⚡ Electric Fields and Electric Potential

Electric Field

An electric field is a vector field that describes the electric force per unit charge at any point in space.

$\vec{E} = \frac{\vec{F}}{q_0}$

where:

• $\vec{E}$ = electric field (N/C or V/m)
• $\vec{F}$ = force on test charge
• $q_0$ = small positive test charge

Point Charge Field:

$E = k\frac{|q|}{r^2}$

Direction:

• Positive charge → field points away
• Negative charge → field points toward

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Electric Field Lines

Visual representation of electric fields:

Rules:

1. Lines start on positive charges, end on negative charges
2. Density of lines ∝ field strength
3. Lines never cross
4. Tangent to line gives field direction
5. Perpendicular to conductor surfaces

Uniform field: Parallel, evenly spaced lines (e.g., parallel plates)

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Superposition of Fields

$\vec{E}_{total} = \vec{E}_1 + \vec{E}_2 + \vec{E}_3 + ...$

Calculate field from each charge, then vector sum.

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Electric Potential Energy

Work done moving charge in electric field:

$U_E = k\frac{q_1 q_2}{r}$

• Same sign charges: $U > 0$ (repulsive, stored energy)
• Opposite sign charges: $U < 0$ (attractive, bound state)

Change in PE: $\Delta U = -W_{field} = W_{external}$

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Electric Potential (Voltage)

Electric potential (V) is potential energy per unit charge:

$V = \frac{U}{q_0} = k\frac{q}{r}$

Units: Volt (V) = J/C

Potential Difference:

$\Delta V = V_f - V_i = -\int \vec{E} \cdot d\vec{r}$

For uniform field: $\Delta V = -Ed$

where $d$ is distance in field direction.

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Relationship: E and V

$\vec{E} = -\frac{dV}{dr}$

Electric field points from high to low potential (downhill).

For uniform field: $E = \frac{\Delta V}{d}$

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Parallel Plate Capacitor

Uniform field between plates:

$E = \frac{V}{d} = \frac{\sigma}{\epsilon_0}$

where:

• $V$ = potential difference
• $d$ = plate separation
• $\sigma$ = surface charge density

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Equipotential Surfaces

Surfaces where $V =$ constant

• No work to move charge along equipotential
• Always ⊥ to electric field lines
• Closer spacing → stronger field

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Electron Volt (eV)

Energy gained by electron moving through 1 V:

$1 \text{ eV} = 1.60 \times 10^{-19} \text{ J}$

Useful for atomic/particle physics.

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Problem-Solving Strategy

For Fields:

1. Calculate $E$ from each charge
2. Determine directions (away from +, toward -)
3. Use components if needed
4. Vector sum

For Potential:

1. Calculate $V$ from each charge (scalar!)
2. Algebraic sum (watch signs)
3. Or use $\Delta V = -Ed$ for uniform field

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Common Mistakes

❌ Treating potential as vector (it's scalar!) ❌ Wrong field direction from negative charge ❌ Forgetting $\Delta V = V_f - V_i$ (order matters) ❌ Sign errors in potential energy ❌ Confusing E (field) with V (potential)