7.13 Fields and their consequences - Moving charges in a magnetic field – Questions

Q1.A cyclotron has two D-shaped regions where the magnetic flux density is constant.The D-shaped regions are separated by a small gap.An alternating electric field between the D-shaped regions accelerates charged particles. The magnetic field causes the charged particles to follow a circular path.

The diagram shows the path followed by a proton that starts from O.

(a)     Explain why it is not possible for the magnetic field to alter the speed of a proton while it is in one of the D-shaped regions.

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(b)     Derive an expression to show that the time taken by a proton to travel round one semi-circular path is independent of the radius of the path.

(3)

(c)     The maximum radius of the path followed by the proton is 0.55 m and the magnetic flux density of the uniform field is 0.44 T.

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Ignore any relativistic effects.

Calculate the maximum speed of a proton when it leaves the cyclotron.

maximum speed = ____________________ m s–1

(2)(Total 6 marks)

Q2.The diagram below shows a diagram of a mass spectrometer.

 

(a)     The magnetic field strength in the velocity selector is 0.14 T and the electric field strength is 20 000 V m–1.

(i)      Define the unit for magnetic flux density, the tesla.

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(ii)     Show that the velocity selected is independent of the charge on an ion.

 

 

 (2)

(iii)     Show that the velocity selected is about 140 km s–1.

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 (1)

(b)     A sample of nickel is analysed in the spectrometer. The two most abundant isotopes

of nickel are Ni and Ni. Each ion carries a single charge of +1.6 × 10–19 C.

mass of a proton or neutron = 1.7 × 10–27 kg

The Ni ion strikes the photographic plate 0.28 m from the point P at which the ion beam enters the ion separator.

Calculate:

(i)      the magnetic flux density of the field in the ion separator;

 

 

 

 (3)

(ii)     the separation of the positions where the two isotopes hit the photographic plate.

 

 

 

 (2)

(Total 10 marks)

 

 

Q3.Read the following passage and answer the questions that follow.

A mass spectrometer is an instrument for measuring the masses of isotopes. The main working parts of the instrument are shown in Figure 1.

Figure 1

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Figure 2 shows the components in more detail. Positive ions are created in the ionizer. Some of these ions enter the accelerator where they are accelerated by a potential difference VA. The ions emerge from the accelerator with different speeds and enter the velocity selector.

The velocity selector contains a region where there is a uniform magnetic field at right angles to an electric field. This electric field is formed between two parallel plates held at a potential difference VD. This combination of fields only allows ions of a particular velocity to enter the mass separator. Here ions of different mass are separated by a uniform magnetic field. Finally the ions are detected.

Figure 2

 

(a)     Explain what is meant by ionisation.

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(b)     Discuss the energy transfers that take place in the accelerator as the ion passes through it. Assume the ions are in a perfect vacuum.

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(c)     Figure 3 shows the path taken by an ion that moves through the velocity selector at a velocity v.

Figure 3

 

Discuss how the path changes when an ion enters the velocity selector with a velocity greater than v.

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(d)     Draw, on Figure 3, the path of the ion that is suggested by your answer to part (c).(1)

(e)     Ions created in the ioniser may have the same charge but a different number of nucleons.

Discuss how the path of an ion in the mass separator is affected when it has more nucleons.

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(f)     Some ions are created with the same mass but a double charge. The path of the ions shown in Figure 2 is that of a singly charged ion.

Compare, with justification, the path of a doubly charged ion through the mass spectrometer with that of a singly charged ion of the same mass.

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(Total 13 marks)

Q4.(a)     Explain why a particle is accelerating even when it is moving with a uniform speed in

a circular path.

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(b)     Figure 1 shows a schematic diagram of a proton synchrotron. This is a device for accelerating protons to high speeds in a horizontal circular path.

 

Figure 1

In the synchrotron the protons of mass 1.7 × 10–27 kg are injected at point A at a speed of 8.0 × 106 m s–1. The diameter of the path taken by the protons is 400 m.

