An ideal capacitor of 261 μF is at 80 V. Calculate its stored energy in millijoules. Round the final result to 4 decimal places if needed. Answer in mJ.
Answer and explanation
835.2 mJ
U = (261 × 10⁻⁶)(80)²/2 J. Multiply joules by 1000 for mJ.
Subjects
Electromagnetism
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Sample exercises
An ideal capacitor initially at 20 V discharges through 18 kΩ. C=130 μF. Find the capacitor voltage at t=0.1 s. Round the final result to 4 decimal places if needed. Answer in V.
Answer and explanation
19.1633 V
RC = (18×1000)(130×10⁻⁶) = 2.34 s; substitute into V₀ exp(−t/RC).
A closed surface encloses 28 nC. Using ε₀=8.854×10⁻¹² F/m, find the net electric flux in N·m²/C. Round the final result to 4 decimal places if needed. Answer in N·m²/C.
Answer and explanation
3162.4125 N·m²/C
Convert charge to 28×10⁻⁹ C and divide by ε₀. The sign follows the enclosed charge.
Study notes
Stored energy in an ideal capacitor
Convert microfarads to farads before using SI units. This is a paper exercise, not hardware guidance.
UniversityStored energy in an ideal capacitor · formula reference
Convert microfarads to farads before using SI units. This is a paper exercise, not hardware guidance.
UniversityRC discharge voltage
Ideal RC discharge with no driving source. Convert kilohms and microfarads consistently.
UniversityRC discharge voltage · formula reference
Ideal RC discharge with no driving source. Convert kilohms and microfarads consistently.
UniversityGauss-law electric flux
Net flux through the entire closed surface depends on enclosed charge, not on external charges.
UniversityGauss-law electric flux · formula reference
Net flux through the entire closed surface depends on enclosed charge, not on external charges.
UniversityMagnetic force magnitude
The stated velocity is perpendicular to the magnetic field. Magnetic force does no work on the point charge.
UniversityMagnetic force magnitude · formula reference
The stated velocity is perpendicular to the magnetic field. Magnetic force does no work on the point charge.
UniversityVacuum electric-field energy density
This asks for the electric contribution only. A magnetic field would add B²/(2μ₀).
Advanced universityVacuum electric-field energy density · formula reference
This asks for the electric contribution only. A magnetic field would add B²/(2μ₀).
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