Magnetism and Magnetic Poles

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Questions and Answers

What is the significance of the pattern of iron filings around a bar magnet and a current-carrying solenoid?

The pattern of iron filings suggests that the magnet has two poles similar to the positive and negative charge of an electric dipole, and permits us to plot the magnetic field lines.

What is the direction of the net magnetic field B at a given point on a magnetic field line?

The tangent to the field line at that point.

What can be inferred about the strength of the magnetic field B from the number of field lines crossing per unit area?

The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B.

Why do magnetic field lines not intersect?

Because if they did, the direction of the magnetic field would not be unique at the point of intersection. Signup and view all the answers

How can one plot the magnetic field lines in a variety of ways?

One way is to place a small magnetic compass needle at various positions and note its orientation. Signup and view all the answers

What is the similarity between the poles of a magnet and an electric dipole?

One pole is designated the North pole and the other, the South pole, similar to the positive and negative charge of an electric dipole. Signup and view all the answers

What is the shape of the magnetic field lines of a magnet or a solenoid?

Continuous closed loops. Signup and view all the answers

How do the magnetic field lines around a bar magnet and a current-carrying solenoid compare to electric field lines of an electric dipole?

Magnetic field lines form continuous closed loops, unlike electric field lines, which begin from a positive charge and end on the negative charge or escape to infinity. Signup and view all the answers

How can a bar magnet be thought of in comparison to a solenoid?

A bar magnet can be thought of as a large number of circulating currents, similar to those in a solenoid. Signup and view all the answers

What happens when a bar magnet is cut in half?

When a bar magnet is cut in half, it results in two smaller magnets with weaker magnetic properties. Signup and view all the answers

What is the significance of the magnetic field lines around a solenoid and a bar magnet?

The magnetic field lines are continuous, resembling each other, indicating that they produce similar magnetic fields. Signup and view all the answers

How does the axial field of a finite solenoid compare to that of a bar magnet?

At large distances, the axial field of a finite solenoid resembles that of a bar magnet. Signup and view all the answers

What is the formula for the torque on a compass needle in a magnetic field?

The torque τ on a compass needle is given by τ = m × B, with its magnitude equal to τ = mB sinθ. Signup and view all the answers

What is the expression for the magnetic potential energy U_m of a magnetic needle?

The magnetic potential energy is expressed as U_m = -mB cosθ. Signup and view all the answers

Why can the zero of potential energy be fixed at one's convenience?

The zero of potential energy can be fixed conveniently to simplify calculations, like setting it at θ = 90°. Signup and view all the answers

In what way does a small compass needle demonstrate the properties of magnetic fields?

When allowed to oscillate in a magnetic field, the compass needle exhibits behavior that reveals the strength and direction of the field. Signup and view all the answers

Study Notes

Magnetic Fields and Iron Filings

Iron filings sprinkled on a sheet of glass over a short bar magnet show a pattern suggesting the magnet has two poles, similar to an electric dipole.
One pole is designated the North pole and the other, the South pole, which point towards the geographic north and south poles when suspended freely.

Magnetic Field Lines

The pattern of iron filings permits us to plot the magnetic field lines, which are a visual and intuitive realization of the magnetic field.
Properties of magnetic field lines:
- Form continuous closed loops
- The tangent to the field line at a given point represents the direction of the net magnetic field B at that point
- The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B
- Do not intersect, for if they did, the direction of the magnetic field would not be unique at the point of intersection

Magnetic Field Lines of a Bar Magnet and a Solenoid

The resemblance of magnetic field lines for a bar magnet and a solenoid suggests that a bar magnet may be thought of as a large number of circulating currents in analogy with a solenoid.
Cutting a bar magnet in half is like cutting a solenoid, resulting in two smaller solenoids with weaker magnetic properties.
The field lines remain continuous, emerging from one face of the solenoid and entering into the other face.

Analogy between a Bar Magnet and a Solenoid

Moving a small compass needle in the neighbourhood of a bar magnet and a current-carrying finite solenoid shows similar deflections of the needle in both cases.
The axial field of a finite solenoid resembles that of a bar magnet at large distances, making the analogy more firm.

Magnetic Moment of a Bar Magnet and a Solenoid

The magnetic moment of a bar magnet is equal to the magnetic moment of an equivalent solenoid that produces the same magnetic field.
The magnetic moment of a bar magnet can be demonstrated experimentally.

Torque and Magnetic Potential Energy

The torque on a small compass needle in a magnetic field is given by τ = m × B, and in magnitude, τ = mB sinθ.
The magnetic potential energy U_m is given by U_m = -m.B, where m is the magnetic moment of the needle and B is the magnetic field.

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Description

This quiz covers the basic principles of magnetism, including the arrangement of iron filings around a bar magnet and the concept of North and South poles.