Electric Potential

SPH4U – Grade 12 Physics

Unit 1

Quick Review Mini Quiz

The symbol stands for: _______________

Field lines show the direction of the electric force on a positive/negative test charge placed at every point in the field.

Electric field strength is measured in: _____

The electric field between two parallel plates of charge is uniform/non-uniform and parallel/perpendicular to

the plates.

Quick Review Mini Quiz

The symbol stands for: Electric Field

Field lines show the direction of the electric force on a positive/negative test charge placed at every point in the field.

Electric field strength is measured in: N/C

The electric field between two parallel plates of charge is uniform/non-uniform and parallel/perpendicular to

the plates.

Electric Potential Videos by Derek Owens Electricity concepts can be difficult for us to wrap our

minds around. Today we will watch a few videos by Derek Owens to get more of a visual concept of what is happening.

It is probably a good idea to take notes of these videos, because it might help your understanding. All of these video links will be posted to our Wiki-classroom for you to view again later.

http://www.youtube.com/watch?v=wT9AsY79f1k http://www.youtube.com/watch?v=LkIai_KXGxg http://www.youtube.com/watch?v=NrpFMyVLSck http://www.youtube.com/watch?v=A9TDzsnYkHo

Electric Potential and Fields

Recall: An electric field is defined as the region in which a force is exerted on an electric charge.

We know that a charged particle will feel a force if it is in the presence of an electric field (either repulsion or attraction). The magnitude of this force is given by:

qFE

Electric Potential and Fields

Several things are implied by this: If there is a net force acting on a charge, that charge

must be accelerating. This is due to Newton’s second law.

If there is a force being exerted on the charge over some distance, then by we know that work is done on the charge.

If work is being done on the charge, the charges must be given energy, because energy is the ability to do work.

dFW

Electric Potential and Fields

Let us define electric potential energy EE as the energy stored in a system of two charges a distance apart.

We can also define EE as the energy stored in an electric field that can do work on a positively charged particle.

d

Electric Potential and Fields

We also know that if the electric force does an amount W of work on a charged particle, the change in the electric potential energy is

Combining everything we get:

WEE dFE EE

dqEE

Electric Potential and Fields

This gives us the change in the potential energy as the charge moves through a displacement in a region where the electric field is parallel to the displacement. Note that the path taken does not effect .It is dependent on the starting and ending locations only.

d

EE

Electric Potential and Fields

For ΔEE > 0, work is done against the field. (-

W). This results in energy stored in the field.

Electric Potential and Fields

For ΔEE < 0, work is done by the electric field (+W) on the particle, which typically increases the potential energy of the particle.

Example 1

An electron enters a uniform electric field of 145N/C pointed toward the right. The point of entry is 1.5m to the right of a given mark, and the point where the electron leaves the field is 4.6m to the right of that mark. (a) Determine the change in the electric potential energy

of the electron(b) The initial speed of the electron was 1.7 x 107m/s

when it entered the electric field. Determine its final speed.

Example 1

An electron enters a uniform electric field of 145N/C pointed toward the right. The point of entry is 1.5m to the right of a given mark, and the point where the electron leaves the field is 4.6m to the right of that mark. (a) Determine the change in the electric potential energy

of the electron (Ans: 7.2 x 10-17 J)(b) The initial speed of the electron was 1.7 x 107m/s

when it entered the electric field. Determine its final speed. (Ans: 1.1 x 107 m/s)

Electric Potential

Electric Potential (V) is the value in volts of the potential energy EE per unit positive charge for a given point in an electric field.

Electric potential is also a measure of how much electric potential energy is associated with a specific quantity of charge at a particular location in an electric field.

Electric Potential

Electric Potential is measured in volts. 1V = 1J/C.

Electric Potential is calculated using the equation:

q

EV E V = Electric Potential (Volts)

EE = Electric potential Energy (Joules)

q = charge (Coulombs)

Electric Potential

Electric Potential difference ΔV is the amount of work required per unit charge to move a positive charge from one point to another in the presence of an electric field. It is often referred to as “voltage”.

It is given by the equation:q

EV E

Electric Potential

If we have a uniform electric field ε, we can also use the following equation for an electric potential difference: dV

Electric Potential

Rearranging this formula, we see how a non-uniform electric field must depend on the electric potential difference and the change in position in the field:

d

V

Electric Potential

The equation tells us that the electric field is largest in regions where V is large and changes rapidly with small changes in displacement.

The electric field is zero in regions where V is constant.

Also, because of the negative sign in the equation, the electric field points from regions of high potential to regions of low potential.

Example 2

An old cathode ray tube creates a potential difference of 1.6 x 104V across the parallel accelerating plates. These plates accelerate a beam of electrons toward the target phosphor screen. The separation between the plates is 12cm. Calculate the magnitude of the electric field.

Example 2

An old cathode ray tube creates a potential difference of 1.6 x 104V across the parallel accelerating plates. These plates accelerate a beam of electrons toward the target phosphor screen. The separation between the plates is 12cm. Calculate the magnitude of the electric field. (Ans: 1.3 x 105 N/C).

Homework

Read Sections 7.4 & 7.5Make additional notes to supplement the

lesson notes. Complete the following questions:

Pg. 354 # 1, 2, 4, 5