Magnetic Fields — A-Level Physics Revision

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What is Magnetic Fields?

This topic introduces magnetic fields, which are produced by moving charges (currents) and permanent magnets. You will learn to represent magnetic fields using field lines and define magnetic flux density (B) as a measure of the field's strength. Key principles include the force on a current-carrying conductor in a magnetic field (F = BILsinθ) and the force on an individual moving charge (F = BQvsinθ), with the direction of the force determined by Fleming's Left-Hand Rule.

Board notes: Magnetic fields are a fundamental topic in all A-Level Physics specifications (AQA, Edexcel, OCR). All boards cover the forces on currents and charges, the concept of magnetic flux density, and Fleming's Left-Hand Rule. The specific contexts, such as the Hall effect (AQA) or detailed analysis of motors, can vary.

Worked example

Common mistakes

• 1Incorrectly applying Fleming's Left-Hand Rule. A common error is to misalign the fingers and thumb, or to use the left hand for positive charges and the right hand for negative charges (the rule is defined for conventional current, so for electron flow, the 'current' finger should point opposite to the electron's velocity).
• 2Confusing magnetic flux and magnetic flux density. Magnetic flux density (B) is the strength of the field per unit area (measured in Tesla, T). Magnetic flux (Φ) is the total amount of field passing through a given area (Φ = BA, measured in Webers, Wb).
• 3Forgetting that the force is a maximum when the charge or current is perpendicular to the field (sinθ = 1) and zero when it is parallel (sinθ = 0).

Magnetic Fields exam questions

Exam-style questions for Magnetic Fields with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What creates a magnetic field?

  Magnetic fields are created by moving electric charges. This can be an electric current in a wire or the motion of electrons in the atoms of a permanent magnet.

• What is Fleming's Left-Hand Rule?

  It is a mnemonic for determining the direction of the force on a current-carrying conductor in a magnetic field. The thumb represents the direction of the Force (or Thrust), the first finger represents the direction of the Field, and the second finger represents the direction of the Current.