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Flashcards

Magnetic Field Definitions

Noah Bryan Physics A Level OCR English 49 cards

Magnetic Field Definitions

Cards in this deck (49)

  1. 1Magnetic Field LinesLines of force drawn to represent a magnetic field pattern.
  2. 2Magnetic Field PatternVisual Representations used in interpreting the direction and strengths of magnetic fields.
  3. 3Magnetic FluxThe product of the component of the magnetic flux density perpendicular to a given area and that cross-sectional area.
    ( (Phi) = BAcos(Theta) )
  4. 4Magnetic Flux DensityThe strength of a magnetic field
    (B = F/IL)
  5. 5Magnetic Flux LinkageThe product of the number of turns in a coil, N, and the magnetic flux, (Phi).
  6. 6What is a Magnetic Field?A field surrounding a permanent magnet or current carrying conductor in which a magnetic object will experience a force.
  7. 7What does the arrow on a magnetic field line represent?The direction in which a free north pole would move (They point North --> South)
  8. 8Describe the Magnetic Field Patterns for the following scenarios: 1- Around a Single Bar Magnet 2 - Between two unlike poles 3- Between two like polesDescribe the Magnetic Field Patterns for the following scenarios: 1- Around a Single Bar Magnet 2 - Between two unlike poles 3- Between t...
  9. 9How is a field created in a wire?When a charged particle moves, it creates a magnetic field in the space it's moving.
  10. 10How do you determine the direction of a magnetic field around a current carrying conductor?Right-Hand Grip Rule
  11. 11Why does a current carrying conductor experience a force when placed in a permanent magnetic field?The wire itself has a magnetic field due to the movement of the charged particles (electrons). These two mag. fields interact and the wire & permanent magnet experience an equal and opposite force.
  12. 12When is the force experienced by a wire in a permanent mag. field maximum?When the current carrying wire is perpendicular to the direction of the permanent mag. field.
  13. 13Experimentally, it was found that the Force experienced was proportional to what four factors? Give an Equation1- Current, I
    2- Length, L, of the wire in the Mag. Field
    3- The Strength of the Mag.Field
    4- sin (Theta) - where Theta is the Angle between the Direction of Current and the Direction of the Mag. Field.
    F = BILsin(Theta)
  14. 14What are the SI units & SI Base Units for Magnetic Flux Density?Tesla (T)
    1T = 1(N)(m^-1)(A^-1)
  15. 15Magnetic Flux Density is a [Vector/Scalar] Quantity?Vector
  16. 16Experiment to Determine Magnetic Flux Density in a Laboratory:Place the magnets atop the balance. The mag. field between the north and south plate is assumed uniform. A stiff (!) copper wire is held in place between the two poles of the magnet. The length of the wire within the mag. field is measured with a ruler. A section of the wire is connected to a power supply with an ammeter in series and a voltmeter in parallel. The balance is zeroed when there is no current in the wire. With a current flowing through the wire, a vertical [upwards/downwards- to be predicted] is experienced by the wire & an equal and opposite force on the magnet is experienced. (Newton's III law). This shows a difference in mass on the balance - the force can be calculated by (F=mg) Then the Magnetic Flux Density, B, can be calculated by (B=F/IL)
  17. 17*Describe what happens in the Aurora [Borealis/Australis] (Northern/Southern Lights)Energetic charged particles from the Sun spiral down the Earth's magnetic field towards a polar region and collide with atoms in the atmosphere, causing photons of light to be emitted.
  18. 18What sort of motion does a charged particle undergo in a mag. field?Circular
  19. 19How can the circular motion of charged particles in a mag field be demonstrated?Using an Electron Deflection Tube. As the electrons enter the mag. field they experience a force. The electrons change direction, but (as seen in the LHR) the force on each electron always remains perpendicular to its velocity. Therefore the speed doesn't change as the force has no component in the direction of motion. Once it leaves the field, the electron continues in a straight line.
  20. 20Why dose a current carrying wire in a uniform mag. field experience a force?Because each electron experiences a small force - therefore the wire will experience a sizeable force as the number density of the wire is great.
  21. 21The equation to find the force acting on a charged particle of charge, Q, moving a speed, v, in at right angles to a uniform mag. field of flux density, B.F=Bev
  22. 22Derive the equation F=BevStart with F=BIL
    L=vt => F=BI(vt)
    Current is the rate of flow of charge; and with N, number of charged particles each with a charge, Q.
    I=NQ/t => F=B(NQ/t)(vt)=NBQv
    Force acting on each charged particle => F=(NBQv)/N = BQv
    Q=e for an electron/proton => F=Bev
  23. 23From two suitable equations, find an equation for the radius of a circle described by a charged particle in a mag. field. State any proportionalities to r.F=BQv=(m(v^2))/r
    r=mv/BQ
    r is:
    1- Proportional to both the mass of the particle and the velocity it's travelling
    2- Inversely proportional to both the charge on the particle and the magnetic flux density of the mag. field it's travelling through.
  24. 24What is a Velocity Selector? What instruments is it used in?A device that uses both Electric and Magnetic Fields to select charged particles with a desired velocity. Used in Mass Spectrometers/ Particle Accelerators.
