Electric and Magnetic Fields

Introduction

Electric and magnetic fields give invisible interactions a measurable structure. The guide builds this topic from charge, Coulomb’s law, field diagrams, Millikan’s experiment, parallel plates, magnetic field patterns, and HL electric potential.

Guide Focus

• Use Coulomb’s law and electric field strength.
• Sketch and interpret electric and magnetic field lines.
• Apply HL electric potential, potential energy, equipotentials, and work done.

Key Concepts

1. Charge and Coulomb’s law

There are two types of electric charge, and charge is conserved. For point charges, F = kq1q2 / r^2, where k = 1 / (4 pi epsilon0). Millikan’s experiment provides evidence that electric charge is quantized.

2. Charging methods

Charge can be transferred by friction, contact, or electrostatic induction. Grounding provides a path for charge to move to or from Earth.

3. Electric fields

Electric field strength is force per unit positive charge: E = F / q. Between parallel plates, E = V / d. Field line density represents field strength.

4. Magnetic field patterns

The guide requires field patterns for a bar magnet, a straight current-carrying wire, a circular coil, and an air-core solenoid. Right-hand rules determine field direction around currents.

5. HL electric potential

For a point charge, electric potential energy is Ep = kq1q2 / r and potential is Ve = kQ / r. E = -delta Ve / delta r, and work done moving charge is W = q delta Ve. No work is done along an equipotential.

Common Mistakes

• Drawing electric field lines crossing each other.
• Forgetting electric field direction is defined for a positive test charge.
• Mixing electric potential, potential difference, and potential energy.

Exam Tips

• Use symmetry before calculating resultant fields.
• For field-line sketches, show stronger fields with closer line spacing.
• Convert electronvolts to joules when combining electric potential energy with mechanical energy.

Practice Questions

Question 1 (Multiple Choice)

The electric field between two large oppositely charged parallel plates is best described as:

A. Uniform except near edges. B. Radial outward. C. Zero everywhere. D. Circular around the plates.

Question 2 (Structured Paper 2 Style)

Two parallel plates have potential difference 240 V and separation 0.030 m.

(a) Calculate the electric field strength. [2 marks]

(b) Calculate the force on a charge of +2.0 microcoulomb in the field. [2 marks]