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.

Positive and negative are the two signs of electric charge; like charges repel and unlike charges attract.

The electrostatic force between two point charges acts along the straight line joining their centres.

The two charges exert forces on each other that are equal in magnitude and opposite in direction.

A positive test charge is the convention used to define electric field direction.

Before using equations, decide whether the force is attractive or repulsive from the signs.

Label the force-direction features on the two-charge diagram.

Label

Labels

4

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

Giving only the magnitude rule and not saying whether the force is attractive or repulsive.

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

Choose

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.

Coulomb’s law for point charges is F = kq1q2/r^2, where k = 1/(4πε0).

For force magnitude, use |F| = k|q1q2|/r^2 and express the answer in newtons.

The separation r is the distance between the charges, treated as point charges.

The force is repulsive for charges with the same sign and attractive for charges with opposite signs.

Doubling r reduces the force magnitude by a factor of four.

Assemble Coulomb’s law and the sign-direction sentence.

Formula

Target formula F = kq1q2/r^2

F

electrostatic force magnitude

N

k

Coulomb constant, 1/(4πε0)

N m^2 C^-2

q1

first point charge

C

q2

second point charge

C

r

separation between charges

m

1Use the product of charge magnitudes and the inverse-square separation.|F| = k|q1q2|/r^2

2Name the electrostatic constant.k = 1/(4πε0)

3Use signs to state attraction or repulsion.same signs repel; opposite signs attract

4Check inverse-square scaling.r doubled -> F/4

Build the formula

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

Forgetting that r is squared or failing to describe attraction or repulsion.

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

Choose

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.

Charge is conserved: the total charge of an isolated system remains constant.

Charging processes transfer charge between objects or between an object and Earth; they do not create net charge from nothing.

In most solid charging examples, electrons are the mobile charges that move.

If one object gains negative charge, another part of the system must lose the same amount of negative charge or gain positive charge by electron loss.

Charge is also quantized, so net charge changes in integer multiples of the elementary charge e.

Repair the three charge-conservation statements.

Spot Errors

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

Saying charge is created rather than transferred.

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

Choose

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.

Millikan’s oil-drop experiment measured charges on tiny oil drops.

An electric field was adjusted so electric force could balance the weight of a charged drop.

From the field and drop properties, the charge on each drop could be inferred.

The measured charges were integer multiples of the elementary charge e.

The key syllabus conclusion is charge quantization: q = ne, where n is an integer.

Match each Millikan cue to its role in the argument.

Match

Reasons

0/5

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

Describing only that drops float, without stating charge quantization.

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

Choose

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.

Charging by friction transfers electrons when two materials are rubbed together.

Charging by contact transfers charge when a charged object touches another object.

Charging by induction separates charge within a conductor without direct contact from the charged object.

Earthing provides a path for electrons to flow between the object and Earth.

In induction, the final charge of the conductor is opposite to the nearby inducing charge if the grounding step is completed before the charged object is removed.

Sort each charging cue into the correct mechanism.

Sort

Unsorted

5

friction

0

contact

0

induction

0

earthing

0

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

Removing the rod before disconnecting Earth, or saying the rod transfers charge by touching.

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

Choose

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.

Electric field strength E is the force per unit positive test charge: E = F/q.

The unit N C^-1 is equivalent to V m^-1.

The direction of E is the direction of the force on a positive test charge.

For a point charge Q, the field magnitude is E = k|Q|/r^2.

Field direction is away from a positive source charge and toward a negative source charge.

Build E from force per unit positive test charge.

Formula

Target formula E = F/q

E

electric field strength

N C^-1 or V m^-1

F

force on the test charge

N

q

positive test charge used to define the field

C

Q

source charge for a radial field

C

r

distance from the source charge

m

1Start with force per unit positive test charge.E = F/q

2Convert the unit.N C^-1 = V m^-1

3For a point source charge, combine with Coulomb’s law.E = kQ/r^2 for magnitude use k|Q|/r^2

4State direction using the positive-test-charge convention.away from +Q, toward -Q

Build the formula

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

Defining field direction using the force on an electron.

