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

R I

CONVERSIONS

• Divide by 1000 to convert mA (m = milli) A

• Times by 1000 to convert A mA

Example:

5 mA ? A

5 mA/1000 = 0.005 A

0.095 A ? mA

0.095 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 ΩThe 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 = IRV = (0.80 A)(15 Ω)

V = 12 VThe 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/RI = 60 V/ 15 Ω

I = 4.0 AThe 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 = 12 000 000V)

• 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)

(Accuracy)

RESISTOR COLOUR CODEExample: 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%