6.3: Step 1 — Define
- Page ID
- 132122
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The Define step is the most commonly skipped — and the most important. A complete Define step includes:
- Known quantities with values and units
- Unknown quantities with symbols and expected units
- Governing constraints (equilibrium, conservation laws, circuit laws)
- Explicit assumptions — anything you are simplifying or ignoring
- A free-body diagram or schematic for physical problems
The diagram is underused by students and consistently present in professional engineering work. Drawing the physical situation before writing equations forces spatial thinking about the problem and catches sign errors, missing forces, and incorrect geometry before they propagate.
"Setting up the equation" without a Define step is usually where unit errors and sign errors enter. If you cannot write down every known quantity with its unit and every unknown with its expected unit before touching an equation, you do not yet have a full mental model of the problem. The equation will not save you — it will amplify the gap.
Practice 6.1 — Write the Define Step Only
A 12 V battery is connected to a 330 Ω resistor. Determine the current and power. Write only the Define step — known quantities, unknowns with expected units, and two assumptions. No equations yet.
Compare these two approaches
| Weak define | Complete define |
|---|---|
V = 12, R = 330 |
V = 12 V (source voltage), R = 330 Ω (resistance), I = ? A, P = ? W Assumptions: ideal resistor (constant R), DC source, steady-state. |
The complete version assigns units at the point of definition. This takes 30 extra seconds and prevents the majority of common errors.