21.1 MAGNETIC FIELD
21.1.1 Concept of Magnetic Field
1 Meaning
A region in which a magnet placed in it would experience a force is known as a magnetic field.
2 Sources
The following can produce magnetic field:
(a) A permanent magnet.
(b) Electric charges moving in space or in an electric conductor.
This means that a current flowing in a wire can produce a magnetic field in the space surrounding the wire. This phenomenon of a magnetic field being produced by an electric current is known as the magnetic effect of current.
Demonstration of magnetic effect of current:
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| Figure 21.1 A straight copper wire is placed horizontally above a compass and aligned parallel to the needle to observe the magnetic effect produced when current flows through the wire. |
(i) A small compass is placed on a table and a straight copper wire is placed horizontally and directly above the compass, as shown in Figure 21.1. The wire is aligned until it is parallel to the magnetised compass needle.
(ii) A current is allowed to flow through the wire. It is observed that at the moment when current flows, the compass needle begins to deflect through an angle. Finally it is at a rest at a new position.
(iii) When current stops flowing the compass needle returns to the initial north-south position.
(iv) Deduction:
When current starts to flow in the wire, the compass needle deflects, indicating that the compass is in the presence of a magnetic field. This field must have been produced by the current.
21.1.2 Magnetic Field Strength B
1. Magnetic field strength
Magnetic field has ‘strength’ at any point in the field. The strength is known as the magnetic induction, magnetic flux density or magnetic field strength (Note: a definition will be given later).
2. Quantity
Magnetic field strength is a vector quantity. It is represented by the symbol B.
3. Units of B
B has units of weber per m2 (Wb m−2) or tesla (T).
21.1.3 Pictorial Representation of Magnetic Field
1. An example
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| Figure 21.2 Magnetic field lines illustrating the pattern of the magnetic field surrounding a bar magnet, showing the direction and shape of the field outside the magnet. |
A magnet field can be represented by a set of lines or curves, which are referred to as the magnetic field lines. Figure 21.2 shows how the pattern of the magnetic field outside a bar magnet can be represented by these lines.
2. Some features of the magnetic field lines
The magnetic field lines have the following features:
(a) They never cross each other.
(b) They form closed loops.
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Figure 21.3 Magnetic field lines with arrowheads indicating the direction of the magnetic field B at each point, where the field vector is tangent to the lines. |
(c) An arrow head is attached to each line, as shown in Figure 21.3. The direction of the arrow head indicates the direction of the magnetic field strength, B, at a point on the field line (not indicating the direction of magnetic force). The vector representing B at the point must be drawn as a tangent to the line.
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| Figure 21.4 Magnetic field lines showing that regions with closely spaced lines have stronger magnetic fields, while widely spaced lines indicate weaker fields. |
(d) The closeness of the lines indicates the strength of the field. The strength of the magnetic field is high in a region where the number of lines crossing unit perpendicular area is large. The strength is low in another region where the number of lines crossing unit perpendicular area is low, as shown in Figure 21.4.
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| Figure 21.5 Straight, parallel, and equally spaced magnetic field lines representing a uniform magnetic field with constant magnitude throughout the region. |
If the field lines are straight, parallel to each other and equally spaced, as shown in Figure 21.5, the field represented by these lines is a uniform field. This means that the magnitude of the field strength is the same at any point in the field.
3. Uniform magnetic field pointing into or out of page
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| Figure 21.6 Magnetic fields perpendicular to the page are represented by symbols: dots indicate a field coming out of the page, while crosses indicate a field going into the page. |
Quite often we find that it is convenient to consider a uniform magnetic field which points perpendicularly either out of a horizontal page or into the page. The field that
(a) points perpendicularly out of the page is represented by a set of dots (Figure 21.6(a))
(b) points perpendicularly into the page is represented by a set of crosses (Figure 21.6(b)).
Applications of Magnetic Field
Magnetic fields have many important applications in daily life and modern technology. The relationship between electricity and magnetism enables the development of various useful devices and systems.
- Electric Motors
Magnetic fields are used to convert electrical energy into mechanical energy. When current flows through a coil in a magnetic field, a force is produced that causes rotation. - Electric Generators
Generators use magnetic fields to convert mechanical energy into electrical energy through electromagnetic induction. - Transformers
Magnetic fields are used to transfer electrical energy between coils, allowing voltage to be increased or decreased efficiently. - Magnetic Storage Devices
Devices such as hard disks store data using magnetic fields to represent information. - Compass Navigation
A compass uses Earth’s magnetic field to determine direction, helping in navigation. - Electromagnets
Magnetic fields produced by electric current are used in cranes to lift heavy metal objects and in devices like relays and doorbells. - Medical Applications (MRI)
Magnetic Resonance Imaging (MRI) uses strong magnetic fields to produce detailed images of the human body. - Particle Accelerators
Magnetic fields are used to control and guide charged particles in research and experiments.
In summary, magnetic fields play a crucial role in various technologies, making them essential in science, engineering, and everyday applications.
Conclusion – Magnetic Field
A magnetic field is a region in space where a magnet or a moving electric charge experiences a force. Magnetic fields can be produced by permanent magnets as well as by moving electric charges, such as an electric current flowing through a wire. This demonstrates the magnetic effect of current, where electricity produces magnetism.
The strength of a magnetic field is described by a vector quantity known as magnetic field strength (B), which has both magnitude and direction. Its SI unit is the tesla (T) or weber per square meter (Wb m−2).
Magnetic fields can be represented using magnetic field lines, which help visualize the field pattern. These lines have several important properties:
- They never intersect and always form closed loops.
- Arrowheads on the lines indicate the direction of the magnetic field (B).
- The closeness of the lines represents the strength of the field (closer lines indicate a stronger field).
- Straight, parallel, and equally spaced lines represent a uniform magnetic field.
Magnetic fields perpendicular to a surface are represented symbolically:
- Dots (•) indicate a field coming out of the page.
- Crosses (×) indicate a field going into the page.
In conclusion, magnetic fields illustrate the close relationship between electricity and magnetism, forming the foundation for many applications such as electric motors, generators, and other electromagnetic devices.
Frequently Asked Questions (FAQ) – Magnetic Field
1. What is a magnetic field?
A magnetic field is a region in space where a magnet or a moving electric charge experiences a force.
2. What are the sources of a magnetic field?
Magnetic fields can be produced by permanent magnets and by moving electric charges, such as electric currents flowing through a conductor.
3. What is meant by the magnetic effect of current?
It is the phenomenon where an electric current flowing through a conductor produces a magnetic field around it.
4. What is magnetic field strength (B)?
Magnetic field strength, denoted by B, is a vector quantity that represents the strength and direction of a magnetic field.
5. What are the units of magnetic field strength?
The SI unit of magnetic field strength is tesla (T) or weber per square meter (Wb m−2).
6. What are magnetic field lines?
Magnetic field lines are imaginary lines used to represent the direction and pattern of a magnetic field.
7. What are the characteristics of magnetic field lines?
- They never intersect.
- They form closed loops.
- They have direction indicated by arrowheads.
- Their spacing shows the strength of the field.
8. What does the spacing of magnetic field lines indicate?
Closely spaced lines indicate a strong magnetic field, while widely spaced lines indicate a weak magnetic field.
9. What is a uniform magnetic field?
A uniform magnetic field is a field where the strength and direction are constant throughout the region, represented by straight, parallel, and equally spaced lines.
10. How are magnetic fields into and out of the page represented?
- Dots (•) represent a magnetic field coming out of the page.
- Crosses (×) represent a magnetic field going into the page.
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