The answer is the fourth choice because there are 7 represents in a coefficient.
Answer:
(a): The magnitude of the electric force on the small sphere = 
(b): Shown below.
Explanation:
<u>Given:</u>
- m = mass of the small sphere.
- q = charge on the small sphere.
- L = length of the silk fiber.
= surface charge density of the large vertical insulating sheet.
<h2>
(a):</h2>
When the dimensions of the sheet is much larger than the distance between the charge and the sheet, then, according to Gauss' law of electrostatics, the electric field experienced by the particle due to the sheet is given as:

<em>where,</em>
is the electrical permittivity of the free space.
The electric field at a point is defined as the amount of electric force experienced by a unit positive test charge, placed at that point. The magnitude electric field at a point and the magnitude of the electric force on a charge q placed at that point are related as:

Thus, the magnitude of the electric force on the small sphere is given by

The sheet and the small sphere both are positively charged, therefore, the electric force between these two is repulsive, which means, the direction of the electric force on the sphere is away from the sheet along the line which is perepndicular to the sheet and joining the sphere.
<h2>
(b):</h2>
When the sphere is in equilibrium, the tension in the fiber is given by the resultant of the weight of the sphere and the electric force experienced by it as shown in the figure attached below.
According to the fig.,

<em>where,</em>
= electric force on the sphere, acting along left.
= weight of the sphere, acting vertically downwards.
<em />

g is the acceleration due to gravity.
Answer:
Intermolecular forces
Explanation:
The force of attractions that act between molecules are called intermolecular forces.
Their nature is electromagnetic, this means that they are just an expression of the electromagnetic force.
One example of intermolecular force is the ionic bond: this type of bond occurs when there are two ions, one positively charged and the other one negatively charged, and they are attracted by each other due to the electrostatic force, which therefore creates a bond between them.
Other types of intermolecular forces include:
Hydrogen bond
Ion-dipole forces
Van der Waals forces
The strength of these intermolecular forces determine the state of the substance. In fact, in solids, these forces are very strong, so that the molecules are strongly bond to each other and they cannot move freely, but only vibrate about their fixed position. On the other hand, in gases, these forces are very weak, therefore the molecules are able to move freely away from each other.
Explanation:
this is due to the roughness of the metal like you mentioned aluminum and iron are rough and don't provide a reflective surface as good as a mirror