May/June 2026 Paper 43

Define magnetic flux density.

Figure 6.1 shows a beam of charged particles in a device known as a mass spectrometer.

The shaded region contains a uniform magnetic field that is out of the plane of the page. The particles enter the region of the magnetic field at point X and strike a detector at point Y. Explain whether the sign of the charge on the particles is positive or negative.

Explain why the path followed by the beam of particles inside the magnetic field is circular.

Each of the particles in 6(b) has a mass of $2.0\times 10^{-26}\text{ kg}$ and a charge of magnitude $3.2\times 10^{-19}\text{ C}$. All of the particles have a speed of $7.3\times 10^4{\text{ m s}}^{-1}$. The distance XY is $8.2\text{ cm}$. Show that the centripetal acceleration of one of the particles is $1.3\times 10^{11}{\text{ m s}}^{-2}$.

Use the information in 6(c)(i) to determine the flux density $B$ of the uniform magnetic field. Give a unit with your answer.

The particles in 6(b) are replaced with particles that have half of the charge and a quarter of the mass of the particles in 6(b). The sign of the charge and the speed of the particles are both unchanged. On Figure 6.1, draw the path of this beam of particles.