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

Warmup

Go to menti.com and use code 90 84 11 What is the current i3 and voltage V3?

• 100mA

V3 i3

1V

5V 20mA

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 find unknown voltages and currents. Calculate the power supplied or dissipated by a device.

How much power does the source absorb?

• 0.5A

120V

This is called the "passive sign convention".

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.

✔ ✗ ✔ ✗

Which of these are absorbing electrical power?

A B C D E F

1A

• 1A
• 1A

2V 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 source B) The battery C) Both D) Neither, this circuit is broken. 500mA 4V

woodgears.ca

Marble-track resistor

R is the resistance, measured in Ohms

I = V R

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 current if 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

• f your kit, and put (only!) two
• f them in the holder.

(Keep the third one handy.)

Happy Friday! Do this now:

Take your digital multimeter

• ut 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

(𝑗 = 0)

Short circuit

• Direct connection
• Zero resistance
• No potential difference

(𝑤 = 0)

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 4

• pen

circuit short circuit

Modes of your digital multimeter (DMM)

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 5

Voltage meter Voltmeter (V) Resistance meter Ohmmeter (Ω) Current meter Ammeter (A)

Other modes are beyond the scope

• f 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

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!

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 7

• What happens if you

get the probes the “wrong way round”?

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?

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 9

• How should you

choose the scale?

Using your multimeter: scales

• Your multimeter has 3½ di

digi gits: The 4th digit can

• nly 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)

• 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

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

A voltmeter goes in parallel

But this changes the circuit! How should a voltmeter behave to avoid changing 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
• pen circuit.

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 13

Using your ohmmeter

• Resistance is measured

wit ithout

• ut an

any po power er

• For res

esis istor

• rs, polarity

doesn’t matter

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 21

Do now: w:

• 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 avoid changing the circuit? 4.5V 1kΩ

A) 4.5mA B) 0mA C) "1 ." (out of range) D) Nothing, your meter will burn up. 4.5V 1kΩ

If you connect your meter like this, while in current-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.

Apr 14, 2017 ENGR 40M Spring 2017 — C.Z. Lee, J. Plummer, R. Howe 16

How do you feel about this material?