Electric Fields


This image is of a plasma ball at a science museum. It shows electricity flowing out of a charged ball. The electricity follows the lines of the electric field. What is an electric field? Read on to find out.


What Is an Electric Field?

An electric field is a space around a charged particle where the particle exerts electric force on other charged particles. Because of their force fields, charged particles can exert force on each other without actually touching. Electric fields are generally represented by arrows which show the direction that a small positive charge would travel, as you can see in the Figure below. The arrows show the direction of electric force around a positive particle and a negative particle. 


Interacting Electric Fields

When charged particles are close enough to exert force on each other, their electric fields interact. This is illustrated in the Figure below. The lines of force bend together when particles with different charges attract each other. The lines bend apart when particles with like charges repel each other. Remember, the arrows show the direction that a small positive test charge would travel, if it were placed at that point.


Q: What would the lines of force look like around two negative particles?

A: They would look like the lines around two positive particles, except the arrows would point toward, rather than away from, the negative particles.  This would show that a positive test charge would be attracted to the negative particles.


The Intensity of an Electric Field

Coulomb’s Law gives us the formula to calculate the force exerted on a charge by another charge. On some occasions, however, a test charge suffers an electrical force with no apparent cause. That is, as observers, we cannot see or detect the original charge creating the electrical force. Michael Faraday dealt with this problem by developing the concept of an electric field. According to Faraday, a charge creates an electric field about it in all directions. If a second charge is placed at some point in the field, the second charge interacts with the field and experiences an electrical force. Thus, the interaction we observe is between the test charge and the field and a second particle at some distance is no longer necessary.

The strength of the electric field is determined point by point and can only be identified by the presence of test charge. When a positive test charge, q, is placed in an electric field, the field exerts a force on the charge. The field strength can be measured by dividing the force by the charge of the test charge. Electric field strength is given the symbol E and its unit is Newtons/coulomb.

E=\frac{F_{onq_t}}{q_t}

The test charge can be moved from location to location within the electric field until the entire electric field has been mapped in terms of electric field intensity.


Example Problem: A positive test charge of 2.0 \times 10^{-5} \ \text{C} is placed in an electric field. The force on the test charge is 0.60 N. What is the electric field intensity at the location of the test charge?

Solution: E=\frac{F}{q}=\frac{0.60 \ \text{N}}{2.0 \times 10^{-5} \ \text{C}}=3.0 \times 10^4 \ \text{N/C}


Practice

The following video covers electric fields. 



Vocabulary

  • electric field: Space around a charged particle where the particle exerts electric force.


Summary

  • An electric field is a space surrounding a charged particle where the particle exerts electric force.
  • When charged particles are close enough to exert force on each other, their electric fields interact. Particles with opposite charges attract each other. Particles with like charges repel each other.
  • An electric field surrounds every charge and acts on other charges in the vicinity.
  • The strength of the electric field is given by the symbol E, and has the unit of Newtons/coulomb.
  • The equation for electric field intensity is E=\frac{F}{q}.