Electric Force and Field Ranking

Instructions for Students & Teachers

Learning goal: combine the forces and fields from several charges to compare their sizes, using how charge and distance scale each one and how vectors add or cancel.
Each set shows four cases, A to D, on identical grids. Rank the size of the electric force on the charge q, or the size of the electric field at point P, from greatest to least.
No calculator is needed. Every size is a multiple of kq²/s² for force or kq/s² for field, where s is one grid space, so the reasoning is all in the charges and the distances.
After each Check, every case shows its force or field arrows drawn to scale and the reasoning written out step by step.
The Sketch level switches from ranking to drawing: sketch each force or field arrow and the net arrow for one case.
Note for teachers: The tasks follow the ranking task format from physics education research and match AP Physics 2 topics 10.1 and 10.3. Every set is generated, so students never run out of practice.

Usage Steps

Levels

Level 1 has one interaction: q and one other charge. Only the size of the charge and the distance change, so this level is about scaling.
Level 2 puts two or three charges on a line with q, so forces in the same direction add and forces in opposite directions partly cancel.
Level 3 places charges along lines at right angles through q, so the net comes from combining perpendicular parts.
Level 4 uses equilateral triangles: q at one corner and two equal charges at the others.
Sketch switches from ranking to drawing arrows (see Sketch below).
Mastery tip: On Level 1, notice that the sign of the other charge never changes the size of the force. Sign only tells you the direction.

Question Type

Force asks for the net electric force on the charge q. Field asks for the net electric field at point P. Mixed picks Force or Field at random for each set.
Some Field sets place a charge at point P. That charge does not count: the field at P comes only from the other charges.
Each question type keeps its own streak, so practicing Force does not change your progress in Field.
Note for teachers: Mixed mode is the best check that students know the difference between force and field, since the question type changes from set to set.

Ranking the Cases

Drag each letter into a slot from Greatest to Least, or click a letter and then click a slot.
If two or more cases tie, put their letters in the same slot.
If every case is equal, choose All the same. If every case is zero, choose All zero.
Mastery tip: Write each case as charge divided by distance squared before you rank. Twice the distance gives one fourth the force, not one half.

Show Hint

Show hint gives a reminder for the current level before you check your answer. Using it does not affect your streak.

Check and Reasoning

Check grades your ranking and shows the correct answer.
Each case then shows an arrow for each charge, drawn to scale, with a ring around each charge in the same color as its arrow. The bold arrow is the net force or field.
The Reasoning lists each charge's contribution (for example, +2q at 1 space: 2/1² = 2, pushes q left) and then how they combine.
If your ranking matches a common mistake, such as forgetting to square the distance or adding forces that point in opposite directions, a hint names that mistake.
Mastery tip: When you miss one, compare your ranking to the arrows. The arrows usually show exactly which case you misjudged and why.

Next Set and Streak

Three correct sets in a row masters a level. A wrong answer resets the streak to 0.
After a wrong answer, the next set uses the same kind of layout with new charges and distances, so you get another try at the idea you missed.
When a level is mastered, continue to the next level or keep practicing. Reset progress clears your streaks.
Note for teachers: Progress is saved in the student's browser, not in an account.

Sketch

One case is shown. Draw an arrow for the force or field from each charge, then the net arrow.
Pick an arrow from the buttons (From +2q, Net force, and so on), then drag from the dot on the pad. Arrows snap to the grid lines and the diagonals.
If the arrows cancel, choose Net is zero instead of drawing a net arrow.
Only directions and relative lengths are graded, not the overall size. A force twice as large should be drawn twice as long.
After Check, the correct arrows appear dashed beside yours, with the reasoning.
Note for teachers: This is the same skill as AP free-response prompts that ask students to draw force or field vectors with correct relative lengths.

Before a quiz or test: assign Mixed mode on Levels 1 to 3 as review. Three in a row on fresh sets is hard to reach by guessing, so a mastered level is a fair signal that the student is ready.
Sign versus size: start Level 1 by asking whether a −4q pulls on q harder or softer than a +4q at the same distance. The tool treats them as a tie, which separates direction from magnitude before any vector addition begins.
Inverse square: many students halve the force when the distance doubles. A quick warm-up on Level 1 shows them the pattern 1, 1/4, 1/9, 1/16 again and again.
Vector addition: on Level 2, have students predict the direction of each force before adding anything. Most errors come from adding sizes that point in opposite directions.
Perpendicular parts: Level 3 connects to vector addition from mechanics. Equal parts at right angles give a net along the diagonal that is about 1.4 times one part.
Symmetry: on Level 4, ask students why two equal pushes at 60° give a larger net than a push and a pull at 120°. Answering it without trig is the point of the level.
Field versus force: use the Field sets with a charge at P to start a discussion. Ask what would change about the field at P if the charge there doubled, then what would change about the force on it.
Before a free-response question: use the Sketch level to practice drawing vectors with correct relative lengths, then have students write one sentence justifying the direction of the net arrow.