Electrical Quantities in Circuits

• In a circuit that is a closed-loop, such as a series circuit, the current is the same value at any point

  • the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part
  • all components in a closed-loop have the same current

  
• The amount of current flowing around a series circuit depends on two things:

  • The voltage of the power source
  • The resistance of the components in the circuit
• Series circuit - same current flows through each part of the circuit

  • A1 = A2
• Series circuit - PD is shared between the component of the circuit so the voltages around the circuit will always add up to the source voltage

  • V = V1 + V2 + V3
• Series circuit - resistance adds up and the bigger the resistane of a component, the bigger the share of the PD

  • Combined resistance of resistors in a series circuit is the sume of all the individual resistors
  • R = R1 + R2 + R3
• Series circuit - the sum of the current entering a junction in a circuit is equal ttot he sum of the currents leaving junction

  • I = I1 + I2 + I3
• Total PD across the components in a series circuit is equal to the sum of the individual PDs across the component

  • E = IR1 + IR2
• Cells are connected in series circuits

  • total PD across them is the sum of the individual PDs
• Increasing the voltage of the power source drives more current 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 so 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

  A) more

  B) more

  C) more

  D) less

  
• A parallel circuit consists of two or more components attached along separate branches of the circuit

  
• Parallel circuit - current is shared between branches and the total current is equal to the total of current in separate branches

  • A = A1 + A2
• Parallel circuit - PD and voltage is the same across all components

  • V1 = V2 = V3
• Parallel circuit - resistance is dependent on the amount of current through each component
• Parallel circuit - the reciprocal of the combined resistance of resistors in parallel is the sum of reciprocals of all individual resistors

  • 1/R = 1/R1 + 1/R2 + 1/R3
• 2 identical resistor in parallel have a combined resistance equal to a half of the value of one resistor
• Resistors in parallel have a combined resistance less than the value of the smallest individual resistance
• Combined resistance in a parallel circuit = resistances multiplied / resistance added
• The advantages of parallel circuits are:

  • The components can be individually controlled, using their own switches
  • If one component stops working the others will continue to function
• In a parallel circuit, the current splits up

  • some of it going one way and the rest going the other
• The current in each branch will be smaller than the current from the power supply
• At a junction in a parallel circuit (where two or more wires meet), the current is conserved

  • the amount of current flowing into the junction is equal to the amount of current flowing out of it causing charge to be conserved
• Current does not always split equally – 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
• Current don’t always split equally because of the flow of electrons:

  • Electrons are physical matter – they cannot be created or destroyed
  • The total number of electrons aka current going around a circuit must remain the same
  • When the electrons reach a junction, however, some of them will go one way and the rest will go the other

  
• The direction of current flow is super important when considering junctions in a circuit

  • remember that current flows from the positive terminal to the negative terminal of a cell / battery - helps determine the direction current is flowing 'in' to a junction and which way the current then flows 'out'
• When several cells are connected together in series, their combined EMF is equal to the sum of their individual EMFs

  
• • In a series circuit, the sum of potential differences across the components is equal to the total EMF of the power supply

  
• A parallel circuit consists of two or more components attached along separate branches of the circuit
• The advantages of a parallel circuit are:

  • The components can be individually controlled, using their own switches
  • If one component stops working the others will continue to function
• The potential difference across each component connected in parallel is the same

  • This is the opposite of the current, which is different in each branch
  • VTotal = V1 = V2 = V3...

  
• • When two or more components are connected in series:
  • The combined resistance of the components is equal to the sum of individual resistances

  A) v1

  B) v2

  C) v3

  D) r1

  E) r2

  F) r3

  
• • When resistors are connected in parallel, the combined resistance decreases and is less than the resistance of any of the individual components
  • If two resistors of equal resistance are connected in parallel, then the combined resistance will halve

  
• More generally, to determine the combined resistance of any combination of two resistors, you must use the equation:

  A) 1/r1

  B) 1/r2

  
• What is the voltage across components in a parallel circuit like?

  Same
• What is the current across components in a series circuit like?

  Same
• What happens to resistors in parallel?

  Resistance decreases