(i)      Show on Figure 1 the direction of the force required to make a proton move in

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the circular path when the proton is at the position marked P.(1)

(ii)     Calculate the force that has to be provided to produce the circular path when the speed of a proton is 8.0 × 106 m s–1.

(2)

(iii)    Sketch on Figure 2 a graph to show how this force will have to change as the speed of the proton increases over the range shown on the x-axis. Insert an appropriate scale on the force axis.

 

Figure 2

(c)     Before reaching their final energy the protons in the synchrotron in part (b) travel around the accelerator 420 000 times in 2.0 s.

acceleration of free fall, g = 9.8 m s–2

(i)      Calculate the total distance travelled by a proton in the 2.0 s time interval.(2)

(ii)     Unless a vertical force is applied the protons wold fall as they move through the horizontal channel.

Calculate the distance a proton would fall in two seconds.(2)

(iii)    Determine the force necessary to prevent the vertical movement.(1)

(Total 12 marks)

Q5.Figure 1 shows the plan view of a cyclotron in which protons are emitted in between the dees. The protons are deflected into a circular path by the application of a magnetic field. Figure 2 shows a view from in front of the cyclotron.

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Figure 1

 

Figure 2

(a)     (i)      Mark on Figure 2 the direction of the magnetic field in the region of the dees such that it will deflect the proton beam in the direction shown in Figure 1.

(2)

(ii)     Show that the velocity of the proton, v, at some instant is given by:

v =

where m is the proton mass, r the radius of its circular path, B the magnetic flux density acting on the proton and +e the proton charge.

 

 

 

 

 

 (2)

(iii)     Write down an equation for the time T for a proton to make a complete circular path in this magnetic field.

 

 

 

 

 

 

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(2)

(iv)    Explain how your equation leads to the conclusion that T is independent of the speed with which the proton is moving.

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(b)     In addition to this magnetic field there is an electric field provided between the dees. This accelerates the proton towards whichever dee is negatively charged. An alternating potential difference causes each dee to become alternately negative and then positive. This causes the proton to accelerate each time it crosses the gap between the dees.

(i)      Describe and explain the effect the acceleration has on the path in which the proton moves.

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(ii)     In terms of T, write down the frequency with which the p.d. must alternate to match the period of motion of the proton.

 

 

 

 (1)

(c)     (i)      Calculate the velocity of a proton of energy 0.12 keV.

the proton mass, m = 1.7 × 10–27 kgthe magnitude of the electronic charge, e = 1.6 × 10–19 C

 

 

 

 

 (3)

(ii)     Calculate the de Broglie wavelength of the 0.12 keV proton.

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         the Planck constant, h = 6.6 × 10–34 J s

 

 

 

 (3)

(iii)     Name the region of the electromagnetic spectrum which has an equivalent wavelength to that of the proton.

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(Total 17 marks)

Q6.A narrow beam of electrons is directed into a uniform electric field created by two oppositely-charged parallel metal plates at right angles to the field lines. A fluorescent screen is used to make the beam give a visible trace.

 

(a)     (i)      Explain why the beam curves towards the positive plate.

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(ii)     How does the trace show that, on entry to the electric field, all the electrons have the same speed?

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(b)     The beam is produced as a result of accelerating electrons between the filament and a metal anode.

(i)      Explain why the wire filament must be hot.

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(ii)     Write down an equation relating the speed of the electrons, υ, to the potential difference, VA, between the anode and the filament.

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(c)     The deflection of the beam due to the electric field can be cancelled by applying a suitable uniform magnetic field in the same region as the electric field.

(i)      What direction should the magnetic field be in to do this?

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(ii)     Write down an equation relating the speed of the electrons υ to the plate voltage Vp, the plate separation d, and the magnetic flux density B necessary to make the beam pass undeflected between the plates.

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(iii)    The following measurements were made when the beam was undeflected.

VA = 3700 V      Vp = 4500 V      d = 50 mm      B = 2.5 mT

Use the two equations you have written down and the given data to calculate the specific charge, e/m, of the electron.

