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AP Physics 2: Algebra-Based · Unit 2 of 7
Electric Force, Field, and Potential
15–18% of the exam4 lessons · 51 min20 terms
What this unit covers
The topics below follow the published Physics 2 course framework for Unit 2. This unit is worth 15–18% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Coulomb’s lawElectric fieldsElectric potentialCharge distributions
Lessons in this unit
- Electric Charge & Coulomb’s Law12 min · 3 objectivesDescribe electric charge as quantized and conserved, and distinguish conductors from insulators · Use Coulomb’s law to find the force between two point charges · Predict how the electric force changes as charge or separation changes
- The Electric Field13 min · 3 objectivesDefine the electric field as force per unit charge and state its units · Calculate the field produced by a point charge and the force it exerts on a test charge · Interpret electric field lines and the direction of the field around charges
- Electric Potential & Potential Energy14 min · 3 objectivesDistinguish electric potential (a scalar, in volts) from the electric field (a vector) · Relate electric potential energy to charge and potential difference · Compute the potential of a point charge and the energy to move a charge through a potential difference
- Conductors & Charge Distributions12 min · 3 objectivesDescribe the electric field and charge arrangement inside and on a conductor in equilibrium · Explain why charge concentrates at sharp points on a conductor · Analyze the uniform field between parallel charged plates
Formulas in Unit 2
Coulomb’s law
F = k · |q₁ · q₂| / r²
k = 8.99 × 10⁹ N·m²·C⁻² (often rounded to 9.0 × 10⁹). r is the center-to-center distance. The force acts along the line joining the charges: repulsive for like charges, attractive for opposite.
Electric field: definition and point-charge value
E = F / q and E = k · |Q| / r²
Units are N·C⁻¹ (equivalently V·m⁻¹). The first form gives the force felt by a charge q in a field; the second gives the field a source charge Q creates at distance r.
Potential of a point charge & energy of a charge
V = k · Q / r and ΔU = q · ΔV
Keep the sign of Q in V = kQ/r: a positive charge makes V positive, a negative charge makes V negative. ΔU is the energy change when a charge q moves through a potential difference ΔV.
Uniform field and potential
E = V / d
Between two parallel plates a distance d apart with a voltage V across them, the field is uniform with magnitude V/d. This is why field units can be written as V·m⁻¹.
Uniform field between parallel plates
E = V / d
Two large parallel plates with a voltage V across a gap d produce a nearly uniform field V/d, pointing from the positive plate to the negative plate. This is the geometry of a parallel-plate capacitor.
Every term in Unit 2
All 20 terms we publish for Electric Force, Field, and Potential, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Uniform field between parallel plates
- E = V/d, constant between the plates. A charge released there experiences constant force and therefore constant acceleration.
- Charge conservation and quantization
- Charge is never created or destroyed, only transferred, and comes in multiples of the elementary charge 1.6 × 10⁻¹⁹ C.
- Coulomb's law
- F = kq₁q₂/r² with k = 8.99 × 10⁹ N·m²/C². Like charges repel, opposite attract, and force falls with the square of separation.
- Charging by friction, conduction and induction
- Friction transfers electrons between materials; conduction requires contact and shares charge; induction polarizes then grounds, leaving opposite charge without contact.
- Electric field
- E = F/q, force per unit positive test charge, in N/C. Points away from positive charge and toward negative.
- Field of a point charge
- E = kq/r². Field lines never cross, start on positive charge and end on negative, and their density represents field strength.
- Electric potential energy
- U = kq₁q₂/r for two point charges, and includes sign. Like charges have positive potential energy that decreases as they separate.
- Electric potential
- V = U/q, energy per unit charge, in volts. A scalar, so potentials from several charges add algebraically without vectors.
- Potential difference and work
- W = qΔV. Moving a charge along an equipotential surface requires no work, since ΔV is zero.
- Equipotential lines
- Always perpendicular to field lines. Closely spaced equipotentials indicate a strong field.
- Field vs potential
- Field is a vector and can be zero where potential is not; potential is a scalar and can be zero where field is not. Between two equal opposite charges, the midpoint has V = 0 but E ≠ 0.
- Conductors vs insulators
- Conductors have mobile charge carriers that redistribute freely; insulators hold charge where it is placed, which is why only conductors can be charged by induction.
- Grounding
- Connecting to a large reservoir of charge, allowing electrons to flow in or out until the object is neutral or at the ground's potential.
- Electroscope behavior
- Leaves diverge because like charges repel. They diverge for either sign of charge, so an electroscope alone cannot tell you which.
- Field line rules
- Start on positive and end on negative charge, never cross, and are denser where the field is stronger. They are perpendicular to a conductor surface.
- Superposition of fields
- Add contributions as vectors, resolving into components first. The point where two fields cancel lies nearer the smaller charge.
- Field inside a conductor
- Zero at equilibrium, because any field would move charges until it was canceled. All excess charge sits on the surface.
- Why potential is a scalar
- It is energy per charge, and energy has no direction. This is why V from several charges is a simple sum with signs.
- Sketching equipotentials
- Draw them perpendicular to field lines and at equal potential intervals; closer spacing indicates a stronger field.
- Charged particle released in a uniform field
- Experiences constant force and therefore constant acceleration, so the kinematics are identical to projectile motion.
What examiners penalize here
- When only ratios change (charge or distance scaled up or down), you rarely need to plug into Coulomb’s law fully. Track the proportionality: F ∝ q₁q₂/r². Double one charge → ×2; triple the distance → ×1/9.
- Two different formulas share the letter E. Use E = F/q when a charge and the force on it are given; use E = kQ/r² when a source charge and a distance are given. Mixing them up is a classic slip.
- Remember the different distance dependences: the field of a point charge falls off as 1/r², but its potential falls off as 1/r. Energy questions use potential (scalar, add algebraically); force questions use the field (vector).
- Lock in the conductor facts: field zero inside, excess charge on the surface, field perpendicular just outside, and charge densest at sharp points. These appear on nearly every electrostatics free-response question.
Practice Physics 2
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Physics 2: Algebra-Based exam is Unit 2?
Unit 2, Electric Force, Field, and Potential, is worth 15–18% of the Physics 2 multiple-choice section according to the published course framework. Across all 7 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in Physics 2 Unit 2?
Electric Force, Field, and Potential covers Coulomb’s law, Electric fields, Electric potential and Charge distributions. We publish 20 terms with definitions for this unit, all of them on this page.
How should I study Physics 2 Unit 2?
Read the 4 lessons below first — about 50 minutes — then drill the 20 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 7 units of AP Physics 2: Algebra-Based
Unit names, topics and exam weights follow the published College Board course framework for AP Physics 2: Algebra-Based. AP® is a trademark registered by the College Board, which does not endorse this site.