Chapter 8

Ohm’s Law  George Ohm experimented with circuits and came up with a mathematical relationship relating voltage, current and resistance in a circuit.  The relationship is called OHM’S LAW. V = I · RI = V/RR = V/I

Using Ohm’s Law  A light bulb with a resistance of 2 Ω is connected to a 1.5 V battery. Calculate the current that will flow through the circuit. V = IR I = V R I = 1.5 V 2 Ω I = 0.75 A

Using Ohm’s Law  A light bulb requires 3 amps to produce light. If the resistance of the bulb is 1.5 ohms, How many batteries will you need? (Each battery is 1.5 volts) V = IR V = (3 A)(1.5 Ω) V = 4.5 V Batteries = 4.5 V 1.5 V Batteries = 3

Graphing Ohm’s Law  Ohm’s Law tells you how much current flows based on the voltage, if the device has the same resistance.  If a device has a variable resistance, it does not follow Ohm’s Law directly. An example is a light bulb.

Graphing Ohm’s Law A current vs. voltage graph shows us if resistance changes. Often, these graphs have both positive and negative values of current and voltage. These positive and negative values are just a way to refer to the direction of current in a wire. You can apply voltage two ways across a wire. How you apply voltage determines current direction. One direction is positive and the other negative.

Graphing Ohm’s Law A simple resistor obeys Ohm’s law—its current vs. voltage graph is a straight line. Resistance is the same at all values of voltage and current. For a diode, the graph is not a straight line. A diode only allows charge to flow in one direction! This is why current is zero when voltage is negative. Diodes do not obey Ohm’s law. Diodes, like computer chips, are made from semiconductors.