Electrical Chart

Ohm's Law

Say that you’re wiring a circuit. You know the amount of current that the component can withstand without blowing up and how much voltage the power source applies. So you have to come up with an amount of resistance that keeps the current below the blowing-up level.

In the early 1800s, George Ohm published an equation called Ohm’s Law that allows you to make this calculation. Ohm’s Law states that the voltage equals current multiplied by resistance, or in standard mathematical notation.

V = I x R

you can rearrange its elements so that if you know any two of the three values in the equation, you can calculate the third. So, here’s how you calculate current: current equals voltage divided by resistance, or

I = V/R

You can also rearrange Ohm’s Law so that you can calculate resistance if you know voltage and current. So, resistance equals voltage divided by current, or

R = V/I

For example using a circuit with a 10-volt battery and a light bulb (basically, a big flashlight). Before installing the battery,you measure the resistance of the circuit with a multimeter and find that it’s 100ohms. Here’s the formula to calculate the current:

I = V/R =  10 volts/100 ohms =0.1 amps (or100mA)

The VIR triangle

You can use the VIR triangle to help you remember the three versions of Ohm's Law.Write down V, I and R in a triangle like the one in the yellow box on the right.

• To calculate voltage, V: put your finger over V,this leaves you with I R, so the equation is V = I × R
• To calculate current, I: put your finger over I,this leaves you with V over R, so the equation is I = V/R
• To calculate resistance, R: put your finger over R,this leaves you with V over I, so the equation is R = V/I
  
  

  Understanding Electrical Power

  Electrical power is what drives a motor or produces sound through speakers or provides light through a light bulb. The current alone can not produce energy as the current is the movement of electrons and when the voltage is absent (V=0) current is static (in fact it does not exist). On the other hand voltage alone without current is static and can not be beneficial for driving electrical appliances, actually a million volts static voltage won't harm you. Hence power is directly proportional to both current (I) and voltage (V) of a circuit. It is inversely proportional to the impedance (resistance) (R) of the circuit. For DC circuits, power can be calculated as follows:
  
  
  P = V x I
  or
   P = V²/R
  or
  P = I² x R