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DP IB Physics: SL

Topic Questions

Home / IB / Physics: SL / DP / Topic Questions / 5. Electricity & Magnetism / 5.4 Magnetic Effects of Electric Currents / Structured Questions


5.4 Magnetic Effects of Electric Currents

Question 1a

Marks: 3

Alpha particles travel in a vacuum at speed v and enter an area where there is a uniform magnetic field of flux density B. In this area, it begins to move in a circular trajectory. 

(a)
Show that the momentum of a single alpha particle is given by:

p equals 2 e B r

where e is the elementary charge and r is the orbital radius.   
[3]
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    Question 1b

    Marks: 3

    An alpha particle moves across the Earth's equator towards the east. At this point, the Earth's magnetic field has a direction due north and is parallel to the surface.

    (b)
    Deduce the direction of the force acting on the alpha particle at this instant. 
    [3]
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      Question 1c

      Marks: 3

      Charged particles from the sun, carried by the 'solar wind', may become trapped in the Earth's magnetic field near its poles, causing the sky to glow. Some of these charged particles travel in a circle of radius 45 km in a region where the flux density is 6.0 × 10–5 T. 

      (c)
      Show that these charged particles cannot be electrons. 
      [3]
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        Question 2a

        Marks: 3

        An electron is incident normally into a region of uniform magnetic flux density 0.50 T at a speed of 1.3 × 107 m s–1

        (a)
        Show that the percentage reduction in the magnetic force exerted on the electron is 50% when the electron is incident at an angle of 30°. 
        [3]
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          Question 2b

          Marks: 2

          Two electrons, X and Y, travel in a uniform magnetic field. X has kinetic energy EX and Y has kinetic energy EY

          (b)
          Calculate the ratio E subscript X over E subscript Yif X is incident at 30° to the field but Y is incident normally to it. 
          [2]
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            Question 2c

            Marks: 3

            A cylindrical aluminium bar XY of mass 6.0 g rests on two horizontal aluminium rails, separated by 5.0 cm. 

            sl-sq-5-4-hard-q2c

            The rails are connected via a switch to a cell that can drive a current of 4.5 A through XY. A magnetic field of flux density 0.20 T acts into the screen. 

            (c)
            Calculate the angle to the horizontal to which the rails must be tilted in order to keep XY stationary. 
            [3]
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              Question 2d

              Marks: 3

              A similar metal rod is suspended in a magnetic field by two identical, vertical springs. The cell and the rod have negligible internal resistance. 

              sl-sq-5-4-hard-q2d

              When the switch S is closed, the metal rod is displaced a distance x from its starting point. 

              (d)
              Show that, when both the spring constant and electrical resistance of each spring is doubled, closing S would cause the rod to be displaced by x over 4
              [3]
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                Question 3a

                Marks: 2

                A beam of electrons, each travelling at various speeds, passes through a hole in plate P1. Pis parallel to P1, also with a hole in it. The region between the plates contains a uniform electric field and a uniform magnetic field. Both the electric field strength and the magnetic flux density B are adjustable. 

                sl-sq-5-4-hard-q3a

                Electrons that are undeviated travel with a particular speed v along the straight line joining the holes in P1 and P2

                (a)
                Deduce the direction of the electric field between the plates.
                [2]
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                  Question 3b

                  Marks: 2
                  (b)
                  Mark with an X a position on P2 that would indicate where electrons with a speed greater than v may strike P2
                  [2]
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                    Question 3c

                    Marks: 3

                    The equipment is adjusted such that a single electron is shot with kinetic energy K through the hole in P1. The distance between the plates d is fixed, and electric field is switched off, such that the electron is incident in a region of uniform magnetic flux density only. 

                    (c)
                    Show that the maximum magnetic flux density Bmax that ensures the electron reaches P2 is given by:

                    B subscript m a x end subscript equals fraction numerator square root of 2 m subscript e K end root over denominator e d end fraction

                    where me is the rest mass of the electron and e is its charge. 
                    [3] 
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                      Question 4a

                      Marks: 2

                      Very small cracks in some metals can be detected by a method which includes the use of magnetism. In a particular method for steel pipes, a coil of wire is wrapped around it, and a current passed through the coil. This magnetises the pipe and cracks in the direction shown in the image can be found by sprinkling iron filings on the pipe. 

                      sl-sq-5-4-hard-q4a

                      Cracks along or parallel to the length of the pipe do not show up. 

                      (a)
                      Deduce why this method cannot be used for copper pipes. 
                      [2]
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                        Question 4b

                        Marks: 2
                        (b)
                        Explain why iron filings cluster around the crack shown in the image in part (a). 
                        [2]
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                          Question 4c

                          Marks: 3

                          The crack only shows up if it is across the direction of the field. 

                          (c)
                          Describe and explain how the coil in the image in part (a) should be arranged so that the magnetic field it produces will show cracks cracks that are along the pipe. 
                          [3] 
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                            Question 5a

                            Marks: 2

                            The image shows the main features of a loudspeaker L. A current-carrying coil is positioned within the magnetic field provided by a permanent magnet, and the current directions in the coil at a particular instant is shown. 

                            sl-sq-5-4-hard-q5a

                            The dust cap D prevents dust from blocking the gap between the cardboard tube and the south pole of the magnet. 

                            (a)
                            Identify, on the diagram, the direction of the force on the coil at this particular instant with the current directions shown. 
                            [2]
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                              Question 5b

                              Marks: 2
                              (b)
                              Describe how the magnitude and direction of the force on the coil varies over a complete cycle. 
                              [2]
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                                Question 5c

                                Marks: 3

                                The coil consists of 200 turns, each of average diameter 2.0 cm. The magnetic flux density created by the permanent magnet is 0.40 mT. The peak current in the coil is 0.48 mA.

                                (c)
                                Calculate the maximum magnetic force on the coil. 
                                [3]
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