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College Physics (Urone)
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Short Answer

Explain in your own words why equipotential lines and surfaces must be perpendicular to electric field lines.

As the work done in moving a charge on an equipotential surface is zero but the electric field and charge is non zero, the electric field must be perpendicular to the direction of motion of charge.

See the step by step solution

Step by Step Solution

Step 1: Definition of Equipotential surface

On the equipotential surface, the potential has the same value at every point. The equipotential lines are 2-D manifestations of the surfaces, on which potential is the same all along the line.

Step 2: Explanation of why equipotential lines and surfaces must be perpendicular to electric field lines

The work required (W) to move a charge along an equipotential line is zero;

W=-q×dV=-q×(0)=0

where q is electric charge and dv is potential difference.

This work is related to the electric field as:

W=Fe×d×cosθ=qE×d×cosθ

where Fe = qE is the force on charge q due to electric field E .

Since neither charge q nor electric field strength E is zero, cosθ must be zero.

Hence, θ must be 90° .

This means that the equipotential lines and surfaces are perpendicular to the electrical field lines.

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