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Magnetic Fields (AQA A Level Physics): Flashcards

Exam code: 7408

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  • Define magnetic field.

    A magnetic field is a region of space in which a magnetic pole will experience a force.

  • What two things can create a magnetic field?

    • A moving electric charge

    • A permanent magnet

  • Define magnetic flux density, B.

    Magnetic flux density is the number of magnetic flux lines passing through a region of space per unit area.

  • One tesla is defined as the flux density that causes a force of 1 N on a 1 m wire carrying a current of .......... at right angles to the flux.

    One tesla is defined as the flux density that causes a force of 1 N on a 1 m wire carrying a current of 1 A at right angles to the flux.

  • True or False?

    A stationary electric charge produces a magnetic field.

    False.

    A magnetic field is produced by a moving electric charge or a permanent magnet; a stationary charge does not produce a magnetic field.

  • In a magnetic field, in which direction do flux lines point, outside the magnet?

    From the north pole to the south pole.

  • How does the spacing of magnetic flux lines relate to field strength?

    • Flux lines drawn closer together represent a stronger field

    • Flux lines drawn further apart represent a weaker field

  • What is the equation for the force F on a current-carrying conductor of length L carrying current I at angle θ to a magnetic field of flux density B?

    F = B I L \sin \theta

  • Name three ways to increase the force on a current-carrying conductor in a magnetic field.

    • Increasing the strength of the magnetic field

    • Increasing the current flowing through the conductor

    • Increasing the length of the conductor in the field

  • A current-carrying conductor experiences the .......... force when it is perpendicular to the magnetic field.

    A current-carrying conductor experiences the maximum force when it is perpendicular to the magnetic field.

  • What is the equation for the force on a current-carrying conductor when it is perpendicular to the magnetic field?

    F = B I L

  • True or False?

    A current-carrying conductor experiences its maximum force when placed parallel to a magnetic field.

    False.

    A conductor experiences zero force when parallel to the field; the maximum force occurs when the conductor is perpendicular to the field.

  • Define L in the equation F = BIL.

    L is the length of the conductor that is within the magnetic field.

  • In Fleming's left-hand rule, what does the thumb represent?

    The direction of motion, or force F, of the conductor.

  • In Fleming's left-hand rule, what does the first finger represent?

    The direction of the applied magnetic field, B.

  • In Fleming's left-hand rule, what does the second finger represent?

    The direction of the flow of conventional current, I (from positive to negative).

  • A .......... represents a magnetic field directed out of the page, while a .......... represents a magnetic field directed into the page.

    A dot represents a magnetic field directed out of the page, while a cross represents a magnetic field directed into the page.

  • True or False?

    Fleming's left-hand rule is used to find the direction of induced current in a generator.

    False.

    That is Fleming's right-hand rule. Fleming's left-hand rule is used to find the force on a current-carrying conductor and is sometimes called the rule for motors.

  • Define Fleming's left-hand rule.

    A rule used to determine the directions of force, magnetic field and current, which are mutually perpendicular, using the thumb, first finger and second finger respectively.

  • What is the equation for the magnetic force F on an isolated particle of charge Q moving at speed v at angle θ to a magnetic field of flux density B?

    F = B Q v \sin \theta

  • A charged particle moving .......... to a magnetic field experiences no magnetic force.

    A charged particle moving parallel to a magnetic field experiences no magnetic force.

  • For a negative charge, in which direction does conventional current point relative to the particle's motion?

    In the opposite direction to its motion.

  • True or False?

    The equations F = BIL and F = BQv can be used interchangeably.

    False.

    F = BIL is for a current-carrying conductor, while F = BQv is for an isolated moving charge (which may be inside a conductor).

  • Define the condition for maximum magnetic force on a moving charged particle.

    Maximum force occurs when the particle travels perpendicular to the field (θ = 90°), giving F = BQv.

  • A beam of electrons enters a magnetic field directed into the page, moving from right to left. Using Fleming's left-hand rule, in which direction do the current and the force point?

    • Current: to the right (opposite to the electrons' motion)

    • Force: upwards

  • What provides the centripetal force on a charged particle moving through a uniform magnetic field?

    The magnetic force on the particle.

  • What is the equation for the radius r of the circular path of a charged particle of mass m, charge Q and speed v in a magnetic field of flux density B?

    r = \frac{m v}{B Q}

  • A faster-moving particle in a magnetic field travels in a .......... circle.

    A faster-moving particle in a magnetic field travels in a larger circle.

  • True or False?

    A particle with a greater charge moves in a larger circle in a magnetic field.

    False.

    The radius is inversely proportional to charge (r \propto \frac{1}{Q}), so a greater charge produces a smaller circle.

  • Define a cyclotron.

    A type of particle accelerator that uses a magnetic field to keep charged particles moving in a circular path and an alternating electric field to accelerate them to high speeds.

  • In a cyclotron, what is the role of the magnetic field?

    To supply the centripetal force needed to keep the particles moving in circular motion.

  • In a cyclotron, what is the role of the alternating electric field?

    To accelerate the particles between the dees.

  • What is the aim of the required practical investigating magnetic fields in wires?

    To calculate the magnetic flux density of a magnet by measuring the force on a current-carrying wire placed perpendicular to the field.

  • In this practical, what are the independent and dependent variables?

    • Independent: current, I

    • Dependent: mass, m

  • In this practical, a graph of mass .......... current is plotted, and the magnetic flux density is calculated from the gradient.

    In this practical, a graph of mass against current is plotted, and the magnetic flux density is calculated from the gradient.

  • What is the equation used to calculate the magnetic flux density B from the gradient of the mass–current graph?

    B = \frac{g \times \text{gradient}}{L}

  • True or False?

    In this experiment, the top-pan balance directly measures the upward magnetic force on the wire.

    False.

    By Newton's third law, the balance measures the downward reaction force, which is equal in magnitude to the upward magnetic force on the wire.

  • Define the systematic error to avoid in this practical.

    A zero error, avoided by resetting the top-pan balance to read zero before any current flows.

  • Why should currents above 6 A be avoided in this experiment?

    They cause the wire to heat up, increasing its resistance and affecting the results.

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