OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Magnetic fields: (i) force on a current-carrying conductor; F = ILB sin θ
A straight conductor carrying current I in a uniform magnetic flux density B experiences a force F given by F = ILB sin θ, where L is the length of conductor within the field and θ is the angle…
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 7
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A straight conductor carrying current I in a uniform magnetic flux density B experiences a force F given by F = ILB sin θ, where L is the length of conductor within the field and θ is the angle between the current direction and the magnetic field lines. The force is greatest when the conductor is perpendicular to the field (θ = 90°, sin θ = 1) and zero when it lies parallel to the field (θ = 0°, sin θ = 0). The force direction is given by Fleming's left-hand rule.
For example: A 0.20 m wire carrying 3.0 A at 90° to a 0.50 T field feels F = 3.0 × 0.20 × 0.50 × 1 = 0.30 N. If the wire were at 30°, F = 3.0 × 0.20 × 0.50 × sin 30° = 0.15 N.
What earns the marks
- State the equation F = ILB sin θ and identify each symbol with its unit: F in newtons (N), I in amperes (A), L in metres (m), B in tesla (T).
- Explain that θ is the angle between the current direction and the magnetic field lines, so the force is maximum at 90° and zero at 0°.
- Apply Fleming's left-hand rule to determine the direction of the force on the conductor.
- Substitute values correctly, including taking sin θ, and give the force with the correct unit and sensible significant figures.
Common mistakes and how to put them right
Using the angle between the wire and the horizontal instead of the angle between the current and the magnetic field.
Put it right: θ is always measured between the current direction and the field lines.
Forgetting the sin θ factor and treating every orientation as perpendicular.
Put it right: Include sin θ, which equals 1 only at 90°.
Mixing up Fleming's left-hand rule with the right-hand rule for induced current.
Put it right: Use the left hand for the force on a current-carrying conductor in a motor effect.
Exam tips
- Sketch the field lines and the current arrow before substituting, so θ is unambiguous.
- Check that L is the length of conductor inside the field, not the whole wire.
- Give the force in newtons and round to the significant figures justified by the data.
Try the lesson
The start of the Magnetic fields: (i) force on a current-carrying conductor; F = ILB sin θ lesson, made from checked teaching notes for this specification point.
Listen
A straight conductor carrying current I in a uniform magnetic flux density B experiences a force F given by F = ILB sin θ, where L is the length of conductor within the field and θ is the angle between the current direction and the magnetic field lines. The force is greatest when the conductor is perpendicular to the field (θ = 90°, sin θ = 1) and zero when it lies parallel to the field (θ = 0°, sin θ = 0). The force direction is given by Fleming's left-hand rule. For example, a 0.20 m wire carrying 3.0 A at 90° to a 0.50 T field feels F = 3.0 × 0.20 × 0.50 × 1 = 0.30 N. If the wire were at 30°, F = 3.0 × 0.20 × 0.50 × sin 30° = 0.15 N. A strong answer earns marks for these points. State the equation F = ILB sin θ and identify each symbol with its unit: F in newtons (N), I in amperes (A), L in metres (m), B in tesla (T). Explain that θ is the angle between the current direction and the magnetic field lines, so the force is maximum at 90° and zero at 0°. Apply Fleming's left-hand rule to determine the direction of the force on the conductor. Substitute values correctly, including taking sin θ, and give the force with the correct unit and sensible significant figures. A common slip: Using the angle between the wire and the horizontal instead of the angle between the current and the magnetic field. Instead, θ is always measured between the current direction and the field lines. Exam tip: Sketch the field lines and the current arrow before substituting, so θ is unambiguous.
See it
Key words
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Magnetic flux density
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Fleming's left-hand rule
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See every definition
- Magnetic flux density
- The quantity B in the equation F = ILB sin θ, measured in tesla (T).
- Fleming's left-hand rule
- The rule that gives the direction of the force on a current-carrying conductor.
Label the diagram
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