Module 21 - Electric charge, force, & fields Flashcards
Learn about electric charges, electric forces, and electric fields. Columbs law and electric field equations.
GENERAL CONCEPTS
What is the relationship between electric fields and electric forces?
They are related, but not the same thing.
An electric field results from the effects of one or more charged objects (called source charges), whereas electric force must involve the interaction between at least two charged objects.
GENERAL CONCEPTS
Define an electric dipole.
An arrangement of electric charge with two equal but opposite amounts of charge separated by some distance.
ELECTRIC CHARGE
Objects A and B are electrically repelled from each other. Can we tell what sign the charge of object A is?
No, we cant tell.
If two charges repel we can only tell that the charges are either both positive or both negative. We need more information to determine which is which.
ELECTRIC CHARGE
What are the similarities and differences between protons and electrons.
Same amount of charge (magnitude)
Different charge signs (positive vs negative)
Different masses
ELECTRIC CHARGE
Oxygen is the element that has 8 protons in its nucleus. If you start out with a neutral oxygen atom and it then loses 2 electrons, what is a good way to describe the resulting object and its charge?
An oxygen ion with a charge of +2e.
Since the oxygen atom is losing electrons (negative charges), it is becoming more positively charged, thus +2. The identity of the atom does not change, since it is the number of protons that determines this. So it is still oxygen, but we refer to it as an ION.
ELECTRIC CHARGE
Suppose an object starts out electrically neutral. Through some process, 11 electrons are removed from the object. What is the electric charge of the object afterward?
+11 protons
Since the object started out neutral, and now has lost 11 electrons. We can say that it has a positive charge of 11 protons. This is because the charge of protons and electrons are equal.
ELECTRIC CHARGE
If a composite object is electrically neutral it must be made of:
-Objects with all zero electrical charge
-Objects with equal amounts of positive and negative charge
The net charge of all objects must be zero in order for the composite to have a charge of zero and be neutral.
ELECTRIC CHARGE
If a composite object is electrically positive it must be made of:
-Objects with all positive electrical charge
-Objects with more positive electrical charge than negative electrical charge
The net charge of all objects must be positive in order for the composite to have a positive charge overall.
ELECTRIC FORCE
Suppose object A is electrically charged and is experiencing forces from three other charged objects. How should the total effects of those three objects be combined in order to find the total electric force?
Electric force vectors are calculated for object A and each of the other three objects. These three force vectors are then added using vector addition to obtain the total electric force.
ELECTRIC FORCE
The electric force between objects A and B is F. If the charge of object A were twice as large as it is, but everything else was kept the same, what would be the new electric force between objects A and B?
2F, or double the original force.
Look at Coulomb’s equation. If one of the q’s in the numerator is doubled and all other variables remain the same, the final result will be doubled. (To test this, set all the variables equal to 1 and solve, then redo the equation but set one of the q variables to 2).
ELECTRIC FORCE
The electric force on object A by object B is F. If the charge of object B were only half as large as it is, but everything else was kept the same, what would be the new electric force on object B by object A?
1/2F, or half the original force.
Look at Coulomb’s equation. If one of the q’s in the numerator is doubled and all other variables remain the same, the final result will be halved. (To test this, set all the variables equal to 1 and solve, then redo the equation but set one of the q variables to 0.5).
ELECTRIC FORCE
The electric force on object B by object A is F. If the distance between the objects were tripled, but everything else was kept the same, what would be the new electric force on object B by object A?
1/9F, or one ninth of the original force.
Look at Coulomb’s equation. If the r^2 in the denominator is tripled and all other variables remain the same, the final result will be one ninth due to the ^2. (To test this, set all the variables equal to 1 and solve, then redo the equation but set the r variable to 3).
ELECTRIC FORCE
How do you calculate the electric force by object A exerted on object A (itself)?
Trick question, an object never experiences electric force due to itself.
Look at Couloumb’s equation. If the distance between the object and itself is 0, r = 0. Since r is in the denominator, you cant use the equation at all. (Macisaac wont let us divide by 0).
