Ohm's+Law

[[image:Ohm.jpg align="right" link="http://www.uoguelph.ca/%7Eantoon/gadgets/resistors/resistor.htm"]]
proportional to the voltage applied across it, and also inversely proportional to the resistance. The relationship between Voltage, Current and Resistance in any d.c. electrical circuit was firstly discovered by the German physicist **Georg Ohm**, (1787 - 1854). This relationship between the Voltage, Current and Resistance forms the bases of **Ohm's Law**
 * Georg Ohm** found that, at a constant temperature, the electrical current flowing through a fixed linear resistance is directly

It can be written in three ways:
 * Ohm’s law** allows you to relate voltage (v), current (I) and resistance (R). To make a current flow through a resistance there must be a voltage across that resistance.
 * ** V = I x R **
 * ** R = V / I **
 * ** I = V / R **

For example look at the diagram opposite: When connected to a battery there is a current of 100mA in a 100R resistor. What is the voltage across the resistor? Using the equation **V = I x R** Firstly convert the current to Amps from milliamp I = 100/1000 therefore I = 0.1 Amps Then calculate the voltage across the resistor V = 0.1 x 100 therefore V = 10 Therefore the voltage V is 10 volts Also known as voltage dividers, they are widely used in electronic circuits for setting and adjusting voltages - e.g. in radios, games and toys. They can be used to split the voltage of a circuit, and very often in circuits you will see two resistors connected together as show below Each of the resistors has the same value of current flowing through it. For example if there is 10mA flowing through R1 then there will be 10mA flowing through R2. However a portion of the supply voltage is dropped across each resistor.
 * Potential Dividers**

//**NOTE**// In some textbooks and some websites and
 * V**s is know as the **voltage supply** and sometimes as **V**in
 * V**o is known as the **signal voltage** and as **V**out

If each resistor is 100R and the supply voltage **V**s is 9V then because each resistor is of the same value they will each drop half the supply voltage i.e. 4.5V. Indeed if you measured the voltage across either of the resistors you would find it was half the supply voltage.

But what happens when the two resistors are of different values. The **current** **(I)** through each resistor is the same but to find the voltage at **V**o the formula below is used.


 * Vo = Vs x R2 / (R1 + R2)**

For example find the output voltage:
 * If R1 is 5R, R2 is 10R and the supply voltage **V**s is 9V.
 * **Vo =** **Vs x R2/ (R1 + R2)**
 * **V**o = 9 x 10 / (5 + 10)
 * **V**o = 6 volts

Although we have considered two fixed resistors for our potential divider they could be any two resistive components. For example one of the two resistors could be a preset and the other could be a light or heat sensor. You will see practical applications of the potential divider when we cover the unit on sensors.