Potential difference (voltage) is the change in electric potential energy per unit charge between two points in a circuit, measured in volts (V). It tells you how much work is done per coulomb to move charge between those points, and it's what pushes current through resistance via Ohm's law (V = IR).
Potential difference, usually just called voltage, is the difference in electric potential energy per unit charge between two points in a circuit. One volt means one joule of energy per coulomb of charge. So when a battery is labeled 9 V, it's telling you that every coulomb of charge gains 9 joules of energy as it moves through the battery.
Think of it as the "pressure" that pushes charge through a circuit. A battery sets up a potential difference across its terminals, and that difference drives current through whatever is connected to it. Across any circuit element (a resistor, a lightbulb, a motor), the potential difference tells you how much energy each coulomb of charge gives up as it passes through. That energy-per-charge framing is the whole reason voltage shows up in power equations like P = IV. You're literally multiplying charge per second (current) by energy per charge (voltage) to get energy per second (power).
Potential difference lives in Unit 9 of AP Physics 1, where it underpins Topic 9.2 Resistivity and everything that follows. You can't use Ohm's law (V = IR), analyze series and parallel circuits, or reason about power without it. It's also one of the best bridges back to earlier units, because voltage is fundamentally an energy idea. When the exam asks you to track electrical energy turning into mechanical energy or heat, you're really applying Unit 4 energy conservation with a new vocabulary word. If you can say "potential difference is energy per charge" and actually mean it, half of circuit analysis stops being mysterious.
Keep studying AP Physics 1 Unit 9
Ohm's Law (Unit 9)
V = IR is the relationship you'll use constantly. Potential difference across a resistor equals the current through it times its resistance, so if you know any two of the three, you can find the third.
Electric Current (Unit 9)
Potential difference is the cause; current is the effect. A voltage across a conductor pushes charge through it, and with no potential difference there's no sustained current. Keeping cause and effect straight saves you on conceptual MCQs.
Resistivity and Resistance (Unit 9)
For a fixed potential difference, the current you get depends on resistance, which depends on the material's resistivity, length, and cross-sectional area. The 2018 conductive-dough FRQ tested exactly this chain, applying a known voltage and measuring current to find resistivity.
Energy Conservation (Unit 4)
Voltage is joules per coulomb, so circuit problems are secretly energy problems. The 2019 motor FRQ asked you to connect electrical energy (from current and potential difference) to the gravitational potential energy of a lifted block. Same conservation logic, new context.
Potential difference shows up two main ways. First, quantitatively, where you apply V = IR and P = IV to find voltages, currents, or power in circuits. Second, conceptually, where you reason about how voltage distributes across series and parallel elements. The 2017 short FRQ is a classic example, asking you to compare brightness of identical bulbs in different circuit arrangements, which forces you to track how the battery's potential difference splits up (series) or stays the same across branches (parallel). Lab-design questions are also fair game. The 2018 long FRQ had students apply a potential difference across dough cylinders and use measured current to determine resistivity, so know that a voltmeter measures potential difference across an element (in parallel), not through it. In energy questions like the 2019 motor SAQ, you multiply potential difference by current to get electrical power, then connect it to mechanical energy output.
Electric potential energy is the total energy a charge has at a point, measured in joules. Potential difference is energy per unit charge between two points, measured in volts (joules per coulomb). A 9 V battery doesn't give every charge 9 joules. It gives each coulomb of charge 9 joules. Dividing by charge is what makes voltage a property of the circuit rather than of any particular charge moving through it.
Potential difference is the change in electric potential energy per unit charge between two points, and one volt equals one joule per coulomb.
Voltage is the cause and current is the effect; a potential difference across a conductor is what drives charge to flow.
Ohm's law (V = IR) connects the potential difference across a resistor to the current through it and its resistance.
In a series circuit the battery's potential difference splits among the elements, while in parallel each branch gets the full potential difference.
Power delivered to a circuit element is P = IV, which works because current is charge per second and voltage is energy per charge.
A voltmeter measures potential difference and must be connected in parallel across the element you're measuring.
It's the difference in electric potential energy per unit charge between two points, measured in volts. A 9 V battery does 9 joules of work on every coulomb of charge that passes through it.
Yes. Voltage is just the everyday name for potential difference, and both are measured in volts. The College Board uses both terms interchangeably, so don't let the wording throw you on the exam.
Potential difference is energy per charge (volts) and current is charge flowing per second (amperes). Voltage is the push, current is the flow, and Ohm's law (V = IR) connects them through resistance.
No. Charge is conserved and flows all the way around the circuit. What gets transferred is energy. As charge moves through a potential difference, it gains energy in the battery and delivers it to resistors, bulbs, or motors.
With a voltmeter connected in parallel across the element you're measuring. This setup appears in AP lab-design FRQs, like the 2018 question where students applied a known potential difference across dough cylinders and measured current to calculate resistivity.