warmup - stanford university resistance &xuuhqwfdq ... place the ammeter to have the same...

Warmup

Go tomenti.com and use code 90 84 11

What is the current i3 and voltage V3?

-100mA V3i3

1V

5V 20mA

ENGR 40M, Lecture 3:Ohm's law, power, and multimeters

Steven Bell30 June 2017

LogisticsLab 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!

ObjectivesBy the end of class, you should be able to:

Use Ohm's law with KVL/KCL to find unknown voltages and currents.

Calculate the power supplied or dissipated by a device.

Howmuch power does the source absorb?

-0.5A 120V

This is called the "passive sign convention".

Let's make a dealWhen calculating power,current references should match voltage references:

Then positive power is always power absorbed, andnegative power is always power supplied.

✔ ✗ ✔✗

Which of these are absorbing electrical power?

A B C

D E F

1A -1A -1A2V 2V 2V

1A 1A -1A-2V 2V -2V

Watch the signs!

A strange circuit

Which device is providing elctrical power?(i.e., which has negative power absorbed?)

A) The current sourceB) The batteryC) BothD) Neither, this circuit is broken.

500mA 4V

woodgears.ca

Marble-track resistor

R is the resistance, measured in Ohms

I = VR

For a resistor (and only a resistor):Ohm's Law

Represented with the schematic symbol:

What is the current i?

Challenge: what would happen to the currentif we added a second 1kΩ resistor in parallel?

5V 1kΩ

i

Breaking break:My wife's hair dryer

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 1

Take all three AA batteries out of your kit, and put (only!) two

of them in the holder.

(Keep the third one handy.)

Happy Friday!

Do this now:

Take your digital multimeter

out of its packaging, as well

as the probes in the box

Open circuit and short circuit

Open circuit

• No connection

• Infinite resistance

• Current can’t flow (𝑖 = 0)

Short circuit

• Direct connection

• Zero resistance

• No potential difference(𝑣 = 0)


open

circuit

short

circuit

Modes of your digital multimeter (DMM)


Voltage meter

Voltmeter (V)

Resistance meter

Ohmmeter (Ω)

Current meter

Ammeter (A)

Other modes are

beyond the scope

of ENGR 40M—

please ask if

you’re interested

Connecting your multimeter’s probes

• Red probe → VΩmA, normally

• “Volts, ohms and milliamps”

• Black probe → COM

• “Common”

• 10A(DC) is used for higher current


Using your voltmeter (1)

Do now:

• Use the 2000mV 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”?


Do now:

• Change to the 200mVscale. Measure the same battery. What does it say?

• How about on the 20Vscale?


• How should you choose the scale?

Using your multimeter: scales

• Your multimeter has 3½ digits: The 4th digit can only show a “1”.

• Numbers indicate the maximum 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.



• You’ll need to choose the appropriate scale.

• Are the batteries in series or in parallel?


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

A voltmeter goes in parallel

But this changes the circuit!

How should a voltmeter behave to avoidchanging the circuit?

4.5V 1kΩ

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.


Using your ohmmeter

• Resistance is measured without any power

• For resistors, polarity doesn’t matter


Do now:

• Measure the resistance of the resistor you just read

• Note: No battery!

• Does it match?

Resistance modes

An ammeter goes in series

This also changes the circuit!

How should an ammeter behave to avoidchanging the circuit?

4.5V 1kΩ

A) 4.5mAB) 0mAC) "1 ." (out of range)D) Nothing, your meter will burn up.

4.5V 1kΩ

If you connect your meter like this, while incurrent-measuring mode, what will you measure?

Don't try it yet - analyze this with what you know.

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.


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