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Electricity and Magnetism - Magnetic Field

Grade 7ICSE

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

🔑Concepts

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A magnetic field is the region around a magnet where its magnetic force (attraction or repulsion) can be experienced by other magnetic materials or magnets.

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Magnetic field lines are continuous closed loops that originate from the North Pole (NN) and end at the South Pole (SS) outside the magnet, and move from SS to NN inside the magnet.

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The density of magnetic field lines indicates the strength of the field; lines are closest at the poles where the magnetic field strength BB is maximum.

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Two magnetic field lines never intersect each other because at the point of intersection, the magnetic field would have two different directions, which is impossible.

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The Earth behaves like a giant bar magnet with its magnetic South Pole located near the geographic North Pole and its magnetic North Pole near the geographic South Pole.

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Electromagnetism is the production of a magnetic field by passing an electric current II through a conductor. This was first discovered by Hans Christian Oersted.

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The strength of an electromagnet depends on the magnitude of the current II, the number of turns nn in the coil, and the nature of the core material (e.g., soft iron).

📐Formulae

B∝I (Magnetic field strength is directly proportional to current)B \propto I \text{ (Magnetic field strength is directly proportional to current)}

B∝n (Magnetic field strength is directly proportional to the number of turns per unit length)B \propto n \text{ (Magnetic field strength is directly proportional to the number of turns per unit length)}

Total Strength≈k⋅n⋅I\text{Total Strength} \approx k \cdot n \cdot I

💡Examples

Problem 1:

An electromagnet has n=50n = 50 turns and a current of I=2 AI = 2\text{ A} flowing through it. If the current is increased to 4 A4\text{ A} while keeping the turns constant, how does the magnetic field strength change?

Solution:

Since B∝IB \propto I, if the current is doubled from 2 A2\text{ A} to 4 A4\text{ A}, the magnetic field strength BB will also double.

Explanation:

In an electromagnet, the magnetic field intensity is directly proportional to the current. Mathematically, B1B2=I1I2\frac{B_1}{B_2} = \frac{I_1}{I_2}. Thus, doubling the current results in doubling the magnetic field.

Problem 2:

Why does a magnetic compass needle get deflected when placed near a wire carrying current II?

Solution:

The current II flowing through the wire creates a magnetic field around it. This field exerts a force on the magnetic compass needle, causing it to deflect.

Explanation:

This phenomenon is known as the magnetic effect of electric current. The compass needle is a small magnet, and it aligns itself with the resultant magnetic field produced by the wire and the Earth.