Everything Science Grade 11

CHAPTER 17. ELECTROSTATICS 17.4

Then for Q 2 :

E = k

Q

r^2

=

(8. 99 × 109 )(4× 10 −^9 )

(0,3)^2

= 2, 70 × 102 N.C−^1

We need to add the twoelectric fields because both are in the same direction.

The field is away from Q 1 and towards Q 2. Therefore,

Etotal= 6, 74 × 102 + 2, 70 × 102 = 9, 44 × 102N.C−^1

17.4 Electrical potentialenergy and potential

ESBHK

The electrical potential energy of a charge is the energy it has because of its position relative to other

charges that it interacts with. The potential energyof a charge Q 1 relative to a charge Q 2 a distance r

away is calculated by:

U =

kQ 1 Q 2

r

See video: VPlsc at http://www.everythingscience.co.za

Example 7: Electrical potential energy 1

QUESTION

What is the electric potential energy of a 7nC charge that is 2 cm froma 20nC charge?

SOLUTION

Step 1 : Determine what is required

We need to calculate the electric potential energy (U).

Step 2 : Determine what is given

We are given both charges and the distance between them.

Step 3 : Determine how to approach the problem

We will use the equation:

U =

kQ 1 Q 2

r

Step 4 : Solve the problem