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(Total 10 marks)

Q7.The diagram shows an arrangement in a vacuum to deflect protons into a detector using a magnetic field, which can be assumed to be uniform within the square shown and zero outside it.

The motion of the protons is in the plane of the paper.

 

(a)     Sketch the path of a proton through the magnetic deflector. At any point on this path draw an arrow to represent the magnetic force on the proton. Label this arrow F.

(2)

(b)     State the direction of the uniform magnetic field causing this motion.

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(c)     The speed of a proton as it enters the deflector is 5.0 × 106 m s–1. If the flux density of the magnetic field is 0.50 T, calculate the magnitude of the magnetic force on the proton.

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(d)     If the path were that of an electron with the same velocity, what two changes would need to be made to the magnetic field for the electron to enter the detector along the same path?

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(Total 7 marks)

Q8.(a)     (i)      State two differences between a proton and a positron.

difference 1 _____________________________________________

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difference 2 _____________________________________________

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(ii)     A narrow beam of protons and positrons travelling at the same speed enters a uniform magnetic field. The path of the positrons through the field is shown in Figure 1.

Sketch on Figure 1 the path you would expect the protons to take.

 

Figure 1

(iii)    Explain why protons take a different path to that of the positrons.

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(b)     Figure 2 shows five isotopes of carbon plotted on a grid in which the vertical axis represents the neutron number N and the horizontal axis represents the proton number Z.Two of the isotopes are stable, one is a beta minus emitter and two are positron emitters.

 

Figure 2

(i)      Which isotope is a beta minus emitter?

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(ii)     Which of the two positron emitters has the shorter half-life? Give a reason for your choice.

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(c)     A positron with kinetic energy 2.2 MeV and an electron at rest annihilate each other. Calculate the average energy of each of the two gamma photons produced as a result of this annihilation.

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(Total 10 marks)

Q9.Protons and pions are produced in a beam from a target in an accelerator. The two types of particles can be separated using a magnetic field.

(a)     State the quark composition of

(i)      a proton,

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(ii)     a positive pion, π+

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(b)     A narrow beam consisting of protons and positive pions, all travelling at a speed of 1.5 × 107 m s–1 , is directed into a uniform magnetic field of flux density 0.16 T, as shown in the diagram.

 

(i)      Calculate the radius of curvature of the path of the protons in the field.

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(ii)     Sketch, on the diagram above, the path of the pions from the point of entry into the field to the point of exit from the field.

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(iii)    If the magnetic field were increased, how would this affect the paths of the particles?

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(Total 9 marks)

Q10.(a)     Figure 1 shows a negative ion which has a charge of –3e and is free to move in a

uniform electric field. When the ion is accelerated by the field through a distance of 63 mm parallel to the field lines its kinetic energy increases by 4.0 × 10–16 J.

Figure 1

 

(i)      State and explain the direction of the electrostatic force on the ion.

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(ii)     Calculate the magnitude of the electrostatic force acting on the ion.

 

 

 

magnitude of electrostatic force ____________________ N(2)

(iii)    Calculate the electric field strength.

 

 

 

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electric field strength ____________________ NC–1

(1)

(b)     Figure 2 shows a section of a horizontal copper wire carrying a current of 0.38 A.A horizontal uniform magnetic field of flux density B is applied at right angles to the wire in the direction shown in the figure.

Figure 2

 

(i)      State the direction of the magnetic force that acts on the moving electrons in the wire as a consequence of the current and explain how you arrive at your answer.

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(ii)     Copper contains 8.4 × 1028 free electrons per cubic metre. The section of wire in Figure 2 is 95 mm long and its cross-sectional area is 5.1 × 10–6 m2.Show that there are about 4 × 1022 free electrons in this section of wire.

 

 

 (1)

(iii)     With a current of 0.38 A, the average velocity of an electron in the wire is5.5 × 10–6 m s–1 and the average magnetic force on one electron is 1.4 × 10–25 N.Calculate the flux density B of the magnetic field.