ELECTRIC FIELDS
Can there be a non-zero electric field at a point in space where non charged object is present?
Yes.
The electric field is a property of a location in space. Therefore, the field can have any value (zero or non-zero) totally independent of what object is placed there. The field does not depend on an object experiencing it.
ELECTRIC FIELDS
Can there be an electric field equal to zero at a point where a charged object is present?
Yes.
The electric field is a property of a location in space. Therefore, the field can have any value (zero or non-zero) totally independent of what object is placed there. The field does not depend on an object experiencing it.
ELECTRIC FIELDS
Consider and electric dipole with charges +q and -q. If the dipole and a single point charge (q) are viewed from a large distance, which field will look weaker?
The field due to the dipole will be much weaker than the field due to the point charge.
Since the dipole is made up of two equal and opposite charges, the fields due to each part of the dipole will cancel each other out for the most part. So when it is viewed from afar it will seem like a very weak field.
ELECTRIC FIELDS
Two charges, +4μC and -1μC are very close together. What is the equation that would best represent the electric field at some distance far away from the charges?
E = k · 3μC / r^2
Since we are looking a large distance away from the charges, we can treat them as a single point charge by summing them up (+4μC -1μC = +3μC).
ELECTRIC FIELDS
Two objects, each with a charge of +1μC, are sitting alone in space. If we measure the magnitude of the electric field at a point P at a distance of r, which is equidistant from both objects, what result would we get?
Between E = 0 and E = k · 2μC / r^2
If the point P is between the two charges, they will cancel out, E = 0.
If the point is far away from both charges , they will act as one, E = k · 2μC / r^2
ELECTRIC FIELDS
Consider a conductor at rest in a static electric field. The electric field is perpendicular to the surface of the conductor at what point?
At all points along the surface.
This is because the charges in the conductor are free to move around the surface of it. Thus all points on the surface must be experiencing a field that is perpendicular to it.
ELECTRIC FIELDS
Consider a conductor at rest in a static electric field. What is the electric field inside the conductor?
Exactly zero.
Because the charges move around, they will look for a spot where the field is zero (any spot where the field is not zero will cause them to be pushed around). So once the charges settle into place, the electric field will be zero everywhere inside the conductor.
ELECTRIC FIELDS
Consider a conductor at rest in a static electric field. Where does any net charge reside in regard to the conductor?
The charges reside on the surface of the conductor.
Because the charges are free to move around, they will repel each other to the surface of the conductor (as far apart from each other as possible) before they settle into equilibrium.
ELECTRIC FIELDS
Consider a conductor at rest in a static electric field. How are positive and negative charges affected by the electric field?
Both positive and negative charges will both experience equal magnitude forces due to the electric field, but in opposite directions.
Underlying physics doesn’t favor one type of charge or another, therefore both positive and negative charges will be equally affected.
ELECTRIC FIELDS
Consider a conductor at rest in a static electric field. Which direction will positive and negative charges travel towards?
Positive charges - With the field
Negative charges - Against the field
Fields travel from positive end to negative end, therefore the positive charges within the field will be attracted to the “head” of the field and negative charges to the “tail”.
ELECTRIC FIELDS
Consider a conductor at rest in a static electric field. How does the strength of the field affect the spacing of the field lines? Can the lines ever cross?
Stronger field = Close field lines
Weaker field = More spaced out field lines
***Field lines can never cross
Since field lines indicate the direction of the field, they must always be parallel. If field lines were to cross it would indicate that the field is travelling in two difference directions at once (not possible!).
ELECTRIC FIELDS
Consider a conductor at rest in a static electric field. Which direction do the field lines travel?
Field lines go from positive to negative
ELECTRIC FIELDS
Suppose object A is experiencing an electric field with a magnitude of E at its location. If the charge on object A is doubled, what happens to the electric field it is experiencing?
The field is unchanged.
Since A is experiencing a field and not a force, its magnitude does not matter. The charge experience a field does not affect the magnitude of the field, since it is unrelated.