I Trying to derive Gauss' law using a cylindrical surface

AI Thread Summary
The discussion focuses on deriving Gauss's law for an infinitely long line of charge using a cylindrical surface. The initial attempt incorrectly equates the electric field and area, leading to confusion between vector and scalar quantities. Emphasis is placed on recognizing the symmetry of the electric field, which points radially outward from the line charge. To correctly apply Gauss's law, one should evaluate the integral over the cylindrical surface and account for the charge enclosed within the volume. Understanding these principles is crucial for accurately deriving Gauss's law in this context.
annamal
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When I try to derive Gauss's law with a straight line of charge with density ##\lambda## through a cylindrical surface of length L and radius R,
$$\vec E = \frac{\lambda*L}{4\pi\epsilon*r^2}$$
$$A = 2\pi*r*L$$
$$\vec E*A = \frac{\lambda *L^2}{2\epsilon*r} \neq \frac{q_{enc}}{\epsilon}$$
What am I doing wrong?
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annamal said:
$$\vec E = \frac{\lambda*L}{4\pi\epsilon*r^2}$$
Don't confuse the field from a point charge with that of a line of charge. Look it up!
 
First of all your "syntax checker" should alarm you that you have equated a vector with a scalar!

I guess you are considering an "infinitely long" homogeneous line-charge along the ##z## axis of a cylindrical coordinate system. Now think about the symmetry of the problem, i.e., in which direction must the electric field point, and then use an arbitrary cylindrical volume, ##V##, with the axis along the line-charge distribution and evaluate carefully Gauss's Law,
$$\int_{\partial V} \mathrm{d}^3 \vec{f} \cdot \vec{E}=\frac{1}{\epsilon_0} Q_V,$$
where ##\partial V## is the boundary of the cylinder (with the surface-normal vectors pointing out of this cylinder), and ##Q_V## is the charge inside the considered cylindrical volume, ##V##.
 
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