2.5 - Unit 1: Resistors / Ohm's Law / Circuits

Voltage (V)

an electromotive force or potential difference expressed in volts. Considered a pressure, doesn't move, Relative.

Current

A flow of electric charge.

Resistance (Ohms)

A material's opposition to the flow of electric current.

Bostain's Law for Good Engineers

All results should move the decimal point around in such a way that there is 1, 2, or 3 digits in front of the decimal point, then use a proper prefix to attach to the measurement.

Conventional Current

Current that flows from the positive side of the battery to the negative side. This is the way current is drawn in circuit diagrams, even though it is wrong.

Kirchoff's Voltage Law (KVL)

Around a loop, in a circuit, the sum of the voltages equals zero.

Formula for Current

I = V/R

Anode

On a battery, it is the Negative end. Everything else, it is the Positive end.

Cathode

On a battery, it is the Positive end. Everything else, it is the Negative end.

Formula for Power

P = I * V

Formula for Resistance

R = V/I

Battery symbol

The universal symbol in a circuit diagram that represents a battery.

Ohm's Law

V=I*R

Ground (Zero Volts)

normally understood to be a place of return for electricity in a circuit. In a circuit, you have a certain voltage at different parts. Ground, in most circuits, is where the voltage is zero, and that's where everything ends its journey. Without it, no current would actually flow. Electric current moves through these places of voltage from highest to lowest like a reverse "connect the dots".

Power (watts)

the rate at which work is done

Ohms (Ω)

unit for resistance

Ampere (Amp) [I]

unit of electric current

How to select Resistors

⦁ Find your Power using P = I * V in Watts. ⦁ Use Bostain's Law to find the engineering significant value. ⦁ Resistors come in 1/2 W (500 mW), 1/4th W (250 mW), and 1/8th W (125 mW). ⦁ Determine which wattage your resister can safely operate in using the above 3 types. ⦁ You CAN'T find the Wattage on the resistor, only on the package they come in. ⦁ Next, go to Standard Resistor Values, and select your 5% or 10% that most closely matches your calculated Resistance in Ω Ohms. ⦁ Next, use the resistance color selectors to find the color bands needed. ⦁ The first two bands tell you the first two significant digits in the resistance. 100 Ω would have a 1 and a 0. ⦁ The third band is the multiplier, which tells you how many more 0's you have. 100 Ω would have another 1 to indicate 1 more zero. ⦁ Look up the colors. ⦁ The fourth band is tolerance. If there is no fourth band, it could be 80% to 120% the indicated resistance. Silver is 90% to 110%, and Gold is 95% to 105%.