CHSE Odisha Elective Physics common mistakes
CHSE Odisha Elective Physics · Understand the traps and how to avoid them.
When adding capacitors in a mixed series-parallel network, students apply the series formula 1/C = 1/C1 + 1/C2 to branches that are actually in parallel, or they add parallel branches directly, then compute total charge with the wrong equivalent capacitance.
The reciprocal rule for series capacitors looks like the parallel rule for resistors, and candidates memorise formulas without checking which plates share the same node. Under exam pressure they pattern-match to whichever formula was revised last.
What to do instead
Before substituting numbers, redraw the network and label each junction; capacitors sharing two common nodes are in parallel, capacitors sharing exactly one node in a chain are in series. Work from the innermost pair outward, and sanity-check that C_eq is smaller than the smallest series element and larger than the largest parallel element.
In potentiometer and metre-bridge numericals, students write the balance condition with resistances inverted (R/S instead of S/R, or l/(100-l) instead of (100-l)/l) and then report a physically impossible unknown resistance without noticing.
The balance condition is memorised as a ratio without a picture of which side carries the unknown. Students also forget that the bridge wire is exactly 100 cm, so the second length is 100 minus the measured length.
What to do instead
Always sketch the circuit, mark which arm holds the unknown, and derive the ratio from 'potential drop is proportional to resistance in the two arms'. Check the answer against the measured lengths: if the balance point is left of centre, the unknown must be smaller than the known resistance. For the potentiometer, verify that the emf of the driver cell exceeds every emf being compared.
When finding the magnetic field of a current-carrying loop, solenoid, or toroid, candidates plug the straight-wire formula B = mu0*I/(2*pi*r) into the centre of a coil, or use the axis field of a solenoid at its end without the factor 1/2.
The straight-wire result is the most practised formula, so it becomes the default. The distinction between field at the centre of a circular loop (mu0*I/2R), on its axis, at the end of a solenoid (half the interior value), and inside a toroid is a set of special cases that are rarely contrasted in revision.
What to do instead
Build a one-page table listing geometry, formula, and the point where it applies (centre, axis, interior, end, outside). Before solving, state in words where the field is being evaluated, then select the matching row. Use Ampere's law for the toroid and solenoid interior to re-derive the result rather than recalling it.
In AC circuits, students compute impedance as the algebraic sum R + X_L + X_C instead of the phasor sum, and then report a power factor or resonance frequency that contradicts their own impedance value.
Reactance and resistance are all measured in ohms, so they look additive. The 90-degree phase difference between the voltage across a resistor and across an inductor or capacitor is a conceptual point that gets lost when only formulas are memorised.
What to do instead
Draw the phasor (impedance) triangle every time: R on the horizontal, X_L - X_C on the vertical, Z as the hypotenuse. Then compute cos(phi) = R/Z and check it lies between 0 and 1. At resonance confirm X_L = X_C, so Z equals R and the current is maximum.
For a two-lens or lens-mirror combination, students apply the thin-lens formula to each lens using the same object distance for both, ignoring that the image of the first element is the object of the second and may be virtual.
Single-lens problems dominate practice, so the sign convention is applied mechanically without tracking where the intermediate image falls. The virtual-object case (image formed beyond the second lens) is rarely drilled.
What to do instead
Draw a ray diagram and mark the intermediate image position numerically before starting the second lens. Apply the Cartesian sign convention consistently (distances measured from the pole, against incident light negative). State explicitly whether the intermediate image is real or virtual and set the sign of the second object distance accordingly before substituting.
In Young's double-slit numericals, students substitute the slit separation d and screen distance D interchangeably, or use the fringe width formula with the distance to the nth bright fringe instead of the spacing between adjacent fringes.
The symbols d and D differ by a single letter and both appear in the same formula, beta = lambda*D/d. Fringe width and position of the nth fringe are also easily confused because both contain D/d.
What to do instead
Write the formula with words attached: fringe width = (wavelength x distance to screen) / slit separation. Before substituting, write the numerical values with units beside each named quantity. Check the answer's order of magnitude: for visible light with millimetre slits and a metre screen, fringe width should be a fraction of a millimetre.
Students compute the de Broglie wavelength of an electron accelerated through a potential difference using the non-relativistic formula without checking validity, and they mix up the work function, threshold frequency, and stopping potential in photoelectric-effect problems.
The photoelectric equation and the de Broglie relation are both short formulas that are memorised as strings of symbols. The physical meaning of each term (work function is an energy, threshold frequency a frequency, stopping potential a voltage) is not fixed in memory.
What to do instead
For each photoelectric problem, write the energy balance in words first: photon energy = work function + maximum kinetic energy, and maximum kinetic energy = e times stopping potential. Attach units to every symbol before substituting. For the de Broglie wavelength, use lambda = h/sqrt(2meV) and verify that the electron energy is well below about 100 keV before trusting the non-relativistic result.
In nuclear physics, students treat the mass defect as the mass of the products minus the mass of the reactants in the wrong order, and they forget to convert atomic mass units to energy using 931.5 MeV per u, reporting binding energy in the wrong unit or with the wrong sign.
Mass defect and binding energy are defined with opposite-looking subtractions, and the conversion factor is easy to drop. The convention that a stable nucleus has positive binding energy is not linked to the sign of the mass difference.
What to do instead
Always compute mass defect as (mass of separated nucleons) minus (mass of nucleus), which is positive for a bound nucleus. Convert with 1 u = 931.5 MeV and state the unit in the answer. As a check, binding energy per nucleon should peak near 8.8 MeV around iron, so any value far outside 1 to 9 MeV per nucleon signals an arithmetic or sign error.
In semiconductor problems, students draw or interpret a transistor circuit without checking the bias polarity of both junctions, and they apply the ideal diode equation to a reverse-biased diode as if it conducted.
The forward and reverse characteristics look like two halves of one graph, and the biasing rules for the emitter-base and collector-base junctions are easily swapped. Zener operation in reverse breakdown is also confused with ordinary reverse bias.
What to do instead
For every diode or transistor circuit, first label each junction as forward or reverse biased using the rule that the p-side must be at higher potential for forward bias. Then state which region the device is in (cut-off, active, saturation) before calculating currents. Remember a Zener conducts only beyond its breakdown voltage, in reverse.
In rotational motion and gravitation numericals, students use the moment of inertia of a body about its centre of mass when the axis of rotation is at the edge or some other point, and they equate gravitational field strength to gravitational potential without accounting for the difference in dimensions.
Standard results such as the moment of inertia of a disc or rod are memorised for the centre only, and the parallel-axis theorem is not automatically applied. Field and potential both describe gravity, so their different units are overlooked.
What to do instead
Before computing torque or angular acceleration, mark the actual axis on a sketch and apply the parallel-axis theorem I = I_cm + M*d^2 whenever the axis is displaced. For gravitation, write the dimensions of the quantity being asked for (N/kg for field, J/kg for potential) and check the answer matches. Verify limiting cases such as the field inside a uniform shell, which is zero while the potential is not.