Physics Chapter 21
electric potential difference
the work done moving a positive test charge between two points in an electric field divided by the magnitude of the test charge
T or F: The direction of an electric field between two parallel conducting plates is *from the positive plate to the negative plate*
true
The potential increases in the direction *opposite* the electric field direction.
true
law of conservation of energy
allows you to predict the motion of objects without knowing the forces in detail
What produces an electric field?
an electric charge
Why must you use a test charge to observe an electric field?
an electric field can only be observed when it produces forces on other charges
the potential difference between 2 points does *NOT* depend on _________
magnitude of the test charge
electric field strength equation
E = F/q
volt
1 j/c
how can i detect field lines if i can't see them? (3 things)
1. place test charge at that point and determine if theres a force on it 2. divide force by test charge to find magnitude 3. to choose magnitude of test charge, pick one that's small compared to the magnitude of the charges producing the field
What charge would experience a 1.2x10^3 N force when in a uniform electric field of 4.4x10^6 N/C?
2.7x10^-10 C *process*: use formula E=F/q E= 4.4 x 10^6 N/C F= 1.2 x 10^3 N q=what you are solving for plug in numbers: (4.4x10^6)=(1.2x10^3)/q multiply q by both sides: (4.4 x10^6)q = (1.2 x 10^3) divide and get answer
It takes 1.22 J to move a charge of 3.22 C between two points in an electric field. What is the potential difference between these two points?
3.8 x 10^-1 *process*: use formula ΔV=W/q ΔV=what you are solving for W= 1.22 J q= 3.22 C plug in numbers: ΔV=1.22/3.22
Two plates of a capacitor are 12 cm apart and have an electric field of 350 N/C. What is the potential difference between the plates?
4.2 x 10^3 V *process*: use formula ΔV=Ed ΔV= what you are solving for E= 350 N/C d= 12 cm plug in numbers: ΔV=(350)(12) multiply and get answer
A negative charge of 1.4 x 10^7 C experiences a force of 1.2 N. What is the electric field magnitude at this location?
8.6 x 10^6 N/C *process*: use formula: E=F/q E=what you are solving for F= 1.2 N q= 1.4 x 10^7 C plug in numbers: E=1.2/(1.4x10^7) divide and get answer
capacitance equation
C= q/ΔV
explain the Van de Graaf generator (and be able to draw it)
It builds up a large amount of net charges in its metal dome. A belt is driven by two rollers, one attached to point A and the other is in the metal dome (B). When it's turned on, the roller builds up a negative charge and the belt builds up a positive charge. The positive ions are transported to the metal dome.
How can a Van De Graaff generator be used to show field lines?
It transfers large amounts of charge form one party of the machine to the top. If someone touches it, it is charged electrically. The charges on the person's hair repel one another and follow the field lines.
A positive test charge is located at Point A. If the test charge is moved to a Point B and then back to Point A, what is the change in electric potential?
The electric potential does not change
capacitor
a device with a specific capacitance that is used in electric circuits to store electrical energy
field lines must NEVER __________
cross
The electric potential ____ when a positive charge is moved toward a negative charge
decreases
Only ____ electric potential can be measured.
differences in
Do field lines and electric fields actually exist? How are field lines and electric fields useful?
electric fields do, but field lines do not. Field lines provide a model of an electric field but electric fields provide a method of calculating the force on a charged body.
field lines must be tangent to the direction of the _______ at any point.
field
the potential difference between two points ONLY depends on ____ and ______
field and displacement
what does the potential difference depend on?
field and displacement
what is the difference between field and force?
field does NOT depend on the test charge used to measure it, force does
the greater the line density, the ________ the magnitude of the field.
greater
The potential is *lower* near the positively charged plate.
higher
When you do positive work on a two-charge system, the electric potential energy ____
increases
Why should an electric field be measured by a small test charge?
it should be small so it does not not move the charge to a new location or change the force
what is electric potential difference is measured in?
joules per coloumb j/c (or volts)
If arrows represent electric field vectors in a picture of an electric field, how are the magnitude and the direction of the field shown?
length of the arrow: magnitude direction of the arrow: field direction
the greater the line density, the greater the ______ of the field.
magnitude
Two large, flat conducting plates parallel to each other can create a constant electric force and field. *Both are charged negatively.*
one is negative and one is positive
What do positive electric field lines look like?
pointing *away* from the center
What do negative electric field lines look like?
pointing *towards* the center
field lines always start from _________ charged objects and end on _________ charged objects.
positively ; negatively
The *electrical* difference between two points separated by a distance d in a uniform field is represented by the equation ΔV=Ed.
potential
electric potential difference ONLY depends on ________ and ________
starting and final positions on the path
the closer electric field lines are, the ________ they are
stronger
field lines must be _______ to the direction of the field at any point.
tangent
How does Coulomb's law relate to test charges?
the force exerted on a test charge in an electric field is proportional to the size of the test charge
electric field line
the lines providing a picture of the size and strength of the field around a charged object
electric potential difference does NOT depend on __________
the path used to go from one position to another
equipotential
the potential difference of zero between two or more positions in an electric field
electric field
the property of the space around a charged object that exerts a force on other charged objects
capacitance
the ratio of the net charge on one plate to the potential difference, measured in farads, between two plates of a capacitor
How can you find the electric field from two charges?
use the vector sum of the 2 charges
one j/c = one ____
volt
electric potential difference equation
ΔV= W/q
electric potential difference equation (distance)
ΔV=Ed