20. Magnetic fields
A section of Physics, 9702
Listing 10 of 214 questions
Positive ions are travelling through a vacuum in a narrow beam. The ions enter a region of uniform magnetic field of flux density B and are deflected in a semi-circular arc, as shown in . 12.8 cm detector beam of positive ions uniform magnetic field The ions, travelling with speed 1.40 × 105 m s–1, are detected at a fixed detector when the diameter of the arc in the magnetic field is 12.8 cm. By reference to , state the direction of the magnetic field. The ions have mass 20 u and charge +1.6 × 10–19 C. Show that the magnetic flux density is 0.454 T. Explain your working. Ions of mass 22 u with the same charge and speed as those in are also present in the beam. On , sketch the path of these ions in the magnetic field of magnetic flux density 0.454 T. In order to detect these ions at the fixed detector, the magnetic flux density is changed. Calculate this new magnetic flux density. magnetic flux density = T
9702_w10_qp_41
THEORY
2010
Paper 4, Variant 1
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Positive ions are travelling through a vacuum in a narrow beam. The ions enter a region of uniform magnetic field of flux density B and are deflected in a semi-circular arc, as shown in . 12.8 cm detector beam of positive ions uniform magnetic field The ions, travelling with speed 1.40 × 105 m s–1, are detected at a fixed detector when the diameter of the arc in the magnetic field is 12.8 cm. By reference to , state the direction of the magnetic field. The ions have mass 20 u and charge +1.6 × 10–19 C. Show that the magnetic flux density is 0.454 T. Explain your working. Ions of mass 22 u with the same charge and speed as those in are also present in the beam. On , sketch the path of these ions in the magnetic field of magnetic flux density 0.454 T. In order to detect these ions at the fixed detector, the magnetic flux density is changed. Calculate this new magnetic flux density. magnetic flux density = T
9702_w10_qp_42
THEORY
2010
Paper 4, Variant 2
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The poles of a horseshoe magnet measure 5.0 cm × 2.4 cm, as shown in . A pole piece of magnet direction of movement of wire copper wire 5.0 cm 2.4 cm The uniform magnetic flux density between the poles of the magnet is 89 mT. Outside the region of the poles, the magnetic flux density is zero. A stiff copper wire is connected to a sensitive ammeter of resistance 0.12 Ω. A student moves the wire at a constant speed of 1.8 m s–1 between the poles in a direction parallel to the faces of the poles. Calculate the magnetic flux between the poles of the magnet. magnetic flux = Wb Use your answer in to determine, for the wire moving between the poles of the magnet, the e.m.f. induced in the wire. e.m.f. = V Show that the reading on the ammeter is approximately 70 mA. By reference to Lenz’s law, a force acts on the wire to oppose the motion of the wire. The student who moved the wire between the poles of the magnet claims not to have felt this force. Explain quantitatively a reason for this claim.
9702_w10_qp_43
THEORY
2010
Paper 4, Variant 3
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A stiff straight copper wire XY is held fixed in a uniform magnetic field of flux density 2.6 × 10−3 T, as shown in . 4.7 cm 34° stiff wire uniform magnetic field flux density 2.6 × 10–3 T X Y 34° The wire XY has length 4.7 cm and makes an angle of 34° with the magnetic field. Calculate the force on the wire due to a constant current of 5.4 A in the wire. force = N The current in the wire is now changed to an alternating current of r.m.s. value 1.7 A. Determine the total variation in the force on the wire due to the alternating current. variation in force = N
9702_w14_qp_41
THEORY
2014
Paper 4, Variant 1
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A stiff straight copper wire XY is held fixed in a uniform magnetic field of flux density 2.6 × 10−3 T, as shown in . 4.7 cm 34° stiff wire uniform magnetic field flux density 2.6 × 10–3 T X Y 34° The wire XY has length 4.7 cm and makes an angle of 34° with the magnetic field. Calculate the force on the wire due to a constant current of 5.4 A in the wire. force = N The current in the wire is now changed to an alternating current of r.m.s. value 1.7 A. Determine the total variation in the force on the wire due to the alternating current. variation in force = N
9702_w14_qp_42
THEORY
2014
Paper 4, Variant 2
