Throughout this course you have had opportunities to observe electrostatic forces acting on charged objects. In many cases, the force was able to produce an observable effect even though the objects were not touching.

How were the positive charges on the sweater able to exert an electrostatic force on the negative charges on the balloon, even though the sweater was not touching the balloon? How was the negatively charged globe of the Van de Graaff generator able to exert forces on the soap bubbles, even though the Van de Graaff generator was not touching the bubbles?

 

 



These questions illustrate an important property of the electrostatic force: the force can act on a distant object without any apparent contact.  This property has been called "action at a distance." But naming this property is not the same as explaining it.  After all, how does "action at a distance" work?  What mechanism enables the electrostatic force to act through space?  The answer to these riddles relates to an idea that you studied in previous courses-the concept of a field .


Field: a region of influence surrounding an object through which a forces operates.


Read Fields
Learn more about how the concept of a field explains "action at a distance" by reading page 545, then answer Self-Check 1

 
Before you continue your study of electric fields, you may find it helpful to review what you have learned about gravitational fields from previous courses. A satellite in orbit around Earth is a good example of gravitational fields at work.

The Hubble Space Telescope moves at about 8 km/s as it orbits about 600 km above Earth's surface.  Have you ever wondered why an object moving so quickly, so far from the planet, stays in orbit?  Shouldn't this satellite just fly off into space?

To answer the question above, think about Earth's gravitational field.  If you need help remembering how Earth's gravitational field acts on objects in orbit, read pages 200 and 201. Then answer Self-Check 2

 

Image by WikiImages from Pixabay

 Electrostatics is not the only branch of physics that utilizes the concept of a field.

  1. Identify another type of force that uses the field concept to explain "action at a distance."
  2. Support your answer to the previous question by briefly describing an example of how the concept of a field is used to explain action at a distance for the force you identified.


  1. The field concept is used to explain how the force of gravity is able to act across empty space to keep distant objects in orbit.
  2. This is one possible approach. As a space module approaches the moon, the path of the space module changes due to the influence of the moon's gravitational force of attraction. Even though the moon is not touching the space capsule, the moon's gravitational field acts upon the space capsule. The moon's gravitational field is a property of the space around the moon that enables it to exert a gravitational force on the space capsule.



 Earth's gravitational field can be described by a field diagram and by an equation.

  1. Describe the direction of Earth's gravitational field.

  2. Although Earth's gravitational field is invisible, it can be represented with a diagram showing gravitational field lines.  Sketch a diagram representing Earth's gravitational field.  Use this diagram to compare the strength of Earth's gravitational field at locations close to the planet and far from the planet.

  3. Compare the direction of the gravitational force of Earth acting on the Hubble Space Telescope to the direction of Earth's gravitational field at the same location.

  4. Define gravitational field strength.

  5. State the equation for gravitational field. How does this equation support your answer to part c?

  6. Explain the difference between gravitational force and gravitational field.

Earth's gravitational field is directed toward the centre of the planet.



  • The gravitational force of Earth exerted on the Hubble Space Telescope is directed toward the centre of the planet. The gravitational field of Earth points in the same direction at this location.

  • Gravitational field strength is defined as the gravitational force per unit mass at a specific location.

  • The equation is  «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mover»«mi»g«/mi»«mo»§#8594;«/mo»«/mover»«mo»=«/mo»«mfrac»«msub»«mover»«mi»F«/mi»«mo»§#8594;«/mo»«/mover»«mi»g«/mi»«/msub»«msub»«mi»m«/mi»«mrow»«mi»t«/mi»«mi»e«/mi»«mi»s«/mi»«mi»t«/mi»«/mrow»«/msub»«/mfrac»«/math»

    This equation is a vector equation, with the gravitational field and the gravitational force pointing in the same direction.

  • Gravitational force is the attractive force between any two objects due to their masses. The magnitude of the gravitational force can be experienced as a pull that can be measured with a spring scale calibrated in newtons.

    Gravitational field is the invisible region of influence that surrounds a source mass. The strength of the gravitational field is the ratio of the gravitational force per unit of test mass. The strength of the gravitational field can only be observed indirectly by observing the behavior of test bodies.