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Bar magnet is an object that produce magnetic fields around itself. It is rectangular in shape and made up of iron, steel or any ferromagnetic substance that have permanent magnetic properties. In a bar magnet, there are two poles – north pole and south pole. If a magnet is suspended freely, the north pole of the magnet will align itself towards the magnetic north pole of the earth.
The like poles of two magnets repel each other, while the unlike poles attract. This means that the north pole of a bar magnet will get attracted towards the south pole of another magnet or vice-versa.
Check Also: Magnetism and Matter
| Table of Content |
Key Terms: Bar Magnet, Ferromagnetic Substances, Magnetic Fields, Magnetic Poles, Alnico Bar Magnet, Neodymium Bar Magnet, Electromagnet
What is a Magnet?
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A magnet is a kind of material that has permanent magnetic properties. Due to its magnetic nature, it attracts substances made up of iron like iron scraps, nails etc. Depending upon their magnetic abilities, magnets are classified into two groups – natural and artificial.
Classification of Magnets
Magnets are divided into two broad categories:
- Natural Magnets – The magnets that are found in nature are termed as natural magnets. These magnets have weak magnetic fields. For example, Lodestones.
- Artificial Magnets – They include man-made magnets. These magnets possess stronger magnetic fields than natural magnets.
Read more :
| Read More about Magnets | ||
|---|---|---|
| Magnetic Poles | Magnetic Declination | Magnetometer |
| Permanent Magnets and Electromagnets | Derivation of Biot Savart Law | Unit of Magnetic Field |
Types of Bar Magnet
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Bar magnets fall under the category of artificial magnets. The artificial magnets can be made in different shapes. Depending upon the shape of the magnet, bar magnets are of two types:
- Cylindrical Bar Magnet: This type of magnet is mostly used in educational institutions and research centers. A cylindrical bar magnet is also known as rod magnet. It has a thickness larger than the diameter of the magnet providing it high magnetism.
- Rectangular Bar Magnet: The bar magnets having rectangular shape are called rectangular bar magnets. They are used mainly in the engineering and manufacturing industries due to their high magnetic field strength.
Alnico and Neodymium Bar MagnetAlnico and Neodymium bar magnets are two very strong bar magnets. However, both the magnets are brittle in nature. Alnico Bar Magnet: This type of bar magnet releases strong magnetic fields and retain its magnetic power even under extreme heat. The primary components of Alnico bar magnet are iron, cobalt, nickel and aluminium. Neodymium Bar Magnet: This type of bar magnet is made up of a mixture of boron, iron, and neodymium. |
Properties of Bar Magnet
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The properties of a bar magnet are as follows:
- A bar magnet has two poles same as other magnets, North Pole and the South Pole. If you break a magnet from the middle, the broken parts will still have the north and south pole.
- The magnetic force of these magnets is strongest at the poles and weak in the middle of the magnet.
- If two bar magnets are placed facing N-S or S-N, they will attract each other. If they are placed N-N or S-S, they will repel each other.
- Ferromagnetic particles like iron, nickel, and cobalt gets easily attracted by the bar magnet.
- If the bar magnets are hung in the air freely, they will not come to rest until the poles are aligned in a North-South position.
Pole StrengthThe strength of a magnetic north or south pole to attract magnetic substances towards itself is measured as pole strength. It is a scalar quantity and is denoted by ‘P’. P = W / I where;
The SI unit of pole strength is N.Tesla-1 and its dimensional formula is [LA]. |
Uses of Bar Magnet
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A bar magnet can be used for various purposes including:
- They are used in laboratories for conducting magnetic experiments.
- They are also employed in medical procedures.
- Bag magnets are used in electronic devices like radio, television, telephones, etc.
- Some industries use bag magnets to retain the power of other magnets and to collect loose metals.
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Magnetic Field Lines around Bar Magnet
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Every magnetic substance produces magnetic fields around itself. The imaginary lines that can be drawn along these magnetic fields are called magnetic field lines. The magnetic field lines around a bar magnet shows the following properties:
- These lines around the magnet forms closed continuous loops.
