Trace the field and poles of a current-carrying solenoid
Draw the bar-magnet-like field of a coil and determine a viewed end pole from conventional current.
Section 4.5.3 · Magnetic effect of a current
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Trace external bar-magnet field lines.
- Use clockwise and anticlockwise from a defined face.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which way do field lines run outside a magnet?
What you will learn
- Draw the bar-magnet-like field of a coil and determine a viewed end pole from conventional current.
- Use an explicit face view to determine poles, then draw closed internal and external field paths.
A solenoid is a succession of coil turns
A solenoid is a coil of wire with multiple turns arranged along an axis. Its current produces a field resembling a bar magnet’s pattern. In a simple sufficiently long solenoid, central internal lines run roughly parallel to the axis; outside, lines curve from the north end toward the south end.
Magnetic field lines form closed loops: inside the solenoid the direction is south toward north, continuing the external north-to-south path. Do not draw the field ending at one pole or charge travelling along the field lines. The exact end-region pattern differs from the simple central approximation.
Define the viewed end
Looking directly at one solenoid end, anticlockwise conventional current around that face makes it north; clockwise makes it south. Equivalently, curled right fingers follow winding current and the thumb points toward the north end. Current sense must be given from the stated viewing side.
If the right end is north and the left end south, the central internal field points rightward. Looking from the opposite end reverses the apparent clockwise sense of the same winding. A two-dimensional side-view coil without a winding/current convention cannot by itself determine its poles.
Pause and explain
A solenoid has right N and left S. Which way is its central internal field?
Worked example
The right end is viewed directly and has anticlockwise conventional current. Name the poles and central internal field direction.
Show the worked solution
- The viewed right face is north.
- The opposite left face is south.
- Internal field runs south to north, left to right.
Answer Right N; left S; central internal field rightward.
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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.
Sketch a solenoid with right N and left S, showing internal and external field arrows.
Give me a hint
Use a closed-loop bar-magnet-like pattern.
Compare my reasoning
- Draw roughly axial lines through the central interior.
- Arrow them from left S to right N.
- Curve outside arrows from right N back toward left S.
Internal S→N and external N→S in closed loops.
Look for these in your work
- Show both regions.
- Do not terminate field lines at poles.
Viewed from its right end, current is clockwise. Name that end, the opposite end and central internal direction.
Give me a hint
Clockwise makes the viewed face south.
Compare my reasoning
- Right face is south.
- Left face is north.
- Inside S→N therefore points leftward.
Right S; left N; central internal field leftward.
Look for these in your work
- State the view.
- Use the internal direction rule.
Two observers look at opposite ends of one solenoid and disagree about clockwise current. Explain how both readings can fit one field.
Give me a hint
Apparent rotation sense depends on the viewing side.
Compare my reasoning
- The observers face opposite directions along the axis.
- The same winding appears clockwise from one side and anticlockwise from the other.
- The corresponding face poles are opposite.
Opposite views reverse apparent current sense and refer to opposite pole faces.
Look for these in your work
- Identify the viewing difference.
- Keep one consistent physical winding.
Common mistakes
- State the viewed end.
- Inside S to N differs from outside N to S.
- An unlabeled winding alone does not fix polarity.
What can you explain now?
Which face pole corresponds to anticlockwise current when viewed directly at that face?
Compare with the explanation
North
The right-hand coil convention makes that viewed face the north end.
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Make it stick
Use an explicit face view to determine poles, then draw closed internal and external field paths.
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.