Electrical Measurement, Ohm’s Law, & Watt’s Law

CVSDElectronics 1

Resistance A measure of the opposition to the flow

of electrons (current) through a material

Unit of Measurement: Ohm (Ω)

Symbol: R = Resistance

Current A measure of the total amount of

electrons flowing in a circuit in a given unit of time

Unit of Measurement: Ampere (A)

Symbol: I = Intensity

Voltage A measure of the Electromotive Force

(EMF) or pressure that pushes electrons through a circuit

Unit of Measurement: Volt (V)

Symbol: E = Electromotive Force

Power A measurement of the rate of consuming

energy, transferring energy, or doing work

Unit of Measurement: Watt (W)

Symbol: P

Ohm’s Law Describes the mathematical relationship

between resistance, current, and voltage

Voltage (E) = Amperage (I) x Resistance (R)

With two of these values, you can use Ohm’s Law to find the third

Variations of Ohm’s Law E = IxR I = E/R R = E/I

Ohm’s Law Example E = 24 VDC I = ? R = 12 Ω

E=IR24=Ix12I = 24/12I = 2 A

Calculations

Watt’s Law Describes the mathematical relationship

between power, current, and voltage

Power (P) = Amperage (I) x Voltage (E)

With two of these values, you can use Watt’s Law to find the third

Variations of Watt’s Law P = IE I = P/E E = P/I

Watt’s Law Example P = 36 W I = ? E = 12 V

P=IE36=Ix12I = 36/12I = 3 W

Calculations

Combinations of Ohm’s and Watt’s Laws

Measuring Resistance with the DMM Must select Ohmmeter Range (Ω) Pos (+) lead must be in Volt/ Ohm pin Sat meter to proper range Must wire in parallel with circuit Power must be removed from the circuit

Measuring Voltage with the DMM Must select DC Voltage Range Pos (+) lead must be in Volt/ Ohm pin Set meter to proper range Must wire in parallel with the circuit Circuit must be have power

Measuring Current with the DMM Must select DC Current Range Set meter to proper range. (meter can be

damaged if not set to the proper range) Must wire in series with the circuit Circuit must be have power

Measured Tolerance Predicted values, and measured

values may not always be exactly the same There is unaccounted for resistance in the

wires etc. Applied voltage may fluctuate Human error may disturb measurements Components have tolerance (not always exact

values) As components change temperature,

resistance may also change

Review State Ohm’s Law State Watt’s Law How is voltage measured? How is resistance measured? How is current Measured? Why do predicted and calculated

values differ?