 

 

 

flux density ____________________ T

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(2)(Total 10 marks)

Q11.(a)    

 

The diagram above shows a doubly-charged positive ion of the copper isotope  that is projected into a vertical magnetic field of flux density 0.28 T, with the field directed upwards. The ion enters the field at a speed of 7.8 × 105 m s–1.

(i)      State the initial direction of the magnetic force that acts on the ion.

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(ii)     Describe the subsequent path of the ion as fully as you can.Your answer should include both a qualitative description and a calculation.

mass of  ion = 1.05 × 10–25 kg

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(b)     State the effect on the path in part (a) if the following changes are made separately.

(i)      The strength of the magnetic field is doubled.

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(ii)     A singly-charged positive  ion replaces the original one.

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(Total 8 marks)

Q12.When travelling in a vacuum through a uniform magnetic field of flux density 0.43 m T, an electron moves at constant speed in a horizontal circle of radius 74 mm, as shown in the figure below.

 

(a)     When viewed from vertically above, the electron moves clockwise around thehorizontal circle. In which one of the six directions shown on the figure above,+x, –x, +y, –y, +z or –z, is the magnetic field directed?

direction of magnetic field ______________________(1)

(b)     Explain why the electron is accelerating even though it is travelling at constant speed.

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(c)     (i)      By considering the centripetal force acting on the electron, show that its speed is 5.6 × 106 m s–1.

 

 

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 (2)

(ii)     Calculate the angular speed of the electron, giving an appropriate unit.

 

 

 

 

answer = ______________________(2)

(iii)     How many times does the electron travel around the circle in one minute?

 

 

 

 

answer = ______________________(2)

(Total 9 marks)

Q13.(a)     The equation F = BQv may be used to calculate magnetic forces.

(i)      State the condition under which this equation applies.

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(ii)     Identify the physical quantities that are represented by the four symbols in the equation.

F ____________________________________________________________

B ____________________________________________________________

Q ____________________________________________________________

v ____________________________________________________________(1)

(b)     The figure below shows the path followed by a stream of identical positively charged ions, of the same kinetic energy, as they pass through the region between two

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charged plates. Initially the ions are travelling horizontally and they are then deflected downwards by the electric field between the plates.

 

While the electric field is still applied, the path of the ions may be restored to the horizontal, so that they have no overall deflection, by applying a magnetic field over the same region as the electric field. The magnetic field must be of suitable strength and has to be applied in a particular direction.

(i)      State the direction in which the magnetic field should be applied.

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(ii)     Explain why the ions have no overall deflection when a magnetic field of the required strength has been applied.

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(iii)     A stream of ions passes between the plates at a velocity of 1.7 x 105ms–1. The separation d of the plates is 65 mm and the pd across them is 48 V. Calculate the value of B required so that there is no overall deflection of the ions, stating an appropriate unit.

 

 

 

 

 

 

answer = ______________________

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(4)

(c)     Explain what would happen to ions with a velocity higher than 1.7 x 105ms–1 when they pass between the plates at a time when the conditions in part (b)(iii) have been established.

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(Total 11 marks)

Q14.The Large Hadron Collider (LHC) uses magnetic fields to confine fast-moving charged particles travelling repeatedly around a circular path. The LHC is installed in an underground circular tunnel of circumference 27 km.

(a)     In the presence of a suitably directed uniform magnetic field, charged particles move at constant speed in a circular path of constant radius. By reference to the force acting on the particles, explain how this is achieved and why it happens.

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(b)     (i)      The charged particles travelling around the LHC may be protons. Calculate the centripetal force acting on a proton when travelling in a circular path of circumference 27 km at one-tenth of the speed of light. Ignore relativistic effects.

 

 

 

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answer = ____________________ N(3)

(ii)     Calculate the flux density of the uniform magnetic field that would be required to produce this force. State an appropriate unit.

 

 

 

 

answer = ____________________ unit __________(3)

(c)     The speed of the protons gradually increases as their energy is increased by the LHC.State and explain how the magnetic field in the LHC must change as the speed of the protons is increased.

