In previous classes, potential energy was introduced as the energy stored when an external force does work against a force, such as gravitational or spring force. The Coulomb force between two stationary charges is also a conservative force. Like gravitational potential energy, we can define the electrostatic potential energy of a charge in an electrostatic field.
Imagine bringing a test charge q from point R to point p against the repulsive force due to a charge Q at the origin. If both Q and q are positive, an external force equal to the negative of the electric force is applied, ensuring no net force or acceleration on q. The work done by the external force is stored as potential energy. Thus, the work done in moving a charge q from R to p is given by:
WRP=∫RPFext⋅dr=−∫RPFE⋅dr
The potential energy difference between points p and R is:
ΔU=UP−UR=WRP
Did You Know?
The concept of potential energy is only meaningful for conservative forces, where the work done is path-independent.
Mnemonic:
"CAP" - Conservative forces, like Coulomb and gravitational forces, have Associated Potential energy.
Electrostatic potential Vat a point is defined as the work done per unit charge in bringing a positive test charge from infinity to that point. For a charge configuration, the potential difference between points P and R is:
VP−VR=−∫RPE⋅dr
If we choose the potential to be zero at infinity, the potential V at a point due to a charge Q at the origin is:
V(r)=4πϵ0rQ
Real-life Application:
Electrostatic potential is crucial in designing electronic components like capacitors and transistors, which are the building blocks of modern electronic devices.
NEET Tip:
Always remember that the potential at infinity is taken as zero unless otherwise specified.
For a point charge Q at the origin, the potential V at a distance r is:
V(r)=4πϵ0rQ
This expression holds for both positive and negative charges, with the potential being positive for positive charges and negative for negative charges.
NEET Problem-Solving Strategy:
For calculating potential due to multiple point charges, use the principle of superposition, adding the potentials due to individual charges algebraically.
An electric dipole consists of charges +q and −qseparated by a distance 2a. The potential at a point P with position vector r due to a dipole is:
V(r)=4πϵ01r3p⋅r
where p=q⋅2a is the dipole moment vector.
Common Misconception:
The potential due to a dipole decreases as 1/r2 at large distances, not as1/rlike a single charge.
Equipotential surfaces are surfaces where the potential is constant. For a point charge, these surfaces are concentric spheres centered on the charge. For a uniform electric field, they are planes perpendicular to the field lines.
The electric field is always perpendicular to equipotential surfaces and points in the direction of decreasing potential. The magnitude of the electric field E is related to the potential difference ΔV and the distanceΔl by:
E=−ΔlΔV
Did You Know?
No work is done in moving a charge along an equipotential surface, as the potential difference is zero.
Concept Connection:
In chemistry, the concept of equipotential surfaces is similar to the idea of contour lines on a map, where each line represents a constant potential energy.
A capacitor is a device that stores electric charge and energy. The capacitance C is defined as the ratio of the charge Q on one plate to the potential difference V between the plates:
C=VQ
The unit of capacitance is the farad (F).
For a parallel plate capacitor with plate areaA and separation d, the capacitance is:
C=dϵ0A
where ϵ0 is the permittivity of free space.
Real-life Application:
Capacitors are used in various electronic circuits, including filters, timers, and memory storage devices.
NEET Exam Strategy:
In problems involving capacitors, ensure to account for the dielectric constant if a dielectric material is present between the plates.