Ampere'S Law
Ampere’s Law
Who Was AndréMarie Ampère?
AndréMarie Ampère was a French physicist and mathematician who made significant contributions to the field of electromagnetism. He is best known for his development of Ampère’s law, which describes the magnetic field created by an electric current.
Early Life and Education: AndréMarie Ampère was born on January 20, 1775, in Lyon, France. He showed an early aptitude for mathematics and physics, and by the age of 18, he had already mastered calculus and mechanics. Ampère’s father was a wealthy merchant, but he lost his fortune during the French Revolution, forcing Ampère to support himself through teaching and tutoring.
Contributions to Electromagnetism: Ampère’s most important contribution to science was his work on electromagnetism. In 1820, he published his groundbreaking memoir, “On the Mathematical Theory of Electrodynamic Phenomena,” in which he presented Ampère’s law. This law states that the magnetic field around a currentcarrying wire is proportional to the strength of the current and inversely proportional to the distance from the wire.
Ampère’s law was a major breakthrough in the understanding of electromagnetism, and it laid the foundation for much of the subsequent work in this field. It allowed scientists to calculate the magnetic fields produced by various current configurations and to design electromagnets, which are essential components of many electrical devices.
Other Contributions: In addition to his work on electromagnetism, Ampère also made contributions to other areas of physics and mathematics. He developed a theory of probability and statistics, and he also studied the elasticity of solids. Ampère was a prolific writer, and he published numerous papers and books on his research.
Recognition and Legacy: Ampère’s contributions to science were widely recognized during his lifetime. He was elected to the French Academy of Sciences in 1814, and he served as its president in 1836. Ampère also received numerous awards and honors, including the Copley Medal from the Royal Society of London in 1827.
AndréMarie Ampère died on June 10, 1836, in Marseille, France. He left behind a legacy of scientific achievement that continues to influence the fields of electromagnetism and physics to this day. In his honor, the unit of electric current, the ampere (A), is named after him.
What Is Ampere’s Law?
Ampere’s Law
Ampere’s law is a fundamental law of electromagnetism that relates the magnetic field around a currentcarrying wire to the electric current flowing through the wire. It was discovered by AndréMarie Ampère in 1820 and is one of the four Maxwell equations that form the foundation of classical electromagnetism.
Mathematical Formulation
Ampere’s law states that the magnetic field (B) around a currentcarrying wire is proportional to the current (I) flowing through the wire and inversely proportional to the distance (r) from the wire. Mathematically, it can be expressed as:
∮B⋅dl = μ₀I
where:
∮B⋅dl represents the line integral of the magnetic field around a closed loop μ₀ is the permeability of free space (4π × 10^7 H/m) I is the current flowing through the wire dl is a differential length vector along the closed loop
Explanation
Ampere’s law essentially states that a magnetic field is created whenever an electric current flows. The direction of the magnetic field is determined by the righthand rule. If you wrap your right hand around the wire with your thumb pointing in the direction of the current, your fingers will curl in the direction of the magnetic field lines.
The strength of the magnetic field is directly proportional to the current flowing through the wire. This means that the more current that flows, the stronger the magnetic field will be.
The magnetic field is also inversely proportional to the distance from the wire. This means that the closer you are to the wire, the stronger the magnetic field will be.
Examples
Here are a few examples of Ampere’s law in action:

Solenoid: A solenoid is a coil of wire that creates a magnetic field when an electric current flows through it. The magnetic field inside a solenoid is strong and uniform, and it can be used to create various electromagnetic devices such as motors, generators, and transformers.

Electromagnet: An electromagnet is a device that uses an electric current to create a magnetic field. Electromagnets are used in a wide variety of applications, including lifting heavy objects, separating metals, and generating electricity.

