P2 - Electricity

• Current equation

  Charge = Current x time

  Q=It
• Ohms Law

  Voltage=Current x Resistance

  V=IR
• Series Circuit calculations

  I1 = I2 = I3

  VT = V1 + V2 + V3

  RT = R1 + R2 + R3
• Parallel Circuit calculations

  V1=V2=V3

  Current is shared equally between each loop of the circuit

  Resistance is lower than the lowest value resistor
• Filament Lamp - Current Voltage graph

  When the component becomes hotter, the atoms in the metal lattice vibrate more. More collisions occur between the free electrons and atoms, increasing the resistance.

  An increase in the potential difference will allow the current to increase, but only up to a certain point.

  
• Diode - Current Voltage graph

  A diode has a very high resistance in one direction. This means that current can only flow in the other direction.

  
• LDR and Thermistor - Current Voltage graphs

  Light dependent resistors or LDRs are made of semiconductor material. Their resistance decreases as the light intensity increases. LDRs can be used to switch lights on or off automatically, such as stadium lights which come on when it gets dark.

  Thermistors are also made of semiconductor material. Their resistance decreases as the temperature increases. Thermistors are used to control temperatures in many devices, such as ovens and central heating systems.

  
• Circuit Symbols

  1. Open Switch
  2. Closed Switch
  3. Bulb
  4. Voltmeter
  5. Ammeter
  6. Resistor
  7. LDR
  8. Thermistor
  9. Variable Resistor
  10. Cell
  11. Battery
  12. Fuse

  
• LDR - Resistance vs Light Level Graph

  • LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
  • In the dark, its resistance is very large (millions of ohms)
  • In bright light, its resistance is small (tens of ohms)

  
• Thermistor - Resistance V Light graph

  Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers

  • As the thermistor gets hotter, its resistance decreases
  • As the thermistor gets cooler, its resistance increases

  
• 3 Pin plug

  A plug connects a device to the mains electricity supply.

  The cable between the device and the three-pin plug contains three copper wires that are coated with plastic.

  • copper wires are good conductors;
  • plastic is a good insulator.

  
• Direct and Alternating Potential Difference

  • Potential difference leads to a current. An alternating potential difference will lead to an alternating current, whilst a direct potential difference leads to a direct current.
  • Alternating current changes direction. An alternating current (ac) changes direction all the time. The direction of current is constantly alternating between positive and negative.
  • Direct current does not change direction. A direct current (dc) does not change direction. By this, we mean that the current flows in one direction only, never reversing.
• National Grid

  The National Grid distributes electricity across the country. The National Grid connects power stations to homes, workplaces and public buildings all around the country. The electricity may be produced by a conventional power station turning a generator or by another method.

  Transformers are used to change voltages and currents in transmission lines. A transformer is formed from two coils of wire around a magnetic core. The number of coils determines whether the transformers will step-up or step-down the voltage.

  
• When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.
• When two electrically charged objects are brought close together they exert a force on each other. Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract. Attraction and repulsion between two charged objects are examples of non-contact force.
• A charged object creates an electric field around itself. The electric field is strongest close to the charged object. The further away from the charged object, the weaker the field.A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases.