Series and parallel

• Components in Series & Parallel Circuits

  • Current in Series & Parallel
  • Voltage in Series & Parallel
  • Resistors in Series
  • IV Graphs
  • Electrical Components
• Current in Series Circuits

  • The current is the same value at any point in a closed-loop circuit
  • All components in a closed-loop have the same current
  • The amount of current flowing around a series circuit depends on the voltage of the power source and the number (and type) of components in the circuit
  • Increasing the voltage of the power source drives more current around the circuit
  • Decreasing the voltage of the power source reduces the current
  • Increasing the number of components in the circuit increases the total resistance, hence less current flows through the circuit
• Current will increase if the voltage of the power supply increases, and decreases if the number of components increases (because there will be more resistance)
• Current in Parallel Circuits


  • A junction in a parallel circuit is where two or more wires meet
  • The current is always split at a circuit junction
  • The current is conserved, meaning the amount of current flowing into the junction is equal to the amount flowing out of it
  • The current in each branch will only be identical if the resistance of the components along each branch is identical
  • Current behaves in this way because it is the flow of electrons, which cannot be created or destroyed
  • When electrons reach a junction, some will go one way and the rest will go the other
• Worked example

  1. Step 1: Recall that at a junction, the current is conserved

  2. Step 2: Consider the first junction in the circuit where current splits

  3. Step 3: Calculate the missing amount of current
• The question says that 6 A flows through ammeter A
• The remaining current flowing through ammeter A must be: 10 A - 6 A = 4 A
• Therefore, 4 A flows through ammeter A
• The direction of current flow is super important when considering junctions in a circuit. Current flows from the positive terminal to the negative terminal of a cell/battery
• Series Circuit

  • The current is the same at all points through each component
  • The total potential difference of the power supply is shared between the components
• Series Circuit


  • Lamps connected in a series circuit
• If the battery is marked 12 V, then the potential difference would be 12 ÷ 2 = 6 V across each lamp in a series circuit
• Parallel Circuit

  • The total current through the whole circuit is the sum of the currents through the separate components
  • The potential difference across each component is the same
• Parallel Circuit


  • Lamps connected in a parallel circuit
• If the battery is marked 12 V, then the potential difference would be 12 V across each lamp in a parallel circuit
• Advantages of Series Circuits

  • All of the components can be controlled by a single switch
  • Fewer wires are required
  • Components cannot be controlled separately
  • If one component breaks, they will all stop working as well
• Disadvantages of Series Circuits

  • All components have the same voltage as the supply, so harder to control if components need to have different voltages
• Advantages of Parallel Circuits

  • The components can be individually controlled, using their own switches
  • If one component breaks, then the others will continue to function
• Disadvantages of Parallel Circuits

  • Many more wires involved so much more complicated to set up
  • All components have the same voltage as the supply, so harder to control if components need to have different voltages
• The current does not always split equally in a parallel circuit – often there will be more current in some branches than in others
• The current in each branch will only be identical if the resistance of the components along each branch are identical
• The voltage across two components connected in parallel is always the same
• Increasing the number of resistors in series increases the overall resistance, as the charge now has more resistors to pass through
• Resistors in Series

  1. The total resistance is equal to the sum of their individual resistances

  2. The total voltage is also the sum of the voltages across each of the individual resistors

  3. In a series circuit, the voltage of the power supply is shared between all components
• Three resistors connected in series. The total voltage is the sum of the individual voltages, and the total resistance is the sum of the three individual resistances
• The combined resistance R in the following series circuit is 60 Ω. The resistance value of R is 20 Ω
• In a series circuit, current is conserved, so it is the same size if measured anywhere in the series loop
• The cell supplies a current of 2 A to the circuit, and the fixed resistor has a resistance of 4 Ω. The current flowing through the fixed resistor is 2 A
• The voltmeter reads 8 V across the fixed resistor
• As the potential difference across a component is increased, the current also increases because potential difference and current are proportional
• The precise relationship between voltage and current is different for different components and can be shown on an IV graph
• IV Graphs


  Graphs that show the relationship between current (I) and voltage (V) in electrical components
• As the potential difference across a component is increased, the current also increases
• Potential difference and current are proportional
• IV graph


  Shows the relationship between voltage and current for different components
• Components shown on an IV graph


  • Fixed resistors & wires
  • Filament lamps
  • Diodes
• Fixed Resistors & Wires

  • The current through a fixed resistor or a wire increases as the potential difference (or voltage) across it increases
  • Current is directly proportional to the potential difference for a fixed resistor (or a wire)
  • The resistance of the fixed resistor (or wire) stays constant
• IV graph for a fixed resistor shows a straight line through the origin, indicating direct proportionality between current and potential difference
• Filament Lamps

  • Current and voltage are not directly proportional
  • Resistance of the filament lamp increases as the temperature of the filament increases
  • IV graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference
• Resistance increases as the graph curves in a filament lamp IV graph