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Unit 2: & Lesson 2 – Electric Fields Two types of - Contact and Non-contact

Contact – friction, normal , pushing, etc. Objects touch.

Non-contact – gravity, electric force etc. Objects do not have to touch. Forces act over a distance.

Electric – Area around a charged object where another charge experiences an electric force ie. electric field exerts a force on any object in its vicinity.

The closer a charged object is brought to the charged object creating the field, the greater the force exerted on it.

How can we tell if an electric field exists?

By placing a small test charge (+) at the location to be tested.

Test charge: A positive charge of very small magnitude, used to determine the direction of the electric field.

(+): because of convention small: so that it does not interfere with the electric field we are trying to measure

Electric field lines: Lines of electric force that represent electric field.

 Density measures strength of electric field (closer field lines = greater field strength or )

 Direction is same as the direction a small positive test charge would move if in the field.

 Electric field lines never cross (if they crossed, it would mean that a (+) test charge could be moving in multiple directions simultaneously - not possible)

 Field lines are directed from (+) charge to (-) charge.

Unlike charges

Equipotential Lines (curves)

Lines of constant Perpendicular to the electric field lines Have no direction Represent the amount of at a given region

http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html

Electric field strength (or intensity) E: ratio of force to the amount of test charge; force per unit charge at that point

SI unit: Newtons/ (N/C)

E = F / qo

E - is electric field strength (N/C)

F - Force produced on charge (N)

qo - Charge on object in the field (test charge) - (C)

Show that electric field exists regardless of the presence of the test charge qo ie.,

E - is electric field strength (N/C) q - source charge (charge producing the E field) (C) d - distance of test charge from source charge (m)

By definition,

Example:

How strong would an electric field have to be to produce a force of 1.00 N if the charge in the field was 1.00 x 103μC?

Electric Potential

Two points are said to differ in electric potential if is done to move a charge from one point to another point in an electric field.

The Potential Difference (PD - measured in , V) between two points in an electric field is the work done per unit charge as a charge is moved between these two points.

W ΔV = /q

ΔV = Voltage, Potential Difference

W = Work done to move charge between two points

PE = Potential due to position (separation) of charge q = Amount of charge

How is potential difference (voltage) related to E?

∴The electric field strength between 2 points can be found by dividing the pot. diff. between them by the distance separating them.

Example Problems:

1. 30 J of work is done to move a +5 C charge from one point to another in an electric field. What is the difference in potential between the two points?

2. A test charge of +5 x 10-4 C is in an electric field which exerts a force of 2.5 x 10-4N upon it. What is the strength of the electric field?

3. A test charge of +80 μC is placed in a 50 N/C field. What force does it experience?

4. A +19 μC charged sphere produces a 1.7 x 10 3 N/C upon a 0.5 μC test charge.What force is the charge subjected to? How far away is the test charge from the sphere?

5. What is the charge on an object that experiences a force of 0.6 N in an electric field, between two plates separated by 12 cm, and having a potential difference of 225 V?

6. A +4.9 μC charge produces an electric field of 3.6 x 104 N/C upon a (+) test charge. How far away is the charge? (1.11 m)

7. How much energy is needed to move a charge of +5μC from A to B if the potential of A relative to is +100 V and the potential of B relative to ground is +112 V?