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Atomic Structure

Q16. The diagram below represents an ion propulsion unit used on a spacecraft launched in 1998.

Atoms of xenon-131 were injected from a storage tank into a chamber where they became ionised due to collisions with electrons.

A negatively charged grid attracted the xenon ions, accelerating them out of the back of the ion propulsion unit and causing the spacecraft to be propelled forward.

(a) The mass of a Xe+ ion is 2.18 × 10–25 kg.

Calculate the specific charge of a Xe+ ion.

Give an appropriate unit for your answer.

[2 marks]

(b) The storage tank contained 79 kg of xenon.

When the ion propulsion unit was switched on it had an average power output of 2.1 kW.

Each xenon ion gained 1300 eV of energy as it was accelerated and ejected out of the propulsion unit.

Calculate the energy, in J, gained by each xenon ion.

[1 mark]

(c) Determine the length of time the ion propulsion unit operated before all of the 79 kg of xenon was used up.

mass of xenon atom = 2.18 × 10–25 kg

[3 marks]

(d) Ion propulsion units could use helium ions instead of xenon ions.

An ion of helium-4 has a much higher specific charge than an ion of xenon-131.

Both ions would gain 1300 eV of kinetic energy in being accelerated and ejected from an ion propulsion unit.

Suggest whether helium ions or xenon ions are better to use as a propellant in future space programmes.

In your answer you should compare the relative speeds and momentum changes of the ions.

[3 marks]

(Total 9 marks)