Magnetic Resonance Imaging (MRI): MRI is a medical imaging technique that uses magnetic fields and radio waves to create detailed images of the inside of the body. MRI scanners use powerful electromagnets to generate strong magnetic fields that align the protons in the body’s tissues. The radio waves then excite these protons, causing them to emit signals that are used to create images.
Ampere’s law is a fundamental law of electromagnetism that has numerous applications in various fields of science and technology. It provides a deep understanding of the relationship between electric currents and magnetic fields, enabling us to design and develop a wide range of electromagnetic devices and systems.
What Is Ampere’s Circuital Law?
Ampere’s circuital law is a law of electromagnetism that relates the magnetic field around a currentcarrying wire to the electric current flowing through the wire. It was discovered by AndréMarie Ampère in 1820.
The law states that the magnetic field around a currentcarrying wire is proportional to the current flowing through the wire and inversely proportional to the distance from the wire. The direction of the magnetic field is given by the righthand rule.
Ampere’s circuital law can be used to calculate the magnetic field around a variety of different currentcarrying conductors, such as straight wires, coils, and solenoids. It can also be used to calculate the force between two currentcarrying wires.
Here are some examples of how Ampere’s circuital law can be used:
 To calculate the magnetic field around a straight wire, we can use the following formula:
B = μ₀I/2πr
where:

B is the magnetic field strength in teslas (T)

μ₀ is the permeability of free space (4π × 10^7 T·m/A)

I is the current flowing through the wire in amperes (A)

r is the distance from the wire in meters (m)

To calculate the magnetic field around a coil of wire, we can use the following formula:
B = μ₀NI/2πr
where:

B is the magnetic field strength in teslas (T)

μ₀ is the permeability of free space (4π × 10^7 T·m/A)

N is the number of turns in the coil

I is the current flowing through the coil in amperes (A)

r is the radius of the coil in meters (m)

