8.3Resistance and Ohm’s LawName:
Date:
Block:
(Reference: BC Science 9 pp. 290 - 301)
RESISTANCE AND THE FLOW OF ELECTRONS
- resistance: ______
______
- Ex. The ______resistance of the filament of a light bulb causes the electrons’ electrical energy to be transformed into light and heat energy.
- In the lab last class, the filament of the light bulbsresisted the flow of electrons and therefore ______current.
- You observed that the higher the voltage of the bulb, the ______the current.
- The conducting wire has very ______resistance and not much energy is lost to the wire.
RESISTANCE AND CURRENT
Remember, ______isthe difference in potential energy per unit of charge between two different points in a circuit, and ______is the amount of charge passing a point in a conductor per second.
Voltage across the source is ______proportional to current.
- Ex. A 6V battery will supply more current than a 1.5 V battery.
Voltage across the load is ______proportional to current.
- Ex. The 6 V bulb will slow down current more than the 1.5 V bulb.
Resistance is ______proportional to current.
- Ex. If a 1.5 V battery is connected to a circuit with high resistance, less current will flow compared to if the same battery was connected to a circuit with lower resistance.
OHM’S LAW
By measuring the amount of current that a given voltage produces, German physicist Georg Ohm was able to calculate a circuit’s resistance.
- electrical resistance: ______
- unit = ______(___)
- measured using a device called an ______
- Ohm’s Law: the mathematical relationship comparing______(V), ______(I), and ______(R).
SAMPLE PROBLEMS
- An electrical motor has an operating resistance of 25 Ω when a 4.8 A current is flowing through it. What is the potential difference of the outlet that the motor is plugged into?
- A car headlight has an average resistance of 24 Ω. Car batteries provide a potential difference of 12 V. What amount of current passes through the headlight?
- A 9.0 V battery maintains a current of 3.0 A through a portable radio. What is the resistance of the conductor?
Converting prefixes
- milli (m) represents ______
- Ex. 200 mA =
- kilo (k) represents ______
- Ex. 2.5 kΩ =
- mega (M) represents ______
- Ex. 15 MV =
MORE SAMPLE PROBLEMS
- The current through a load in a circuit is 120 mA. If the potential difference across the load (bulb) is 6 V, what is the resistance of the load?
- A 12 kΩ load is connected to a 90 V power supply. What is the current through the load in milliamperes (mA)?
DETERMINING RESISTANCE
Method 1: Use Ohm’s Law
Method 2: Use an ohmmeter
RESISTORS
- resistor: ______
______
- Controls current or voltage to provide the right amount for other components of the circuit.
- energy not really “lost” it is ______
- circuit symbol:
RESISTOR COLOUR CODE
- A band of colour that have a corresponding numerical value that indicates the resistance of the resistor.
- Refer to Table 8.2 on p. 297
The four bands of the colour code represent the following:
1st band = the first digit of the resistance
- Ex. brown =
2nd band = the second digit of the resistance
- Ex. orange =
3rd band = the multiplier or power of 10
- Ex. red =
4th band = the percentage of accuracy
(gold = 5%, silver 10%, and no colour =20%)
- Ex. gold =
- The resistance of the above resistor is: ______or ______and is accurate within _____%.
SUMMARY: