliances casing from becoming live. It doesn't usually carry a current, only when there is a fault. It is also at 0V
• The national grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers. The national grid transfers electrical power from power stations anywhere on the grid to anywhere else on the grid where it's needed e.g. homes and industry
• in a step-up transformer the potential difference decreases while the current decreases. This is to reduce energy dissipated into the surroundings.
• A step-up transformer has more turns on secondary coil than primary coil.
• Power of primary coil is power = pd * current
• Transformers are nearly 100% efficient so power in primary coil = power in secondary coil
• When certain insulating materials are rubbed together, negatively charged electrons will be scraped of one and are dumped onto another. This will leave the materials electrically charged, one with a positive static charge and the other with a equal negative static charge. Which way the electrons are transferred depends on materials used.
• The forces of attraction are known as electrostatic attraction or repulsion. This is a non contact force.
• An electrical field is created around any electrically charged object. The closer you are, the stronger the field.
• When a charged object is placed in an electric field of another object, it feels a force. This causes electrostatic attraction or repulsion. The force is caused by the electrical fields of each charged object as they interact with each other/ The force of an object is linked to the strength of the electrical field, As you increase the distance between charged objects, the strength of the field decreases and the force between them gets smaller
• Sparks are caused when there is a high enough potential difference between a charged object and the earth. A high potential difference causes a strong electrical field between the charged object and the earthed object. The strong electrical field causes electrons in the air particles to be removed (ionised). Air is normally an insulator, but when it is ionised it is much more conductive, so a current can flow through. This is a spark.