Chapter 13 - Voltage

Electric Potential

• Due to the gravitational field of the Earth, when things are raised above the ground level, they have potential energy

  • That is, they have “stored” energy that can be calculated (and changed to kinetic) if the object is allowed to fall

  • The amount of potential energy depends on:

    • Mass of the object (more mass = more PE)

    • Height of the object (more height = more PE)

    • Gravitational field (more height = less PE)

• When an object falls, it is doing work against gravity

  • That work is equal to the amount of PE_gravity it has

    W = FD

    W = ΔPE_g

• Similarly, a charge ‘placed’ in an E-Field also has potential energy, called the Electric Potential Energy

  • Just like a mass in a gravitational field, it has the ‘potential’ to have work done is ‘let go’

• Equations:

  • V = EPE / q

    • This doubles as a definition, as it is the electric potential between two points which is the potential difference, also known as voltage

  • V = kq / r

• At any given distance from the Earth, the potential energy per unit mass is the same

  • Where distance from the Earth is the height

• At any given distance from a point charge, the potential energy per charge is the same

  • These distances are called equipotential surfaces, surfaces that have the same amount of electric potential

• How much WORK does it take to carry a textbook from 2m above the ground at one spot to another spot, 5m over but still 2m above the ground?

  • Well, equipotential surfaces around charges work the same way

    • Answer: It takes zero work (or energy) to move a charge along an equipotential surface

      

• More equations:

  • V = Ed

    • Where E is the magnitude of the electric field (constant between plates)

    • Where d is the distance between the plates