Solution
Related Formula
The change in potential energy Δ U of a charge q moved between two points of potentials VC and VD is:
Δ U = q₃ (VD - VC)Potential due to a point charge q at distance r is:
V = (1)/(4πε₀) (q)/(r)Core Logic
Let point A hold charge q₁ and B hold q₂ separated by 30 ~cm = 0.3 ~m.
- The path of q₃ is a circular arc of radius R = 40 ~cm = 0.4 ~m centered at A. Thus, distance of C and D from q₁ is constant:
- Distance of C from q₂ (B):
- Distance of D from q₂ (B):
Step 1: Calculate Potentials
Potential at C due to q₁ and q₂:
VC = (1)/(4πε₀) ( (q₁)/(0.4) + (q₂)/(0.5) )Potential at D due to q₁ and q₂:
VD = (1)/(4πε₀) ( (q₁)/(0.4) + (q₂)/(0.1) )Step 2: Difference in Potential Energy
The potential difference is:
VD - VC = (1)/(4πε₀) ( (q₂)/(0.1) - (q₂)/(0.5) ) = (q₂)/(4πε₀) [10 - 2] = (8q₂)/(4πε₀)(Notice that the potential contribution of q₁ cancels out because C and D are equidistant from q₁).
The change in potential energy is:
Δ U = q₃ (VD - VC) = (q₃ (8q₂))/(4πε₀)Comparing with (q₃ K)/(4πε₀) yields K = 8q₂.
Pattern Recognition
Sees: Arc path centered on one of the charges. Shortcut: Since the path is circular about q₁, q₁ contributes nothing to the potential difference between the end points. The entire change in potential energy is due to q₂. The distance difference translates to potential difference Δ V = q₂ ((1)/(0.1) - (1)/(0.5)) = 8q₂, so K = 8q₂.
Chapter Mix
Class 12 Physics: Electrostatic Potential and Capacitance