Electric Fields

1. Electric Fields

2. Fields: Action at a distance • Remember in the lab, a charged balloon was able to attract small pieces of paper without touching them?

3. Non-contact forces: • The electric force (and the gravitational force) are non-contact forces. (Can you name some contact forces?) • Gravity can attract objects without touching them. • The electric force can attract or repel objects without touching them.

4. Electric Field • Two charged balloons • How can one balloon reach across space and push a second balloon away? • The best explanation to this question involves the concept of electric field.

5. The Electric Field • There is something different about the space surrounding a charged object. • Any other charged object that is in that space feels the effect of the charge. • A charged object creates an electric field - an alteration of the space in the region that surrounds it. • Other charges in that field would feel the alteration of the space. • Whether another charged object enters that space or not, the electric field exists.

6. Electric Field An electric field is defined as being present in any region where a charged object experiences an electric force. This is a fancy way of saying that the only way we can tell if a field exists is to place a test charge at that spot and see if it feels a force. (In other words, it takes one to know one.)

7. A stinky analogy…. • Anyone who has ever been near a skunk that has sprayed his foul-smelling scent has experienced a stinky field. There is something about the space surrounding a skunk that exerts an influence upon people who enter that space. • When you get close enough, your detector (i.e., your nose) begins to detect the presence of a stinky field. As you move closer and closer to the skunk, the stinky field becomes more and more intense.

8. Our analogy… • In the same manner, an electric charge creates an electric field - it has altered the nature of the space surrounding it. And if another charge gets near enough, that charge will sense the effect. • An electric field is sensed by a detector charge in the same way that a nose senses the stinky field. • The strength of the stinky field is dependent on the distance from the skunk and the amount of spray. • Similarly, the strength of the electric field is dependent on the amount of charge that creates the field and the distance from the charge.

9. . P m F The Concept of a Field A field is defined as a property of space in which an object experiences a force. Above earth, we say there is a gravitational field at point P. A mass m experiences a downward force at that point. The direction of the field is determined by the force.

10. The Concept of a Field . P A field is defined as a property of space in which an object experiences a force. Important note: The field exists, whether or not another mass is present.

11. F +q + + E + + r Q + + + Electric Field + The Electric Field 1. An area of influence around a charged object. . + P 2. The magnitude of the field is proportional to the amount of charge creating the field, and on the distance from the charge. 3. Electric field is a vector quantity. 4. The magnitude of E is given by the formula:

12. Calculating the strength of an electric field: From Coulumb’s Law, the force on a charge q when it’s near another charge Q that is creating a field is: The electric field E is therefore:

13. Check your understanding The electric field strength in a certain region of space is 15 Newtons/Coulumb. What is the force experienced by a 5.0 C charge placed at that point in the field?

14. Like gravity and electric force, field strength is calculated with an inverse squareformula: • Electric field strength is location dependent, and its magnitude decreases as the distance from a location to the source increases. • If separation distance increases by a factor of 2, the electric field strength decreases by a factor of 4 (22). If the separation distance increases by a factor of 3, the electric field strength decreases by a factor of 9 (32).

15. Check your understanding: Charge Q acts as a point charge to create an electric field. Its strength, measured a distance of 30 cm away, is 40 N/C. What is the magnitude of the electric field strength that you would expect to be measured at a distance of ... • 60 cm away? • 15 cm away? c. 3 cm away?

16. The Direction of Electric Field The direction of an electric field at any point is the same as the direction that a positive charge would move IF it were placed at that point.

17. E = ? . P r 3 m -Q ExampleWhat is the electric field at point P, a distance of 3.0 m from a negative charge of –8.0 x 10-9 C? First, find the magnitude: E = 8.00 N/C The direction is the same as the force on a positive charge if it were placed at the point P: toward –Q. E = 8.00 N, toward -Q

18. Sample Problem • What is the electric field strength 0.30 m away from Van de Graff generator with a charge of 1.2 x 10-6 C ? • What is the force on a 0.1 x 10-6 C test charge at this point from .30 m away from the generator ?

19. - + + - + - + - Q + - + - + - + - -Q Electric Field Lines Electric Field Lines are imaginary lines drawn in such a way that their direction at any point is the same as the direction of the field at that point. Field lines go away from positive charges and towardnegative charges.

20. Electric Field Lines It is very important that you notice that the map lines have a direction to them. The direction represents the motion of the positive test charge when placed at different points around the field.  You should also notice that field lines never cross each other.  The closer the field lines are to each other, the greater the field intensity or field strength.

21. Check your understanding: True or False: Every electric charge is surrounded by an electric field.

22. Check your understanding: • Draw arrows (NOT charges) on this diagram to indicate that the charge on the left is negative, and the charge on the right is positive: