Physic Labs

Problem 2

An ideal capacitor of capacitance C is charged to voltage V₀ and connected at t=0 to a resistor R initially carrying no current. Define discharge current as positive in the discharge direction and v(t) with the initial capacitor polarity; neglect inductance and wire resistance. (a) Establish the discharge differential equation and time constant τ. (b) Find v(t) and current i(t) for t≥0. (c) Calculate heat dissipated from 0 to t and check its limit as t approaches infinity. Use SI units throughout and treat components as ideal exactly as specified. State sign conventions when writing equations and retain units in final results.
i(t)=v/R=(V0/R)e−t/RCi(t)=v/R=(V_0/R)e^{-t/RC}
problems.proof.analysis

The next part uses the preceding result as input rather than treating parts independently. This keeps conventions and physical state consistent throughout.

problems.proof.pitfall. Pitfall: do not change sign conventions between equations; a negative sign may encode direction rather than an invalid result.