Question

What is a good rule of thumb for designing a simple magnetic coil? Specifically, given a circular coil of radius \(\sim 1 \mathrm{~cm},\) what is the approximate magnitude of the magnetic field, in gauss per amp per turn? (Note: \(1 \mathrm{G}=0.0001 \mathrm{~T}\).) a) \(0.0001 \mathrm{G} /(\mathrm{A}\) -turn \()\) b) \(0.01 \mathrm{G} /(\) A-turn \()\) c) \(1 \mathrm{G} /(\mathrm{A}\) -turn \()\) d) \(100 \mathrm{G} /(\mathrm{A}\) -turn \()\)

Short answer

Answer: c) 1 G/A⋅turn

Step-by-step solution

Relevant Formula

The magnetic field, B, at the center of a circular coil carrying a current I is given by the following formula, derived from the Biot-Savart law: \(B = \frac{\mu_0 I}{2r}\) where \(\mu_0\) is the permeability of free space (with a value of \(4\pi\times10^{-7} \,\mathrm{Tm/A}\)), I is the current in Amperes, and r is the radius of the coil in meters.

Convert units to gauss

We will use the conversion factor: \(1\mathrm{G} = 0.0001\mathrm{T}\) \(1\mathrm{T} = 10000\mathrm{G}\)

Calculate the magnetic field per amp per turn

Now, let's calculate the magnetic field for 1 Amp of current and a radius of 1 cm. \(B = \frac{\mu_0 \times 1\,\mathrm{A}}{2 \times 0.01\,\mathrm{m}} = \frac{4\pi\times10^{-7}\,\mathrm{Tm/A}\times 1\,\mathrm{A}}{2\times 0.01\,\mathrm{m}} = 2\pi\times10^{-5}\,\mathrm{T}\) Let's convert the magnetic field to Gauss: \(B = 2\pi\times10^{-5}\,\mathrm{T} \times 10000\,\mathrm{G/T} = 2\pi\times10^{-1}\,\mathrm{G}\) \(B\approx 0.63\,\mathrm{G/A\cdot turn}\)

Choose the correct answer

Comparing our computed value to the choices: a) \(0.0001\,\mathrm{G/A\cdot turn}\) b) \(0.01\,\mathrm{G/A\cdot turn}\) c) \(1\,\mathrm{G/A\cdot turn}\) d) \(100\,\mathrm{G/A\cdot turn}\) Our calculated value, approximately \(0.63\,\mathrm{G/A\cdot turn}\), is closest to option (c) \(1 \,\mathrm{G/A\cdot turn}\). Therefore, the correct answer is: c) \(1\,\mathrm{G/A\cdot turn}\).

Key concepts

Biot-Savart Law

The Biot-Savart Law is a fundamental principle in electromagnetism that describes how currents produce magnetic fields. Primarily used for calculating the magnetic field generated in a space due to any current distribution, it states that each infinitesimal segment of current produces a tiny magnetic field at a point in space, and the total magnetic field is the vector sum of all these tiny fields.

Mathematically, it's expressed as:
\[d\textbf{B} = \frac{\text{μ}_0}{4π} \frac{I d\textbf{l} × \textbf{r̂}}{\textbf{r}^2}\]
where \(d\textbf{B}\) is the differential magnetic field produced by a small segment of current I, \(d\textbf{l}\) is the length element through which the current flows, \( \textbf{r̂} \) is the unit vector from the current element to the point where the field is being calculated, and \(\textbf{r}\) is the distance from the current element to the point.

This law is pivotal for understanding how to calculate the magnetic field around a coil or a wire, which is essential when dealing with electromagnetic applications like motors, generators, and inductors.

Magnetic Field Calculation

When it comes to calculating the magnetic field of a coil, a few key elements determine the characteristics of the field it creates. The type of coil, the number of turns, the current flowing through it, and the coil's radius all come into play.

In the specific case of a circular coil of radius \(r\), carrying current \(I\), the magnetic field at the center of the coil can be calculated using a simplified version of the Biot-Savart Law:
\[B = \frac{\text{μ}_0 I}{2r}\]
This equation reveals that the magnetic field (B) is directly proportional to the current (I) and inversely proportional to the radius (r). The constant \( \text{μ}_0 \) represents the permeability of free space, which is an inherent property of space defining how much resistance there is to the formation of a magnetic field within it. Understanding this calculation is crucial for students because it enables them to determine the strength of the magnetic field generated by different coil configurations, assisting in the design and analysis of electromagnetic devices.

Permeability of Free Space

Permeability of free space, denoted as \( \text{μ}_0 \), is a physical constant that represents how a magnetic field can permeate the vacuum of free space. It is crucial in the calculation of the magnetic field because it forms the proportionality constant in the Biot-Savart Law and Ampère's Law.

The value of permeability of free space is exactly defined as:
\[ \text{μ}_0 = 4π × 10^{-7} \text{ Tm/A} \]
This constant indicates the extent to which a vacuum allows for the passage of magnetic lines of force or flux. The higher the permeability, the less resistance a material or space opposes to the formation of a magnetic field. In everyday units, one Tesla (T) per Ampere (A) per meter (m) is equivalent to the force a charge would feel moving through a magnetic field. Applied to problems like the one in the exercise, knowing the permeability of free space is essential to correctly calculate the strength of the magnetic field produced by current-carrying conductors, such as coils.