Electronics basics: use resistance

Start with a stated model

Today we use an ideal resistor whose resistance stays constant. That assumption matters. Not every electronic component behaves this way. Our examples are diagrams and calculations, not instructions to connect parts. We will use voltage, current, and resistance to make testable predictions.

Three quantities, one relationship

For our resistor, voltage equals current times resistance. We write V equals I times R. Voltage is measured in volts, current in amps, and resistance in ohms. To find current, divide voltage by resistance. Keep the units beside the numbers.

Calculate a small current

Put five volts across one thousand ohms in our ideal model. Five divided by one thousand is zero point zero zero five amps. A milliamp is one thousandth of an amp, so this is five milliamps. Small decimals deserve patient reading.

Pause and predict

Keep the voltage at five volts, but change the resistance to two thousand ohms. Before calculating, predict the direction of the change. Will current increase, decrease, or stay the same? Pause and remember that voltage, not current, is being held fixed.

Calculate and compare

Five divided by two thousand is zero point zero zero two five amps, or two point five milliamps. Current is half as large. Doubling resistance at the same voltage halves current. Our resistor did not negotiate; the result follows from the stated model.

Know when the model changes

Do not treat every component as a constant resistor at every voltage. An LED has a different current and voltage relationship. Real parts also have limits and may heat up. A correct calculation starts by choosing a model that fits the component and conditions.

Another path changes resistance

Fun fact: adding a resistor in parallel gives current another path and lowers the equivalent resistance. It does not simply add the two resistance numbers. Connections matter as much as part values. We will explore those different arrangements in the worksheet collection.

Show your reasoning

Continue to the worksheet. Write the relationship, substitute the values, and keep track of amps and milliamps. Then explain what changes when resistance changes. Use calculations only: never short a battery, use an outlet, open powered equipment, or handle charged capacitors.