1. 8.3 Resistance and Ohm’s Law

2.  Resistance is the property of any material that slows down the flow of electrons and converts electrical energy into other forms  Example – filament in light bulbs have a high resistance that causes electrical energy to convert to heat and light energy  A large resistance means less current flow  Electrical resistance is the ratio of the voltage to the current  Unit of measurement is the Ohm (Ω)  Relationship is called Ohm’s law and the usual form is: V=IR where I is current

3.  For Ohm’s problems you need to make sure that the units are in A, V and Ω  Common prefixes  milli (m) represents one-thousandth ex. mV is 0.001 V  kilo (k) represents one thousand ex. kV is 1000 V  Mega (M) represents one million ex. MV is 1 000 000 V

4. Try the problems on the next page before you forward to the answer! Steps 1. What do I know? What do I need to know? 2. Are all the numbers in V, A or Ω form? 3. What form of Ohm’s law do I need? 4. Plug the numbers in and do the calculation. 5. Don’t forget to put the unit at the end!

5. 1. What is the resistance of a bulb if there is a current of 0.75 A through the bulb when connected to a 3.0 V battery? 2. The resistance of a car headlight is 400 Ω. If there is a current of 15 mA through the headlight, what is the voltage? 3. A 60 V potential difference is measured across a load that has a resistance of 15 Ω. What is the current through this load?

6. 1. What is the resistance of a bulb if there is a current of 0.75 A through the bulb when connected to a 3.0 V battery? Formula : V = IR…we know I and V so R = V/I 3.0 V / 0.75 A = 4 Ω 2. The resistance of a car headlight is 400 Ω. If there is a current of 15 mA through the headlight, what is the voltage? Formula: V = IR…we know I and R but I is in mA so I need to convert that first 15 mA x (1 A / 1000 mA) = 0.015 A 0.015 A x 400 Ω = 6 V 3. A 60 V potential difference is measured across a load that has a resistance of 15 Ω. What is the current through this load? Formula: V = IR…we know V and Ω so I = V/R 60 V / 15 Ω = 4 A

7.  Experimentally use a voltmeter and a ammeter to measure the voltage and current then calculate resistance using Ohm’s law  Or an ohmmeter is a device that measures resistance directly

8.  A resistor is an electrical component of a circuit that has a specific resistance  Controls current or potential difference to provide the correct amount of voltage or current to other components of the circuit  Circuit symbol is:  Energy lost through a resistor is generally transformed into heat

9.  Marked with coloured bands to indicate the resistance of each resistor (table in notes)  First band is the first digit of the resistance  Second band is the second digit  Third band represents the number of zeros that follow the second number  Fourth band indicates the percentage of accuracy  Gold 5%  Silver 10%  No colour 20%