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Edexcel A Level (IAL) Physics-4.27 Magnetic Force on a Current–Carrying Conductor- Study Notes- New Syllabus

Edexcel A Level (IAL) Physics -4.27 Magnetic Force on a Current–Carrying Conductor- Study Notes- New syllabus

Edexcel A Level (IAL) Physics -4.27 Magnetic Force on a Current–Carrying Conductor- Study Notes -Edexcel A level Physics – per latest Syllabus.

Key Concepts:

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Edexcel A level Physics-Study Notes- All Topics

Force on a Current-Carrying Conductor in a Magnetic Field

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This effect is the basis of the operation of electric motors.

 Magnetic Force on a Current-Carrying Conductor

The magnitude of the force on a straight conductor in a magnetic field is given by:

\( F = BIl\sin\theta \)

  • \( F \) = force on the conductor (N)
  • \( B \) = magnetic flux density (T)
  • \( I \) = current in the conductor (A)
  • \( l \) = length of conductor in the magnetic field (m)
  • \( \theta \) = angle between the conductor (current direction) and the magnetic field

 Key Features of the Force

  • The force acts only when the conductor carries current.
  • The force is maximum when the conductor is perpendicular to the field.
  • The force is zero when the conductor is parallel to the field.
  • The force is perpendicular to both the current direction and the magnetic field.

Maximum force: \( F = BIl \) when \( \theta = 90^\circ \)

Zero force: \( \theta = 0^\circ \)

 Fleming’s Left-Hand Rule

Fleming’s left-hand rule is used to determine the direction of the force on a current-carrying conductor.

  • First finger → direction of magnetic field (N to S)
  • Second finger → direction of current (positive to negative)
  • Thumb → direction of force (motion of the conductor)

Important:

  • The rule applies to conventional current.
  • Reversing the current reverses the direction of the force.
  • Reversing the magnetic field also reverses the force.

 Physical Explanation

  • Current consists of moving charges.
  • Moving charges experience a magnetic force.
  • The combined effect produces a force on the conductor.

 Applications

  • Electric motors
  • Loudspeakers
  • Moving-coil meters

Example (Easy)

A wire of length \( 0.40\ \mathrm{m} \) carries a current of \( 5.0\ \mathrm{A} \) at right angles to a magnetic field of flux density \( 0.30\ \mathrm{T} \). Calculate the force on the wire.

▶️ Answer / Explanation

Since the wire is perpendicular, \( \sin\theta = 1 \).

\( F = BIl = 0.30 \times 5.0 \times 0.40 = 0.60\ \mathrm{N} \)

Example (Medium)

A conductor is placed parallel to a magnetic field. Explain what happens when current flows.

▶️ Answer / Explanation
  • Angle between current and field is \( 0^\circ \).
  • \( \sin 0^\circ = 0 \).
  • No magnetic force acts on the conductor.

Example (Hard)

Explain how Fleming’s left-hand rule predicts the direction of motion of a wire in a d.c. motor.

▶️ Answer / Explanation
  • First finger shows magnetic field direction.
  • Second finger shows current direction.
  • Thumb gives direction of force on the wire.
  • This force causes the coil to rotate.
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