ACT Science: Ohm's Law and Circuits—V=IR Basics Made Simple

Voltage (V) is electrical potential energy per unit charge (measured in volts). Current (I) is the flow of charge (measured in amperes or amps). Resistance (R) is opposition to current flow (measured in ohms). Ohm's Law: V=IR. Higher voltage drives more current; higher resistance reduces current. This single equation is the foundation of circuit analysis on the ACT. If you know any two quantities, you can calculate the third.

Example: A bulb has 12-volt supply and 3-ohm resistance. Current I=V/R=12/3=4 amps. The bulb draws 4 amps.

Type 1 (Find current): V=120 volts, R=40 ohms. I=V/R=120/40=3 amps. Type 2 (Find voltage): I=2 amps, R=50 ohms. V=IR=2(50)=100 volts. Type 3 (Find resistance): V=24 volts, I=0.5 amps. R=V/I=24/0.5=48 ohms. Know all three forms: V=IR, I=V/R, R=V/I.

Series circuits add resistances: R_total=R₁+R₂+R₃. Parallel circuits use: 1/R_total=1/R₁+1/R₂+1/R₃ (or R_total=R₁R₂/(R₁+R₂) for two resistors).

Problem 1: A circuit has a 9-volt battery and a 3-ohm resistor. Find current. I=V/R=9/3=3 amps. Problem 2: A resistor draws 2 amps at 15 volts. Find resistance. R=V/I=15/2=7.5 ohms. Problem 3: A 10-ohm and 5-ohm resistor in series are powered by a 30-volt battery. Find total resistance and current. R_total=10+5=15 ohms. I=V/R_total=30/15=2 amps. Complete three problems daily until you solve each in under 60 seconds.

Verify: I×R should equal V. 2×15=30 ✓.

Circuit and electricity questions appear in 1-2 ACT Science passages. They are computational; once you know Ohm's Law and series/parallel rules, solving is mechanical. Investing 20 minutes in this topic yields 1-2 guaranteed points because the concept is simple and formulas are provided.

Master Ohm's Law two days before the test. By test day, circuit problems become routine calculations.