Components in series and parallel circuits

• Current in series Circuits

  • In a circuit that is a closed-loop, such as a series circuit, the current is the same value at any point
  • This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part
  • This means that all components in a closed-loop have the same current

  
• what amount of current flowing around a series circuit depends on

  • The amount of current flowing around a series circuit depends on two things:
  • The voltage of the power source
  • The number (and type) of components in the circuit
  • Increasing the voltage of the power source drives morecurrent around the circuit
  • So, 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 in Parallel circuits

  • A junction in a parallel circuit is where 2 or more wires meet
  • current is always split at a circuit junction
  • current is conserved
  • means amount of current flowing into junction=amount of current flowing out as charge is conversed
  • current doesn’t always split equally
  • current in each branch will only be identical if resistance of components along each branch is identical as current is the flow of electrons
  • Electrons=physical matter – can’t be created or destroyed so total number of electrons (hence current) going around a circuit is the same

  
• Voltage in Series circuit

  • The current is the same at all points ie. through each component
  • The total potential difference of the power supply is shared between the components
  • circuit below:
  • The current from the power supply is the same as the current in both lamps I = I1 = I2
  • If the battery is marked 12 V, then the potential difference would be 12 ÷ 2 = 6 V across each lamp

  
• Voltage in parallel circuits

  • The total current through the whole circuit is the sumof the currents through the separate components
  • The potential difference across each component is the same

  circuit below:

  • Because the current splits up, the sum of currents in each branch will equal the current from the power supply I = I1 + I2
  • If the battery is marked 12 V, then the potential difference would be 12 V across each lamp

  
• advantages and disadvantages of a series circuit

  • A series circuit consists of a string of two or more components connected in a loop
  • The advantages of a series circuit are:
  • All of the components can be controlled by a singleswitch
  • Fewer wires are required
  • The disadvantages of a series circuit are:
  • The components cannot be controlled separately
  • If one component breaks, they will all stop working as well
• Advantages and disadvantages of a parallel circuit

  • A parallel circuit consists of two or more components attached across different branches of the circuit
  • The advantages of a parallel circuit are:
  • The components can be individually controlled, using their own switches
  • If one component breaks, then the others will continue to function
  • The disadvantages of a parallel circuit are:
  • 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
• Resistors in series

  • When 2 or more resistors are connected in series, the total (or combined) resistance is equal to sum of their individual resistances
  • Eg, for 3 resistors of resistance R1, R2 + R3, total resistance is calculated by:

  Combined Resistance in Series= R1+R2+R3

  • R=total resistance, in Ohms (Ω)
  • Increasing number of resistors increases overall resistance, as charge now has more resistors to pass through
  • total voltage is the sum of the voltages across each of the individual resistors
  • series circuit: voltage of power supply is shared between all components
• IV graphs explanation

  • As the potential difference across a component is increased, the current also increases
  • This is 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, including in:
  • Fixed resistors & wires
  • Filament lamps
  • Diodes
• fixed resistors and wires

  • The current through a fixed resistor or a wire increases as the potential difference (or voltage) across it increases
  • In other words, current is directly proportional to the potential difference for a fixed resistor (or a wire)
  • This relationship is true because the resistance of the fixed resistor (or wire) stays constant
  • An IV graph shows that the line is straight and goes through the origin, as shown in the image below:

  
• Filment lamps

  • For a filament lamp, current and voltage are not directly proportional
  • This is because the resistance of the filament lamp increases as the temperature of the filament increases
• IV graph for filmament lamp


  shows current increasing at a proportionally slower rate than voltage

  • because:
  • As current increases, temperature of filament in lamp increases
  • higher temperature causes atoms in metal lattice of filament to vibrate more
  • causes an increase in resistance as it becomes harder for free electrons ( current) to pass through
  • Resistance opposes current, causing current to increase at a slower rate
  • Where graph=straight line, resistance is constant
  • resistance increases as graph curves
  • Reversing the voltage reverses the current+ makes no difference to shape of curve

  
• Diodes

  • allows current to flow in one direction only
  • This is called forward bias
  • In the reverse direction, the diode has very high resistance, + thus no current flows
  • This is called reverse bias
  • IV graph for a diode is slightly different:
  • When current is in direction of arrowhead symbol, this is forward bias
  • shown by the sharp increase in voltage+current on the right side of the graph
  • When diode is switched around, this is reverse bias
  • shown by a zero reading of current or voltage on left side of graph

  
• Investigating IV Graphs Experimentally- equipment

  • In order to investigate the relationship between current and voltage different components, the following equipment is required:
  • An ammeter - to measure the current through the component
  • A voltmeter - to measure the voltage across the component
  • A variable resistor - to vary the current through the circuit
  • Power source - to provide a source of potential difference (voltage)
  • Wires - to connect the components together in a circuit

  
• Investigating IV graphs experimentally- variables

  • The current is the independent variable 
  • The variable resistor is used to change the current flowing through the filament lamp / diode
  • The voltage is the dependent variable
  • The voltmeter is used to measure the voltage across the filament lamp / diode
  • Recording measurements of current and voltage as the current increases enables an IV graph to be plotted for each component
• Resistance

  • Resistance is the opposition to the flow of current
  • The higher the resistance of a circuit the lower the current
  • Resistors come in two types:
  • Fixed resistors
  • Variable resistors
  • Fixed resistors have a resistance that remains constant
  • Variable resistors can change the resistance by changing the length of wire that makes up the circuit
  • A longer length of wire has more resistance than a shorter length of wire

  
• Electrical Components: Thermistors

  • A thermistor is a temperature-dependent resistor
  • The resistance of a thermistor changes depending on its temperature
  • As the temperature increases the resistance of a thermistor decreases and vice versa
  • low temperature= high resistance
  • high temperature=low resistance

  
• electrical components: LDRs

  • The resistance of an LDR changes depending on the light intensity on it
  • As the light intensity increases the resistance of an LDR decreases and vice versa

  
• Lamps and LEDs

  • Lamps illuminate (light up) when a current flows in a circuit
  • LEDs are types of diodes
  • This means they only allow current to pass in one direction through them and will only light if the current passes in that direction
  • LEDs also illuminate when a current flows in a circuit (provided the LED is placed in the correct direction)
  • Since both electrical components have a visual response to current, they can be used to indicate the presence of a current in a circuit