IB Physics HL D.2 Electric and magnetic fields Notes

Electric charge force questions are sign-and-direction questions before they are calculation questions. The magnitude may come from Coulomb’s law, but the arrow comes from the charge signs: like charges push apart, opposite charges pull together.

State the force direction between two charged particles for like and unlike signs.

Coulomb’s law is the electric-force analogue of an inverse-square law. In exam answers, build the magnitude cleanly and then add the direction in words. The sign product is useful for reasoning, but the physical arrow should be described as attraction or repulsion.

Two point charges are separated by distance r. State Coulomb’s law and explain how charge signs affect direction.

A conservation explanation is a before-and-after account. You decide what system is isolated, add the charge before, add the charge after, and make the totals match. This is separate from quantization, which says the allowed amounts come in packets of e.

Explain why charging an object by rubbing does not violate conservation of charge.

The exam-worthy point is the evidence chain. A charged drop in a known electric field has a measurable force balance; repeated measurements reveal that the charge values are not continuous. They occur as multiples of one elementary charge.

State what the Millikan oil-drop experiment demonstrated about electric charge.

Charge-transfer questions are process-order questions. Look for contact, for a nearby charged object causing separation, and for an earthing connection. The answer should name the method and track electrons through the steps.

Describe how a neutral conducting sphere can be charged by induction using a negatively charged rod.

Electric field strength tells you what force each coulomb of positive test charge would feel. That convention matters: a negative charge placed in the field experiences force opposite to E.

Define electric field strength and state the direction of an electric field.

A field-line diagram is not decoration; it encodes field direction and relative field strength. The arrows come from the positive-test-charge convention. Density represents strength. Crossed lines would imply two different field directions at the same point.

Sketch electric field lines for a positive and a negative point charge and explain what line spacing shows.

Reading a field-line diagram means making claims from the drawing: direction comes from the local tangent and arrows, while strength comes from line density. The diagram does not show a particle track unless a question explicitly says a charge follows it.

Use an electric field-line diagram to identify where the field is strongest and state the force direction on an electron.

Parallel plates are the cleanest electric-field model. The field is treated as constant in magnitude and direction between the plates, so potential changes linearly with distance and E = V/d.

A potential difference is applied across two parallel plates. State the expression for the electric field and describe the field-line pattern.

Magnetic field-line questions are pattern recognition plus direction rule. For magnets, use north-to-south outside the magnet. For wires and coils, use the right-hand grip rule with conventional current.

Sketch the magnetic field around a straight current-carrying wire and state the rule used to determine its direction.

Potential energy belongs to a particular charge, while potential belongs to the field point. Once V is known, multiply by q to get E_p. Be careful with negative charges: the same potential change gives the opposite sign of energy change.

A charge moves between two electric potentials. State how to calculate the change in electric potential energy.

This formula is the energy version of the two-charge interaction. The sign matters physically: positive potential energy for repelling like charges, negative potential energy for attracting unlike charges when zero is at infinity.

State the electric potential energy of two point charges and explain why it can be negative.

Potential tells you energy per coulomb at a point. Unlike electric field, it does not point anywhere. This makes multi-charge problems easier: calculate each potential with its sign and add the numbers.

Define electric potential and explain how potentials from several charges combine.

Point-charge potential looks like the energy-per-charge version of Coulomb’s interaction. Keep the sign of Q and add contributions as scalars. The result is a voltage value at a point, not an arrow.

Calculate the electric potential at a point due to several point charges. State the rule used to combine contributions.

The potential-gradient relation connects graph shape to field. A flat potential region has zero field. A steep potential change has large field. The negative sign is directional: a positive test charge is pushed toward lower potential.

A graph of electric potential against distance is provided. Explain how to determine the electric field strength from the graph.

Work questions need a named agent. The electric field’s work is negative the potential-energy change. An external agent moving the charge slowly does work equal to the energy change. Along an equipotential, the potential difference is zero, so the work is zero.

A charge is moved between two points of different electric potential. Explain how to find the work done by the electric field.

Equipotentials are the contour lines of electric potential. They show equal voltage values. Moving along one does not change potential energy, so the electric field does no work along that path.

Define an equipotential and explain why no work is done moving a charge along it.

Equipotentials and field lines are two representations of the same field. Equipotentials show constant voltage; field lines show direction of steepest decrease in V for a positive test charge. Their perpendicular relationship is one of the safest diagram checks in D.2.

Explain the relationship between electric field lines and equipotential surfaces.

This summary card is a retrieval net for SL D.2. It asks students to move between force rules, charge conservation, transfer mechanisms, electric field diagrams, uniform-field equations, and magnetic field-line patterns without importing gravitational or motion language.

Summarize the core D.2 electric and magnetic field models.

HL D.2 is mostly about energy and representation discipline. Keep potential scalar, field vector, signs explicit, and the work agent named. Equipotentials then become easy: constant V means zero ΔE_p and field lines must be perpendicular.

Summarize the HL D.2 potential and equipotential model.