Saturday, 21 June 2014

Build Your First Robot - Resistors and Math

In the last post we came to know what a resistor is and why it is used in electronics. In today's post we will learn about the math that is associated with these resistors. The unit of resistance is called ohms.

Voltage is directly proportional to current, which means more the voltage, more will be the current. Hence
V/I = R

where V is potential difference (voltage)across the resistor,I is current and R is the resistance.

We will know apply the formula we just saw.
1. Say you have a resister wired up in a closed circuit. The potential difference is 6 volts and the current flowing through the circuit is 2 amperes. Calculate the resistance of the resistor.
Ans- We are given that the potential difference across the 2 ends of the resistor is 6v. We also know that the current flowing through the circuit is 2 amps. Hence by using the formula V/I =  R
6/2 = 3 ohms.

Note:- While using the formula, all the units must be in SI system.

2. An led operates at 3 v and requires current not more than 20 milli-amperes. Calculate the value of the resistor if the led is connected to a circuit having a potential difference of 6 v.
Ans- This problem is a little different . You cannot directly apply the formula here. First we need to know the potential difference across the 2 ends of the resistor. We know that the led requires 3v. Hence from 6v, 3v must be reserved for the led and therefore we are left with 3v. This is the potential difference across the resistor. Now we need the current to not exceed 20 milli-amperes. Hence by using the formula
V/I = R
3/0.02 = 150ohms