A uniform field of 4 N/C is parallel to a planar area's outward normal. For area 3 m², find flux.
Answer and explanation
12 N m²/C
Flux EA cos(0)=12.
Subjects
University electromagnetism
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Sample exercises
What charge enters the right side of Gauss law for a closed surface?
- Net enclosed charge
- Only charge on the surface
- Only positive charge inside
- All charge in the universe
Answer and explanation
Net enclosed charge
The enclosed algebraic charge controls total flux.
Use ε0=2 in stated model units. A closed surface encloses charge 10. Find total electric flux in the matching units.
Answer and explanation
5
Gauss law gives Qenclosed/ε0=10/2; the units are deliberately rescaled.
Does zero net flux imply zero electric field at every point on the surface?
- Yes, in every geometry
- Only if charge exists outside
- Yes, if the area is nonzero
- No; contributions can cancel
Answer and explanation
No; contributions can cancel
A uniform external field can enter and leave a closed surface with zero net flux.
Explain why a point charge outside a Gaussian surface can affect E without changing its total flux.
Answer and explanation
It contributes a local field, but its field lines enter and leave the closed surface with cancelling oriented contributions. It adds no enclosed charge.
Credit the distinction between field and integral flux.
A field makes an angle of 60 degrees with the normal. If EA=10 N m²/C, find flux.
Answer and explanation
5 N m²/C
Multiply EA by cos 60°=1/2.
Study notes
Electric flux and Gauss symmetry
Electric flux is the oriented surface integral of E·dA. A negative flux reflects direction relative to the chosen normal, not negative area.
UniversityElectric flux and Gauss symmetry · revision notes
Electric flux is the oriented surface integral of E·dA. A negative flux reflects direction relative to the chosen normal, not negative area.
UniversityCapacitors, RC transients and field energy
For a linear capacitor Q=CV, with stored energy CV²/2. Potential difference and capacitance must refer to the same device.
UniversityCapacitors, RC transients and field energy · revision notes
For a linear capacitor Q=CV, with stored energy CV²/2. Potential difference and capacitance must refer to the same device.
UniversityElectromagnetic waves and energy flow
In vacuum, plane electromagnetic waves have transverse electric and magnetic fields with E0=cB0. Both fields are perpendicular to propagation.
UniversityElectromagnetic waves and energy flow · revision notes
In vacuum, plane electromagnetic waves have transverse electric and magnetic fields with E0=cB0. Both fields are perpendicular to propagation.
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