GCSEPhysicsAQAFlashcardsMagnetism & ElectromagnetismInduced Potential, Transformers & the National Grid

Induced Potential, Transformers & the National Grid (AQA GCSE Physics): Flashcards

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  • Define the generator effect.

    If an electrical conductor moves relative to a magnetic field, or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor.

  • What happens if a conductor is part of a complete circuit when a potential difference is induced in it?

    A current is induced in the conductor.

  • List four factors that affect the size of the induced potential difference.

    • The speed at which the wire, coil or magnet is moved

    • The number of turns on the coil

    • The size of the coil

    • The strength of the magnetic field

  • Increasing the ______ of the magnetic field will increase the induced potential difference.

    Increasing the strength of the magnetic field will increase the induced potential difference.

  • True or False?

    The direction of the induced potential difference depends on the orientation of the magnet's poles.

    True.

    Reversing the direction in which the wire, coil or magnet is moved, or reversing the orientation of the magnet's poles, reverses the direction of the induced potential difference.

  • Define Lenz's law.

    The direction of an induced potential difference always opposes the change that produces it.

  • A magnet is pushed north pole first into a coil of wire. What pole does the end of the coil closest to the magnet become?

    A north pole, so that it repels the magnet and opposes it being pushed in.

  • A magnet is pulled away from a coil of wire. What pole does the end of the coil closest to the magnet become?

    A south pole, so that it attracts the magnet and opposes it being pulled away.

  • Define an alternator.

    A device which converts energy from motion into an electrical output with an alternating current.

  • Define a dynamo.

    A device which converts energy from motion into an electrical output with a direct current.

  • What is the function of the slip rings in an alternator?

    To allow the alternating current to flow between the coil and the external circuit.

  • What component does a dynamo use instead of slip rings, and what does it do?

    A split-ring commutator, which changes the connections between the coil and the brushes every half turn to keep the current leaving the dynamo in the same direction.

  • When is the induced potential difference in an alternator at a maximum?

    When the position of the coil is horizontal and its motion is perpendicular to the magnetic field.

  • A bicycle dynamo consists of a rotating ______ placed inside or next to a magnet.

    A bicycle dynamo consists of a rotating coil placed inside or next to a magnet.

  • True or False?

    A dynamo produces direct current.

    True.

    A dynamo uses a split-ring commutator, so it produces direct current — unlike an alternator, which produces alternating current.

  • What component do an alternator and a dynamo have in common that provides the electrical connection between the coil and the external circuit?

    Carbon brushes.

  • What shape is the p.d.–time graph for an alternator that starts from a horizontal (maximum p.d.) position?

    A cosine curve.

  • What shape is the p.d.–time graph for an alternator that starts from a vertical (zero p.d.) position?

    A sine curve.

  • What shape is the p.d.–time graph for a dynamo?

    A sine curve that is always in the same direction.

  • When the coil of an alternator has rotated 90° from vertical, what is happening to the induced p.d.?

    The coil is horizontal and moving perpendicular to the magnetic field, so the induced p.d. is at a maximum.

  • Why does a dynamo's induced p.d. never become negative, unlike an alternator's?

    The split-ring commutator changes the connections between the coil and the brushes every half turn, so the induced potential difference varies from zero to a maximum twice each cycle but never changes polarity.

  • The magnitude of the induced p.d. can be increased by inserting a ______ into the coil.

    The magnitude of the induced p.d. can be increased by inserting a soft iron core into the coil.

  • True or False?

    Increasing the frequency of rotation of the coil increases the magnitude of the induced p.d.

    True.

    The induced p.d. can also be increased by increasing the number of turns on the coil, increasing the strength of the magnet, or inserting a soft iron core.

  • At what angles of rotation is the induced p.d. zero for both an alternator and a dynamo?

    At 0°, 180° and 360°, when the coil is vertical and moving parallel to the magnetic field.

  • What do microphones convert sound waves into?

    Electrical current.

  • What effect does a moving-coil microphone use to work?

    The generator effect.

  • What happens to the diaphragm of a moving-coil microphone when sound waves reach it?

    The pressure variations cause the diaphragm to vibrate.

  • What happens to the coil of a moving-coil microphone as the diaphragm vibrates?

