PHY C17: Electric Forces & Fields (AS)

If you read my post about fields, you should remember that fields and forces are influenced by certain properties of matter. In this chapter, we look at fields affected by the ELECTRIC CHARGE.

  • Electric Forces
  • Electric Fields
  • Electric Conductors & Insulators
    • Charging objects

What is an electric force?
A force that acts on electrically-charged objects. You should know that like charges repel, while opposites attract. Head over to my A2 coverage of this chapter to see the formula describing electric force (hint: it’s almost identical to gravitational force).

The so-called “Law of Charges”

We should really be calling this the “electrostatic force”, since it only applies when the charges are not moving relative to each other. This is because moving charges experience an additional force: the magnetic force. However, since the syllabus uses the term “electric force”, we will assume that charges are stationary or have low magnetic effects IN THIS CHAPTER ONLY. In a future chapter, we will investigate the magnetic force.

What is an electric field?
A region in space where electric forces act on electrically-charged objects.

How do we represent electric fields?
Electric field lines. There are a few rules you must obey when drawing these:

By convention, field lines point from positive to negative charges  The lines represents the force acting on a small positive test charge
Field lines CANNOT overlapIf they did, it would mean that a charge located at the point of intersection of field lines could go into 2 directions   See superposition of fields
Field lines must be perpendicular to the charged surfaces, when the charged object is a conductor  If they are not perpendicular, there is a component of force parallel to the surface. Since it is a conductor, mobile charges will move along the surface until they are equally distributed. In the end, the field lines will be perpendicular.
Strength of the field is shown by how close the lines are Weak Field: This image has an empty alt attribute; its file name is weak-fieldpng-1.png
Strong Field:
This image has an empty alt attribute; its file name is strong-field-3.png
Direction of the electric force at any point is tangent to the field line at that point 

As we’ve seen in the fields post, there are 2 types of basic electric fields:

Radial FieldUniform Field
This image has an empty alt attribute; its file name is strong-field-5.png This image has an empty alt attribute; its file name is uniform-field-1.png

What are conductors & insulators?
These terms refer to the ability for charges to travel within a material.

allow charges to travel easily through themdo not allow charges to travel easily through them

With that in mind, what happens to these materials when they are placed near a charge?

If a neutrally-charged object is placed near an electric charge, the particles in that object will react accordingly:

Since charges can travel easily through the conductor, they will arrange themselves to oppose the external field. Negative charges are attracted to external positive charges & vice versa.

Inside a conductor, there is 0 electric field!
Since charges cannot travel easily, they will mostly stay where they are. The external field exists inside the insulator.

Inside an insulator, the electric field is (almost) identical to the external field.

How do you charge objects?

Charge by Friction  We know that by rubbing a balloon on hair or a cloth on a rod, we can charge both objects.
This happens via rubbing as some charged particles are transferred between the objects, leaving a net charge imbalance.*

Insulators are able to maintain this electrostatic charge because the charges cannot easily redistribute themselves within the object.
Charge by inductionA charged object approaches a neutral object (& does not touch it).
If the neutral object is a conductor, charges move freely within the object. If the neutral object is an insulator, charges move slightly within the atoms.
Net charge of the second object is still 0.

*Why don’t they distribute equally?
Some insulators have a higher affinity for electrons than others. See this explanation on charging by friction – AKA triboelectric charging.

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One thought on “PHY C17: Electric Forces & Fields (AS)

  1. Pingback: PHY C17: Electric Fields (A2) – ProDuckThieves

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