Compare straight-wire field strength at different positions
Describe weaker field with greater distance and stronger field with greater current while keeping the comparison controlled.
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Draw straight-wire circular fields.
- Distinguish magnitude from direction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which quantity describes field direction at a point?
What you will learn
- Describe weaker field with greater distance and stronger field with greater current while keeping the comparison controlled.
- Make qualitative strength comparisons at controlled distances and currents without treating drawing or compass angle as calibration.
Compare distance at fixed current
For the isolated long straight-wire model, the magnetic field is stronger nearer the wire and weaker farther away. Closely spaced representative lines indicate stronger field only when the drawings use a consistent convention and scale. The number of lines an artist chooses is not a count of physical objects.
A compass direction alone is not a calibrated strength measurement. A weak wire contribution can be masked by background fields, so comparisons require the same current, geometry and background or a suitable strength-sensitive method. This objective requires a qualitative variation, not a numerical distance law.
Compare current at fixed position
Increasing conventional-current magnitude increases the wire field strength at the same position under unchanged geometry and other conditions. Reversing current direction reverses field direction; at the same magnitude it does not make the isolated field vanish. Direction and strength are different properties.
Real total field is the vector sum of wire, Earth and other contributions. Equal and opposite contributions can cancel at a particular point, while the wire still produces its own field. Do not infer an exact strength ratio from a few line spacings unless a calibrated physical relation is separately supplied.
Pause and explain
Same current magnitude, reversed direction, isolated wire: what changes?
Worked example
At unchanged wire current, compare positions 2 cm and 6 cm from the stated long straight wire qualitatively.
Show the worked solution
- Keep current and wire geometry fixed.
- The 2 cm position is nearer.
- The wire contribution is stronger nearer than farther.
Answer Stronger at 2 cm than at 6 cm; no numerical ratio is asserted.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare the wire contribution at 2 cm and 6 cm from the same current-carrying long straight wire.
Give me a hint
Hold current fixed and use the qualitative distance trend.
Compare my reasoning
- The 2 cm point is nearer.
- Nearer wire contribution is stronger.
- The syllabus trend alone does not assign a numerical ratio.
2 cm has the stronger wire field; no exact ratio follows from the qualitative statement.
Look for these in your work
- Keep controls fixed.
- Avoid an invented numerical law.
Compare the same point with current magnitudes 0.20 A and 0.40 A, geometry and background unchanged.
Give me a hint
Use current magnitude, not signed order.
Compare my reasoning
- Position is unchanged.
- 0.40 A is the greater magnitude.
- The wire contribution is stronger for 0.40 A.
Stronger wire contribution at 0.40 A.
Look for these in your work
- State fixed position.
- Do not infer an unsupported measured ratio.
A compass barely changes direction when wire current increases, and a student concludes strength cannot increase. Explain the missing distinction.
Give me a hint
Direction and resultant-field dominance affect compass observations.
Compare my reasoning
- The compass shows local resultant direction.
- Wire strength can increase without a large angular change.
- A calibrated strength comparison needs appropriate evidence and background control.
A small angle change does not disprove a larger wire-field magnitude.
Look for these in your work
- Separate strength from direction.
- Account for the resultant field.
Common mistakes
- Use fixed current for a distance comparison.
- A compass angle is not a strength calibration.
- Direction reversal is not disappearance.
What can you explain now?
At fixed position and geometry, what does greater current magnitude do to the wire field strength?
Compare with the explanation
Increases it
The field contribution becomes stronger under the stated unchanged comparison conditions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Make qualitative strength comparisons at controlled distances and currents without treating drawing or compass angle as calibration.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.