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).
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 overlap||If 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: |
|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 Field||Uniform Field|
What are conductors & insulators?
These terms refer to the ability for charges to travel within a material.
|allow charges to travel easily through them||do 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 induction||A 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.
- PhysicsClassroom – Charging by Induction: https://www.physicsclassroom.com/class/estatics/Lesson-2/Charging-by-Induction
- PhysicsClassroom – Static Electricity Interactives: https://www.physicsclassroom.com/Physics-Interactives/Static-Electricity