Introductory physics: circuits

Electrical devices (TVs, stereos etc.) are connected to a voltage supply by an electrical circuit. The only difficult thing about circuits is that devices can be connected either in series or in parallel.

If connected in series, the same current runs through each device since there is no alternative path. However, the voltage across each device is different: from V = IR, the largest voltage drop will be across the largest resistance (just as the largest energy drop occurs across the largest waterfall in a river). As you might expect, the total resistance (or load) of the circuit is the sum of the individual resistances.

On the other hand, electrical devices can also be connected in parallel. In this case, each device is connected directly to the terminals of the voltage source and hence experiences the same voltage. Here, there will be a different current through each device since I = V/R. A counter-intuitive aspect of parallel circuits is that the total resistance of the circuit is lowered as you add in more devices (the physical reason is that you are increasing the number of alternate paths the current can take).

Parallel circuit: each device is connected directly to the battery terminals

Which is more useful? Household electrical devices are connected in parallel because it is easier (for the manufacturer) if every device sees the same voltage and it also turns out to be more efficient from the point of view of power consumption.

A more complicated type of circuit is the combination circuit: here some resistors are connected in series, others in parallel. In order to calculate the current through a given device, the trick is to replace any resistors in parallel with the equivalent resistance in series and analyse the resulting series circuit.

Combination circuit


Assuming a resistance of 100 Ohms for each of the resistors in the combination circuit above, calculate the current through each if a voltage of 12 V is applied.

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Filed under Introductory physics, Teaching

10 responses to “Introductory physics: circuits

  1. cormac

    It’s all made up, I swear

  2. I want message for solving physic problem.

  3. cormac

    I will delay the solutions until my students have sat their exams!

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  5. Hello,

    I have a website for electric circuits simulation, its based on Flash 10, I will be glad if you just go and see it:

    Also, you can submit lessons and save new circuits

    I will be glad to hear from you!
    Would you be interested to write about my lab ? it has Ohmmeter, Voltmeter, and Amper meter as well withe realistic sticks :)


  6. shubhi

    Thanks it was useful to me

  7. A cirtain ckt is composed of two parallel resistor. Tha total resistance is 1403 ohm one of the resistor is 2k ohm the other resistor will have……..value.

  8. Grant

    Your picture is wrong, Electrons flow from the negative terminal of the battery NOT the positive. Please check for simple mistakes like this before misinforming other.

    • cormac

      The convention, since 1900, is to draw circuits this way. Surprising, I agree. Try not to state your opinions as facts, very irritating