To calculate the force between two currentcarrying wires, we can use the following formula:
F = μ₀I₁I₂L/2πd
where:
 F is the force between the wires in newtons (N)
 μ₀ is the permeability of free space (4π × 10^7 T·m/A)
 I₁ and I₂ are the currents flowing through the wires in amperes (A)
 L is the length of the wires in meters (m)
 d is the distance between the wires in meters (m)
Ampere’s circuital law is a powerful tool that can be used to calculate the magnetic field around a variety of different currentcarrying conductors. It is also used to calculate the force between two currentcarrying wires.
Determining Magnetic Field by Ampere’s Law (Example)
Ampere’s Law
Ampere’s law is a fundamental law of electromagnetism that relates the magnetic field around a currentcarrying wire to the electric current flowing through the wire. It states that the magnetic field at a point is proportional to the current flowing through a loop of wire that encloses the point.
Example
To calculate the magnetic field at a point 1 cm from a long, straight wire carrying a current of 1 A, we can use Ampere’s law. We imagine a circular loop of wire with a radius of 1 cm centered on the wire. The current flowing through the loop is 1 A.
The magnetic field at the center of the loop is given by:
$$B = \frac{\mu_0 I}{2\pi r}$$
where:
 B is the magnetic field in teslas (T)
 μ0 is the permeability of free space (4π × 10^7 T·m/A)
 I is the current in amperes (A)
 r is the radius of the loop in meters (m)
Plugging in the values we know, we get:
$$B = \frac{(4\pi \times 10^{7} \text{ T}\cdot\text{m/A})(1 \text{ A})}{2\pi (0.01 \text{ m})}$$
$$B = 2 \times 10^{5} \text{ T}$$
So the magnetic field at a point 1 cm from a long, straight wire carrying a current of 1 A is 2 × 10^5 T.
Applications of Ampere’s Law
Ampere’s law is used in a variety of applications, including:
 Designing electromagnets
 Calculating the magnetic field around currentcarrying wires
 Determining the force between two currentcarrying wires
 Measuring the current flowing through a wire
Ampere’s law is a powerful tool for understanding and calculating magnetic fields. It is one of the fundamental laws of electromagnetism.
Applications of Ampere’s Law
Ampere’s law is a fundamental law of electromagnetism that relates the magnetic field around a currentcarrying wire to the electric current flowing through the wire. It provides a mathematical framework for calculating the magnetic field generated by various current configurations and has numerous applications in different fields of science and engineering. Here are some applications of Ampere’s law:
1. Calculating the Magnetic Field of a Straight Wire: Consider a long, straight wire carrying a current I. Ampere’s law states that the magnetic field (B) at a distance r from the wire is given by:
B = (μ₀ * I) / (2π * r)
where μ₀ is the permeability of free space (4π × 10^7 T·m/A). This equation allows us to determine the strength and direction of the magnetic field at any point around the wire.
2. Solenoids and Electromagnets: A solenoid is a coil of wire that, when carrying a current, creates a uniform magnetic field inside the coil. Ampere’s law can be used to calculate the magnetic field inside a solenoid, which is given by:
B = μ₀ * n * I
where n is the number of turns per unit length of the solenoid. Solenoids are widely used in various devices such as electromagnets, electric motors, and MRI machines.
3. Magnetic Force between Parallel Wires: Ampere’s law can also be used to determine the magnetic force between two parallel wires carrying currents. The force per unit length (F) between two long, parallel wires separated by a distance d is given by:
F = (μ₀ * I₁ * I₂) / (2π * d)
where I₁ and I₂ are the currents in the wires. This equation is crucial in understanding and designing electrical circuits, transformers, and other electromagnetic devices.
4. Magnetic Field of a Toroid: A toroid is a doughnutshaped coil of wire. Ampere’s law can be applied to calculate the magnetic field inside a toroid, which is found to be:
B = μ₀ * n * I
where n is the number of turns per unit length of the toroid. Toroids are often used in transformers and inductors due to their ability to create a concentrated magnetic field.
5. Magnetic Field of a Bar Magnet: Although Ampere’s law is primarily applicable to currentcarrying conductors, it can also be used to understand the magnetic field of a bar magnet. By considering the bar magnet as a collection of tiny current loops, Ampere’s law helps explain the magnetic field pattern around the magnet.
These are just a few examples of the applications of Ampere’s law. It is a powerful tool that enables scientists and engineers to analyze and design a wide range of electromagnetic devices and systems.
Frequently Asked Questions – FAQs
State Ampere’s law.
Ampere’s Law
Ampere’s law is a fundamental law of electromagnetism that relates the magnetic field around a currentcarrying wire to the electric current flowing through the wire. It was discovered by AndréMarie Ampère in 1820.
Mathematical Form of Ampere’s Law
The mathematical form of Ampere’s law is given by:
∮B⋅dl=μ0I
where:
 B is the magnetic field (in teslas)
 dl is a differential length vector (in meters) along a closed loop
 μ0 is the permeability of free space (4π×10^7 T·m/A)
 I is the electric current (in amperes) flowing through the loop
Explanation of Ampere’s Law
Ampere’s law states that the magnetic field around a currentcarrying wire is proportional to the electric current flowing through the wire. The direction of the magnetic field is given by the righthand rule.
Example of Ampere’s Law
One example of Ampere’s law is the magnetic field around a long, straight wire. The magnetic field at a distance r from the wire is given by:
B=μ0I/2πr
where:
 B is the magnetic field (in teslas)
 μ0 is the permeability of free space (4π×10^7 T·m/A)
 I is the electric current (in amperes) flowing through the wire
 r is the distance from the wire (in meters)
The magnetic field around a long, straight wire is shown in the figure below.
[Image of a long, straight wire with a magnetic field around it]
Applications of Ampere’s Law
Ampere’s law is used in a variety of applications, including:
 Designing electromagnets
 Calculating the magnetic field around electrical devices
 Determining the force between two currentcarrying wires
Ampere’s law is a fundamental law of electromagnetism that has a wide range of applications.
