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Circuits and Magnetism

163 questions

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Question 161Question →

Two identical bar magnets are placed end-to-end on a workbench with their north poles facing each other across a narrow gap. Which of the following best describes the behavior of the magnetic lines of force in the space between these two poles?

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Answer: The flux lines curve away from each other without crossing, exerting lateral pressure that pushes the two poles apart.

Answer

The flux lines curve away from each other without crossing, exerting lateral pressure that pushes the two poles apart.
Magnetic lines of force (flux lines) have two fundamental physical rules: they emerge outward from north poles into external space, and they can never intersect or cross. When two north poles face each other, the emerging flux lines are forced to deflect and curve away from each other into surrounding space. This lateral repulsion between parallel lines of force creates the physical repulsive force pushing the two like poles apart.

Step-by-Step Solution

1
Identify the polarity and fundamental rule of magnetic interaction.
Both facing ends are north poles, meaning they are like magnetic poles that inherently repel each other.
The fundamental law of magnetism states that like poles repel and opposite poles attract.
2
Determine the direction and properties of magnetic flux lines outside the magnets.
Magnetic flux lines emerge from north poles and seek south poles; flux lines never cross or intersect.
A magnetic field at any given point has a unique direction, making intersecting lines physically impossible.
3
Analyze the field line geometry in the gap between the two facing north poles.
The emerging lines of force from both north poles bend outward into surrounding space, exerting lateral repulsive pressure on one another.
Because lines of force running in the same direction repel each other and cannot enter a like pole, they diverge outward and create magnetic repulsion.

Key Concept

Magnetic Field Line Properties and Like-Pole Repulsion
Estimated Time:1m 0s
Question 162Question →

Soft iron is commonly used for the cores of temporary electromagnets because it combines high magnetic permeability with high retentivity.

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Answer: False

Answer

False. Soft iron has high permeability but low retentivity.
The statement is false because soft iron has high permeability but low retentivity. High permeability allows soft iron to readily channel magnetic flux when current flows through the coil, while low retentivity ensures that the core demagnetizes rapidly as soon as current flow stops.

Step-by-Step Solution

1
Identify the operational requirements of a temporary electromagnet core.
A temporary electromagnet core must concentrate magnetic flux easily when energized and release its magnetism immediately when power is cut.
Devices like relays and solenoids must switch on and off rapidly without residual magnetism holding the mechanism closed.
2
Analyze the magnetic properties of soft iron.
Soft iron possesses high permeability (high ease of magnetization) and low retentivity (minimal residual magnetism).
Retentivity measures the ability of a material to retain magnetism after the magnetizing force is removed; high retentivity is a property of permanent magnets (such as hard steel), not temporary cores.
3
Evaluate the truth value of the statement.
The statement incorrectly claims soft iron has high retentivity, making the overall statement false.
Because soft iron has low retentivity, the claim is inaccurate.

Key Concept

Magnetic permeability and retentivity in ferromagnetic materials
Question 163Question →

A technician places a solid block of pure copper into a uniform magnetic field between two magnetic poles. How do the magnetic lines of flux interact with the copper block?

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Answer: They pass straight through the block with virtually no distortion because copper has a relative permeability nearly equal to that of air.

Answer

They pass straight through the block with virtually no distortion because copper has a relative permeability nearly equal to that of air.
Magnetic flux lines pass directly through non-magnetic substances like copper with virtually no distortion because copper has a relative permeability nearly identical to that of air or a vacuum (μr≈1\mu_r \approx 1). Unlike ferromagnetic materials such as soft iron, copper does not offer a lower-reluctance path to concentrate magnetic field lines, nor does it act as a barrier to steady magnetic flux.

Step-by-Step Solution

1
Identify the magnetic classification and permeability of copper.
Copper is a non-magnetic (diamagnetic) substance with a relative magnetic permeability (μr\mu_r) approximately equal to 1.01.0.
The degree to which a material alters or concentrates an external magnetic field is determined by its magnetic permeability relative to free space.
2
Determine how magnetic flux lines behave in a material with a relative permeability of 1.
The lines of flux maintain their original spacing and direction, passing directly through the copper block without concentration or obstruction.
Only ferromagnetic materials (such as iron, nickel, and cobalt) provide a low-reluctance path that bends and concentrates flux lines. Non-magnetic materials offer the same reluctance as air.

Key Concept

Magnetic Permeability and Flux Behavior in Non-Magnetic Materials
Estimated Time:1m 0s
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