Control current comparisons in wire and solenoid fields
Keep geometry, material and current magnitude separate when interpreting direction and strength changes.
Section P4.5.3 · Magnetic effect of a current
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish field strength from direction.
- Use ammeter readings and controlled variables.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why use an ammeter in a current-magnitude comparison?
What you will learn
- Keep geometry, material and current magnitude separate when interpreting direction and strength changes.
- Match the controlled factor and measurement to a direction or strength claim without confusing current, voltage and field.
Define the factor being investigated
To investigate current magnitude, use the same wire or solenoid, observation point, orientation, core condition and current direction while changing the measured magnitude. A variable current-limited supply and ammeter can establish the stated currents in a teacher-supervised setup. A suitable strength-sensitive observation is needed for a strength claim.
To investigate direction, reverse current at the same magnitude with winding, geometry and viewing side fixed. Then compare local compass north-tip directions or pole labels. Reversing current and turning the whole coil at the same time does not isolate the effect of current alone.
Judge the evidence and scope
A given report at a fixed point can support a qualitative stronger-field trend if the controlled larger current produces a stronger measured contribution. A direction-only reading cannot automatically establish a calibrated strength ratio. Two arbitrary field sketches with different line counts are not numerical experimental results.
School current limits and short observation intervals prevent unnecessary heating; do not use mains for a bare-wire demonstration. Temperature-related resistance changes can affect current unless the ammeter confirms it. A written comparison checks the experimental reasoning, not practical competence or instrument calibration.
Sources for this lesson
Pause and explain
A comparison doubles current and turns the coil around. What can it isolate?
Worked example
A solenoid comparison doubles the measured current but also inserts a core. Does it isolate the current-strength effect?
Show the worked solution
- Current magnitude changes.
- Magnetic material changes too.
- The observation combines at least two influences.
Answer No; hold the core condition fixed to isolate current magnitude.
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.
Design a comparison of solenoid field strength for current magnitudes 0.20 A and 0.40 A.
Give me a hint
Keep core, geometry, direction and observation position unchanged.
Compare my reasoning
- Measure the two currents with a suitable ammeter.
- Keep winding, core and position fixed.
- Use an appropriate strength-sensitive comparison and account for background.
A current-magnitude comparison with other conditions controlled.
Look for these in your work
- State the current evidence.
- Choose an observation suitable for strength.
Design a comparison for reversal of straight-wire field direction.
Give me a hint
Change current sign but keep the magnitude and card view fixed.
Compare my reasoning
- Record the original conventional-current and compass directions.
- Reverse current at the same magnitude.
- Compare settled compass north-tip directions at the same marked points.
A fixed-geometry same-magnitude reversal isolates current direction.
Look for these in your work
- Use consistent page convention.
- Keep the observation points fixed.
A supply dial is unchanged while the wire warms, and a report assumes current is unchanged. Explain how to improve the control.
Give me a hint
Wire resistance can depend on its temperature.
Compare my reasoning
- Warming can change resistance.
- An unchanged dial need not give unchanged current.
- Measure and control current with suitable apparatus while limiting heating.
Use measured current and thermal controls rather than assuming the dial fixes current.
Look for these in your work
- Name the confounding resistance change.
- Identify the needed current measurement.
Common mistakes
- Use measured current, not supply dial alone.
- A core change is a second factor.
- Compass direction is not a calibrated strength ratio.
What can you explain now?
Which comparison best isolates current direction?
Compare with the explanation
Reverse current at the same magnitude with geometry and view fixed.
It changes the sign while controlling magnitude, winding, observation point and viewing side.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Match the controlled factor and measurement to a direction or strength claim without confusing current, voltage and field.
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.