D.2
Electric & Magnetic Fields
Electric charges, Coulomb's law, field lines, magnetic field patterns and current-carrying conductors. HL extends to electric potential, equipotential surfaces and field-potential relationships.
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D.2 Electric & Magnetic Fields
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Key Concepts, Electric & Magnetic Fields
Coulomb's Law and Electric Charge
Coulomb's Law states that the electric force between two point charges is F = kq₁q₂/r², where k = 8.99 × 10⁹ N m² C⁻² (sometimes written as k = 1/4πε₀), q₁ and q₂ are the charges and r is the separation. Like charges repel; unlike charges attract. The force is directed along the line joining the charges. The law of conservation of electric charge states that the total electric charge of an isolated system is always conserved. Charge is quantised: all charges are integer multiples of the elementary charge e = 1.6 × 10⁻¹⁹ C. Millikan's oil drop experiment demonstrated this quantisation by showing that measured charge values were always whole-number multiples of e.
Electric Field Strength and Field Lines
Electric field strength E is defined as the force per unit positive charge: E = F/q, with units N/C or V/m. For a uniform field between parallel plates, E = V/d where V is the potential difference and d is the plate separation. For a radial field around a point charge Q, E = kQ/r² (from Coulomb’s law). Electric field lines start on positive charges and end on negative charges, never crossing. The density of field lines indicates field strength. Arrows on field lines show the direction of force on a positive test charge. Inside a conducting sphere, the field is zero; outside, it is the same as if all the charge were at the centre.
Charging Methods
Charging by friction: when two different materials are rubbed together, electrons transfer from one to the other, leaving one positively and one negatively charged. Charging by contact: a charged object is touched to a neutral one, sharing charge (same sign results). Charging by induction: a charged object is brought near (not touching) a neutral conductor; free electrons redistribute, leaving the near side with opposite charge and the far side with same charge as the inducing object. Earthing (grounding) allows charge to flow to or from the Earth, neutralising an object or making the induction permanent.
Magnetic Field Patterns
Magnetic fields are produced by magnets and by moving charges (electric currents). The field around a long straight current-carrying wire forms concentric circles: use the right-hand grip rule (curl the fingers of the right hand in the direction of the field, with the thumb pointing in the direction of conventional current). The field inside a solenoid is uniform and parallel to the axis; outside, it resembles a bar magnet’s field. A single current-carrying loop produces a field that bulges outward through the loop and returns around the outside. Field line density indicates field strength; arrows show direction.
Electric Potential and Equipotentials (HL)
Electric potential V at a point is defined as the work done per unit positive charge in bringing a small test charge from infinity to that point: V = kQ/r for a point charge Q (positive for positive charges, negative for negative charges). It is a scalar with units of volts (J/C). Electric potential energy of a charge q at potential V is E_p = qV. Equipotential surfaces are surfaces of constant potential; no work is done moving a charge along them, and they are always perpendicular to field lines. The electric field strength is the rate of change of potential with distance: E = ΔV/Δr (the gradient of a V-r graph gives field strength).
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Frequently Asked Questions, IB Physics Electric & Magnetic Fields
What is Electric & Magnetic Fields in IB Physics? ↓
Electric charges, Coulomb's law, field lines, magnetic field patterns and current-carrying conductors. HL extends to electric potential, equipotential surfaces and field-potential relationships. This topic is part of Theme D (Fields) in the current IB Physics syllabus.
Is Electric & Magnetic Fields SL or HL in IB Physics? ↓
Electric & Magnetic Fields is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Electric & Magnetic Fields? ↓
The key equations for Electric & Magnetic Fields are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Electric & Magnetic Fields? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Electric & Magnetic Fields for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.