- The direction of the net magnetic field (B) at a point can be shown with the help of a tangent to the magnetic field line at that point.
- These lines never intersect each other.
- The magnitude of the magnetic field is directly proportional to the number of magnetic field lines crossing per unit area.
Difference Between Electromagnet and Bar Magnet
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The key differences between electromagnet and bar magnet are tabulated below:
| Electromagnet | Bar Magnet |
|---|---|
| Electromagnets are coils of wire surrounded by ferromagnetic core. | Bar magnets are blocks of ferromagnetic substances. |
| They generate the magnetic fields with the help of electric current from external sources. | They generate their own magnetic fields. |
| They do not have a constant magnetic pull. | They have a constant magnetic pull as they are permanent magnets. |
| The magnetic force of the electromagnets changes if the electric current flowing through it is changed. | The magnetic force of the bar magnet depends upon the material of the magnet and remains constant. |
Things to Remember
- Bar magnets are the magnetic substances that produce magnetic field lines and attract other magnetic substances.
- There are two poles in a magnet – north and south pole.
- The like poles of the magnets repels each other whereas unlike poles attract.
- The imaginary lines along the magnetic fields around a bar magnet are called magnetic field lines.
- The bar magnets can be used for various purposes like in laboratories, medical equipments, electronic devices etc.
- Electromagnets are the magnets formed by coils of wire carrying current.
Sample Questions
Ques. Select the incorrect statement from the following: (1 Mark)
a) Magnetic fields intersect each other
b) Magnetic lines are closed curves
c) If the magnetic field lines are parallel in a region of space then it means that the region has homogeneous field strength.
d) None of the above
Ans. (A) Magnetic fields intersect each other.
The magnetic field lines never intersect each other. They form continous closed loops around the magnet.
Ques. How can you make a magnet lose its magnetic property? (1 Mark)
Ans. A magnet can be made to lose its magnetic property by passing electric current through it.
Ques. What are magnetic field lines? (1 Mark)
Ans. The magnetic field lines are the imaginary lines that can be drawn along the magnetic field produced around a magnet.
Ques. In what direction does the magnetic field lines move inside the magnet? (1 Mark)
Ans. The magnetic field lines inside a magnet from north to south pole.
Ques. How is a compass used to find direction? (2 Marks)
Ans. A magnetic compass tells the direction with the help of the magnetic needle present in it. The magnetic needle can rotate freely inside the compass. When the compass is kept at a place, the needle in the compass aligns itself in the north-south direction. The red arrow of the needle represent north pole and the other end indicates south pole.
Ques. A magnetic dipole of length 10 cm has pole strength of 20 Am. Find the magnetic moment of the dipole. (2 Marks)
Ans. We know, Magnetic Dipole Moment = Pole Strength × Magnetic Length
Magnetic Dipole Moment = 20 × 0.1
Magnetic Dipole Moment = 2 Am2
Ques. If there are no indication of north and south pole of a magnet, how can the north pole of the magnet be determined? (2 Marks)
Ans. The north pole of the magnet can be determined by suspending it freely. A freely suspended bar magnet aligns itself along with the magnetic north and south poles of the earth. The north pole of the magnet will be in the direction of the north pole of the earth.
Ques. List two important properties of a bar magnet. (2 Marks)
Ans. The two main properties of a bar magnet are:
- A bar magnet has two poles (north and south).
- The like poles of the magnets attracts while the unlike poles repels.
Ques. A magnetized needle of magnetic moment 20 JT-1 is placed at 60º with the direction of the uniform magnetic field of magnitude 9T. What is the torque acting on the needle? (3 Marks)
Ans. Torque (\(\tau\)) = mBsin\(\theta\)
m = 20 J/T
B = 9T
\(\theta\) = 60º
\(\tau\) = 20 × 9 × sin60º
\(\tau\) = 155.8 J
Ques. A bar magnet of the magnetic moment 5 Am2 has poles 20 cm apart. Calculate the pole strength. (3 Marks)
Ans. Magnetic dipole moment = Pole strength × magnetic length
Pole strength = Magnetic dipole moment / magnetic length
Pole strength = 5 / 0.2
Pole strength = 25 Am
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