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Physics - Magnetism and Electromagnetism

Grade 7Cambridge (IGCSE)

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

🔑Concepts

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Magnetic Poles: All magnets have two poles, North (NN) and South (SS). Like poles repel, and opposite poles attract.

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Magnetic Materials: Materials that can be magnetized or are attracted to magnets include Iron, Steel, Cobalt, and Nickel. These are often referred to as ferromagnetic materials.

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Magnetic Field: The region around a magnet where magnetic forces can be detected. Field lines move from the North pole to the South pole (N→SN \rightarrow S).

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Induced Magnetism: When a piece of unmagnetized magnetic material (like an iron nail) is placed in a magnetic field, it becomes a temporary magnet itself.

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Electromagnets: A temporary magnet produced by passing an electric current (II) through a coil of wire, known as a solenoid.

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Hard and Soft Magnets: Soft magnetic materials (like Iron) lose magnetism easily, while hard magnetic materials (like Steel) retain magnetism to become permanent magnets.

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Strength of an Electromagnet: The magnetic field strength can be increased by: 1. Increasing the current (II), 2. Increasing the number of turns in the coil (NN), or 3. Inserting a soft iron core.

📐Formulae

V=I×RV = I \times R

Strength∝I×N\text{Strength} \propto I \times N

F∝1d2F \propto \frac{1}{d^2}

💡Examples

Problem 1:

An electromagnet is created using a 9V9\text{V} battery and a coil of wire with 5050 turns. If a student replaces the battery with a 18V18\text{V} source (assuming resistance stays constant), how does the magnetic field change?

Solution:

The magnetic field strength will double.

Explanation:

According to Ohm's Law, V=I×RV = I \times R. If the voltage (VV) doubles from 9V9\text{V} to 18V18\text{V}, the current (II) also doubles. Since the magnetic field strength is directly proportional to the current (II), doubling the current doubles the strength of the electromagnet.

Problem 2:

A student has an iron nail wrapped with 2020 turns of copper wire. They want to make the electromagnet stronger without changing the battery. What should they do?

Solution:

Increase the number of turns of wire (NN) from 2020 to a higher number, such as 4040 or 6060.

Explanation:

The strength of the magnetic field in a solenoid is proportional to the number of turns (NN). By adding more loops of wire around the nail, the magnetic fields of each loop add together, resulting in a stronger overall magnetic field.

Problem 3:

Draw the direction of the magnetic field lines for a bar magnet.

Solution:

Lines emerge from the NN pole and enter the SS pole.

Explanation:

By scientific convention, magnetic field lines represent the direction a North pole of a compass would point. Therefore, the lines always flow from N→SN \rightarrow S outside the magnet.