Electronics basics: voltage and current

Two different quantities

Voltage and current are related, but they are not two names for the same thing. We will keep their meanings and units separate. A number without its unit can be confusing, like asking for three and forgetting to say three of what.

Voltage compares two points

Voltage describes an energy difference per unit charge between two points. Its unit is the volt. In our ideal source example, three volts means three joules of energy transferred per coulomb of charge. A coulomb is a unit used to measure charge.

Current is a rate

Current tells us how quickly charge passes a point. Its unit is the ampere, often called an amp. One amp means one coulomb per second. We are counting a rate of charge flow, not measuring how many joules each coulomb can transfer.

Work through a rate

Suppose two coulombs pass a point evenly in four seconds. Divide charge by time: two divided by four equals one half. The steady current is half an amp. This calculation needs charge and time. It does not give us a voltage.

Pause and choose the unit

Now a source transfers three joules per coulomb. Is that statement describing three volts or three amps? Pause and explain which meaning matches. Look for the words per coulomb. Compare them with per second before choosing your answer.

Match the meaning

Three joules per coulomb means three volts. Current would need charge per time instead. Also, a voltage can exist across an open gap with no steady current. That is another reason not to swap the two words or assume voltage always means flow.

A direction convention

Fun fact: conventional current uses the direction positive charge would move. In a metal wire, the electrons drift in the opposite direction because they are negatively charged. That convention is useful, but you do not need to chase an electron to solve our rate example.

Keep units with answers

Summarize the difference: voltage is energy per unit charge, and current is charge per time. Continue to the worksheet to match units and calculate a rate. This is a diagram activity only. Do not use outlets, short batteries, or open powered equipment.