8.3 Resistance and Ohm’s Law Resistance is the property of any material that slows down the flow of electrons, and converts electrical energy into other forms. Ohm’s Law The greater the amount of resistance, the lower the current Resistance = Voltage ÷ Current Unit of resistance is the ohm (Ω) Resistance can be measured in a circuit with an ohmmeter Switch: turns circuit on or off Circuit resistors are marked with coloured bands that allow you to determine the ohm value of the resistor. Take the Section 8.3 Quiz See pages 290 - 297 (c) McGraw Hill Ryerson 2007

Ohm’s Law 1827, German mathematician and physicist Georg Simon Ohm determined that the ratio of voltage to current was constant for a given conductor this ratio is called the resistance of the conductor the resistance (R) of a conductor is defined as the ratio of the voltage across a conductor to the current through the conductor resistance is a measure of how much the material of a conductor slows down (resists) the flow of the current (“birdies” under water)

V = IR (or V = I X R) Ohm’s Law: where R = resistance in ohms (Ω) I = current in amperes (A) V = voltage in volts (V)

Ohm’s Law Triangle: How to use the triangle: 1) to calculate voltage (V) use index finger tip and cover “V”, you are left with IR (or I X R) Example: circuit with 1 single cell; 1 resistor 9Ω; ammeter reads 1 A; what is the voltage V = IR = (1A ) (9Ω) = 9 V

2) to calculate current (I) use index finger tip and cover “I”, you are left with V/R (or V ÷ R) Example: circuit with 1 single cell; voltmeter shows 9 V; 1 resistor 9Ω; what is the current (I)? I = V/R = (9V)/(9Ω) = 1 A

3) to calculate resistance (R) use index finger tip and cover “R”, you are left with V/I (or V ÷ I) Example: circuit with 1 single cell; voltmeter shows 9 V; ammeter shows 1 A; what is the resistance (R)? R = V/I = (9V)/(1A) = 9 Ω