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Capacitors

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Question 4

An exploratory deep-space scientific probe uses a small nuclear fission reactor to continuously generate an electrical power of 250 kW250\text{ kW}250 kW. To achieve high propulsion, the probe is equipped with a pulsed electromagnetic plasma thruster that requires a massive, rapid release of energy.

To power each pulse, energy from the reactor is accumulated over time in a heavy-duty capacitor bank. When this capacitor bank is fully charged:

  • potential difference across the bank = 15 kV15\text{ kV}15 kV
  • total energy stored in the bank = 180 kJ180\text{ kJ}180 kJ
a.

Calculate the total capacitance, CCC, of the capacitor bank.

[2]
b.

Explain why the individual capacitors in the bank must be connected in parallel in order to produce this total capacitance.

[1]
c.

The total stored energy must be fully discharged into the thruster's electromagnetic coils in a time of less than 0.15 ms0.15\text{ ms}0.15 ms.

Explain, using a calculation, why the thruster is powered by the capacitor bank instead of being connected directly to the probe's 250 kW250\text{ kW}250 kW nuclear reactor.

[2]
d.

The onboard nuclear reactor relies on the following fission reaction:

n+235U→141Ba+92Kr+3n \text{n} + {}^{235}\text{U} \rightarrow {}^{141}\text{Ba} + {}^{92}\text{Kr} + 3\text{n} n+235U→141Ba+92Kr+3n

Calculate the number of individual fission reactions that must occur to release a total amount of nuclear energy equal to the electrical energy stored in the capacitor bank.

  • mass of neutron, mn=1.008665 um_{\text{n}} = 1.008665\text{ u}mn​=1.008665 u
  • mass of uranium-235, mU=235.043929 um_{\text{U}} = 235.043929\text{ u}mU​=235.043929 u
  • mass of barium-141, mBa=140.914411 um_{\text{Ba}} = 140.914411\text{ u}mBa​=140.914411 u
  • mass of krypton-92, mKr=91.926156 um_{\text{Kr}} = 91.926156\text{ u}mKr​=91.926156 u
  • 1 u=1.661×10−27 kg1\text{ u} = 1.661 \times 10^{-27}\text{ kg}1 u=1.661×10−27 kg
  • speed of light, c=3.00×108 m s−1c = 3.00 \times 10^8\text{ m s}^{-1}c=3.00×108 m s−1
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Capacitors Questions

  1. A Level
  2. /Physics
  3. /Capacitors