RESISTANCE AND OHM’S LAW

RESISTANCE AND THE FLOW OF ELECTRONS

RESISTANCE: property of any material that slows down the flow of electrons and converts electrical energy into other forms of energy

EXAMPLE: the filament of a light bulb highly resistance filament causes the electrons’

electrical energy to be converted into heat and light energy

The wire connecting the battery to the bulb has little resistance, which means that the electrons travelling through the wire lose almost no electrical energy

RESISTANCE AND CURRENT VOLTAGE: the difference in potential

energy per unit of charge between 2 points in a circuit

Voltage is directly proportional to current The greater the voltage, the greater

the current

GEORG OHM Studied the relationship between

voltage and current Discovered that resistance is an

important factor that can affect current Different objects can have the same

potential different BUT different currents depending on the amount of resistance

the greater the resistance, the lower the current!

OHM’S LAW Ohm was able to use the amount of current

that a voltage produces to calculate the circuit’s resistance.

ELECTRICAL RESISTANCE: the ratio of voltage to current

OHM (): the unit of measurement for electrical resistance

OHM’S LAW: the mathematical relationship comparing voltage (V), current (I), and resistance (R)

PRACTICE QUESTION 1: What is the resistance of a flashlight

bulb if there is a current of 0.75A through the bulb when connected to a 3.0V battery?

OHM’S LAW (CONTINUED)We add prefixes to indicate the magnitude of

value! Milli (m) = one thousandanth

25mA 25/1000 A 0.025A Kilo (k) = one thousand

5.0k 5000 Mega (M) = 1 million

12MV 12 000 000V

HINT: convert the prefixes BEFORE you do your calculations!

PRACTICE QUESTION 2: What is the voltage across a 12k load

that allows a current of 6.0mA?

DETERMINING THE RESISTANCEMethod 1: Experimentally measure the resistance Use a voltmeter to measure the voltage across the

load and an ammeter to measure the current through the load

Use Ohm’s Law to calculate the resistance

Method 2: Use an ohmmeter to measure resistance The meter uses its internal battery to provide a

voltage across the load It then measures the current leaving the battery

and calculates the resistance

THE RESISTOR REVIEW: Any electrical component that has

electrical resistance slows down current and transforms electrical energy into other forms of energy!

Resistor: an electrical component that has a specific resistance

Can be used to control current or potential difference in a circuit to provide the correct voltage and current to the other components of the circuit

Symbolized by

When a charge encounters resistance, some of the electrical energy that is stored will be transformed into a different type of energy like heat! To be clear, this electrical energy isn’t actually lost. It’s just been turned into a new kind of energy!

RESISTOR COLOUR CODEResistors are marked with coloured bands that

indicate the resistance on a scale from 0-9. First band: 1st digit of the resistance Second band: 2nd digit of the resistance Third band: the multiplier OR power of 10 factor of

the resistance (the # of 0s that follow the second digit)

Fourth band (optional): the % of accuracy between the indicated value and actual value Gold = 5% Silver = 10% No colour = 20%

PRACTICE QUESTION 3:

What is the resistance of the above resistor?