  25. 25What is a Velocity Selector comprised of?Two horizontal parallel plates connected to a power supply. They produce a uniform Electric Field of field strength, E, between the plates. A uniform mag. field of flux density, B, is also applied across the plates perpendicular to the E. field. The charged particles get sorted through a narrow slit at the other end of the parallel plates.
  26. 26What's the theory behind the Velocity Selector?The Magnetic and E. fields deflect the charged particles in opposite directions. F=EQ=BQv. Therefore v=E/B.
    If the Force (due to the E. field - F(E)) is equal to the Force (due to the Mag. Field - F(M)) then the particle will travel straight through the slit. This, however, is dependant on the velocity. As the F(M) is proportional to the velocity.
  27. 27A Velocity Selector DiagramA Velocity Selector Diagram
  28. 28Mass Spectrometer DiagramMass Spectrometer Diagram
  29. 29Give the Equation used to predict the radius of a deflected ion in a mass spectrometer. How does this help the analysis of a compound?r=mv/BQ
    r is proportional to m => Heavier ions/ ionic fragments from a larger compound have different circular paths and so get detected at different positions on the screen before being analysed by a computer.
  30. 30What is a Hall Probe?A device used to measure magnetic flux density directly.
  31. 31What is the Hall Voltage?The potential difference between the two sides of a semiconductor when incoming electrons are deflected and accumulate on one side of the semiconductor walls due to a mag. field. Since there is a potential difference - an electric field is generated through the probe; where E=V(Hall)/d
    The V(Hall) is given by V(Hall)=BI/nte
    (n=number density;t=thickness of the probe; e=elementary charge)
  32. 32How can you induce an Emf in a coil of wire?Moving a magnet towards and away from the coil of wire induces an alternating emf and hence and alternating current in the coil. The faster it's moved, the larger the induced emf is.
  33. 33How to induce an Emf using a DC motor.Attach a weight onto the motor to cause the coil to rotate between the poles of the stationary magnet.
  34. 34How to induce an Emf by use of a wireWhen a copper wire is moved perpendicular to a mag. field. The faster it's moved, the larger the induced Emf.
  35. 35Where does the electrical energy come from when an Emf is induced?Some of the work done on the wire is converted into electrical energy. This is because the coil moves relative to the Mag. field and so the electrons within the coil experience a magnetic force. The movement of electrons constitutes a current and so a current in induced in the coil of wire.
  36. 36The unit for magnetic flux (Phi)The Weber
    1Wb=1T(m^2)
  37. 37The unit for Magnetic Flux LinkageThe Weber
    MFL=Number of Turns x MF
    Number of Turns has no units => MFL & Mf have the same units.
    It may also be Weber-turns.
  38. 38When is an Emf induced?When there is a change in the magnetic flux linkage.
  39. 39State Faraday's Law. With its equationThe magnitude of the induced Emf is directly proportional to the rate of change of flux linkage.
    Emf (Proportional) (Delta)(N)(Phi) / (Delta)(t)
  40. 40What is the Constant of Proportionality equal to in Faraday's law and why?-1 - Lenz's Law: The direction of the induced Emf / Current is always such as to oppose the change producing it.
  41. 41What is a Transformer made of?A Soft Laminated Iron Core; a Primary (input) coil and a Secondary (output) coil.
  42. 42How do Transformers Work?An alternating Current is supplied to the primary coil.This produces a varying magnetic flux in the soft iron core. The secondary coil, which is also attached to the same soft iron core is linked by this alternating magnetic flux. The iron core ensures that all the magnetic flux created in the primary coil is linked to the secondary coil. According the Faraday's Law, a varying Emf is produced across the ends of the Secondary coil.
  43. 43State the Turn-Ratio Equationn(s)/n(p)=V(s)/V(p)
  44. 44What do Step Up transformers do?Increase the Voltage across the wire and decrease the current through the wire.
    n(s)>n(p) therefore V(s)>V(p)
  45. 45What equation can you use to calculate power in an electrical circuit? How can this be used to calculate the efficiency of a transformer?P=VI
    V(s)I(s)=V(p)I(p)
    => I(p)/I(s)=V(s)/V(p)
  46. 46How can you make a transformer more efficient?Use low-resistance windings to reduce energy losses due to the heating effect of a current through a wire.
    Use an insulator between the thin iron layers in the laminated core- this prevents eddy currents from occurring which too heats the core.
    The core is made of soft iron which is easily magnetised and demagnetised.
  47. 47What is the standard voltage and frequency for all A.C generators?25kV and 50Hz
  48. 48Electrical power is transmitted at high [Voltages/Currents] to minimise heat losses in the transmission cables.Voltages
  49. 49The power lost due to heat in the cables is given by this equation: P(Loss)=.... Give any conclusions about what the voltage through cables should be.(I^2)R=(P(initial)^2)(R)/(V^2)
    Therefore the Higher the Voltage, the smaller the power losses are through heating.

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A Level OCR Physics Flashcards on Magnetic Field Definitions, created by Noah Bryan on 02/06/2017.
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Noah Bryan
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