What is meant by a loaded potential divider?

A loaded potential divider is one that has a further component or circuit connected across its output, so that the output has to supply current to that load as well as to the divider chain.

Describe the procedure for calculating the output of a loaded potential divider.

First combine the load resistance in parallel with the resistance of the arm across which the output is taken, then apply the potential divider equation using that combined value in place of the original arm resistance.

A 12 V supply feeds a divider of two 6.0 kΩ resistors and a 6.0 kΩ load is connected across the output. Calculate the loaded output potential difference.

The lower arm becomes 6.0 kΩ in parallel with 6.0 kΩ, which is 3.0 kΩ, so the output is 12 × 3.0 ÷ (6.0 + 3.0) = 4.0 V, compared with 6.0 V unloaded.

A 9.0 V supply feeds a divider with a 1.0 kΩ upper arm and a 2.0 kΩ lower arm, and a 2.0 kΩ load is connected across the output. Calculate the loaded output and compare it with the unloaded value.

The lower arm becomes 2.0 kΩ in parallel with 2.0 kΩ, which is 1.0 kΩ, so the output is 9.0 × 1.0 ÷ 2.0 = 4.5 V, whereas unloaded it would have been 9.0 × 2.0 ÷ 3.0 = 6.0 V.

What condition must the load satisfy if the loading effect on a potential divider is to be negligible?

The load resistance must be much larger than the resistance of the arm it is connected across, in practice at least about ten times as large, so that the parallel combination is close to the arm's own value.

CCEA A-Level Physics 1210

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Internal Resistance and Potential Dividers

Loaded potential dividers and output voltage calculations

3 things to know
Unit AS 1

Overview

Recall questions on what loading means and why it lowers the output, the procedure of combining the load in parallel with the lower arm first, worked calculations for several load resistances, the condition for the loading effect to be negligible, why low-value resistors reduce it and what that costs, and the loading effect of a voltmeter.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How does a voltmeter of finite resistance connected across the lower arm of a potential divider affect the reading it gives?

Key term

Why is a plain potential divider unsuitable for supplying a load whose resistance changes while it is operating?

Key term

Why does connecting a load across the output of a potential divider reduce the output potential difference?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by a loaded potential divider?
Model answer
A loaded potential divider is one that has a further component or circuit connected across its output, so that the output has to supply current to that load as well as to the divider chain.
2
Why does connecting a load across the output of a potential divider reduce the output potential difference?
Model answer
The load is in parallel with the lower arm of the divider, which reduces the resistance of that arm, so a smaller fraction of the supply potential difference now appears across it.
3
Describe the procedure for calculating the output of a loaded potential divider.
Model answer
First combine the load resistance in parallel with the resistance of the arm across which the output is taken, then apply the potential divider equation using that combined value in place of the original arm resistance.
4
A 12 V supply feeds a divider of two 6.0 kΩ resistors and a 6.0 kΩ load is connected across the output. Calculate the loaded output potential difference.
Model answer
The lower arm becomes 6.0 kΩ in parallel with 6.0 kΩ, which is 3.0 kΩ, so the output is 12 × 3.0 ÷ (6.0 + 3.0) = 4.0 V, compared with 6.0 V unloaded.
5
A 9.0 V supply feeds a divider with a 1.0 kΩ upper arm and a 2.0 kΩ lower arm, and a 2.0 kΩ load is connected across the output. Calculate the loaded output and compare it with the unloaded value.
Model answer
The lower arm becomes 2.0 kΩ in parallel with 2.0 kΩ, which is 1.0 kΩ, so the output is 9.0 × 1.0 ÷ 2.0 = 4.5 V, whereas unloaded it would have been 9.0 × 2.0 ÷ 3.0 = 6.0 V.
6
What condition must the load satisfy if the loading effect on a potential divider is to be negligible?
Model answer
The load resistance must be much larger than the resistance of the arm it is connected across, in practice at least about ten times as large, so that the parallel combination is close to the arm's own value.

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