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Electricity section 5: Part 5.4 Investigating the properties of a parallel circuit compared to a series circuit

Doc Brown's Physics exam study revision notes: There are various sections to work through, after 1 they can be read and studied in any order.

INDEX for notes comparing the structure & function of series & parallel circuits


5.4 Investigating the properties of a simple parallel circuit

(compare with the series circuit 34)

  • Know and understand that for components connected in parallel:

    • the potential difference across each component is the same,

    • the current flow is split, but the total current through the whole circuit is the sum of the currents through the separate components,

    • and the current flow through each resistor is different, unless any of the resistors have an identical resistance - in which case they will both experience the same current flow.

    • each component wired in parallel can act independently, i.e. each one can be switched on and off irrespective of the others AND if a component fails, the others still work!

    • Reminder:  parallel circuit and  series circuit

    • 1 ammeter, 1 switch, 2 cells in series with pairs of ammeters and bulbs wired in parallel.

In an exam you might not be told whether the components are wired in series or parallel - you are expected to know!

In a parallel circuit, each component is connected separately to the terminals of the power supply e.g. the +ve and -ve terminals of a dc supply (as in the circuit diagram 35 below).

If you disconnect one component, e.g. one of the resistors, the other components will keep working.

That is why most electrical appliances include parallel circuit systems and enables you to switch individual things 'on and off' without affecting other devices.

Practically everything in a car is wired in parallel and is independently controllable e.g. heater, windscreen wiper, headlamps etc. It would be bad news if one thing failed and the rest did the same!

In reality, most circuits are a mixture of series and parallel wiring.

diagram of circuit to investigation resistors wired in parallel

PARALLEL circuit  of two resistors

You can include a variable resistor in the circuit to vary the p.d. and current and make a varied series of readings to verify the pattern of resistance - which should stay constant if they don't heat up ...

... and check that the potential difference across each resistor is the same.

R1 and R2 are resistors (e.g. any device/resistance) wired in parallel and again I'm assuming the circuit is powered by a 12.0 V d.c. supply from batteries or power pack.

Reminder: Apart from the resistors wired in parallel, if any other part of the circuit is disconnected, then the whole circuit fails - no current can flow

Reminder: Wiring in parallel means each component is independently connected to the positive and negative terminals of the power supply - in this case two resistors on separate loops ...

... therefore if either resistor 1 or 2 fails or removed-disconnected breaking that part of the circuit, the other resistor will continue to function. This is one reason why parallel circuits tend to be more useful than series circuits.

It also means you can control each component independently of the other.

For example, look at circuit 14 on the right (ignoring the ammeters!).

This is a simple way to control two lamps wired in parallel.

After closing switch s3 (power supply on) you can close s1 or s2 to select the lamp to be lit b1 or b2.

Actually, the vast majority of circuits of practical use include a mixture of series and parallel wiring.

Ammeters: Each resistor R1 and R2 is wired in series with its own ammeter A1 and A2.

Ammeter A3 is wired in series with everything else.

 Therefore the possible ammeter readings are:   I1, I2 and I3 A (amps).

Ammeters are always wired in series with any component whose current is being measured.

Voltmeters V1 and V2 are wired in parallel with each resistor to measure the potential difference (p.d.) across it.

Reminder: Wiring in parallel means each component is independently connected to the positive and negative terminals of the power supply - i.e. on separate loops ...

AND this is how you must always wire a voltmeter in a circuit,

and ammeters are always wired in series, at any point in any circuit.

Using some basic rules and Ohm's Law (V = IR) I'll show you how to work out all sorts of things from just only the three pieces of data given.

diagram of circuit to investigation resistors wired in parallel

(a) The current flowing through each resistor and the rest of the circuit

Since the resistors are wired in parallel, they have the same maximum source p.d. across them (V1 = V2 = 12.0 V in this case).

(You couldn't say this for the series circuit where the total p.d. is divided up between the resistors.)

Another consequence, of wiring components in parallel, is that the current is split between the two or more component sections ('loops') of that part of the circuit.

Calculations using Ohm's Law equation: I = V/R

I1 = Vtotal/R1 = 12.0/6.0 = 2.0 A    and    I2 = Vtotal/R2 = 12.0/2.0 = 6.0 A

You can add together the separate currents in the parallel part of the circuit to obtain the total current, which must be the same in the rest of the circuit.

I3 = I4 = I1 + I2 = 2.0 + 6.0 = 8.0 A

Note:

(i) The current is shared between the two branches in the circuit.

(ii) The larger the resistance, the smaller the current:

The 2 ohm allows a flow of 6.0 A and the 6 ohm allows 2.0 A.

(iii) Comparison with the series circuit 34 shows that the two same resistances wired in parallel, offer a much smaller resistance than when they are wired in series and the current is greater.

(iv) If the two resistors R1 and R2 had the same resistance, ammeter readings I1 and I2 would be equal, for example if they were two identical bulbs, they would glow with the same brightness.

INDEX-notes comparing structure & function of series & parallel circuits


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