Topic Menu

Cyber-chess a chess instruction site

Steps to analyse a circuit

Step 1: Split it into strands

A ‘strand’ is a route to/from the power supply that does not branch off midway. Therefore if you have parallel components in series with other components you have to simplify that arrangement before you can do this step.

In the above example:

Strand 1 has two resistors in series they will therefore share the voltage drop from the supply.

Strand 2 only has one resistor it will therefore get the entire voltage drop from the supply.

Strand 3 poses a problem. The equivalent resistance of the two in parallel has to be found first. Once this has been done the strand will be like strand 1 – two in series.

 

is equivalent to a single resistor of R / 2 (see the ‘tip’ above) we can therefore re-sketch the circuit or (to save time in an exam!) mark on it in such a way as to indicate this.

 

Step 2: Share out the voltages

Pure parallel components can be split into separate routes back to the power supply; therefore they each become strands of the circuit. You need to simplify resistor arrangements until you have a simple parallel circuit.

Each strand has access to the full potential difference provided by the power supply. Whatever a voltmeter around the power supply reads is what a voltmeter around the components on that strand will read. We say that the potential or voltage drop across the components is the same as the potential drop across the power supply.

That means that the whole of strand 1 (the one with R 1 in it) will have the same voltage across it as the battery does – the reading on the voltmeter. The ammeter and resistor together share the potential drop from the battery.

The same is true for each of the three strands in our circuit (from above).

Rather than draw in lots of voltmeters get into the habit of putting arrows across each strand to show how the voltage is distributed. It is a good idea to have a marker pen (not green or red!) with you in the exam to do this when you have circuit diagrams to deal with.

Strand 1 is easy – two equal resistors so they get half each.

Strand 2 is easy too, as the entire potential drop is across one resistor.

Strand 3 is a bit trickier. V is shared across 1.5 R, therefore each R gets V / 1.5 = 2 / 3V .

That makes R get 2 / 3V and R / 2 get 1 / 3V

Step 3: Work out the current through a component

Once we have the value of each resistance and the p.d. across each one it is easy to work out the current passing through a component.

We just use V =I R.

 

Basic ideas and terminology for circuit analysis

When it is okay to ignore the current through a voltmeter

Why it is okay to take the voltage drop across an ammeter as zero

Calculating the heat dissipated in a component

 

Ready for some questions?

Click on the graphic - Short Response questions and answers at A level standard await you!

For more topics that have practice questions associated with them see the upper left menu bar