Common mistake: Not drawing the field direction before applying the right-hand rule

Not drawing the field direction before applying the right-hand rule — direction errors cost full marks

Common mistake: Forgetting that for a negative charge, the force from the right-hand rule is reversed

Forgetting that for a negative charge, the force from the right-hand rule is reversed

Common mistake: Confusing gravitational PE = −GMm/r (which is negative) with the usual mgh approximation

Confusing gravitational PE = −GMm/r (which is negative) with the usual mgh approximation

Common mistake: Using diameter instead of radius in circular motion or orbital formulas

Using diameter instead of radius in circular motion or orbital formulas

VCE Units 1–4 · Physics

VCE Physics — Fields and Motion

Fields and Motion covers Unit 3 of VCE Physics — the highest-stakes unit in the course. Gravitational, electric and magnetic fields each require you to understand field direction, force on a charge or mass, and the mathematics of circular motion. This is the topic where a clear diagram saves more marks than any formula.

Key Concepts & Formulas

Gravitational field strength: g = GM/r² (N/kg); gravitational force: F = GMm/r²
Electric field: E = F/q = kq/r² (N/C or V/m); force on charge: F = qE
Electric field lines point from positive to negative; field is always perpendicular to equipotentials
Magnetic force on moving charge: F = qvB sin θ (use right-hand rule for positive charge)
Magnetic force on current-carrying wire: F = BIl sin θ
Right-hand rule: fingers point in direction of v (or I), curl toward B, thumb points in direction of F (for positive charge)
Circular motion: centripetal acceleration a = v²/r; centripetal force F = mv²/r (directed toward centre)
Orbital motion: gravitational force provides centripetal force — GMm/r² = mv²/r, giving v = √(GM/r)
Orbital period: T² ∝ r³ (Kepler's third law)
Energy in orbit: total energy = KE + GPE; GPE = −GMm/r (negative for bound orbit)

Practice Questions

4 questions

Attempt each question before reading the hint. These are styled to match VCE exam format.

Q1.A satellite orbits Earth at an altitude of 400 km. Given Earth's mass M = 6.0 × 10²⁴ kg and radius R = 6.4 × 10⁶ m, find the orbital speed.

3 marks

Q2.An electron (charge −1.6 × 10⁻¹⁹ C) moves at 3 × 10⁶ m/s perpendicular to a magnetic field of 0.5 T. Find the magnitude of the force and describe the path.

3 marks

Q3.Two point charges of +4 μC and −2 μC are 0.3 m apart. Find the electric field at the midpoint between them. (k = 9 × 10⁹ N m² C⁻²)

4 marks

Q4.A 500 kg satellite in circular orbit has an orbital period of 90 minutes. Find the orbital radius. (G = 6.67 × 10⁻¹¹, M_Earth = 6.0 × 10²⁴ kg)

4 marks

Common Mistakes to Avoid

These are the errors that VCE students most frequently make in Fields and Motion — and that examiners are specifically watching for.

Not drawing the field direction before applying the right-hand rule — direction errors cost full marks
Forgetting that for a negative charge, the force from the right-hand rule is reversed
Confusing gravitational PE = −GMm/r (which is negative) with the usual mgh approximation
Using diameter instead of radius in circular motion or orbital formulas

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