Electricity & Circuits: Current, Voltage & Resistance

Electricity can feel abstract, so it helps to picture water in pipes. Voltage is like the water pressure pushing the flow; current is the amount of water flowing past a point; and resistance is anything that narrows the pipe and slows things down. With that picture, the central equation of circuits — Ohm's law — makes intuitive sense, and the difference between series and parallel wiring becomes clear.

This guide covers current, voltage, resistance, Ohm's law, and circuit types, with worked and practice questions matched to the level seen in ACT Science and physics at Northside Tutoring.

Why Circuits Matter

Electricity powers modern life and is a frequent ACT Science topic, often shown as circuit diagrams or current-voltage data. Ohm's law is the key relationship to know. (The SAT has no science section.)

Current, Voltage & Resistance

Quantity	Unit	Water analogy
Current (I)	amperes (A)	amount of water flowing
Voltage (V)	volts (V)	pressure pushing the water
Resistance (R)	ohms (Ω)	a narrow pipe slowing flow

A Simple Circuit

A complete circuit needs an unbroken loop from one terminal of the battery, through the components, and back. Break the loop — like an open switch — and current stops.

Ohm's Law

The three quantities are linked by Ohm's law:

Voltage equals current times resistance. Rearranged, current I = V/R, so for a fixed voltage, more resistance means less current. This single relationship answers most basic circuit questions — solve it for whichever quantity you need.

Series vs. Parallel

Components can be wired two ways. In a series circuit, everything is on one loop, so the same current flows through each component and a break anywhere stops it all (like old holiday lights). In a parallel circuit, components are on separate branches, so each gets the full voltage and one can fail without stopping the others (like the outlets in your home).

Electrical Power

The power used by a device — the rate it uses energy — is voltage times current:

Measured in watts, this is why appliances are rated in watts. Combined with Ohm's law, you can find the power, current, or resistance of a device from the other quantities.

Where You'll See This — Test by Test

Electricity supports ACT Science physics passages, often shown as circuit diagrams; the SAT has no science section and the SSAT doesn't test it. It's core high-school and AP Physics.

Watch the Lesson

Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.

Worked Example Problems

These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.

Common Mistakes to Avoid

Practice Problems — You Try

Three problems below. Work each before checking the solution.

The Northside Method — How We Teach This 1-on-1

Reading a blog is a great starting point. But there's a meaningful gap between understanding a concept and reflexively applying it under timed conditions. That gap is exactly what our tutors close.

Every Northside student works through a four-step framework:

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