Physics Past Questions | JAMB, WAEC, NECO and Post UTME Past Questions

When a soft iron bar, like PQ in the diagram, is placed near two magnets, it gets magnetized by a process called magnetic induction. In this case, the north pole of the first magnet is facing point P and the south pole of the second magnet is facing point Q.

During the magnetic induction process, the soft iron bar will develop the opposite polarity at the end closest to each magnet. This means that at point P, which is closest to the north pole of the first magnet, a south pole will be induced. Similarly, at point Q, which is closest to the south pole of the second magnet, a north pole will be induced.

Therefore, the correct polarity at P and Q is Option D: N and S.

Magnets are stored using keepers for the primary reason of reducing self-demagnetization. This is the correct answer indicated in Option A. Let's understand this concept a bit more clearly.

When we talk about self-demagnetization, we're referring to the loss of a magnet's magnetic properties over time. This can occur due to several factors like temperature changes, shocks, or even just the passage of time. However, one significant cause is the lack of a closed magnetic circuit, which happens when a magnet is not in use.

A keeper can be thought of as a simple piece of soft iron that is placed across the poles of a magnet. The purpose of this is to provide a straightforward path for the magnetic field lines. This path, or magnetic circuit, helps to prevent the loss of magnetism, or self-demagnetization, as the magnetic field lines are less likely to stray into the air where they would lose energy.

Option B is incorrect because the keeper doesn't cancel the effect of Earth's magnetic field. While the Earth's magnetic field might have a slight effect on a magnet over a very long period, it's not the primary reason for using a keeper.

Option C is also not correct. The keeper is made of iron and isn't particularly effective at protecting the magnet from stray electric fields. Its purpose is more about creating a closed magnetic circuit.

Finally, Option D is not accurate because the keeper does not increase the strength of the magnet. It merely helps to maintain the existing strength by preventing self-demagnetization.

To calculate the force on a charge moving in a magnetic field, we can use the formula:

\(F = qvB \sin \theta\)

Where \(F\) is the force, \(q\) is the charge, \(v\) is the velocity of the charge, \(B\) is the magnetic flux density, and \(\theta\) is the angle between the velocity and the magnetic field. In this case, the charge is moving at a right angle to the magnetic field, so \(\theta = 90^{\circ}\), and \(\sin \theta = 1\). Thus, the formula becomes:

\(F = qvB\)

Now, we can plug in the given values:

\(q = 10\mu \mathrm{C} = 10 \times 10^{-6} \mathrm{C}\)

\(v = 1.0 \times 10^5 \mathrm{ms^{-1}}\)

\(B = 5.0 \times 10^{-4} \mathrm{T}\)

Substituting these into the formula, we get:

\(F = (10 \times 10^{-6} \mathrm{C}) \times (1.0 \times 10^5 \mathrm{ms^{-1}}) \times (5.0 \times 10^{-4} \mathrm{T})\)

Multiplying the values, we obtain:

\(F = 5.0 \times 10^{-4} \mathrm{N}\)

Thus, the force on the charge is \(5.0 \times 10^{-4} \mathrm{N}\), which corresponds to Option B. The other options are incorrect because they do not result from the proper application of the formula and given values.

Option A is not correct because steel does not generally become more magnetized than soft iron. In fact, it is soft iron that can be more readily magnetized and demagnetized than steel. Soft iron has a high permeability and low coercivity, which means it can be easily magnetized and demagnetized under external magnetic fields.

Option B is correct because permanent magnets are usually made of steel or other similar materials with high coercivity, meaning they can maintain their magnetism over time. Option C is correct because soft iron is more readily magnetized due to its high permeability and ability to align its domains to an external magnetic field with ease. Option D is also correct because soft iron loses its magnetism more quickly than steel, as it realigns its domains with little or no external magnetic field.

So, the statement that is not correct about steel and soft iron is option A: steel is more magnetized than soft iron.

The question is asking what determines the polarity (north and south ends) of an electromagnet. The correct answer is the direction of the current in the wire.

An electromagnet is a type of magnet where the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off. Electromagnets usually consist of a large number of closely spaced turns of wire that create the magnetic field.

The polarity of an electromagnet is determined by the direction of the current flowing through the wire. This is due to the Right Hand Rule, which states that if you point your thumb in the direction of the conventional current (from the positive to the negative terminal), your fingers will curl in the direction of the magnetic field. Therefore, changing the direction of the current will also change the polarity of the electromagnet.