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(Total 12 marks)

Q15.(a)     (i)      State two situations in which a charged particle will experience no magnetic

force when placed in a magnetic field.

first situation____________________________________________________

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second situation_________________________________________________

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(ii)     A charged particle moves in a circular path when travelling perpendicular to a

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uniform magnetic field. By considering the force acting on the charged particle, show that the radius of the path is proportional to the momentum of the particle.

 

 

 

 (2)

(b)     In a cyclotron designed to produce high energy protons, the protons pass repeatedly between two hollow D-shaped containers called ‘dees’. The protons are acted on by a uniform magnetic field over the whole area of the dees. Each proton therefore moves in a semi-circular path at constant speed when inside a dee. Every time a proton crosses the gap between the dees it is accelerated by an alternating electric field applied between the dees. The diagram below shows a plan view of this arrangement.

 

(i)      State the direction in which the magnetic field should be applied in order for the protons to travel along the semicircular paths inside each of the dees as shown in the diagram above.

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(ii)     In a particular cyclotron the flux density of the uniform magnetic field is 0.48 T. Calculate the speed of a proton when the radius of its path inside the dee is 190 mm.

 

 

 

speed ____________________ ms–1

(2)

(iii)    Calculate the time taken for this proton to travel at constant speed in a semicircular path of radius 190 mm inside the dee.

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time ____________________ s(2)

(iv)    As the protons gain energy, the radius of the path they follow increases steadily, as shown in the diagram above. Show that your answer to part (b)(iii) does not depend on the radius of the proton’s path.

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(c)     The protons leave the cyclotron when the radius of their path is equal to the outer radius of the dees. Calculate the maximum kinetic energy, in Me V, of the protons accelerated by the cyclotron if the outer radius of the dees is 470 mm.

 

 

 

 

 

maximum kinetic energy ____________________ Me V(3)

(Total 14 marks)

Q16.Two electrons, X and Y, travel at right angles to a uniform magnetic field.X experiences a magnetic force, FX, and Y experiences a magnetic force, FY.

What is the ratio if the kinetic energy of X is half that of Y?

A       

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B       

C      

D       1(Total 1 mark)

Q17.Charged particles, each of mass m and charge Q, travel at a constant speed in a circle of radius r in a uniform magnetic field of flux density B.Which expression gives the frequency of rotation of a particle in the beam?

 

A 

B  

C  

D 

(Total 1 mark)

Q18.An electron moving with a constant speed enters a uniform magnetic field in a direction perpendicular to the magnetic field. What is the shape of the path that the electron would follow?

A       parabolic

B       circular

C       elliptical

D       a line parallel to the magnetic field(Total 1 mark)

Q19.An electron moves due North in a horizontal plane with uniform speed. It enters a uniform magnetic field directed due South in the same plane. Which one of the following statements concerning the motion of the electron in the magnetic field is correct?

A       It continues to move North with its original speed.

B       It slows down to zero speed and then accelerates due South.

C       It is accelerated due West.

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D       It is accelerated due North.(Total 1 mark)

Q20.An alpha particle moves at one-tenth the velocity of a beta particle. They both move through the same uniform magnetic field at right angles to their motion.

The magnitude of the ratio is

A       

B       

C       

D       (Total 1 mark)

Q21.Which line, A to D, correctly describes the trajectory of charged particles which enter, at right angles, (a) a uniform electric field, and (b) a uniform magnetic field?

 

  (a) uniform electric field (b) uniform magnetic field

ABCD

circularcircularparabolicparabolic

circularparaboliccircularparabolic

(Total 1 mark)

Q22.The path followed by an electron of momentum p, carrying charge −e, which enters a magnetic field at right angles, is a circular arc of radius r.What would be the radius of the circular arc followed by an α particle of momentum 2p, carrying charge +2e, which entered the same field at right angles?

A        

B       r

C       2r

D       4r

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(Total 1 mark)

Q23.Protons, each of mass m and charge e, follow a circular path when travelling perpendicular to a magnetic field of uniform flux density B. What is the time taken for one complete orbit?

