AP Physics 2 common mistakes
AP Physics 2 · Understand the traps and how to avoid them.
When a gas undergoes an adiabatic process, the student applies PV = constant (or PV/T = constant) instead of PV^gamma = constant, and then also assumes Q = nC_p dT.
The ideal gas law is over-learned as a universal relation, so it gets substituted into every process. Students do not first classify the process (isobaric, isochoric, isothermal, adiabatic) before choosing the constraint equation.
What to do instead
Before touching algebra, write the process name and its single constraint: isobaric V/T = const, isochoric P/T = const, isothermal PV = const, adiabatic PV^gamma = const. Then use the first law with the correct Q (zero only for adiabatic) and use C_v for any process that is not constant pressure.
The student computes the work done in a cyclic process as the area under the P-V curve without regard to sign, or treats the net work as the sum of the magnitudes of the expansion and compression areas.
Area is always positive, so students add the two loops' magnitudes instead of subtracting, and they forget that clockwise and counterclockwise cycles have opposite net work signs.
What to do instead
Compute W = integral P dV with sign: expansion positive, compression negative, then add algebraically. For a closed cycle, net work equals the signed enclosed area, positive if the cycle runs clockwise on a P-V diagram and negative if counterclockwise.
When two point charges are present, the student finds the field at a point by adding the two field magnitudes as scalars, or computes the potential by adding field vectors instead of scalars.
Students conflate the vector nature of E with the scalar nature of V and default to whichever operation they practiced last. Direction reasoning is skipped because the magnitudes look sufficient.
What to do instead
Adopt a fixed routine: for E, draw both field vectors at the point, decompose into components, and add vectorially; for V, add the signed scalars kq/r with the sign of each charge. Then check that E points from high to low potential while V is a scalar that can cancel to zero between like charges.
The student treats the electric field inside a conductor in electrostatic equilibrium as nonzero, or assumes the potential inside a charged solid conducting sphere varies with radius.
The formula E = kq/r^2 is applied for all r without the conductor boundary condition, and the distinction between field (which is zero inside) and potential (which is constant inside) is not internalized.
What to do instead
Memorize the conductor rules: E = 0 everywhere inside the material, excess charge resides on the surface, and the entire conductor (surface and interior) is an equipotential. Only outside the surface does E = kq/r^2 apply; inside, V equals the surface value.
When solving a multi-loop circuit, the student writes Kirchhoff's loop equation using the same current symbol for branches that carry different currents, or assigns signs to battery and resistor terms by guessing.
Single-loop habits carry over, and sign conventions for emf versus IR drops are applied inconsistently because the student never fixed a traversal direction.
What to do instead
Label a distinct current (with an arrow) on every branch before writing equations. Choose a loop traversal direction, then use one rule consistently: going through a battery from - to + is +emf, through a resistor in the direction of current is -IR. Count unknowns and confirm you have as many independent equations as unknowns.
The student treats the ammeter and voltmeter as ideal in every problem, placing an ammeter in parallel with a component or a voltmeter in series, and ignores their internal resistance when the problem gives it.
The ideal-meter abstraction is learned as a rule rather than a model, so the student does not check whether the problem supplies nonzero meter resistance or asks about measurement error.
What to do instead
State the model first: ideal ammeter has zero resistance and goes in series; ideal voltmeter has infinite resistance and goes in parallel. If internal resistance is given, redraw the circuit with that resistance included in the correct position and recompute the equivalent resistance before analyzing.
The student determines the direction of the magnetic force on a moving charge using the right-hand rule but forgets to reverse the result for a negative charge, or uses the field direction as the force direction.
The right-hand rule is memorized as a single gesture (v cross B) without tracking that F = qv x B flips sign when q is negative, and the perpendicular geometry of the force is not reinforced.
What to do instead
Use the sequence: point fingers along v, curl toward B, thumb gives v x B; then multiply by the sign of q, reversing the thumb for electrons. Always verify the force is perpendicular to both v and B, and that a charge moving parallel to B feels zero force.
The student applies the mirror or thin-lens equation with object and image distances both entered as positive, then reports a real image for a case that should be virtual, or misplaces the focal length sign for a diverging lens or convex mirror.
The sign convention is treated as an afterthought, and the student does not distinguish converging from diverging elements before substituting numbers.
What to do instead
Fix the convention before substituting: object distance positive for a real object; image distance positive for a real image on the opposite side of a lens (same side for a mirror); focal length positive for converging (convex lens, concave mirror) and negative for diverging (concave lens, convex mirror). Check the sign of the computed image distance against the physical expectation.
The student predicts the interference pattern of a double slit using the single-slit width instead of the slit separation, or applies the double-slit maxima condition to a single slit.
The two formulas look similar (d sin theta = m lambda versus a sin theta = m lambda), and the student does not track which physical dimension controls which pattern.
What to do instead
Attach a meaning to each symbol: d is the center-to-center slit separation and governs double-slit bright fringes; a is the single-slit width and its minima satisfy a sin theta = m lambda (m nonzero). Write the symbol's meaning next to the equation before substituting.
The student computes the energy or wavelength of a photon or photoelectron using the wrong combination, such as setting the kinetic energy of the emitted electron equal to the photon energy without subtracting the work function, or mixing up frequency and wavelength in E = hf.
The photoelectric equation is recalled loosely, and the student does not separate incident photon energy from the energy needed to escape the metal.
What to do instead
Write the full energy balance: hf = phi + K_max, with phi = hf_threshold. Convert wavelength to frequency via f = c/lambda before using E = hf, and always subtract the work function before calling the remainder kinetic energy.