Study Module 21: Electric Charge - Force - Electric Fields

Consider a conductor at rest in a static electric field. Which statement is true about how the conductor interacts with the electric field?

(These are all the true statements) 1. The electric field is perpendicular to the surface of the conductor at all points along its surface. 2. The electric field inside the conductor is exactly ZERO 3. Any net charge on the conductor resides on the surface of the conductor -Because the charges in the conductor are free to move around, the charges that reside on the surface of the conductor must be experiencing a field that is perpendicular to the surface of the conductor.

Which of the following is true about electric field lines due to static charge distributions?

(These are all true statements) 1. Electric field lines CANNOT cross 2. EFL are directed AWAY from (+) objects and toward (-) objects 3. Electric field lines will be more closely spaced from each other in regions where the electric field is stronger -Electric field is ALWAYS tangent to the electric field line at a given location. Having two lines cross would be a contradiction. -Electric field lines always begin on a positively charged object and end on a negatively charged object

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?

1/9 F -The electric force between two objects is inversely proportional to the square of the distance between the centers of the two objects. -Increasing the distance by a factor of 3m causes the electric force to decrease by 9.

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 -Coulomb's law shows that the electric force between two objects is proportional to the charge of each object. If the charge of either object changes, the force that each object exerts on the other changes in the same way. -BOTH forces in the force-pair must be the SAME. so both are affected the same way.

Which is the best definition of an electric dipole?

An electric dipole is an arrangement of electric charge with two equal but opposite amounts of charge separated by some fixed distance. -It is important that the two charges be the same amount of charge, but have opposite signs. -Opposite but UNEQUAL charges CANNOT be considered a dipole, thought they can be considered a combination of a dipole and a point charge.

How do you calculate the electric force by object A exerted on object A (itself)?

An object never experiences an electric force due to itself.

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?

An oxygen ion with a charge of +2e - "+3 ion" or a "+3e ion" means it has lost 3 electrons -Changing the number of electrons does not change the atom. Solely defined by the number of protons in the nucleus.

Two objects, each with a charge of +1 nC, are sitting alone in space. If we measure the magnitude of the electric field at a location given by vector r, which is equidistant from both objects, what result would we get?

Between E is equal to 0 and E is equal to 1/(4 times pi e0) times (2nC / r squared) -The electric field each object produces is a vector, the total electric field is the sum of those vectors. -the electric field will be zero in a point midway between the two charges and twice the magnitude in a point far away from both charges

An object with a charge of +4 μC is very close to an object with a charge of -1 μC, each one fixed in place. Which of the following is the best expression for the magnitude of the electric field we would find at a distance vector r far away from the pair of objects?

E= 1/4pi e0 time (+3μC)/r2 -Because we are far away from the pair of objects, it is their net charge that matters most in determining the electric field. A good approximation is therefore to treat them as a single point charge with a net charge of +3 μC.

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. -Principle of superposition: applies to forces of all kinds. The net force on an object can always be found by using vector addition to add up all the forces that it experiences from other objects.

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?

Has the same net charge as 11 protons -Removing 11 electrons means there are now 11 protons whose charge is "unbalanced". This leaves the object with a net charge equal to 11 proton charges.

If a composite object is positively charged that means it

Must be either made entirely from positively charged objects or made from objects that have more total positive charge than negative charge -Determining the net charge that an object has is a matter of adding up the total charge of all its constituents.

Objects A and B are electrically repelled from each other. Can we tell what sign the charge of object A is?

No, we cannot tell.

Consider a point with a large electric field. Are there differences in how positively and negatively charged objects are affected by this electric field?

Positively and negatively charged objects both experience equal magnitude forces due to the electric field, but in opposite directions -The underlying physics doesn't favor one type of charge over another. The only difference in how the two charge types are affected is that they are pushed in opposite directions by an electric field.

Consider an electric dipole, composed of charges +q and -q separated by distance d, that is viewed from a large distance (large compared to d). What do we expect about the electric field due to the dipole viewed from this distance?

The electric field is significantly weaker than the electric field due to a single point charge q viewed from the same distance. -At a large distance the dipole will cancel each other out resulting in a weaker net field -However won't be ZERO. Each part of the dipole cannot exactly cancel out.

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 electric field that object A experiences is unchanged. -The electric field at a given location does not depend on the object. -Doubling the amount of A causes A to experience a larger force due to its interaction with the electric field but it own't change anything about the electric field at that location

Can there be a non-zero electric field at a point in space where no charged object is present? Can there be an electric field equal to zero at a point where a charged object is present?

Yes to both -Electric field can have any value (zero or non-zero) independent of whatever object is placed there.

If a composite object is electrically neutral, that means it

can be made from objects that each have zero electrical charge, or made from charged objects with equal amounts of total positive and negative charge - Electrically neutral objects MUST have a total net charge of zero.

One similarity between protons and electrons is

protons and electrons have the same amount of charge, but protons have positive charge, while electrons have negative charge