J. Pulickeel April 2009 SPH 4U1

Electric Forces An Electric Force is a non-contact force which can act at a distance. For instance a (+) charged ebonite rod will attract a (-) pith ball An electric force can be created by Friction, Contact or Induction (Grounding)

Electric Force Fields A field force is sometimes referred to as an action at a distance The space surrounding a charged object is affected by the presence of the charge. An Electric Force Field is an alteration of the space surrounding the charge

Electric Force Fields Electric Force Fields are vector quantities. Each field line represents a net force acting on a charged particle at that particular point in space.

Electric and Gravitational Forces Gravitational Forces Electric Forces A Force of attraction between two objects. A force between two charged particles. It can be attractive or repulsive.

Gravitational Fields It is often useful to relate the Force experienced to a specific location. For instance a 30 kg person would experience 296 N of Force. A 70 kg person would experience 686 N of force. F = mg We could say that at this particular spot, the Gravitational Field Strength is 9.8 N/kg What force would a 95 kg person experience at this location? Is it valid for other locations?

Electric Fields Similarly, a test charge will exert a force around itself that attracts or repels other charges 7.5 N/kg9.8 N/kg4 N/kg Lower Electric Field High Electric Field At this location, a charge would feel 2N/C A 4 C charge would feel 8 N force A 10 C charge would feel 20 N Force At this location, a charge would feel 1J/C

Electric and Gravitational Fields Gravitational Fields Electric Fields Exists in the space surround an object in which the force of gravity exists A field illustrating the force of attraction between two objects The region in which a force is exerted on an electric charge by another electrical source Similarly, we could define an Electric Field as the electric force per unit charge at a specific location

Gravitational Fields Electrical Fields Depending on our location on the earth we experience a different gravitational field strength We approximately 9.8 N/kg The space around an electrical charge in which an electric force acts. This is similar to Gravitational Fields on a microscopic planet… Electric and Gravitational Fields

Potential Energy Potential Energy is the stored energy of position of an object an it is related to the location of the object within a field. Gravitational Potential Energy (GPE) – the stored energy due to attractive forces of large masses and their relative distances. Electrical Potential Energy (EPE) – the stored energy due to attractive or repulsive forces of charged particles and their relative distances.

Potential Energy Gravitational Potential Energy Electrical Potential Energy When gravity does work on an object, it GPE is lowered. When the electrical charge does work, the EPE is reduced High GPE GPE  KE Lower GPE High EPE EPE  KE Lower EPE High EPE EPE  KE Lower EPE

Gravitational and Electrical Potential Energy It is natural for objects to move from high energy to low energy. Work is required to move an object from low to high energy Moving a (+) charge from A to B will require work and increase the EPE Moving a (+) charge from B to A will naturally occur. Work is not required. The EPE will decrease

Gravitational Potential 1 kg 2 kg 1 m 2 m GPE depends on the mass of an object and its height. An object with twice the mass will have twice the potential energy and an object with twice the distance will have twice the potential energy. An object an a high position does not necessarily mean high potential energy and low positions don’t mean low potential energy

Gravitational Potential Gravitational Potential Energy depends on mass and height Gravitational Potential tells us the amount of potential energy per kilogram at a particular spot. 1 kg, 2m  20J PE 1 kg, 1m  10J PE 2 kg, 2m  40J PE 2 kg, 1m  20J PE Gravitational Potential 20J/kg 10J/kg 0 J/kg

Gravitational Potential Gravitational Potential is a means of rating various locations within a gravitational field in terms of the amount of potential energy per unit of mass. Gravitational Potential is  Mass independent and  Location dependant

Electric Potential (V) Similar to GP, Electrical Potential is dependant on the Charge of the particles Distance between the particle and the source Both (+) charges require work to push them closer to the Van de Graff Sphere. The larger charge requires twice the work because it is twice the charge. If a (3+) charge were placed at the same location it would require 30 units of PE The Electric Potential for this location is 10J/C 10 Joule PE 1 C 20 Joule PE 2 C

Electric Potential (V) A (+) charge would be at a high V when held close to a (+) source charge and at a lower V when held further away. Suppose that the electric potential at a given location is 9 J/C A 2 C object would have 18 J of Potential Energy at that location A 0.5 C object would have 4.5 J of Potential Energy at that location

Electric Potential (V) A (+) charge would be at a high V when held close to a (+) source charge and at a lower V when held further away. This battery powered electric circuit has areas of high and low potential. As a charge moves through the wire it will experience changes in electric potential. Movement of (+) charge from (A) will occur naturally because it does not require work. The charge looses PE. Chemical Energy is transformed into EPE within the battery. The (+) charge will move through the circuit and do work on the light bulb. It will return to the (-) terminal with low EPE and low Potential.

What is Voltage? Voltage is a change in Electric Potential or the Electric Potential Difference If the (+) charge were moved from (A) to (B), it would require work to increase its Potential Energy. As a result of this Potential Energy, there is also an increase in Electric Potential. This change in Electrical Potential is called Voltage BA

Let’s consider Gravitational Potential. Is it possible to have 0 J/kg and a Gravitational Field of 0 N/kg? 7.4 Q#5 The electric potential at a point is 0 J/C. Is it possible for the electric field at that point to be non-zero. High GP 0 GP If I removed the table, would the ball continue to fall? Is there a gravitational field acting on the ball?

If there is no charge, then there is no electric field. In order for an Electric Potential of 0 J/C, we need charges of equal and opposite magnitude. If this were so, then there would be a force of attraction from the (+) to the (-) charge. Therefore it is not possible. Click here for animation 7.4 Q#5 The electric potential at a point is 0 J/C. Is it possible for the electric field at that point to be non-zero. 0 J/C Electric field vector