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Electricity and Magnetism - Law of Magnetism

Grade 7ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A magnet is a material that produces a magnetic field and attracts ferromagnetic materials like Iron (FeFe), Nickel (NiNi), and Cobalt (CoCo).

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Every magnet has two poles: the North Pole (NN) and the South Pole (SS). Magnetic strength is concentrated at these poles.

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The Law of Magnetism states: Like poles repel each other (e.g., NN repels NN, SS repels SS) and unlike poles attract each other (e.g., NN attracts SS).

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Directive Property: A freely suspended magnet always comes to rest in the North-South direction. The end pointing towards the Earth's geographic North is the NN-pole.

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Magnetic poles always exist in pairs. If a magnet is broken into two pieces, each piece will develop its own NN and SS poles; isolated monopoles do not exist.

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Magnetic Induction: A piece of non-magnetized magnetic material (like soft iron) acquires magnetic properties when placed near a strong magnet.

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Repulsion is the only sure test of magnetism, as a magnet can attract both a piece of iron and the opposite pole of another magnet, but it can only repel a like pole.

📐Formulae

Magnetic Force (F)∝m1⋅m2d2\text{Magnetic Force (F)} \propto \frac{m_1 \cdot m_2}{d^2}

Total Length≈Magnetic Length×1.2\text{Total Length} \approx \text{Magnetic Length} \times 1.2

Magnetic Dipole Moment (M)=m×2l\text{Magnetic Dipole Moment (M)} = m \times 2l

💡Examples

Problem 1:

A bar magnet is broken into three equal pieces. How many North poles and South poles will be present in total?

Solution:

There will be 33 North poles and 33 South poles.

Explanation:

According to the property of magnetic dipoles, poles cannot be isolated. Each broken piece acts as an individual magnet possessing both a North (NN) and a South (SS) pole. Therefore, 3 pieces×2 poles/piece=6 poles3 \text{ pieces} \times 2 \text{ poles/piece} = 6 \text{ poles} (3 North and 3 South).

Problem 2:

You are given two identical-looking metal bars, AA and BB. One is a magnet and the other is a simple iron bar. How can you identify the magnet using only these two bars?

Solution:

Bring the end of bar AA to the middle of bar BB. If there is strong attraction, BB is the magnet. If there is no or very weak attraction, AA is the magnet.

Explanation:

The magnetic strength is maximum at the poles (NN and SS) and minimum (zero) at the center or the 'neutral region'. If AA is a magnet and its pole is touched to the center of iron bar BB, it will attract. However, if iron bar AA is touched to the center of magnet BB, there will be no attraction because the center of a magnet has no magnetic force.

Problem 3:

Two magnetic poles of strengths m1m_1 and m2m_2 are placed at a distance dd. If the distance is doubled (2d2d), what happens to the force FF?

Solution:

The force becomes 14\frac{1}{4} of the original force (F′=F4F' = \frac{F}{4}).

Explanation:

Using the inverse square law F∝1d2F \propto \frac{1}{d^2}, if the new distance d′=2dd' = 2d, then F′∝1(2d)2=14d2F' \propto \frac{1}{(2d)^2} = \frac{1}{4d^2}. Thus, the force is reduced by a factor of four.