Lesson 4 – L: How Do I Build a Circuit and Calculate Power & Energy?

L: Launch Your Learning

Build a Circuit, Measure It, and Design to a Target

Now you put the ideas to work. Working with a partner, you will build a simple circuit from a source, a resistor, and a small load, then measure it and use Ohm’s Law to redesign it toward a goal. Record everything in your data table. Your measurements are your evidence.

What You Will Do

Step	What You Do	What You Are Looking For
1. Build a baseline	Connect the source, a resistor, and the load into one complete loop.	A working circuit; the load turns on.
2. Measure V and I	Measure voltage across the load and current in the loop. Record both.	Reasonable values with correct units (V and A or mA).
3. Measure R	With the power off, measure the resistor’s actual resistance and compare to its label.	How close the real resistance is to the labeled value.
4. Check Ohm’s Law	See whether your measured V, I, and R fit V = I × R.	Roughly consistent numbers; discuss any difference.
5. Design to a target	Pick a target current, use I = V ÷ R to choose a resistor, build it, and measure.	How close you got; iterate once if needed.
6. Find power and energy	Calculate P = V × I, pick a run-time, and find energy in watt-hours.	Power in watts and energy in watt-hours for your circuit.

Pause and Think

If you want less current in your circuit, should you choose a resistor with more resistance or less resistance? Use V = I × R to explain your answer.

From Measurements to a Load Profile and a Trade-Off

A single measurement is a start, but engineers think in terms of a whole system over a whole day. Once you know the power your load uses, you can estimate its energy use per day, and then imagine running several loads from one limited source.

Estimate the daily energy for your load by multiplying its power by the hours it runs each day. Then list two or three loads a small system might run — for example a light, a phone charger, and a fan — with rough power values and run-times, and add up the daily watt-hours.

Part of a Load Profile	Question to Answer
Power of each load	How many watts does each device use?
Run-time per day	How many hours per day does each one run?
Daily energy per load	Power × hours = watt-hours per day for that load.
Total daily energy	Add the loads together. How many watt-hours per day in total?

Key Idea

A trade-off does not mean a design is bad. It means every choice has strengths and limits. When the energy you have is less than the energy you want, you decide what to cut, shrink, or reschedule — and you explain what you gain by doing it.

Explain With Evidence

Imagine your source can supply only a fixed amount of energy per day, and your loads want more than that. Using your own numbers, what would you cut, shrink, or reschedule to fit the limit? What trade-off are you accepting, and what do you gain?