Physics Homepage


The chapter on electric circuits can be broken down into thee parts: current, voltage and resistance. In order to explain what happens inside any electrical circuit involving simple resistors we will develop three models around these three concepts.

The first and simplest model is that of current. Current is simply the flow of charges inside a wire. It is abbreviated with the symbol “I”. That there is something inside a wire that has a direction (and therefore constitutes a flow) became clear when we held a compass next to a wire attached to a battery. The needle reacted to the wire and it flipped when we turned the battery around. Right now we consider the battery only as the cause for the current.

Next we hooked up light bulbs into a simple series circuit, one light bulb after another light bulb. Two light bulbs are in series, if when you add the second on into the circuit you need to break the circuit in order to put it in. By analyzing this very simple circuit a few things became clear. We need a complete circuit made out of metal parts in order to light the bulbs. If the circuit is broken anywhere the bulbs do not light up. We used the brightness of the light bulbs as an indicator for the current – more flow, more glow. The brightness of the light bulbs is the same, meaning current does not get used up in the first light bulb and what’s left makes it to the second.

Also, the more bulbs are added in series the dimmer they ALL get. That means a light bulb represents an obstacle to the flow of charges – the current.

The bigger the obstacles (the more light bulbs) the flow there will be. We will call these obstacles resistance, and thus found out that current and resistance are inversely proportional. To the right are four figures showing a single bulb circuit, as well as a series circuit and a parallel circuit. The two bulbs in eries are less bright than the rest, inidcating that less current is flowing through them.


The second experiment involved a different circuit we called parallel. The two light bulbs were both hooked up directly to the battery, you could add a third one in parallel without breaking the circuit. We found that these two light bulbs were shining brighter than the two in series; in fact they were just as bright as a single bulb. That means that all bulbs, the two in parallel and the single one, all have the same current. That means that the more light bulbs are hooked up in parallel the more current will flow through the battery, which means that the resistance is decreasing.

To sum up: If a light bulb is added in series the resistance increases, if one is added in parallel the resistance decreases and current and resistance are inversely proportional.


There are many analogies you can use in order to explain what happens to the current in a circuit. In class we used revolving doors, one behind another, or two next to one another, as well as cashiers in a supermarket, who are always in parallel unless you are talking about Costco. We also used immigrations and customs at the airport. If you understand how the flow of people through doors or going through customs you have an idea of how the current moves inside a circuit. 


Following the flow around inside a circuit allows us to predict the brightness of light bulbs in simple circuits. All we have to know is how current splits up at a junction (a cross of two wires) and that more current will result in a more brightness.


Using the current model allows us to solve most of this problem. The current leaves the battery and all of it goes through A. The current then splits after A, meaning that the remaining bulbs have to get less current. At junction 1 the current splits with less going through the branch with B and D, because this branch has more resistance. More current will go through C, Therefore C is brighter than B and D. The current splits again (but evenly this time) at junction 2 and E and F will get half the current that C got. So, B and D and E and F are all less than C, but using only the current model you cannot figure out which of these two pairs is greater.


Website maintained by Volker Krasemann.