The magnitude of the current, the material of the core of the magnet, or the material of the coil do not influence the polarity of the electromagnet. They can affect the strength of the magnetic field, but not its direction.

A beta particle forms when a neutron changes into a proton and a high-energy electron. The proton stays in the nucleus but the electron leaves the atom as a beta particle. When a nucleus emits a beta particle, these changes happen:

• the mass number stays the same
• the atomic number increases by 1
• the nuclear charge increases by 1

NB: Usually gold is created from platinum, which has one less proton than gold, or from mercury, which has one more proton than gold. Bombarding a platinum or mercury nucleus with neutrons can knock off an neutron or add on a neutron, which through natural radioactive decay can lead to gold.

The process in which two light atomic nuclei combine to form a heavier nucleus, releasing energy in the process, is known as nuclear fusion.

In nuclear fusion, the total mass of the resulting nucleus is less than the sum of the masses of the original nuclei. This difference in mass is converted into energy, according to Einstein's famous equation, \(E=mc^2\), where \(E\) is energy, \(m\) is mass, and \(c\) is the speed of light. This equation shows that even a small amount of mass can be converted into a large amount of energy.

Nuclear fusion is the process that powers stars, including our sun. In the sun, for example, hydrogen nuclei fuse to form helium, releasing a tremendous amount of energy in the process.

To contrast, nuclear fission is a process in which a heavy nucleus splits into two or more smaller nuclei, also releasing energy. Radioactivity is a phenomenon where unstable atomic nuclei lose energy by emitting radiation. A chain reaction refers to a sequence of reactions where a reactive product or by-product causes additional reactions to take place.

A p-type semiconductor is a type of semiconductor material that has been doped with impurities that introduce an excess of 'holes.' Holes are locations in the material where an electron is missing, and they can be thought of as positive charge carriers. The process of adding impurities to a semiconductor is called doping, and the impurities themselves are called dopants.

In a p-type semiconductor, the correct answer is that holes are the majority charge carriers (Option D). This is because the dopants added to the material have one less electron in their outer shell compared to the semiconductor atoms. As a result, they create an excess of holes in the material, making holes the majority charge carriers.

It's important to note that the other options are not accurate for a p-type semiconductor. In Option A, the number of holes is not equal to the number of electrons; there are more holes than electrons. In Option B, the electrical resistivity does not necessarily increase; doping can reduce the overall resistivity of the material because it increases the number of available charge carriers. And in Option C, electrons are not the majority charge carriers in a p-type semiconductor; they are the majority charge carriers in an n-type semiconductor, which is doped with different impurities.

To summarize, in a p-type semiconductor, holes are the majority charge carriers due to the presence of dopants that introduce an excess of holes in the material.

The Heisenberg uncertainty principle is a key principle in quantum mechanics which states that it is impossible to simultaneously measure the exact position and momentum of a particle. In other words, the more precisely one property is measured, the less precisely the other can be controlled, determined, or known.

Mathematically, the Heisenberg uncertainty principle is often stated as follows:

\[\Delta x . \Delta p \geq \frac{h}{4\pi}\]

where \(\Delta x\) is the uncertainty in the position measurement, \(\Delta p\) is the uncertainty in the momentum measurement, and \(h\) is the Planck's constant.

Looking at the options provided in the question:

Option A: \(\Delta x . \Delta p = h\) is incorrect because the principle states that the product of the uncertainties is greater than or equal to \(\frac{h}{4\pi}\), not equal to \(h\).

Option B: \(\Delta x . \Delta p \leq h\) is also incorrect. The uncertainty principle does not set an upper limit for the product of the uncertainties.

Option C: \(\Delta x . \Delta p \geq h\) is marked as correct, but it's not entirely accurate. The product of the uncertainties is greater than or equal to \(\frac{h}{4\pi}\), not \(h\).

Option D: \(\Delta x . \Delta p > h\) is incorrect. The uncertainty principle states that the product of the uncertainties can be equal to \(\frac{h}{4\pi}\), so the product is not strictly greater than \(h\).

In conclusion, none of the options exactly match the correct uncertainty principle equation. However, option C is the closest to the correct answer, despite being slightly inaccurate. For more accurate understanding of the Heisenberg Uncertainty Principle, please refer to your quantum mechanics section in your physics textbook.

F = \(\frac{Gm_1m_2}{r^2} = \frac{6.6 \times 10^{-11} \times 80 \times 50}{0.2 \times 0.2}\)
= 6.6 × 10^-6N