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Modern physics
6 MDCAT Physics questions with answers and explanations.
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The photoelectric effect supports the idea that light behaves as:
- 1 A particle (photons)
- 2 A purely continuous wave
- 3 A longitudinal wave
- 4 A sound wave
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Correct answer: A particle (photons)
Light arrives in packets of energy, called photons. A photon of high enough energy can free an electron at once.
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The energy of a photon is given by:
- 1 E = mc
- 2 E = hf
- 3 E = 1/2 mv2
- 4 E = qV / t
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Correct answer: E = hf
E = hf, where h is Planck's constant and f is the frequency of the light.
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What fraction of a radioactive sample remains after 3 half-lives?
- 1 1/3
- 2 1/6
- 3 1/8
- 4 1/9
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Correct answer: 1/8
Each half-life halves the amount: 1/2 x 1/2 x 1/2 = 1/8.
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In beta-minus decay, a neutron changes into:
- 1 A proton, an electron and an antineutrino
- 2 Two protons
- 3 A proton and a positron
- 4 A helium nucleus
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Correct answer: A proton, an electron and an antineutrino
A neutron becomes a proton and the nucleus emits an electron (the beta particle) and an antineutrino.
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Which type of radiation is the most penetrating?
- 1 Alpha particles
- 2 Beta particles
- 3 Gamma rays
- 4 All penetrate equally
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Correct answer: Gamma rays
Gamma rays pass through the body and need thick lead or concrete to be stopped. Alpha particles are stopped by paper.
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The mass number of an atom is equal to the number of:
- 1 Protons only
- 2 Neutrons only
- 3 Protons plus neutrons
- 4 Electrons plus protons
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Correct answer: Protons plus neutrons
The mass number counts all nucleons, which are the protons and the neutrons.