Ready-to-teach lab
Voltage divider lab: predict each tap, then measure it
In this voltage divider lab three equal resistors split 5 V into thirds, and students predict the voltage at each tap before they put the meter on it.
The lesson
Objective and parts
By the end, students can
- Predict the output of a resistor divider from the ratio of its resistors.
- Measure DC voltage, and resistance with the power off.
- Explain why a meter changes the thing it measures, by a little.
Parts, already on the bench
- A 5 V supply and ground rail (no board in this circuit)
- Three 10 k resistors in a row
- A multimeter
Nothing to hand out or collect: the circuit opens built and wired.
Steps
What students do
Each step is short enough to read out. Students work in the simulator beside this page.
- Before pressing Run, predict the voltage at the tap above the bottom resistor and at the tap above the middle one.
- Press Run. The meter sits on the lower tap and reads about 1.67 V.
- Move the red probe to the middle tap. It reads about 3.33 V.
- Work out the current through the string: 5 V across 30 k.
- Stop the simulation, turn the dial to ohms and measure across one resistor, then across two.
- Change the top resistor to 20 k, predict both taps again and check.
Questions
Questions for students
Hand these out as they are. The expected answers are under the list, for you.
- What fraction of 5 V appears at each tap, and why?
- What is the current through the string?
- The meter shows 1.666 V and not 1.667 V. Where did the missing millivolt go?
- Why must the power be off to measure resistance?
Answer key for the teacher
- One third and two thirds. The same current flows through three equal resistors, so each drops the same voltage.
- About 167 microamps: 5 V divided by 30 000 ohms.
- The meter has a resistance of its own, about 10 megohms, in parallel with the bottom resistor. That lowers the bottom resistance very slightly.
- On the ohms range the meter sends its own small current through the part. A powered circuit adds other currents and the reading means nothing.
Go further
For students who finish early
- Design a divider that gives 2.5 V from 5 V, then one that gives 1 V.
- Hang a 10 k load on the lower tap and explain why the voltage drops.
Your class
Give this lab to every student
Assign to my class puts this circuit on each student’s class page as a starter. They change their own copy and hand it in, and the work comes back to you in one list.
The class tools are free for 30 days for one class of up to 20 students, with no card. Opening and running the lab needs no account and stays free.
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