Electricity: series and parallel circuits

Electricity

What does a circuit in series look like? What are the characteristics?

• Components are connected end to end in one loop.

• Voltage is shared across all components (depending on their resistance bc of V=IR. So components with higher R will have a higher V)

  • Vtotal = V1 + V2 + V3…

• Current is the same everywhere in the circuit

  • Measure current with an Ammeter (place in series). Bc current is te same everywhere, we can place it anywhere.

• Resistance: total resistance is the sum of the R of all components

  • Rtotal = R1 + R2 + R3…

What does current depend on? How? (Lo pongo en un flash card distinto porque es un objective (2.8) en el spec)

Current in a series circuit is the same everywhere; it is not used up

Voltage & the number and nature of components (resistance)

• As voltage increases the current also increases.

  • If a second identical cell is added in series, the voltage will double and so the current will also double (= all bulbs in the circuit are brighter)

• In general, the more components in a circuit, the lower the current.

  • More components = more resistance = less current = less bright

  • By “nature” it means how much resistance that component has by itself

What are characteristics of series circuits (containing bulbs)?

Textbook:

• One switch placed anywhere in the circuit turns all the bulbs on and off (advantage)

• If one component breaks/stops working, the whole circuit stops working as it is incomplete/there is a gap (disadvantage)

  • Very bad downside so very few things are connected in series

• Voltage (energy) is shared between all bulbs, so the more bulbs you add the less bright they become

No textbook:

• More components = more resistance = less current = less bright

• Fewer wires, cheaper and easier to assemble

• Suitable for low power devices

  • Voltage is shared

What does a circuit in parallel look like? What are the characteristics?

• Components are connected to the power supply in separate branches (more than one loop)

• Potential difference/voltage is the same everywhere.

  • If battery has 12V, loop one will have a voltage of 12V, loop two also 12V, etc

• Current is shared between each loop.

  • I total = I1 + I2 + I3…

    • So if a circuit with 2 branches has a total current of 4A, we can expect that the sum of the current flowing in both branches = 4A.

    • So, loop A could = 3A and loop B = 1A or both could be = 2A, depending on their resistances. Bc of formula I = V/R, higher resistance means it has a lower current bc it is harder for charge to flow.

  • Also there are junctions where the current splits or rejoins.

    • So, the total current going into a junction must equal to the total current leaving it.

    • For example, in diagram, current that enters junction P = 0.6A. The current that leaves is 0.4A + 0.2A = 0.6A

    • Also current in each loopy loop is the same (colours). E.g all path in blue has a current of 0.2A

• Resistance in parallel circuits: adding more resistors in parallel = lower total resistance

  • This is bc there are more paths for the current to pass through

Characteristics of parallel circuits

• Switches can be placed in different parts of the circuit to switch each bulb on and off individually or all together

• If one bulb breaks, Only the bulbs on the same branch of the circuit will be affected, the rest continues functioning.

• Each branch of the circuit receives the same voltage, so if more bulbs are added to a circuit in parallel (add a new branch), they all keep the same brightness.

Not textbook:

• Useful for high powered devices.

What uses do we see series or parallel circuits in?

• Serie circuit for decorative lights/fairy lights

  • Each bulb only needs low voltage so even when the V from the mains supply is shared, they still get enough energy to produce a light.

  • All switch on/off at the same time

  • However if the filament in one of the bulbs break, then the rest will also go out

• Parallel circuit for domestic lighting

  • If one bulb stops working, the other light bulbs continue to work as they are still in complete circuits.

  • Also components can be switched on/off independently

  • The brightness of each light does not change when other lights are on/off