Q.Two resistances R1 = (100 ± 3) ohm, R2 = (150 ± 2) ohm are connected in series. What is their equivalent resistance?

Error Propagation: From Intuition to Precision

When you measure something, you never get the exact true value. Every measurement carries an uncertainty — a small range within which the true value probably lies. Now imagine you take that imperfect measurement and plug it into a formula. The result you calculate will also be imperfect. The question is: how imperfect?

That's what error propagation answers. It tells you how the uncertainties in your raw measurements "travel through" a calculation and end up in your final answer.

The Core Intuition

Think of uncertainty like a wobble. If you measure the radius of a circle as $5.0 \pm 0.1$ cm, the radius could be anywhere from 4.9 to 5.1 cm. When you calculate the area $A = \pi r^2$, each possible radius gives a different area. The smallest radius (4.9 cm) gives the smallest area; the largest radius (5.1 cm) gives the largest area. The spread of those possible areas is the uncertainty in your result.

The key insight: the uncertainty in the output depends on how sensitive the formula is to changes in each input. If a small change in a measurement causes a big change in the result, that measurement contributes heavily to the final error. If the result barely budges when you nudge that measurement, its contribution is small.

The Precise Statement

For most practical cases in Indian exams (Class 11/12 Physics, lab reports), we use the following rules. They assume uncertainties are small and independent — meaning the error in one measurement doesn't affect the error in another.

Let a calculated quantity $Z$ depend on measured quantities $A, B, C, \dots$, each with uncertainties $\Delta A, \Delta B, \Delta C, \dots$.

Why These Rules Make Sense

Take addition. If $Z = A + B$, and $A$ could be off by $\pm 2$ and $B$ by $\pm 3$, then the worst case is $Z$ being off by $\pm 5$. That's just the sum of the individual errors. The same logic works for subtraction — if $Z = A - B$, the worst case is still $A$ high and $B$ low (or vice versa), giving a total spread of $\Delta A + \Delta B$.

For multiplication, think in percentages. If $A$ has a 2% uncertainty and $B$ has a 3% uncertainty, then $A \times B$ has roughly a 5% uncertainty. The fractional errors add because multiplication amplifies each error proportionally.

For powers, the exponent acts as a leverage factor. Squaring a number doubles its percentage error because you're effectively multiplying the quantity by itself — each copy contributes its own fractional error.

A Worked Example

You measure the radius of a sphere as $r = 2.0 \pm 0.1$ cm. Find the uncertainty in its volume $V = \frac{4}{3}\pi r^3$.

First, the relative uncertainty in $r$ is $\frac{\Delta r}{r} = \frac{0.1}{2.0} = 0.05$ (or 5%). …