The diagram below shows a very simple series circuit:


You will probably recognise from the previous page that the components in this very simple circuits are:


  • A cell
  • A light bulb (or filament lamp)
  • A voltmeter
  • An ammeter
  • A switch


There are a number of things to consider but simplistically assuming that the cell and the filament lamp have compatible ratings (for example if the cell is 5 V and the filament lamp is rated to take a 5 V input) then closing the switch will simply illuminate the lamp. Opening the switch will cause the lamp to go out.


This is called a series circuit because the different components are connected in a line end to end between the positive and negative terminals of the cell or battery. Any break in the circuit at any point will cause the whole thing to stop working. 


When the switch is closed, electrons will flow from the cell, through the ammeter, through the filament bulb and back to the cell. Convention states that current flows from positive to negative but as we said previously, electrons do in fact flow from negative to positive. However I must reiterate again, that if you are asked this in a question you must state "positive to negative".


Again, as was said previously the size of the current is the rate of the flow of charge. Current is measured in Amperes and charge is measured in Coulombs. With time being measured in seconds I will repeat for convenience the relationship between the three:



Series circuits statement number one - "the current has the same value everywhere in the circuit"


The size of the current in a circuit will determine the rate of flow of charge. When the current in the circuit passes a particular point after a given time, the amount of charge which has flowed is given by the formula above.


Where Q is the amount of charge in Coulombs, I is the current in amperes and t is the time in seconds.


Q. In a particular electrical circuit, 25 A flowed in 10 seconds, what was the charge in Coulombs passing through the circuit?


A. This is about as simple as it gets, substitute I = 25 and t = 10 to arrive at a value of 250 C


Q. A battery charger is connected to a battery/cell and passes a current of 3 A. If the overall charge transferred to the battery/cell is 40,000 C, how long was the charger switched on for?


A. We are given a value for the current and are given a value for the charge in coulombs, therefore I = 3 and Q = 40,000. We need to rearrange the equation we are given so that t becomes the subject:



This works out to roughly 222 minutes or approximately 3.7 hours. The dot above the three in the first decimal place signifies that the three is recurring, i.e. goes on forever to the right.


Electrical charge however will only flow around a complete (closed) circuit if there is a "potential difference". The potential difference can be thought of as being the driving force that pushes the charge around the circuit and is measured in "Volts".


Q. In what direction does the current flow in a circuit?


A. Conventionally current flows from positive to negative and this is the answer that you should state if you are asked this question. However electrons are negatively charged and will transfer from negative to positive in actuality.


>> Questions <<