OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) magnetic field patterns for a long straight current- carrying…
Full specification wording
Magnetic fields: (c) magnetic field patterns for a long straight current- carrying conductor, a flat coil and a long solenoid
Each current-carrying shape produces a characteristic field pattern.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Each current-carrying shape produces a characteristic field pattern. A long straight conductor gives concentric circles around the wire, with direction from the right-hand grip rule and strength decreasing with distance. A flat coil, such as a circular loop, gives a field through the centre perpendicular to the coil plane, with the two faces acting like north and south poles. A long solenoid gives a nearly uniform field inside along its axis, with field lines outside resembling those of a bar magnet. The solenoid field strength increases with current and with number of turns per unit length. Recognising these patterns allows prediction of field direction and relative strength in each case.
What earns the marks
- A long straight conductor produces concentric circular field lines around the wire.
- The right-hand grip rule gives the field direction: thumb along conventional current, fingers curl in the field direction.
- A flat coil produces a field through its centre perpendicular to the plane of the coil, with opposite faces acting as north and south poles.
- A long solenoid produces a strong, nearly uniform field inside along its axis.
- Outside a solenoid the field pattern resembles that of a bar magnet.
- Solenoid field strength increases with current and with the number of turns per unit length.
Common mistakes and how to put them right
Drawing straight field lines around a long straight wire.
Put it right: The lines are concentric circles centred on the wire.
Reversing the right-hand grip rule.
Put it right: The thumb points along conventional current, not electron flow.
Thinking the field inside a solenoid is zero.
Put it right: It is strong and nearly uniform along the axis.
Treating the field of a flat coil as identical to that of a straight wire.
Put it right: The coil produces a field through its centre perpendicular to its plane.
Exam tips
- Sketch the pattern first, then add arrows using the right-hand grip rule for the straight wire.
- For a solenoid, compare the inside field to a uniform field and the outside field to a bar magnet.
- Check whether the question asks about direction, shape or relative strength before selecting an option.
- Remember that field strength falls with distance from a straight wire but is nearly uniform inside a long solenoid.
Try the lesson
The start of the Magnetic fields: (c) magnetic field patterns for a long straight current- carrying conductor, a flat coil and a long solenoid lesson, made from checked teaching notes for this specification point.
Listen
Each current-carrying shape produces a characteristic field pattern. A long straight conductor gives concentric circles around the wire, with direction from the right-hand grip rule and strength decreasing with distance. A flat coil, such as a circular loop, gives a field through the centre perpendicular to the coil plane, with the two faces acting like north and south poles. A long solenoid gives a nearly uniform field inside along its axis, with field lines outside resembling those of a bar magnet. The solenoid field strength increases with current and with number of turns per unit length. Recognising these patterns allows prediction of field direction and relative strength in each case. A strong answer earns marks for these points. A long straight conductor produces concentric circular field lines around the wire. The right-hand grip rule gives the field direction: thumb along conventional current, fingers curl in the field direction. A flat coil produces a field through its centre perpendicular to the plane of the coil, with opposite faces acting as north and south poles. A long solenoid produces a strong, nearly uniform field inside along its axis. Exam tip: Sketch the pattern first, then add arrows using the right-hand grip rule for the straight wire.
Key words
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Right-hand grip rule
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Uniform field inside a solenoid
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Comparison with bar magnet field
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Field patterns of current-carrying conductors
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See every definition
- Right-hand grip rule
- The rule giving the field direction: thumb along conventional current, fingers curl in the field direction.
- Uniform field inside a solenoid
- A long solenoid produces a strong, nearly uniform field inside along its axis.
- Comparison with bar magnet field
- Outside a solenoid the field pattern resembles that of a bar magnet.
- Field patterns of current-carrying conductors
- Each current-carrying shape produces a characteristic field pattern, allowing prediction of field direction and relative strength in each case.
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