Ohms Law

We looked at calculations for series circuits using both Kirchoff and Ohm's Laws.

Consider the circuit below.

First we can find the total circuit resistance. Because the resistors are end to end in a series circuit, to find the total resistance is the sum of all the resistances:

Rt = R1 + R2 + R3
    = 2300 + 7500 + 500
    = 10300 Ω
    = 10.3 KΩ

Now that we know both the total resistance of the circuit and the supply voltage we can work out the current flowing in the circuit using ohm's law.

      V
I = R
      24
   = 10300
   = 2.3mA

Because this is a series circuit the current will be the same through each resistor, so now we know the resistance and current of each resistor we can use ohm's law to calculate the voltage dropped over each resistor.

V1 = IR1
     = 2.3 x 10-3 x 2300
     = 5.29v

V2 = IR2
     = 2.3 x 10-3 x 7500
     = 17.25v

V3 = IR3
      = 2.3 x 10-3 x 500
      = 1.5v

We can then check our answer using Kirchoff's Voltage law (all voltages dropped in a circuit must equal the supply)

Vt = V1 + V2 + V3
    = 5.29 + 17.25 + 1.5
    = 24v                                  So our calculations are correct! :)

You can use a variants of this method to find out missing information from circuits.  You may find it useful to write down all the information that you do know at the side of your paper so that it is more obvious where the gaps are and therefore which calculations you need to do.