**Evans SPH 4U1**

**Physics Grade 12**

**Unit 6: Electricity**

**Note 5: Electric
Potential**

Reference: Chapter 7.4

**Electric
Potential Energy, Ee:**

Recall: The magnitude of the force of gravity between any 2 masses is and

the gravitational potential energy between any 2 masses is (if the zero value of Eg is chosen as when the 2 masses are infinitely apart)

Similarly, electric potential energy stored in the system of 2 charges, q1 and q2 is .

~ If q1 and q2 are opposite charges, they attract, and Ee is negative (as in the gravitational case when the masses attract).

~ If q1 and q2 are similar charges, they repel, and Ee is positive (i.e. energy is stored by moving them closer together).

~ The zero level of electrical potential energy is approached as the separation of the charges approaches infinity.

~ Scalar quantity measured in joules

**
Electric Potential, V:**

**
~ the electric potential energy per unit positive charge (not to be confused with Electrical Potential
Energy, Ee)**

** ~ **Scalar quantity

~ One volt is the electric potential when one joule of work is done in moving a charge of 1 coulomb from infinity to that point.

~ The electric potential of a charge at a point is the work done in bringing a positive charge from infinity to the point.

~ The electric potential at a point due to several point charges is the sum of the electric potentials at that point due to each charge.

Units are Volts (joules/coulomb)

** Example:**

Two charges 5.4 x10^{-7}C
and 3.6 x 10^{-7}C are 36 cm apart. What is the electric potential
midway between these charges? (4.5 x 10^{4} J/C)

**Change in
Electric Potential Energy (Work):**

In moving a charge from point A to point B the work done will be equal to the change in electric potential energy

**Change in
Electric Potential Energy Between 2 Parallel Plates:**

Often we will be interested in the potential difference between 2 points in a uniform electric field (between parallel plates).

Note: From the + plate to the - plate we have a decreasing potential. From the - plate to the + plate we have an increasing potential.

Recall: and therefore

**

**Note that we can only use this in cases
where the Electric Field is constant over a distance such as in our case of
parallel plates where it is a uniform electric field.. We can **not** use it
for point charges where the charge is being moved through a distance where the
Electric Field can vary.

(this is similar to using mgh near the earth's surface but not at farther distances)

Now using substitution:

**Example:**

In the diagram (see
blackboard), sphere A has a charge of 3.0 x 10^{-5}C and sphere B has a
charge of -2x10^{-5 }C. They are placed 160cm apart. Point P is 20
cm from A and point M is midway between the two charges.

Find:

a) the electric
potential difference between M and P. (-1.1 x 10^{6} volts)

b) the work done in
moving a charge of 2.0 x 10^{-8} C from P to M. (-2.2 x 10^{-2}
J)

**Text Questions:**

Page 354 #'s1-4 (warm up questions)

Page 358 Practice Question #6

Page 358-359 #'s 7-9