Electric field definition | Electric charge, field, and potential | Physics | Khan Academy

Understanding Electric Forces and Fields

The Nature of Electric Charges

• The instructor poses a question about the interaction between two positive charges, highlighting that they repel each other despite being separated by empty space.

• This raises a fundamental question about how one charge can exert force on another without direct contact, contrasting it with physical interactions in everyday life.

Historical Context of Force at a Distance

• The issue of forces acting over distances is not new; it was also a concern during Newton's time regarding gravitational forces.

• Newton could calculate gravitational forces but struggled to explain how these forces acted across empty space, leaving this mystery unresolved for centuries.

Michael Faraday's Contribution

• Michael Faraday proposed an explanation for electric forces acting at a distance, emphasizing the role of the electric field created by charges.

• He described that a positive charge generates an electric field around itself, which exists regardless of nearby charges.

Understanding Electric Fields vs. Electric Forces

• Faraday clarified that while an electric field (denoted as E) surrounds a charge, it is distinct from the electric force (F); confusion often arises due to their vector representations.

• It’s important to differentiate between electric fields and electric forces: the former does not exert force on itself but can influence other charges entering its vicinity.

Interaction Between Charges and Electric Fields

• A single charge creates an electric field that remains inactive until another charge enters this region; only then does it exert an electric force on that second charge.

Understanding Electric Fields and Forces

The Concept of Locality in Electric Forces

• The discussion begins with the importance of locality in physics, emphasizing that a charge (Q2) only needs to be aware of its immediate surroundings to determine the electric force acting on it.

• It is explained that Q2 does not need to consider distant charges (like those across the galaxy); it simply responds to the local electric field present at its location.

• Faraday's approach introduces the idea of a mediator: an electric field created by one charge (Q1) that influences another charge (Q2), allowing for interaction without direct contact.

Interaction Between Charges via Electric Fields

• The relationship between two charges is described as a communication through their respective electric fields; each charge creates a field that affects the other.

• This mutual influence illustrates how charges interact locally, reinforcing the concept that they do not need to know about distant forces but rather respond to their immediate environment.

Benefits of Using Electric Fields

• A critical question arises regarding whether this conceptual framework is merely theoretical or if it has practical benefits; indeed, there are significant advantages.

• Mathematically, using electric fields simplifies calculations in physics. Knowing just the electric field allows one to determine forces on any charge within that field without needing information about what created it.

Defining Electric Field

• To clarify what an electric field actually is, it's defined as the amount of electric force per unit charge exerted at a specific point in space.

• A test charge is introduced as a small hypothetical charge used to measure the force experienced due to an existing electric field without altering it significantly.

Calculating Electric Field Strength

• The process for determining an electric field involves measuring the force on a test charge and dividing by its own magnitude; this gives us the strength of the electric field at that point.

• An example calculation demonstrates this: if a test charge (Q2 = 2 coulombs) experiences 10 newtons of force, then the resulting electric field strength would be 5 newtons per coulomb.

Understanding Electric Forces and Fields

The Relationship Between Charge and Force

• When a charge of four coulombs is placed in an electric field with a strength of five newtons per coulomb, the resulting force is calculated as 20 newtons (5 N/C * 4 C). This illustrates how charge magnitude directly influences the force experienced in an electric field.

Electric Field and Its Origin

• The formula for electric force can be rearranged to show that the electric force on a charge (Q2) equals the product of that charge's value and the electric field at its location. It's crucial to note that this electric field (E1) is generated by another charge or collection of charges (Q1), not by Q2 itself.

Clarifying Misconceptions About Electric Fields

• A common misconception is that the charge experiencing the force (Q2) creates the electric field it interacts with. In reality, it is Q1 that generates E1, which then exerts a force on Q2. This distinction emphasizes that forces arise from existing fields rather than being created by interacting charges.

Distinguishing Between Electric Field and Electric Force

• It’s essential to differentiate between electric fields and forces:

• The electric field represents the amount of electric force per unit charge at a specific point in space.

• The electric force refers to the total effect experienced by a particular charge within that field.