Name the scientist who performed experiments with forces that act on currentcarrying wires?
Hans Christian Ørsted
Hans Christian Ørsted was a Danish physicist and chemist who is best known for his discovery of the relationship between electricity and magnetism. In 1820, Ørsted performed a series of experiments that showed that a currentcarrying wire could deflect a compass needle. This discovery was a major breakthrough in the understanding of electromagnetism, and it paved the way for the development of many important electrical devices, such as the electric motor and the generator.
Ørsted’s Experiment
Ørsted’s experiment was a simple but elegant demonstration of the relationship between electricity and magnetism. He set up a wire that was connected to a battery, and he placed a compass needle near the wire. When he turned on the battery, the compass needle deflected from its normal position. This showed that the currentcarrying wire was creating a magnetic field, which was causing the compass needle to move.
The RightHand Rule
The direction of the magnetic field created by a currentcarrying wire can be determined using the righthand rule. To use the righthand rule, point your right thumb in the direction of the current flow. Then, curl your fingers around the wire. Your fingers will point in the direction of the magnetic field.
Applications of Ørsted’s Discovery
Ørsted’s discovery of the relationship between electricity and magnetism has had a profound impact on technology. It has led to the development of many important electrical devices, such as the electric motor, the generator, and the transformer. These devices are essential for the functioning of our modern world.
Conclusion
Hans Christian Ørsted was a brilliant scientist who made a major contribution to our understanding of electromagnetism. His discovery of the relationship between electricity and magnetism has had a profound impact on technology, and it continues to be used in many important electrical devices today.
State true or false: Ampere’s law is used to determine the magnetic field inside a toroid.
Ampere’s law is a law of electromagnetism that relates the magnetic field around a currentcarrying wire to the electric current passing through the wire. It states that the magnetic field around a currentcarrying wire is proportional to the current and inversely proportional to the distance from the wire.
Ampere’s law can be used to determine the magnetic field inside a toroid. A toroid is a doughnutshaped object that is made by winding a wire around a circular core. When a current is passed through the wire, it creates a magnetic field inside the toroid. The magnetic field inside a toroid is uniform and is given by the following equation:
B = μ₀nI
where:
 B is the magnetic field strength in teslas (T)
 μ₀ is the permeability of free space (4π × 10^7 T·m/A)
 n is the number of turns in the coil
 I is the current in amperes (A)
Example:
A toroid has a radius of 10 cm and is made of 1000 turns of wire. A current of 1 A is passed through the wire. What is the magnetic field strength inside the toroid?
B = μ₀nI
B = (4π × 10^7 T·m/A)(1000 turns)(1 A)
B = 0.004π T
Therefore, the magnetic field strength inside the toroid is 0.004π T.
Ampere’s law is a powerful tool that can be used to determine the magnetic field around a variety of currentcarrying conductors. It is a fundamental law of electromagnetism and has many applications in electrical engineering and physics.
State true or false: If the direction of the current is reversed, the direction of the magnetic field reverses.
State true or false: If the direction of the current is reversed, the direction of the magnetic field reverses.
Answer: True.
Explanation:
The direction of the magnetic field around a currentcarrying wire is given by the righthand rule. If you point your right thumb in the direction of the current, your fingers will curl in the direction of the magnetic field.
If you reverse the direction of the current, your right thumb will point in the opposite direction, and your fingers will curl in the opposite direction. This means that the direction of the magnetic field will also reverse.
Here is an example to illustrate this:
Imagine you have a solenoid, which is a coil of wire. When you pass a current through the solenoid, it creates a magnetic field. The direction of the magnetic field is given by the righthand rule.
If you reverse the direction of the current, the direction of the magnetic field will also reverse. This is because the righthand rule will now point in the opposite direction.
You can see this for yourself by experimenting with a solenoid. If you have a multimeter, you can measure the direction of the current and the direction of the magnetic field. You will see that when you reverse the direction of the current, the direction of the magnetic field also reverses.
State Ampere’s circuital law.
Ampere’s Circuital Law
Ampere’s circuital law is a law of electromagnetism that relates the magnetic field around a currentcarrying wire to the electric current flowing through the wire. It states that the magnetic field around a currentcarrying wire is proportional to the current and inversely proportional to the distance from the wire.
Mathematically, Ampere’s circuital law can be expressed as:
∮B⋅dl=μ0I
where:
 B is the magnetic field (in teslas)
 dl is a differential length vector (in meters) along a closed loop
 μ0 is the permeability of free space (4π×10−7 T⋅m/A)
 I is the current (in amperes) flowing through the loop
Example:
Consider a long, straight wire carrying a current of 10 A. We can use Ampere’s circuital law to calculate the magnetic field at a distance of 1 m from the wire.
∮B⋅dl=μ0I
We choose a circular loop of radius 1 m centered on the wire. The differential length vector dl is tangent to the loop at every point, and its magnitude is equal to the circumference of the loop (2πr).
∮B⋅dl=μ0I
∮B⋅2πr=μ0I
B⋅2πr=μ0I
B=(μ0I)/(2πr)
B=(4π×10−7 T⋅m/A)(10 A)/(2π(1 m))
B=2×10−6 T
Therefore, the magnetic field at a distance of 1 m from the wire is 2×10−6 T.
Applications of Ampere’s Circuital Law
Ampere’s circuital law is used in a variety of applications, including:
 Calculating the magnetic field around a variety of currentcarrying conductors, such as straight wires, coils, and solenoids
 Designing electromagnets
 Analyzing the behavior of electric motors and generators
 Studying the properties of magnetic materials