Chapter 3: Problem 44
In the product \(\vec{F}=q \vec{v} \times \vec{B}\), take \(q=2\), $$ \vec{v}=2.0 \hat{\mathrm{i}}+4.0 \hat{\mathrm{j}}+6.0 \hat{\mathrm{k}} \text { and } \vec{F}=4.0 \hat{\mathrm{i}}-20 \hat{\mathrm{j}}+12 \hat{\mathrm{k}} $$ What then is \(\vec{B}\) in unit-vector notation if \(B_{x}=B_{y} ?\)
Short Answer
Expert verified
\(\vec{B} = -3 \hat{\mathrm{i}} - 3 \hat{\mathrm{j}} - 14 \hat{\mathrm{k}}\)
Step by step solution
01
Understand the vectors involved
We have the force vector \( \vec{F} = 4.0 \hat{\mathrm{i}} - 20 \hat{\mathrm{j}} + 12 \hat{\mathrm{k}} \), the charge \( q = 2 \), and the velocity vector \( \vec{v} = 2.0 \hat{\mathrm{i}} + 4.0 \hat{\mathrm{j}} + 6.0 \hat{\mathrm{k}} \). We need to find the magnetic field vector \( \vec{B} \).
02
Relate Force to Velocity and Magnetic Field
According to the equation \( \vec{F} = q \vec{v} \times \vec{B} \), \( \vec{F} \) is the cross product of charge times velocity and the magnetic field. Substitute \( q = 2 \) to get \( \vec{F} = 2 \times (\vec{v} \times \vec{B}) \), so \( \vec{v} \times \vec{B} = \frac{\vec{F}}{2} \).
03
Calculate the Cross Product
Given \( \vec{v} = 2 \hat{\mathrm{i}} + 4 \hat{\mathrm{j}} + 6 \hat{\mathrm{k}} \) and expressing \( \vec{B} \) as \( B_x \hat{\mathrm{i}} + B_y \hat{\mathrm{j}} + B_z \hat{\mathrm{k}} \), compute the cross product \( \vec{v} \times \vec{B} \):\[\begin{vmatrix}\hat{\imath} & \hat{\jmath} & \hat{k} \2 & 4 & 6 \B_x & B_y & B_z \\end{vmatrix}= \hat{\imath}(4B_z - 6B_y) - \hat{\jmath}(2B_z - 6B_x) + \hat{k}(2B_y - 4B_x)\]
04
Set up Equations from Cross Product
To find \( \vec{B} \), equate each component of the cross product \( \vec{v} \times \vec{B} \) with the corresponding components of \( \frac{\vec{F}}{2} = 2 \hat{\mathrm{i}} - 10 \hat{\mathrm{j}} + 6 \hat{\mathrm{k}} \):1. \( 4B_z - 6B_y = 2 \)2. \( 6B_x - 2B_z = 10 \)3. \( 2B_y - 4B_x = 6 \)
05
Solve the System of Equations
Since \( B_x = B_y \), substitute \( B_y = B_x \) in equations 1 and 3 to solve for \( B_x, B_y, \) and \( B_z \):From equation 3:\[ 2B_x - 4B_x = 6 \Rightarrow -2B_x = 6 \Rightarrow B_x = -3 \]\[ B_y = -3 \]Substitute \( B_x = -3 \) in equation 2:\[ 6(-3) - 2B_z = 10 \Rightarrow -18 - 2B_z = 10 \Rightarrow -2B_z = 28 \Rightarrow B_z = -14 \]
06
Compile the Vector \( \vec{B} \)
With the solved values, express \( \vec{B} \) in unit-vector notation:\[ \vec{B} = -3 \hat{\mathrm{i}} - 3 \hat{\mathrm{j}} - 14 \hat{\mathrm{k}} \]
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Cross Product
In vector mathematics, the cross product is a binary operation on two vectors in three-dimensional space. The cross product of two vectors, \(\vec{A} \) and \(\vec{B} \), is denoted as \(\vec{A} \times \vec{B}\). This operation yields a third vector, \(\vec{C}\), which is perpendicular to both \(\vec{A}\) and \(\vec{B}\). Its magnitude is determined by the product of the magnitudes of the two vectors and the sine of the angle between them:\[ |\vec{C}| = |\vec{A}| |\vec{B}| \sin \theta \]where \( \theta \) is the angle between \(\vec{A}\) and \(\vec{B}\). The direction of \(\vec{C}\) is given by the right-hand rule. Place your right hand such that your fingers point along \(\vec{A}\) and curl towards \(\vec{B}\). Your thumb then points in the direction of \(\vec{C}\).
- Critical for calculating forces in electromagnetic fields
- Crucial for understanding rotational physics
Vector Notation
Vector notation is a way to represent vectors, which are quantities characterized by both direction and magnitude. A vector is typically denoted by a letter topped with an arrow, such as \(\vec{v}\). This makes it a more intuitive concept compared to just scalar quantities.Consider a vector \(\vec{v}\), expressed in unit-vector notation as \( a \hat{\imath} + b \hat{\jmath} + c \hat{k} \), representing components along the x, y, and z axes respectively. Here, \(\hat{\imath}, \hat{\jmath}, \hat{k}\) are unit vectors representing directions along the three axes.
- Broken down into components making it easy to handle operations
- Algebraic operations on vectors often involve unit-vector notation
Lorentz Force
The Lorentz force is the force exerted on a charged particle moving through electric and magnetic fields. Derived from the Lorentz force equation, it is vital for understanding interactions in electromagnetism:\[\vec{F} = q (\vec{E} + \vec{v} \times \vec{B})\]where:
- \(\vec{F}\) is the force exerted on the particle
- \(q\) is the charge of the particle
- \(\vec{E}\) is the electric field
- \(\vec{v}\) is the velocity of the particle
- \(\vec{B}\) is the magnetic field
Vector Mathematics
Vector mathematics is a branch of mathematics that deals with vectors and operations that can be performed on them. It is the backbone of various applications in physics, engineering, and computer science.
- Addition and Subtraction: Combine vectors by adding corresponding components.
- Scalar Multiplication: Multiply each component of a vector by a scalar.
- Dot Product: A scalar representing the product of two vectors' magnitudes and the cosine of the angle between them.
- Cross Product: Yields a new vector, crucial for calculating rotational forces.