Electric potential describes how much electric potential energy a unit positive charge would have at a point in space. Voltage is the difference in electric potential between two points, so it tells how much energy is transferred per coulomb of charge. These ideas matter because they connect invisible electric fields to measurable quantities in circuits, batteries, capacitors, and particle motion.
A voltage map helps predict where charges will move and how much energy they can gain or lose.
Understanding Physics: Electric Potential and Voltage
Electric potential is a scalar quantity. It has a value but no direction, unlike an electric field. This makes it useful for describing complicated regions of space.
You can assign a reference value of zero at a chosen location, then compare every other location with it. For isolated charges, zero is often chosen very far away. In circuits, a point called ground is commonly chosen as zero.
Ground is a reference point, not necessarily the Earth. What matters physically is the difference between locations.
The sign of charge changes how potential energy behaves. A positive charge gains electric potential energy when it moves toward a higher potential. Left alone, it tends to move toward lower potential, converting stored electric energy into kinetic energy or other forms.
An electron has negative charge, so its potential energy changes in the opposite way. It tends to move toward higher electric potential.
This can seem confusing because conventional current is defined as the direction positive charges would move. In a metal wire, the mobile particles are electrons, which drift opposite to conventional current.
A battery separates charge through chemical reactions. Its terminals end up at different potentials, and this difference can push charge through a complete circuit. The battery does not create charge.
It supplies energy to move charge internally from lower potential toward higher potential. In a lamp or resistor, charge transfers energy to the material.
Collisions in the material produce heating, light, or motion. A larger voltage difference means more energy is available for each coulomb of charge, though the actual current still depends on the resistance and the circuit layout.
Potential diagrams give a visual way to reason about fields. Closely spaced equipotential lines show that potential changes rapidly over a short distance. This usually means the electric field is strong there.
The field crosses equipotential lines at right angles. A charge can move along an equipotential path without a change in electric potential energy, provided only electric forces are considered.
Moving across the lines changes its energy. This idea is used when studying charged particles between parallel plates, where an almost uniform field can speed up, slow down, or bend a particle.
A voltmeter measures the potential difference between its two probes. It must be connected across a component, not placed in series with it. Good voltmeters have very high resistance, so they draw very little current and disturb the circuit as little as possible.
Pay close attention to the order of the probe connections because reversing them changes the sign of the reading. Students often mix up potential with potential energy.
Potential belongs to a location and does not depend on the test charge chosen. Potential energy depends on both the location and the charge placed there.
Key Facts
- Electric potential is electric potential energy per charge: V = U/q.
- Voltage is potential difference: ΔV = VB - VA = ΔU/q.
- For a point charge, electric potential is V = kQ/r, where k = 8.99 × 10^9 N m^2/C^2.
- Electric field points in the direction of decreasing electric potential for a positive test charge.
- In a uniform electric field, ΔV = -Ed when moving distance d along the field direction.
- Equipotential lines connect points with the same electric potential, so moving a charge along one requires no work.
Vocabulary
- Electric potential
- Electric potential is the electric potential energy per unit charge at a point in an electric field.
- Voltage
- Voltage is the difference in electric potential between two points, measured in volts.
- Equipotential line
- An equipotential line is a line connecting points that all have the same electric potential.
- Electric field
- An electric field is a region where a charge experiences an electric force.
- Volt
- A volt is one joule of energy per coulomb of charge, so 1 V = 1 J/C.
Common Mistakes to Avoid
- Confusing electric potential with electric potential energy is wrong because potential is energy per unit charge, while potential energy depends on the amount of charge present.
- Thinking voltage only exists in circuits is wrong because voltage can be defined between any two points in an electric field.
- Drawing electric field lines parallel to equipotential lines is wrong because electric field lines cross equipotential lines at right angles.
- Ignoring the sign of charge is wrong because positive and negative charges move in opposite directions for the same electric field and potential difference.
Practice Questions
- 1 A charge has 0.060 J of electric potential energy at a point where the electric potential is 12 V. What is the charge?
- 2 What is the electric potential 0.30 m from a +2.0 μC point charge? Use k = 8.99 × 10^9 N m^2/C^2.
- 3 A positive charge is released from rest near a positive point charge. Explain whether it moves toward higher potential or lower potential, and connect your answer to the direction of the electric field.