# How to calculate resistance and voltage using a multimeter

• jumbogala
In summary: This will give you a more accurate reading than measuring when the circuit is open. In summary, the conversation discusses a lab involving voltage dividers and how to properly measure the values of R1 and R2, as well as the current through R2. The correct method for measuring these values is also discussed using the voltage division and current division rules. Additionally, the conversation mentions the importance of measuring the voltage while the circuit is closed for more accurate readings.
jumbogala

## Homework Statement

This is just something I want to know, so it doesn't really have a problem statement.

But I'm doing a lab tomorrow that uses voltage dividers. So it's two resistors R1 and R2 in series with a power supply. The power supplies Vin (the input voltage). A voltmeter is connected across R2 to measure Vout.

The lab also asks me to measure the values of R1 and R2, as well as the current going through R2. I want to make sure I know how to measure each of these properly.

## The Attempt at a Solution

R1 and R2: connect the multimeter with the power supply disconnected, across the resistors and measure?

Vin would just be the amount of voltage supplied by the power supply, correct? I guess I just hook up the multimeter across the terminals of the power supply...

Current across R2: Can't I just connect an ammeter in series with the circuit anywhere and take the reading? Can you measure current with a multimeter (I've never used one). Thanks!

jumbogala said:
R1 and R2: connect the multimeter with the power supply disconnected, across the resistors and measure?

Vin would just be the amount of voltage supplied by the power supply, correct? I guess I just hook up the multimeter across the terminals of the power supply...

Current across R2: Can't I just connect an ammeter in series with the circuit anywhere and take the reading? Can you measure current with a multimeter (I've never used one). Thanks!

All of the above is correct, but to get a more accurate reading on Vin you should measure the voltage while the circuit is closed. The power supply's internal resistance is going to lower the voltage by a non-negligible amount.

I have a suspicion that the teacher wanted you to calculate R1 and R2 by measuring voltages and currents instead of measuring resistance directly. In that case, would you know how to get R1 and R2?

In case this helps, the voltage division and current division rules are:

When a voltage V is divided across two series resistors, the voltage divides in the same proportions as each resistor.
voltage across R1 = V [R1/(R1+R2)]
voltage across R2 = V [R2/(R1+R2)]

However, when a current I divides between two parallel resistors, the current in each resistor divides in proportion with the OTHER resistor:
current through R1 = I [R2/(R1+R2)]
current through R2 = I [R1/(R1+R2)]

What does "measure the voltage while the circuit is closed" mean? (Can you tell I'm new at this? haha) And yep, I could find R1 and R2 theoretically if I needed to.

And thank you mike, that does help =)

jumbogala said:
What does "measure the voltage while the circuit is closed" mean?
Measure the voltage when you have R1 and R2 connected to the power supply.

## 1. What is a multimeter and how does it work?

A multimeter is a versatile electronic measuring instrument that is used to measure voltage, current, and resistance. It works by sending a small amount of electrical current through the component being measured and then measuring the amount of resistance or voltage encountered by the current.

## 2. How do I set up my multimeter to measure resistance?

To measure resistance, you will need to set your multimeter to the ohms (Ω) setting. Make sure the test leads are connected to the correct ports, usually labeled "COM" for the black lead and "VΩmA" for the red lead. Then, touch the test leads to the two points of the component you want to measure and read the resistance value on the display.

## 3. How do I use a multimeter to measure voltage?

To measure voltage, set your multimeter to the volts (V) setting. Again, ensure the test leads are connected correctly and then touch the leads to the two points of the circuit or component you want to measure. The multimeter will display the voltage reading.

## 4. Can a multimeter measure both AC and DC voltage and resistance?

Yes, most multimeters have the ability to measure both AC and DC voltage and resistance. Make sure to set the multimeter to the appropriate setting depending on the type of current or resistance you are measuring.

## 5. How do I convert the resistance reading on my multimeter to the appropriate unit?

The resistance reading on your multimeter will typically be displayed in ohms (Ω). If you need to convert this to a different unit, you can use the following conversions: 1 kiloohm (kΩ) = 1000 Ω, 1 megaohm (MΩ) = 1,000,000 Ω, 1 milliohm (mΩ) = 0.001 Ω. Some multimeters also have a "range" button that allows you to change the unit of measurement on the display.

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