Draw Electrostatics
LO's electric field lines Calculate the strength of an electric field
1/23/12 Aim: What is an Electric Field?
Agenda 1. DNQ 2. Demo 3. Notes 4. Example Problems
Test Yourself
In which direction will charges move in the diagram shown: A)To the left + B)To the right C)The electrons will not move -
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Self Check?
Charges flow from less negative to more negative areas.
True False
True
False
Like charges repel.
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Charge Quiz
What are the charges on the objects shown?
Use the + or symbol to indicate the charge on the picture How can you tell?
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Electric Field
The electric force is a field force, it applies force without touching (like the gravitational force) In the region around a charged object, an Electric Field is said to exist
Electric Fields
Charged particles alter the space around them. This alteration is called an electric field. We use a particle of VERY SMALL POSITIVE CHARGE as a field detector.
Forces on Charges
Applet I Applet II
Electric Fields con't. Rules for Drawing Electric Field Lines
1. The lines point TOWARD negative charges and AWAY from positive charges 2. The number of lines drawn is proportional to the magnitude of the charge. 3. No two field lines can cross each other. 4. The line must be perpendicular to the surface of the charge
Electric Field Applet
Charged Objects
All objects that are charged exhibit an electric field.
Ex. Parallel plates, dipoles, charged spheres
uu r u F r 1 kq0 q E E= =( ) 2 q0 q0 r
Electric Field
or
u kq r E= 2 r
Electric Fields is measured in force per unit of charge. E electric field strength, N/C VECTOR q0 + test charge, C q charge producing field, C r distance between charges, m FE Electric Force, N VECTOR kc coulomb constant, 8.99x109Nm2/C2
Conductors in Electrostatic Equilibrium
1. The electric field is zero everywhere inside a conductor. 2. The electric field just outside the charged conductor is perpendicular to the conductor's surface. 3. On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature is the smallest, i.e. AT SHARP POINTS.
Electric Field Strength
Units: newtons per coulomb Defined as E = F/q E = kQ/r2 is the field strength due to a point charge Q as measured at some point `r' from Q.
Practice Problem #1
Practice Problem #2
What is the magnitude of the electric field intensity at a point where a proton experiences an electrostatic force of magnitude 2.30 1025 newton?
Electrostatic Demonstrations
"Static" Electricity
When comb you your hair and...
... bring your comb over a pile of paper bits
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"Static" Electricity
What will happen? A. B.
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"Static" Electricity
The force felt by the paper bits is due to a difference in charge on the comb compared to the paper. This "force of attraction" was first observed by the Greeks who found that piece of amber ("elektron") attracted other objects when rubbed.
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Match the diagram below with its correct description.
1. 2. 3.
Deficiency of Balanced atom Electrons Net Charge Net Charge Zero Positive
A. B.
Excess of Electrons Net Charge Negative
C.
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"Static" Electricity
Usually charges balance each other out, and nothing happens. But when two objects with like charges (all positive or all negative) come together, the charges repel and the objects move away from each other. Objects with opposite charges attract each other because the different charges want to balance each other. Objects can get a negative charge by picking up electrons from other objects.
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Common Misconceptions about "Static" Electricity
Actually, the thing we call static electricity is an imbalance in the amounts of positive and negative charges found on the surface of an object.
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Methods of Charging Induction Direct contact
Results in a transfer of charges
Usually results in a temporary rearrangement of charges
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Common Misconceptions about "Static" Electricity
Lightning is like static electricity, except on a much bigger scale. Both lightning and static electricity happen because of the attraction between the opposite charges.
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PT III: Charging an Electrophorus Rub the Styrofoam plate with cloth or a paper towel. Set the pie tin down on the picnic plate. Hold it by the plastic cup.
Using an Electrophorus to Store Charge Touch the foilwrapped cardboard and the pie tin at the same time, then let go. Hold the plastic cup and lift the pie tin. Touch it to the nail on the Leyden Jar.
Repeat.
The Leyden Jar
The Leyden jar is a device for storing electric charge invented in 1745 by Pieter van Musschenbroek (17001748). Mr. Muschenbroek's bottle is referred to as the "phial" by Benjamin Franklin. Today, we would call it a capacitor.
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Charging by Induction Quiz
Does the diagram below illustrate permanent or temporary charging by induction?
Step I Step I I Step I I I Step IV
Permanent
Temporary
Electroscope