Warmup What is the current i 3 and voltage V 3 ? 1V -100mA 5V 20mA - PowerPoint PPT Presentation

Warmup What is the current i 3 and voltage V 3 ? 1V -100mA 5V 20mA i 3 V 3 Go to menti.com and use code 90 84 11

ENGR 40M, Lecture 3: Ohm's law, power, and multimeters Steven Bell 30 June 2017

Logistics Lab sections will be posted this weekend Homework 1 is out (pick up a copy) Lab 1 is posted (we'll bring copies Monday) Sign up on Piazza!

Objectives By the end of class, you should be able to: Use Ohm's law with KVL/KCL to fi nd unknown voltages and currents. Calculate the power supplied or dissipated by a device.

How much power does the source absorb? -0.5A 120V

Let's make a deal When calculating power, current references should match voltage references: ✔ ✔ ✗ ✗ Then positive power is always power absorbed, and negative power is always power supplied. This is called the "passive sign convention" .

Which of these are absorbing electrical power ? Watch the signs! 1A 2V -1A 2V -1A 2V A B C 1A -2V 1A 2V -1A -2V D E F

A strange circuit 4V 500mA Which device is providing elctrical power? (i.e., which has negative power absorbed?) A) The current source B) The battery C) Both D) Neither, this circuit is broken.

Marble-track resistor woodgears.ca

Ohm's Law For a resistor (and only a resistor): I = V R R is the resistance, measured in Ohms Represented with the schematic symbol:

What is the current i ? i 5V 1k Ω Challenge: what would happen to the current if we added a second 1k Ω resistor in parallel?

Breaking break: My wife's hair dryer

Happy Friday! Do this now: Take all three AA batteries out of your kit, and put (only!) two of them in the holder. (Keep the third one handy.) Take your digital multimeter out of its packaging, as well as the probes in the box Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 1

Open circuit and short circuit Open circuit Short circuit open short circuit circuit • No connection • Direct connection • Infinite resistance • Zero resistance • Current can’t flow • No potential difference ( 𝑗 = 0 ) ( 𝑤 = 0 ) Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 4

Modes of your digital multimeter (DMM) Voltage meter Voltmeter (V) Current meter Ammeter (A) Resistance meter Other modes are beyond the scope Ohmmeter ( Ω ) of ENGR 40M — please ask if you’re interested Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 5

Connecting your multimeter’s probes • Red probe → VΩ mA, normally • “Volts, ohms and milliamps” • Black probe → COM • “Common” • 10A(DC) is used for higher current Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 6

Using your voltmeter (1) Do now: w: • Use the 2000mV 00mV scale • Grab an AA battery • Put one probe on each end of the battery • Make firm contact! • What happens if you get the probes the “wrong way round”? Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 7

Using your voltmeter (2) Do now: w: • Change to the 200 00mV mV scale. Measure the same battery. What does it say? • How about on the 20V 20V scale? • How should you choose the scale? Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 9

Using your multimeter: scales gits: The 4 th digit can • Your multimeter has 3½ di digi only show a “1”. • Numbers indicate the maxim imum um that a scale can measure. • If you measure something exceeding the scale, the meter shows this: • To maximize precision, choose the smallest scale greater than what you’re measuring. Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 10

Using your voltmeter (3) Do Do now: • Place all three AA batteries in the holder • Don’t let the holder wires touch each other • Measure the voltage across the holder • You’ll need to choose the appropriate scale. • Are the batteries in series or in parallel? Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 11

A voltmeter goes in parallel 4.5V 1k Ω But this changes the circuit! How should a voltmeter behave to avoid changing the circuit?

Voltmeters connect in parallel • To measure a voltage across a device, you must probe on either side of it. • Therefore, voltmeters connect in parallel . • In order to avoid affecting the circuit, the voltmeter takes close to zero current . • We say that the ideal voltmeter looks like an open circuit . Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 13

Using your ohmmeter Do now: w: • Measure the resistance of the resistor you just read • Note: No battery! • Does it match? • Resistance is measured wit ithout out an any po power er • For res esis istor ors, polarity doesn’t matter Resistance modes Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 21

An ammeter goes in series 4.5V 1k Ω This also changes the circuit! How should an ammeter behave to avoid changing the circuit?

If you connect your meter like this, while in current-measuring mode, what will you measure? 4.5V 1k Ω Don't try it yet - analyze this with what you know. A) 4.5mA B) 0mA C) "1 ." (out of range) D) Nothing, your meter will burn up.

Ammeters connect in series • To measure current through a device, you must place the ammeter to have the same current as it. • Therefore, ammeters connect in series . • In order to avoid affecting the circuit, the ammeter takes close to zero voltage . • We say that the ideal ammeter looks like a short circuit . Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 16

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