*provide an obstruction to the flow of electricity around a circuit. A voltage is dropped across a resistor, dependant on the current flowing through the resistor. Here are some useful formulae:*

**Resistors****Voltage Dropped across a Resistor:**

**, where I is in Amps, and R in Ohms**

*V = IR*This can be re-arranged to give Current or Resistance if Voltage is already known (see ohms law).

**Power Dissipated in a Resistor:**

^{2}R or V

^{2}/R where P is in Watts

Always make sure the resistor's power rating is not exceeded!

**Resistors in Series:**

R = R

_{1}+ R

_{2}= R

_{3}.....

Note that the total resistance should be greater than that of any of the individual resistors.

**Resistors in Parallel:**

(1/R) = (1/R

_{1}) + (1/R

_{2}) + (1/R

_{3}) .....

If you have only 2 resistors in parallel you can use:

R = (R

_{1}R

_{2}) / (R

_{1}+ R

_{2})

Note that the total resistance will be less than that of any of the individual resistors.