1. Electric field
Physics 122
11/18/2018
Lecture II

2. Workshops We are adding new workshop times as we speak.
I’ll send today to everyone once they are open.
11/18/2018
Lecture II

3. Website Both workshop modules and homework problems are from Giancoli
Different, but deal with the same concepts
Workshop problems are a bit easier, helpers for your homework problems
Questions and problems are assigned
11/18/2018
Lecture II

4. Lab sign up Labs start this week!
Remember to do a little homework from lab manual before the first class
Sign up at
In case of problems send to
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Lecture II

5. Electric field
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6. Concepts Primary concepts: Secondary concepts: Electric field
Field lines
Shields
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7. Laws
Superposition principle
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8. Skills Calculate electric field of a system of charges 11/18/2018
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9. Coulomb’s law + 1 2 1 2 - +
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10. System of charges Calculate the net electrostatic force on particle 3.
Input:
Q1= - 86mC= C
Q2= + 50mC= C
Q3= + 65mC= C
r13=60cm=0.60m
r23=30cm=0.30m
What if the value of Q3 changes?
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Lecture II

11. Electric field + + F – force between two charges(N)
Q – electric charge (C= Coulomb)
E – electric field created at point 1 by charge 2
Charge 2 has changed the property of space at point 1
Charge 1 is experiencing this change + + 1 2
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12. Electric field
E – electric field is the force F exerted on a small positive test charge, divided by the magnitude of this charge q.
Electric field is a vector.
Electric field is measured in N/C.
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13. Electric field Electric field is like the ski slope Charge – a skier
The slope is there whether you ski down or not
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14. Field lines
Field lines at each point are parallel to the electric field at this point.
Let’s take a sample (+) charge and examine electric field pattern around a point positive charge.
Like signs repel.
Field lines are going radially away from the positive charge + + + + + + + + +
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15. Field lines
Field lines at each point are parallel to the electric field at this point.
Let’s take a sample (+) charge and examine electric field pattern around a point negative charge.
Unlike signs attract.
Field lines are going radially toward the negative charge + + + - + + + + +
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16. Superposition of fields
Principle of superposition:
Net field created by a system of charges is a vector sum of fields created by individual charges:
Positive test charge + + - 1 2
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17. Rules of field lines Field lines Are parallel to the electric field
Point away from positive charges
Point toward negative charges
Never cross
Can be open ( go to infinity)
Their density represents the intensity of the electric field.
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18. Constant field -Q +Q
Electric field is constant between two very large parallel plates,
one with a positive charge +Q,
the other one with a negative charge - Q
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19. Charges in electric fields
Positive charges experience force along the direction of the field.
Negative charges – against the direction of the field. + -
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20. Electric field in conductors
Conductor – a material with abundant free (to move) charge
E=0 in good conductors in the static situation.
E is perpendicular to the surface of conductor.
Metal hollow boxes are used to shield electric fields. + + + + + - - - - -
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21. Test problem #4
Points A, B and C are at various distances from a given point charge.Which statement is most accurate? The electric field strength is
A Greatest at point A.
B Greatest at point B.
C Greatest at point C.
D The same at all three points.
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22. Test problem #5
Which diagram best represents the electric field lines around two oppositely charged particles?
A C
B D
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23. Test problem #1
Two point charges are separated by 10meters. If the distance between them is reduced to 5.0meters, the magnitude of the electrostatic force exerted on each charge
A decreases to half its original value.
B increases to twice its original value.
C decreases to one quarter of its original value.
D increases to four times its original value.
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24. Test problem #2
If the magnitude of the charge on two identical charged bodies is doubled, the electrostatic force between the bodies will be
A doubled.
B halved.
C quadrupled.
D unchanged.
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25. Test problem #3
If the charge of one point charge is doubled and that of the other charge stays the same, the force will be
A doubled.
B halved.
C quadrupled.
D unchanged.
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26. Motion in electric field
Remember F=ma ?
This tells about the effect the force will have on an object with mass m
In electric field F=qE
This tells you about the amount of force electric field exerts on the object
Answer the question: what is the origin of forces? E v d
E=2.0x104N/C
e=1.6x10-19C
m=9.1x10-31kg
d=1.5cm=0.015m
v=?
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27. Motion in electric fields
a is always || to net F
Remember v is not always || to a (projectile, circular motion) vx vy
vx=const
vy=at=tF/m=tEq/m
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