Equipotential Surfaces

The surface at which electric potential is the same at each point is called an equipotential surface.

OR

Any surface over which the electric potential is the same everywhere is called an equipotential surface.

Equivalently, it is the locus of all points in an electric field that have equal electric potential.

Extended Hierarchy:

When a charge q₀​ is moved from point A to point B on the same equipotential surface:

Since V_A = V_B​ on the surface:

Property 1: The potential difference between any two points is zero.

• Every point has identical potential ⟹ ΔV = 0 ⟹ W = q₀ΔV = 0

Property 2: No work is done in moving a charge along the surface.

• Direct consequence of Property 1. The electrostatic force does zero work along any path on the surface.

Property 3: The electric field \[\vec E\] is always perpendicular to the equipotential surface.

• If \[\vec E\] had a component along the surface, it would exert a force on a charge along the surface, doing non-zero work — contradicting Property 2. Hence, the tangential component must be zero, forcing \[\vec E\] surface.

Property 4: Two equipotential surfaces can never intersect.

• If two surfaces intersected, a single point would have two different potential values — mathematically impossible. Also, the electric field at that point would have two directions — physically impossible.

Property 5: Equipotential surfaces are denser (closer) in regions of a stronger electric field.

• Relation: E = −\[\frac {dV}{dr}\] ⟹ where potential changes rapidly (strong field), surfaces are closer.

Property 6: Direction of potential decrease follows the direction of \[\vec E\].

• \[\vec E\] points from high potential to low potential, i.e., downhill on the potential surface.

Property 7: The surface of any conductor in electrostatic equilibrium is an equipotential surface.

• Inside a conductor, \[\vec E\] = 0. No work is done in any displacement. Hence, all points — surface and interior — are at the same potential.

Setup: Let AB be a small displacement of length dl along the equipotential surface. The electric field \[\vec E\] makes an angle θ with this displacement.

Step 1: Work done in moving charge q₀​ through dl:

Step 2: Since A and B are on the same equipotential surface, V_A = _VB, so:

Step 3: Equating:

Since q₀ ≠ 0, E ≠ 0, dl ≠ 0:

Conclusion: The electric field is always perpendicular (normal) to the equipotential surface. 

The electric field is the negative of the potential gradient:

This means:

• Greater E ⟹ potential changes faster ⟹ equipotential surfaces are denser
• Weaker E ⟹ potential changes slowly ⟹ surfaces are farther apart

The particle floats because the upward electric force balances gravity, so qE = mg.

• Field between plates: E = \[\frac {V}{d}\] = \[\frac {4000}{0.1}\] = 4 × 10⁴ V/m
• Balancing forces: m = \[\frac {qE}{g}\] = \[\frac {1.6×10^{−19}×4×10^4}{9.8}\] ≈ 6.53 × 10⁻¹⁶ kg

The negative signs in the source cancel out and don't change the physics — they likely came from treating both charge and field as vectors in the same (downward) direction, which is unnecessary for a magnitude calculation.

A point charge's potential falls off as V = \[\frac {1}{4πε_0}\frac {q}{r}\] = \[\frac {kq}{r}\]​, so solving for r gives the distance at which potential equals a given value.

• Rearranged: r = \[\frac {kq}{V}\] = \[\frac {9×10^9×1×10^{−6}}{90}\] = 100 m

So the 90 V equipotential sphere around this 1 μC charge has a radius of 100 m — meaning every point on a sphere of that radius, centered on the charge, is at exactly 90 V.

Analogy 1 — Topographic Map (Best Analogy)

"Equipotential surfaces are exactly like contour lines on a topographic map. Just as contour lines connect points of the same altitude (height above sea level), equipotential surfaces connect points of the same electric potential. Walking along a contour line requires no work against gravity — just as moving a charge along an equipotential requires no work against the electric force."