Current, Voltage, and Resistance Notes

Electric Current

• Continuous flow of electric charge (primarily electrons) through a conductive material.

• Electron Flow: Actual movement of electrons from the negative terminal to the positive terminal.

• Conventional Current: Movement defined as flowing from the positive terminal to the negative terminal.

• Unit: Measured in amperes ($A$), named after André-Marie Ampère.

• Measurement: Measured using an ammeter connected in series with the circuit components.

Voltage

• Also termed electric potential difference; represents the potential energy per unit charge available to push electrons through a conductor.

• Unit: Measured in volts ($V$).

• Sources: Provide energy to operate circuits, including batteries (converting chemical energy to direct current or DC), power outlets (supplying alternating current or AC), solar panels, and electric generators.

• Measurement: Measured using a voltmeter connected in parallel across the component being tested.

Resistance

• Measure of how much a material opposes the flow of electric current.

• Unit: Measured in ohms ($\Omega$).

• Factors Influencing Resistance:

  • Material: Conductors such as copper and aluminum have low resistance, whereas insulators such as rubber have high resistance.

  • Length: Longer conductors possess higher resistance due to increased electron collisions.

  • Cross-Sectional Area: Thicker conductors possess lower resistance; doubling the area halves the resistance.

  • Temperature: Higher temperature increases atomic vibrations, raising resistance in most conductors.

• Measurement: Measured using an ohmmeter connected in parallel with the component; the circuit must be powered off during measurement.

Resistor Color Coding

• Significant Digits (Bands $1$ and $2$): Color code values are Black ($0$), Brown ($1$), Red ($2$), Orange ($3$), Yellow ($4$), Green ($5$), Blue ($6$), Violet ($7$), Gray ($8$), and White ($9$).

• Multiplier (Band $3$): Indicates power of ten, ranging from Black ($10^0$) to White ($10^9$), Gold ($10^{-1}$), and Silver ($10^{-2}$).

• Tolerance (Band $4$): Percentage range of actual resistance variance, including Brown ($\pm 1\%$), Red ($\pm 2\%$), Green ($\pm 0.5\%$), Blue ($\pm 0.25\%$), Violet ($\pm 0.1\%$), Gold ($\pm 5\%$), Silver ($\pm 10\%$), and None ($\pm 20\%$).

• Calculation Example: A resistor with bands Red ($2$), Violet ($7$), Orange ($10^3$), and Gold ($\pm 5\%$) has a rated resistance of $27,000\,\Omega \pm 5\%$ (actual range from $25,650\,\Omega$ to $28,350\,\Omega$).

Questions & Discussion

• What happens to an LED bulb when a circuit switch is opened or closed? An open switch breaks the circuit so no light is emitted; a closed switch completes the path and illuminates the bulb.

• What current flows through a $3\,\Omega$ resistor connected to a $9\,V$ battery? The current in the circuit is $3\,A$.

• Which change reduces resistance in a long, thin wire most effectively? Shortening the wire and increasing its thickness reduces its resistance.