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Physics entrance exam, 2005 E.C. (2013)

Real questions from the Grade 12 university entrance exam in Physics, as students sat it in 2005 E.C. Every question comes with the correct answer and a worked explanation.

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Natural science stream
Who sits it
2005 E.C. (2013)
Exam year
39 questions with answers
In this set

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Answer each question yourself before opening the answer. The explanations point to the exact textbook section, so you know which page to reread when you miss one.

Question 1

Which of the following is correct about the meaning of Photon?

A.quantum of matter
B.positively charged particle
C.negatively charged particle
D.quantum of light

Show the answer and explanation

Answer: D quantum of light

Why

Light comes in packets of energy called photons, and one photon carries the energy \(E = hf\). A photon has no charge and no rest mass, so it is a packet of electromagnetic energy rather than a piece of matter. That makes D correct. Option A is tempting because the word quantum is used elsewhere for matter as well, but the smallest unit of matter is a particle such as an electron, and a photon is not made of matter.

Question 2

Consider a series RC circuit for which \(R = 1.0\,\mathrm{M\Omega}\), \(C = 5\,\mu\mathrm{F}\) and \(\varepsilon = 30\,\mathrm{V}\). What is the time constant of the circuit and the maximum charge on the capacitor after the switch is closed respectively?

A.5 seconds, 150 \(\Omega\)C
B.7 seconds, 155 \(\Omega\) C
C.10 seconds, 160 \(\Omega\)C
D.9 seconds, 135 Mc

Show the answer and explanation

Answer: A 5 seconds, 150 \(\Omega\)C

Why

The time constant of an RC circuit is \(\tau = RC\), so \(\tau = (1.0\times10^{6}\,\Omega)(5\times10^{-6}\,\mathrm{F}) = 5\) s. Once the capacitor is fully charged no current flows and the whole 30 V sits across it, so \(Q_{max} = C\varepsilon = (5\times10^{-6})(30) = 1.5\times10^{-4}\) C, which is 150 microcoulombs. Both numbers in option A come out of those two rules. Option C is what you reach if the prefixes are not tracked: the \(10^{6}\) of the megohm and the \(10^{-6}\) of the microfarad cancel exactly, so the time constant is 5 s and not 10 s.

Question 3

What is the direction with which a fish must push the water with its fins in order to propel eastward?

A.eastward
B.upward
C.westward
D.downward

Show the answer and explanation

Answer: C westward

Why

Newton's third law says forces come in pairs of equal size and opposite direction. The fins push on the water, and the water pushes back on the fish with the same force the other way. To be pushed east, the fish has to push the water west, so C is correct. Option A is the usual trap: pushing the water east would send the fish west.

Question 6

Two non-zero vectors \(\vec{A}\) and \(\vec{B}\) are related by \(\vec{A} = c\vec{B}\) where c is a scalar. If the two vectors have opposite directions, then one of the following is true about c?

A.c is a positive number
B.c is a negative number
C.\(c = 1\)
D.\(c = 0\)

Show the answer and explanation

Answer: B c is a negative number

Why

Writing \(\vec{A} = c\vec{B}\) means \(\vec{A}\) lies along the same line as \(\vec{B}\). A positive \(c\) keeps the direction and only changes the length, while a negative \(c\) turns the vector around. Opposite directions therefore force \(c\) to be negative. Option D fails because \(c = 0\) would make \(\vec{A}\) the zero vector, and the question says both vectors are non zero.

Question 7

On which of the following does the induced emf in a coil NOT depend when a magnet is inserted into it?

A.resistance of the coil
B.number of turns of the coil
C.speed approach of the magnet
D.magnetic moment of the magnet

Show the answer and explanation

Answer: A resistance of the coil

Why

Faraday's law gives the induced emf as \(\varepsilon = -N\dfrac{\Delta\Phi}{\Delta t}\). That expression contains the number of turns, how fast the flux changes (so the speed of the magnet) and how strong the magnet is, but resistance appears nowhere in it. Resistance decides the induced current through \(I = \varepsilon/R\), not the emf itself, so A is the one the emf does not depend on. Option C looks like a safe thing to rule out, yet a faster magnet changes the flux more quickly and does raise the emf.

Question 8

A wave have a speed of 50m/s and a wavelength of 25,000m. What is the frequency of the wave?

A.\(2.0 \times 10^{-3}\) HZ
B.\(5.0 \times 10^{-2}\) HZ
C.\(6.0 \times 10^{1}\) HZ
D.\(1.8 \times 10^{6}\) HZ

Show the answer and explanation

Answer: A \(2.0 \times 10^{-3}\) HZ

Why

Use \(v = f\lambda\), so \(f = \dfrac{v}{\lambda} = \dfrac{50}{25\,000} = 2.0\times10^{-3}\) Hz. A wavelength of 25 km is enormous next to a speed of 50 m/s, so one whole wave takes about 500 seconds to pass and the frequency has to be a very small number. Options C and D can be dropped on that reasoning alone, since a frequency of 1 Hz or more would mean at least one full wave every second.

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Physics papers from other years

2018 E.C. (2026)2017 E.C. (2025)2016 E.C. (2024)2015 E.C. (2023)2014 E.C. (2022)

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Are these the real 2005 E.C. Physics entrance exam questions?

Yes. They come from the national university entrance exam in Physics that students sat in 2005 E.C. (2013). Our set holds 39 of its questions, each with its answer and an explanation that points back to the textbook.

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