    The coil moves back and forth through the magnetic field produced by the magnet, cutting through the field lines.

  • Why is the potential difference induced in a moving-coil microphone alternating?

    Because the coil is continually changing direction due to the vibrations of the diaphragm.

  • As the coil moves through the magnetic field, it ______ through the field lines, which induces a potential difference.

    As the coil moves through the magnetic field, it cuts through the field lines, which induces a potential difference.

  • True or False?

    A moving-coil microphone induces a potential difference by moving the coil through a magnetic field.

    True.

    This induced potential difference is what allows the microphone to convert sound waves into an electrical signal.

  • Define a transformer.

    A device used to change the value of an alternating potential difference or current, using the generator effect.

  • What three components make up a basic transformer?

    A primary coil, a secondary coil and an iron core.

  • Why is iron used for the core of a transformer?

    Because iron is easily magnetised.

  • Why must the current supplied to the primary coil be alternating rather than direct?

    Because it must be continually changing direction to produce a changing magnetic field around the primary coil.

  • How does the changing magnetic field from the primary coil reach the secondary coil?

    The iron core is easily magnetised, so the changing magnetic field passes through it to the secondary coil.

  • The alternating potential difference induced in the secondary coil has the same ______ as the alternating current supplied to the primary coil.

    The alternating potential difference induced in the secondary coil has the same frequency as the alternating current supplied to the primary coil.

  • True or False?

    A transformer will only produce a current in the secondary coil if it is part of a complete circuit.

    True.

    If the secondary coil is part of a complete circuit, the induced alternating potential difference will cause an alternating current to flow.

  • Define step-up transformer.

    A transformer that increases the potential difference of a power source \left(V_{s} > V_{p}\right) and has more turns on the secondary coil than on the primary coil \left(N_{s} > N_{p}\right).

  • Define step-down transformer.

    A transformer that decreases the potential difference of a power source \left(V_{s} < V_{p}\right) and has fewer turns on the secondary coil than on the primary coil \left(N_{s} < N_{p}\right).

  • State the transformer equation linking potential difference and number of turns.

    \frac{V_{p}}{V_{s}} = \frac{N_{p}}{N_{s}}

    Where V~p~ and V~s~ are the potential differences across the primary and secondary coils, and N~p~ and N~s~ are the numbers of turns on the primary and secondary coils.

  • What does the transformer equation show about the ratio of potential differences and turns?

    The ratio of the potential differences across the primary and secondary coils is equal to the ratio of the number of turns on each coil.

  • If a transformer is 100% efficient, then ______ power = output power.

    If a transformer is 100% efficient, then input power = output power.

  • State the ideal transformer equation linking potential difference and current in the primary and secondary coils.

    V_{p} \times I_{p} = V_{s} \times I_{s}

    Where V~p~ and I~p~ are the potential difference and current in the primary coil, and V~s~ and I~s~ are the potential difference and current in the secondary coil.

  • Why can a transformer not increase the power output of a power source, even though it can increase the potential difference?

    Because of the law of conservation of energy — an ideal transformer conserves power, it does not create it.

  • True or False?

    An ideal transformer is 100% efficient.

    True.

    For an ideal transformer, input power equals output power, so V_{p} \times I_{p} = V_{s} \times I_{s}.

  • What are three roles of transformers in the electricity supply system?

    Transformers are used to:

    • Increase the potential difference of electricity before it is transmitted across the National Grid

    • Lower the high voltage electricity in power lines to the lower voltages used in houses

    • Lower mains voltage in adapters to the lower voltages used by many electronic devices

  • Why does increasing the potential difference at which electricity is transmitted cause the current in the power lines to decrease?

    Because P* = *IV, so to transmit the same power, if V increases, I must decrease.

  • Why does transmitting electricity at a smaller current reduce energy loss in the power lines?

    A smaller current produces less heat in the power lines, which reduces the energy lost as heat.

  • This is because P = IV, so if V increases, I must ______ to transmit the same power.

    This is because P = IV, so if V increases, I must decrease to transmit the same power.

  • True or False?

    Adapters use transformers to increase mains voltage to the higher voltages needed by electronic devices.

    False.

    Adapters use transformers to lower mains voltage to the lower voltages used by many electronic devices.

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