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 R = resistance (ohm - Ω) V = voltage (volts – V) I = current (amperes – A) V RI

CONVERSIONS Divide by 1000 to convert mA (m = milli)  A Times by 1000 to convert A  mA Example: 5 mA  ? A 5 mA/1000 = A A  ? mA A x 1000 = 95 mA

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 1,000,000 to convert V to MV (mega) Times by 1,000,000 to convert MV to V Example: 25 MV  ? V 25 MV x 1,000,000 = 25, 000,000 4,000,000 V  MV 4,000,000 V/1,000,000 = 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/1.5A 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. 25mA = 25/1000 = 0.025A) Kilo (k) represents one thousand (eg. 5.0KΩ = 5000 Ω Mega (M) represents one million (example: 12MV = 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 53 Ω +/- 10% 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%