Electricity

• what circuit symbol is this?

  A) diode

  
• what circuit symbol is this?

  A) variable resistor

  
• what circuit symbols are these?

  A) thermistor

  B) LDR

  
• what circuit symbol is this?

  A) LED

  
• For electrical charge to flow through a closed circuit the circuit must include a source of potential difference.
• Electric current is a flow of electrical charge
• The size of the electric current is the rate of flow of electrical charge.
• Charge flow, current and time are linked by the equation: $Q =$$I t$
• current has the same value at any point in a single closed loop
• The greater the resistance of the component the smaller the current for a given potential difference across the component.
• current is inversely proportional to resistance.
• What is potential difference?

  a measure of energy, per unit of charge, transferred between two points in a circuit.
• What is resistance?

  Resistance is opposition to the flow of current
• Current, potential difference or resistance can be calculated using the equation: $V =$$I R$
• To investigate the factors affecting resistance, it is important to

  • record the length of the wire accurately
  • measure and observe the potential difference and current
  • use appropriate apparatus and methods to measure current and potential difference to work out the resistance
• what is the aim of this experiment?

  A) to investigate the factors affecting resistance

  
• What results would you expect to find when investigating how wire length affects resistance?

  Resistance is directly proportional to length
• The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor.
• What does this diagram show?

  A) ohmic conductor

  
• What does this diagram show?

  A) filament lamp

  
• What does this diagram show?

  A) LDR

  
• The resistance of a thermistor decreases as the temperature increases.
• Some applications of thermistors and LDRs include temperature control and sensing and automatic lighting systems.
• The resistance of an LDR decreases as light intensity increases
• what investigation does this diagram suggest?

  A) investigating IV characteristics

  
• label the diagram.

  A) ammeter

  B) voltmeter

  C) battery

  D) variable resistor

  E) resistor

  
• Investigating IV characteristics:

  1. set up circuit
  2. vary the voltage across the component by changing the resistance of the variable resistor, using a wide range of voltages
  3. For each voltage, record the value of the current from the ammeter 3 times and calculate the average current
• Investigating IV characteristics:

  
  

  4.Increase the voltage further in steps of 0.5 V, repeating steps 2 and

  
  

  5.Make sure to switch off the circuit in between readings to prevent heating of the component and wires

  
  

  6.Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current)

  
  

  7.Replace the fixed resistor with the filament lamp, then the diode, repeating the experiment for each
• For components connected in series:

  • there is the same current through each component

  • the total potential difference of the power supply is shared between the components

  • the total resistance is the sum of all resistances.
• For components connected in parallel:

  • the potential difference across each component is the same

  • the total current through the whole circuit is the sum of the currents going through each branch

  • the total resistance can be calculated using 1/Rt = 1/R1 + 1/R2 + 1/R3 ...
• In a series circuit, resistance will increase because the current is the same in every single component in the circuit, so the more resistors we add, the harder it is for current to flow.
• The more resistors we add in parallel, the more pathways the current has to go through, so it is easier for current to flow through the circuit. This means that the overall resistance has decreased in the circuit. 
• Mains electricity is an ac supply
• domestic electricity supply has a frequency of 50 Hz and is about 230 V
• Direct potential differences produce a constant electric current that flows in one direction, while alternating potential differences produce an alternating current that changes direction regularly.
• live wire – brown
• neutral wire – blue
• earth wire – green and yellow
• The live wire carries the alternating potential difference from the supply.
• The neutral wire completes the circuit