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

Choose

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.

Electric field lines point in the direction of the force on a positive test charge.

Lines leave positive charges and enter negative charges.

Closer spacing of field lines represents a stronger electric field.

Field lines never cross because a point has only one field direction.

Between large parallel plates, the central field is approximately uniform: parallel, equally spaced lines from the positive plate to the negative plate.

Drag the labels to the electric-field-line diagram.

Label

Labels

5

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

Drawing arrows into a positive charge or treating line count as exact numerical field strength.

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

Choose

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.

At any point, the electric field direction is tangent to the field line and follows the arrow.

A positive test charge accelerates in the direction of E if no other forces are considered.

A negative charge feels force opposite to E.

The field is stronger where field lines are more closely spaced.

Field lines are a representation of the field, not the actual paths that charges must follow.

Read strength and direction from the field-line pattern.

Graph

Electric field-line diagram with points A, B, and C. Lines are closest at A, wider at B, and nearly parallel at C.

1identify strongest field from line density

2state direction from arrows or tangent

3compare force on positive and negative charges

Which point has the greatest electric field magnitude, and how would the force on an electron compare with E there?

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

Saying an electron feels force in the field direction.

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

Choose

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.

Between large oppositely charged parallel plates, the central electric field is approximately uniform.

Uniform field lines are straight, parallel, equally spaced, and directed from the positive plate to the negative plate.

For plate potential difference V and separation d, E = V/d.

The unit V m^-1 is equivalent to N C^-1.

The uniform-field model ignores fringing near the plate edges.

Assemble the parallel-plate field model.

Formula

Target formula E = V/d

E

uniform electric field strength

V m^-1 or N C^-1

V

potential difference between plates

V

d

plate separation

m

q

charge placed in the field

C

1State the model condition.large parallel plates, central region, fringing ignored

2Field is potential difference per separation.E = V/d

3State field direction.from positive plate to negative plate

4If a charge is placed in the field, link to force.F = qE

Build the formula

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

Using inverse-square radial-field language for plates.

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

Choose

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.

Magnetic field lines outside a bar magnet go from the north pole to the south pole.

Field lines form closed loops; inside the magnet they continue from south to north.

A straight current-carrying wire has concentric circular magnetic field lines around it.

The right-hand grip rule gives the circular field direction: thumb in conventional current direction, fingers curl with B.

A current loop or solenoid produces a field pattern similar to a bar magnet, with an approximately uniform field inside a long solenoid.

Label the standard magnetic field-line patterns.

Label

Labels

5

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

Using electric-field-line rules such as starting at positive and ending at negative.

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

Choose

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.

Like charges repel and unlike charges attract; Coulomb’s law gives |F| = k|q1q2|/r^2 for point charges.

Total charge is conserved in an isolated system, and charge is quantized in integer multiples of e.

Charging occurs by friction, contact, induction, and earthing through electron transfer or redistribution.

Electric field strength is E = F/q, directed as the force on a positive test charge; point charge fields follow E = kQ/r^2 in magnitude.

Electric field lines go from positive to negative, closer lines mean stronger E, and parallel plates give an approximately uniform field with E = V/d.

Magnetic field diagrams include bar magnets, straight wires, loops, and solenoids; current-field direction uses the right-hand grip rule.

Match each core D.2 cue to the model it should trigger.

Match

Reasons

0/8

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

Listing formulas without conditions, directions, or diagram conventions.

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

Choose

IB Physics SL D.2 Electric and magnetic fields Notes

Map Electric charge forces

Map Coulomb’s law

Map Charge conservation

Map Millikan experiment

Map Charge transfer

Map Electric field strength

Map Electric field lines

Read the Field-Line Pattern

Analyze Parallel-plate field

Map Magnetic field lines

Retrieve the Core D.2 Electric and magnetic fields Model