8 3 RESISTANCE AND OHMS LAW SECTION 8

8. 3 – RESISTANCE AND OHMS LAW

SECTION 8. 3 – RESISTANCE AND OHM’S LAW • Resistance – is the property of any material that slows down the flow of electrons (i. e. a load) and converts electrical energy into other forms of energy (i. e. light or heat). • Wires that connect the battery to a load have very little resistance; therefore, electrons can travel through the wire with almost no loss of electrical energy.

RESISTANCE AND CURRENT • Voltage is directly proportional to current; therefore, as the voltage increases, so does the current (I). • If an electric circuit has a large resistance, less current will flow if a battery with the same voltage is connected to a lower resistance circuit.

OHM’S LAW R = V/I V R = resistance (ohm - Ω) V = voltage (volts – V) I = current (amperes – A) R I

CONVERSIONS • Divide by 1000 to convert m. A (m = milli) A • Times by 1000 to convert A m. A Example: 5 m. A ? A 5 m. A/1000 = 0. 005 A 0. 095 A ? m. A 0. 095 A x 1000 = 95 m. A

CONVERSIONS • Divide by 1000 to convert Ω k Ω (k = kilo) • Times by 1000 to convert k Ω Ω Example: 6000 Ω ? k Ω 6000 Ω/1000 = 6 k Ω 20 k Ω ? Ω 20 k Ω x 1000 20, 000 Ω

CONVERSIONS • Divide by 1, 000 to convert V to MV (mega) • Times by 1, 000 to convert MV to V Example: 25 MV ? V 25 MV x 1, 000 = 25, 000 4, 000 V MV 4, 000 V/1, 000 = 4 MV

SAMPLE CALCULATIONS USING THE OHM’S FORMULA 1. The current through a load in a circuit is 1. 5 A. If the potential difference across the load is 12 V, what is the resistance of the load? R = V/I R = 12 V/1. 5 A R = 8. 0 Ω The resistance of the load is 8. 0 Ω.

SAMPLE CALCULATIONS USING THE OHM’S FORMULA 2. The resistance of a car headlight is 15 Ω. If there is a current of 0. 80 A through the headlight, what is the voltage across the headlight? V = IR V = (0. 80 A)(15 Ω) V = 12 V The voltage across the headlight is 12 V.

SAMPLE CALCULATIONS USING THE OHM’S FORMULA 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? I = V/R I = 60 V/ 15 Ω I = 4. 0 A The current through this load is 4. 0 A.

PRACTICE PROBLEMS • Pg. 293 #1 -3

CONVERTING PREFIXES • Prefixes are used to indicate the magnitude of a value • Mili (m) represents one-thousandth (eg. 25 m. A = 25/1000 = 0. 025 A) • Kilo (k) represents one thousand (eg. 5. 0 KΩ = 5000 Ω • Mega (M) represents one million (example: 12 MV = 12 000 V) • Practice Problems Pg. 294 #1 -3 • Pg. 295 – Calculating Resistance

THE RESISTOR • Resistor – an electrical component that has a specific resistance. • Used to control the current or voltage in a circuit. • When charge encounters resistance, it is transformed into other forms of energy (i. e. heat).

RESISTOR COLOUR CODE TABLE 8. 2 PAGE 297 (Accuracy)

RESISTOR COLOUR CODE Example: Red, blue, brown, gold • 260 Ω +/- 5% Example: Green, orange, black, silver Ω +/10% • 53 • Circuit Diagrams • Pg. 296 Circuit Diagrams

RESISTORS • If a resistor has a fourth band, it represents the percentage of accuracy between the indicated value and actual value • Gold 5% • Silver 10% • No Color 20%