Answer:
if you stretch a spring with k = 2, with a force of 4N, the extension will be 2m. the work done by us here is 4x2=8J. in other words, the energy transferred to the spring is 8J. but, the stored energy in the spring equals 1/2x2x2^2=4J (which is half of the work done by us in stretching it).
The electrostatic force between the two ions is 
Explanation:
The electrostatic force between two charged particle is given by Coulomb's law:

where
is the Coulomb's constant
are the two charges
r is the separation between the two charges
In this problem, the ion of sodium has a charge of

while the ion of chlorine has a charge of

And the distance between the two ions is

Substituting, we find the electrostatic force between the two ions:

where the negative sign simply means that the force is attractive, since the two ions have opposite charge.
Learn more about electrostatic force:
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Given Information:
Resistance of circular loop = R = 0.235 Ω
Radius of circular loop = r = 0.241 m
Number of turns = n = 10
Voltage = V = 13.1 V
Required Information:
Magnetic field = B = ?
Answer:
Magnetic field = 0.00145 T
Explanation:
In a circular loop of wire with n number of turns and radius r and carrying a current I induces a magnetic field B
B = μ₀nI/2r
Where μ₀= 4πx10⁻⁷ is the permeability of free space and current in the loop is given by
I = V/R
I = 13.1/0.235
I = 55.74 A
B = 4πx10⁻⁷*10*55.74/2*0.241
B = 0.00145 T
Therefore, the magnetic field at the center of this circular loop is 0.00145 T