J. PulickeelApril 2009

SPH 4U1

Electric ForcesAn 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 FieldsA field force is sometimes referred to as an

action at a distanceThe 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 FieldsElectric 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 ForcesGravitational Forces Electric ForcesA Force of attraction

between two objects.A force between two

charged particles. It can be attractive or repulsive.

Gravitational FieldsIt 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/kgWhat force would a 95 kg person experience at this location?

Is it valid for other locations?

Electric FieldsSimilarly, a test charge will exert a force

around itself that attracts or repels other charges

7.5 N/kg

9.8 N/kg

4 N/kgLower 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 FieldsDepending 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 EnergyPotential 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 EnergyGravitational 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 EPEHigh EPE

EPE KE

Lower EPE

Gravitational and Electrical Potential EnergyIt 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 Potential1 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 PotentialGravitational Potential Energy depends on mass and height

Gravitational Potential tells us the amount of potential energy per kilogram at a particular spot.

1 kg, 2m20J PE

1 kg, 1m10J PE

2 kg, 2m40J PE

2 kg, 1m20J PE

Gravitational Potential

20J/kg

10J/kg

0 J/kg

Gravitational Potential

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 PE1 C

20 Joule PE2 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

B A

Let’s consider Gravitational Potential. Is it possible to have 0 J/kg and a Gravitational Field of 0 N/kg?

7.4 Q#5The 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#5The 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