A      

B      

C      

D       (Total 1 mark)

Q24.Particles of mass m carrying a charge Q travel in a circular path of radius r in a magnetic field of flux density B with a speed v. How many of the following quantities, if changed one at a time, would change the radius of the path?

•    m

•    Q

•    B

•    v

A       one

B       two

C       three

D       four(Total 1 mark)

Q25.Particles of mass m, each carrying charge Q and travelling with speed v, enter a magnetic field of flux density B at right angles. Which one of the following changes would produce an increase in the radius of the path of the particles?

A       an increase in Q

B       an increase in m

C       a decrease in v

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D       an increase in B(Total 1 mark)

Q26.An electron moves due North in a horizontal plane with uniform speed. It enters a uniform magnetic field directed due South in the same plane. Which one of the following statements concerning the motion of the electron in the magnetic field is correct?

A       It accelerated due West.

B       It slows down to zero speed and then accelerates due South.

C       It continues to move North with its original speed.

D       It is accelerated due North.(Total 1 mark)

Q27.A jet of air carrying positively charged particles is directed horizontally between the poles of a strong magnet, as shown in the diagram.

 

In which direction are the charged particles deflected?

A       upwards

B       downwards

C       towards the N pole of the magnet

D       towards the S pole of the magnet(Total 1 mark)

Q28.Two charged particles, P and Q, move in circular orbits in a magnetic field of uniform flux density. The particles have the same charge but the mass of P is less than the mass of Q. TP is the time taken for particle P to complete one orbit and TQ the time for particle Q to complete one orbit. Which one of the following is correct?

A       TP = TQ

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B       TP> TQ

C       TP< TQ

D       TP – TQ= 1(Total 1 mark)

Q29.A negatively charged particle moves at right angles to a uniform magnetic field. The magnetic force on the particle acts

A       in the direction of the field.

B       in the opposite direction to that of the field.

C       at an angle between 0° and 90° to the field.

D       at right angles to the field.(Total 1 mark)

Q30.When a β particle moves at right angles through a uniform magnetic field it experiences a force F. An α particle moves at right angles through a magnetic field of twice the magnetic flux density with velocity one tenth the velocity of the β particle. What is the magnitude of the force on the α particle?

A       0.2 F

B       0.4 F

C       0.8 F

D       4.0 F(Total 1 mark)

Q31.Charged particles, each of mass m and charge Q, travel at a constant speed in a circle of radius r in a uniform magnetic field of flux density B. Which expression gives the frequency of rotation of a particle in the beam?

A       

B       

C       

D       

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(Total 1 mark)

Q32.A beam of positive ions enters a region of uniform magnetic field, causing the beam to change direction as shown in the diagram.

 

What is the direction of the magnetic field?

A        out of the page and perpendicular to it

B        into the page and perpendicular to it

C        in the direction indicated by +y

D        in the direction indicated by -y(Total 1 mark)

Q33.Two charged particles, P1 and P2, follow circular paths as they move at right angles to the same uniform magnetic field. Both particles are travelling at the same speed.

The radius of the path travelled by P1 is twice the radius of the path travelled by P2.

The mass of P1 is m and its charge is q.

What is the mass of P2 and the charge of P2? 

  Mass of P2 Charge of P2  

A 2m q

B 2m 2q

C m q

D m 2q

(Total 1 mark)

Q34.

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Which line, A to D, in the table correctly describes the trajectory of charged particles which enter separately, at right angles, a uniform electric field, and a uniform magnetic field?

 

  uniform electric field uniform magnetic field

A parabolic circular

B circular parabolic

C circular circular

D parabolic parabolic

(Total 1 mark)

Q35.The path followed by an electron of momentum p, carrying charge –e, which enters a magnetic field at right angles, is a circular arc of radius r.

What would be the radius of the circular arc followed by an α particle of momentum 2p, carrying charge +2e, which entered the same field at right angles?

A       

B        r

C        2r

D        4r(Total 1 mark)

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