A LevelPhysicsEdexcelFlashcards7. Electric & Magnetic FieldsMagnetic Fields

Magnetic Fields (Edexcel A Level Physics): Flashcards

Exam code: 9PH0

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  • Define magnetic flux density.

    The magnetic force per unit current per unit length on a current-carrying conductor at right angles to the field.

    Symbol B, measured in tesla (T), and given by B = \frac{F}{IL}

  • Define magnetic flux.

    The product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density.

    Symbol Φ, measured in webers (Wb), and given by \Phi = BA

  • What is the equation for magnetic flux linkage?

    Flux linkage = ΦN = BAN

    Measured in weber turns (Wb turns), where N is the number of turns of the coil, B the flux density and A the cross-sectional area.

  • In the equation Φ = BA cos θ, the angle θ is measured between the magnetic field lines and the .......... to the plane of the area.

    In the equation Φ = BA cos θ, the angle θ is measured between the magnetic field lines and the normal (perpendicular line) to the plane of the area.

  • Magnetic flux through a coil is maximum when the field lines are parallel to the plane of the coil.

    True or False?

    False.

    Magnetic flux is maximum (Φ = BA) when the field lines are perpendicular to the plane of the area, i.e. θ = 0° between the field and the normal. It is zero when the field lines are parallel to the plane.

  • What three changes to magnetic flux linkage can induce an e.m.f in a circuit?

    A change in:

    • magnetic flux density B

    • cross-sectional area A

    • angle θ between the field and the normal

  • State the units of magnetic flux density, magnetic flux and flux linkage.

    • Magnetic flux density: tesla (T)

    • Magnetic flux: weber (Wb)

    • Flux linkage: weber turns (Wb turns)

  • What is the equation for the magnetic force on a moving charged particle?

    F = BQv\sin\theta

    Where B = flux density (T), Q = charge (C), v = speed (m s-1) and θ = angle between the velocity and the field.

  • The magnetic force on a charged particle is maximum when it moves .......... to the field, and zero when it moves .......... to the field.

    The magnetic force on a charged particle is maximum when it moves perpendicular to the field, and zero when it moves parallel to the field.

  • Why does a charged particle follow a circular path in a uniform magnetic field?

    The magnetic force is always perpendicular to the velocity, so it acts as a centripetal force, producing circular motion.

  • Define conventional current.

    The direction of flow of positive charge.

    For an electron beam, the conventional current is in the opposite direction to the motion of the electrons.

  • In Fleming's Left Hand Rule, what do the thumb, first finger and second finger represent?

    • Thumb = magnetic force

    • First finger = magnetic field, B

    • Second finger = conventional current (velocity of a positive charge)

  • When applying Fleming's Left Hand Rule to an electron, the second finger points in the electron's direction of motion.

    True or False?

    False.

    The second finger shows conventional current (flow of positive charge). For an electron, point it in the opposite direction to the electron's motion.

  • What do dots and crosses represent for magnetic field direction?

    • Dots = field directed out of the page

    • Crosses = field directed into the page

  • What is the equation for the force on a current-carrying conductor in a magnetic field?

    F = BIL\sin\theta

    Where B = flux density (T), I = current (A), L = length of conductor in the field (m) and θ = angle between the conductor and the field lines.

  • A current-carrying conductor experiences the maximum force when the current is .......... to the magnetic flux lines, and zero force when it is .......... to them.

    A current-carrying conductor experiences the maximum force when the current is perpendicular to the magnetic flux lines, and zero force when it is parallel to them.

  • The magnitude of the force on a current-carrying conductor is proportional to which four quantities?

    • the current I

    • the magnetic flux density B

    • the length of conductor in the field L

    • sin θ, where θ is the angle between the conductor and the field lines

  • Why does a current-carrying conductor in an external magnetic field experience a force?

    The conductor produces its own magnetic field, which interacts with the external field, so the external field exerts a magnetic force on the conductor.

  • What does F = BIL sin θ simplify to when the conductor is perpendicular to the field?

    F = BIL

    Because θ = 90°, so sin θ = 1, giving the maximum force.

  • When using Fleming's Left Hand Rule for a current-carrying conductor, what do the first finger, second finger and thumb show?

    • First finger = magnetic field

    • Second finger = conventional current

    • Thumb = magnetic force

  • Conventional current flows in the same direction as the electrons in the wire.

    True or False?

    False.

    Conventional current is the flow of positive charge, which is in the opposite direction to the flow of electrons.

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