Describe simple electric field patterns, including the direction of the field: (a) around a point charge (b) around a charged conducting sphere (c) between two oppositely charged parallel conducting plates (end effects will not be examined)

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – Electric Charge

4.2.1 Electric Charge

Objective

Describe simple electric field patterns, including the direction of the field:

  • around a point charge

  • around a charged conducting sphere

  • between two oppositely charged parallel conducting plates (end effects are not examined)

1. Electric field of a point charge

A point charge \$q\$ creates a radial electric field. The magnitude of the field at a distance \$r\$ from the charge is given by

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

where \$k = 9 \times 10^{9}\,\text{N·m}^{2}\text{/C}^{2}\$.

Direction:

  • If \$q\$ is positive, field lines point away from the charge.

  • If \$q\$ is negative, field lines point toward the charge.

Suggested diagram: Radial field lines emerging from a positive point charge and converging on a negative point charge.

2. Electric field of a charged conducting sphere

A uniformly charged conducting sphere of radius \$R\$ behaves electrically like a point charge located at its centre.

For points outside the sphere (\$r \ge R\$):

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

where \$Q\$ is the total charge on the sphere.

Inside the conducting material (\$r < R\$) the electric field is zero because charges reside on the surface.

Direction:

  • Positive \$Q\$: field lines radiate outward from the surface.

  • Negative \$Q\$: field lines terminate on the surface.

Suggested diagram: Field lines outside a charged conducting sphere, none inside.

3. Electric field between two oppositely charged parallel plates

When two large, flat, parallel conducting plates are given equal and opposite charges, a uniform electric field is produced in the region between them (ignoring edge effects).

The magnitude of the field is

\$E = \frac{\sigma}{\varepsilon_{0}}\$

where \$\sigma\$ is the surface charge density on each plate and \$\varepsilon_{0}=8.85\times10^{-12}\,\text{F·m}^{-1}\$.

Direction:

  • From the positively charged plate toward the negatively charged plate.

  • Field lines are straight, parallel, and equally spaced between the plates.

Suggested diagram: Uniform field lines between two oppositely charged parallel plates.

Summary Table

ConfigurationField magnitude formulaDirection of field linesSpecial notes
Point charge \$q\$\$E = k\frac{|q|}{r^{2}}\$Away from \$q\$ if \$q>0\$, toward \$q\$ if \$q<0\$Radial, decreases with \$r^{2}\$
Charged conducting sphere (outside, \$r\ge R\$)\$E = k\frac{|Q|}{r^{2}}\$Same as point charge of magnitude \$Q\$Inside (\$r) \$E=0\$
Parallel plates (ignoring edge effects)\$E = \dfrac{\sigma}{\varepsilon_{0}}\$From positive plate to negative plateUniform, independent of distance between plates

Key Points for Examination

  1. Identify whether the source is a point charge, a conducting sphere, or parallel plates.

  2. Write the correct expression for the magnitude of \$E\$.

  3. State the direction of the field lines based on the sign of the charge(s).

  4. Remember that inside a conductor in electrostatic equilibrium the field is zero.

  5. For parallel plates, ignore edge effects unless specifically asked.