physics electricity

• Factors affecting current through a component

  Depends on both the resistance (R) of the component and the potential difference (V) across the component
• Relationship between resistance and current

  The greater the resistance of the component, the smaller the current for a given potential difference across the component
• Ohm's Law

  V = IR, where V is the potential difference in volts (V), I is the current in amperes (A), and R is the resistance in ohms (Ω)
• If the resistance is constant, an ohmic conductor, current is directly proportional to the potential difference
• If the resistance of components such as lamps, diodes, thermistors, and LDRs is not constant, the graph is nonlinear
• Resistance increases with current as electrons transfer energy to atoms, causing them to vibrate more
• Resistance changes with temperature, length, light intensity, and voltage
• Total resistance for two resistors in parallel is less than the resistance of the smallest resistor
• Total current into a junction

  Total current in each of the branches
• Potential difference across each "branch"

  Is the same
• The total resistance for two resistors in parallel is less than the resistance of the smallest resistor
• Mains electricity in the United Kingdom has a frequency of 50 Hz and is about 230 V
• AC is alternating current, which comes from the mains
• DC, direct current, is the movement of charge in one direction only
• Wires in a plug

  • Live wire
  • Neutral wire
  • Earth wire
• Live wire

  • Brown, at 230V
  • Carries the alternating potential difference from the supply
  • May be dangerous even if mains circuit is off, as current may still be flowing through it
• Neutral wire

  • Blue, at 0V
  • Completes the circuit
• Earth wire

  • Green and Yellow stripes, at 0V
  • Only carries a current if there is a fault
  • Safety wire to stop the appliance becoming live
  • Connected to the earth and to the casing
  • If the live wire touches the metal casing of the appliance, it will become live (you’ll get a serious electric shock if you touch it, as current flows through you to the ground)
• Power

  • Energy transferred per second
  • Directly proportional to current and voltage
  • Power loss is proportional to resistance, and to the square of the current
• Energy is transferred from chemical potential in batteries to electrical energy in wires to any form of useful energy in the devices they power
• Energy transferred = Power multiplied time
• Energy transferred = Charge × potential difference
• The power, P, in watts W, the potential difference, V, in volts V, the current, I, in amperes A, and the resistance, R, in ohms Ω
• Ways electrical energy may be transferred by appliances

  • Kinetic energy for a motor
  • Thermal energy in a kettle
  • Work done when charge flows through a circuit, and is also equal to energy transfer
• Energy Transfers in everyday appliances
• Ways electrical energy may be transferred by the appliance

  • Kinetic energy for a motor
  • Thermal energy in a kettle
• Work done is when charge flows through a circuit, and is also equal to energy transferred, as all the electrical energy (ideally) gets transferred to the appliance
• Power rating of an appliance shows the power it uses in Watts, so greater power rating means it uses more energy
• The National Grid is a system of cables and transformers linking power stations to consumers across the UK
• Electrical power is transferred from power stations to consumers using the National Grid
• Transformers

  1. Change the potential difference

  2. Step-up Transformers increase the pd from the power station to the National Grid, so as the power is constant, current decreases so less energy is lost

  3. Step-down Transformers decrease the pd from the National Grid to consumers for consumer safety
• If a body has the same amount of positive and negative charge, they cancel out, forming a neutral body (i.e. protons and electrons in a neutral atom)
• Like charges repel
• Opposite charges attract
• Insulators do not conduct electricity, their electrons cannot flow throughout the material, they are fixed
• Conductors can conduct electricity, their electrons can flow, and are not fixed (they are delocalised)
• Static Electricity occurs when two insulators are rubbed together, forming a positive charge on one object and a negative charge on the other
• If conductors were rubbed, electrons will flow in/out of them cancelling out any effect, so they stay neutral
• Sparking occurs when enough charge builds up, and the objects are close but not touching, the “spark” is when the charge jumps through the air from the highly negative object to the highly positive object, to balance out the charges
• The charged objects experience a force – electrostatic force (of attraction/repulsion)