Electrostatic Forces

Homework: Complete handout

Magnitude of Force

According to Coulomb’s Law The magnitude of force exerted on a charge by

another is directly proportional to the product of the two charges inversely proportional to the distance between the

charges

Equation for calculating the magnitude of an electrostatic force

F = k q1 q2

r2

Where, q = the charger = distance between 2 charged objects

k=8.99 x 10 9 Nm2/C2

force between 2 objects may either attract or repel depending on sign of charges

Calculate the Force exerted between a proton and an electron at a distance of 50 cm.

Electric Fields

Definition: the area around a charged particle that it can exert a force on another charge

Fields can be represented showing field lines (imaginary lines showing direction that a positive test charge would move)

Electric Field for Positive and negative charges

Field lines always start at the positive charge and move toward negative charges

Electric Fields between 2 particles

Electric Field Strength

The ratio of the force exerted on a test charge to the charge of the test charge

E = Fe / q

Units: Newtons per coulomb

Electric Potential (V)

the work done or energy acquired moving a positive test charge between two points in an electric field

Potential Difference (Voltage)

potential energy difference between 2 points in an electric field per unit of charge

as charges move through an electric field, they will either gain KE or PE

Consider: If opposite charges move closer to one another

Gain KE

Potential Difference (Voltage)

If opposite charges move farther apartGain PE

If like charges move closer to one anotherGain PE

If like charges move away from one anotherGain KE

Calculating the Potential Difference

DV = W / qWhere, W = work done against the field, or the energy

acquired working in the fieldq = amount of charge moving through the fieldV = potential difference

Units: Volts (V) = Joule / Coulomb

It takes 6 J of work to move 2 C of charge between 2 points in an electric field.

What is the potential energy difference (V) between these points?

DV = W / qV = 6 J/2 C

V = 3 J/C or 3 V

If 4.8 x 10-17 joules of work is required to move an electron between 2 points in an electric field, what is the electric potential difference between these points?

V = W/q

V= 4.8 x 10-17 J / 1.6 x 10-19 C

V = 300 V

Electric Fields between 2 parallel plates

the electric field that exists between two parallel charged plates is uniform

Electric Potential Difference in a Uniform Field

Equal to the product of electric field intensity (E) and the distance moved by the charge

DV = E d

Electric potential increases in the direction opposite the electric field direction. (it is higher near the + charged plate

What work is done when 3.0 C of charge is moved through an electric potential difference of 1.5 V?

A 12 V car battery can store 1.44 x 106 C of charge when it is fully charged. How much work can be done by this battery before it needs recharging?