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Define magnetic flux density. A stiff copper wire is balanced horizontally on a pivot, as shown in . 7.5 cm pivot stiff wire P Q S R Sections PQ, QR and RS of the wire are situated in a uniform magnetic field of flux density B produced between the poles of a permanent magnet. The perpendicular distance of PQRS from the pivot is 7.5 cm. When a current of 2.7 A is passed through the wire, a small mass of 45 mg is placed a distance 8.8 cm from the pivot in order to restore the balance of the wire, as shown in . 7.5 cm 8.8 cm 2.7 A 2.7 A pivot stiff wire P Q S R pole pieces of magnet small mass Explain why, when the current is switched on, the current in the sections PQ and RS of the wire does not affect the balance of the wire. The length of section QR of the wire is 1.2 cm. Calculate the magnetic flux density B. B = T
9702_w18_qp_42
THEORY
2018
Paper 4, Variant 2
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Two long straight parallel wires P and Q carry currents into the plane of the paper, as shown in . P current I Q current 2I The current in P is I and the current in Q is 2I. On , draw an arrow to show the direction of the magnetic field at wire Q due to the current in wire P. Label this arrow B. On , draw another arrow to show the direction of the force acting on wire Q due to the current in wire P. Label this arrow F. State, with a reason, how the magnitude of the force acting on wire P compares with the magnitude of the force acting on wire Q. State how the direction of the force on wire P compares with the direction of the force on wire Q.
9702_w21_qp_42
THEORY
2021
Paper 4, Variant 2
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Define magnetic flux density. An insulated rectangular coil of wire, consisting of 40 turns, is suspended in a cradle from a newton meter, as shown in . 3.00 cm 5.00 cm newton meter cradle coil 40 turns The vertical sides of the coil have a length of 5.00 cm and the horizontal sides have a length of 3.00 cm. The initial reading on the newton meter is 0.563 N. A U-shaped magnet rests on a top-pan balance that is set to a reading of 0.00 g. The lower edge of the coil is lowered into the region between the poles of the U-shaped magnet, as shown in the side view in . poles of magnet newton meter initial reading 0.563 N coil (viewed from the side) top-pan balance initial reading 0.00 g The magnetic field in the region between the poles is uniform. The lower edge of the coil is entirely within the uniform magnetic field. A current of 3.94 A is now passed through the coil. This causes the reading on the top-pan balance to change to 2.16 g. Explain why the current causes a vertical force to act on the coil. Determine, to three significant figures, the flux density B of the uniform magnetic field. B = T Determine what is now the reading on the newton meter. Explain your reasoning. reading = N
9702_w22_qp_42
THEORY
2022
Paper 4, Variant 2
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Define magnetic flux density. A long, straight wire carries a current into the page, as shown in . On , draw four field lines to represent the magnetic field around the wire due to the current in it. Two identical wires X and Y are placed parallel to each other. The wires both carry current into the page, as shown in . X Y Explain why the two wires exert a magnetic force on each other. On , draw an arrow to show the direction of the magnetic force exerted on wire X. Label your arrow F. The current in X is double the current in Y. State how the magnetic force exerted on wire Y compares with the magnetic force exerted on wire X. The direction of the current in both wires is now reversed. State, with a reason, the effect of this change on the direction of the force on wire X.
9702_w24_qp_41
THEORY
2024
Paper 4, Variant 1
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Define magnetic flux density. A long, straight wire carries a current into the page, as shown in . On , draw four field lines to represent the magnetic field around the wire due to the current in it. Two identical wires X and Y are placed parallel to each other. The wires both carry current into the page, as shown in . X Y Explain why the two wires exert a magnetic force on each other. On , draw an arrow to show the direction of the magnetic force exerted on wire X. Label your arrow F. The current in X is double the current in Y. State how the magnetic force exerted on wire Y compares with the magnetic force exerted on wire X. The direction of the current in both wires is now reversed. State, with a reason, the effect of this change on the direction of the force on wire X.
9702_w24_qp_43
THEORY
2024
Paper 4, Variant 3
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Questions Discovered
214