Final Exam Physics 1B true/false

Two objects each having zero charge cannot exert electric force on each other

false (Coulomb’s law states that q1=q2=0 however this does not apply to ALL objects)rce on each other

If two point charges with charges q1 and q2 are in water, the total force magnitude on each will be much lower than is predicted by plugging q1 and q2 into Coulomb’s Law

true (use dielectric equation, dielectric constant in medium is always greater than the force the object would experience in empty space)

The charge of an object depends only on the difference between the number of protons it has and number of electrons it has

true (charge is e(Np-Ne))

atoms contain three types of particles that have charge: protons, neutrons, and electrons

false (neutrons have no charge silly)

If two point charges interact and the sign of one of the charges is changed from + to -, this will change the magnitude of the electric force they exert on each other

false (magnitude of the force depends only upon absolute values)

A metal object is an electrical conductor because many electrons leave individual metal atoms and can move relatively freely across the whole object

true

Electric polarization of an atom refers to an electric force causing the positions of electrons shifting relative to the positions of the protons in the nucleus.

true (this is in fact the description of electric polarization nerd)

If two neutral objects are rubbed together they may become charged, and for ordinary objects in air (e.g., rubber, plastic, metal, glass/ceramic, wood, fabric, wood, etc.) it is usually electrons that transfer

true

Protons in the nucleus of atoms exert repulsive electric force on each other but don’t fly apart because a different fundamental force (the “Strong Force”) holds them together

true

The electric field produced by a negative point charge points away from the point charge

false (electric field points toward negative charges)

If a proton is placed in a uniform electric field and has a certain magnitude acceleration, an electron that is placed in the same field will have a lower magnitude acceleration

false (the force magnitude F=qE will be the same since both the proton and electron have the same absolute value of charge)

If a charged object is immersed in water, it will cause water molecules to become polarized and these water molecule will produce an electric field that adds to that produced by the object

true (the electric field produced by the object itself is not changed but the total field in the medium is altered by the presence of the medium)

The electric field is always zero inside a conductor

false (only zero inside a conductor at equilibrium, meaning there are no charges inside moving across it)

The electric field produced outside a uniformly charged sphere with charge q, at a distance r from the sphere, is the same as that produced by a point charge with charge q at a distance r from the point charge.

true (formulas for E fields are the same for both)

If there is a positive point charge and a negative point charge the magnitude of the total electric field produced by them is equal to the magnitude of the electric field produced by the positive one minus the magnitude of the electric field produced by the negative one.

false (electric fields are vectors that can point in any direction and they must be added as vectors. The sum of two vectors would only be equal to the difference of their magnitudes in a special case where they point in exactly opposite directions)

The electric field produced by a long, straight, uniformly positive charged wire depends on the distance from the wire and points away from the wire.

true (E fields point away from positive charges and the formula we have for E field produced by a long straight wire is 1/r where r is the distance from the wire)

the electric field produced by any object can be calculated by dividing up the object into tiny pieces, calculating the electric field produced by each piece, and then adding all these electric fields

true

the electric field produced by an infinitely long, uniformly charged cylinder that is positive charged points away from the cylinder.

true

A positive point charge is only acted on by a positive electric potential and a negative point charge is only acted on by a negative electric potential

false (sign of the potential doesn’t matter, a positive point charge tends to move toward lower potential regardless the sign of the potential)

A positive point charge tends to move towards a position where the electric potential acting on it is higher and this results in an increase in the potential energy of the system

false (a positive point charge tends to move towards a position where the electric potential acting on it is lower and this results in a decrease in the potential energy of the system)

If an electric field acts on an electric dipole the dipole moment vector will tend to rotate to point in the same direction as the electric field

true (electric forces cause torque to be exerted on the dipole in a direction that causes the dipole moment vector to rotate to point in the same direction as the electric field)

A uniformly charged object can have an electric dipole moment

false (only a non-uniformly charged object has, for example, more positive charge on one side and more negative charge on the other side has a dipole moment)

A capacitor is a conductor with charge Q separated by an insulator from a conductor with charge -Q

true (this is in fact the description of a capacitor)

When two capacitors are in parallel the potential difference across each is the same

true (this is true when any two or more circuit elements are in parallel)

Current characterizes movement of positive and/or negative charged particles through a conductor

true (this is the definition of current)

if a point charge q experiences a force magnitude F=qE this does not mean it produces an electric field of magnitude E

true (if a point charge experiences a force of magnitude F = qE this means that the field E must be one produced by other charged objects, not the field produced by q, because an object cannot exert force on itself)

the magnitude of acceleration of a point charge in a uniform electric field is inversely proportional to the charge of the point charge

false (the magnitude of acceleration is directly proportional to q because F = qE = ma)

molecules can potentially gain or lose electrons and can potentially gain or lose protons

true

the electric force exerted by one point charge on another point charge always points perpendicular to the line connecting two charges

false (experiment’s find, and coulombs law states that the force always points along the line connecting two charges)

if two point charges are interacting in water the magnitude of the total electric force on each will be higher than if they were in empty space

false (the total electric force will be lower because the magnitude of the total electric force is equal to the magnitude of the force that q1 exerts on the q2 divided by the dielectric constant)

an electronic device is said to be electrically grounded when it is isolated from Earth so it cannot transfer charge to Earth

false (grounding refers to a connection to the Earth)

a approximately uniform electric field is produced in the region close to a long, straight, uniformly charged wire

false

metals are conductors in which many protons are relatively free to move through the entire material

false

an approximately uniform electric field is produced in the region close to a large, uniformly charged, flat surface

true

molecules can potentially gain or lose electrons but cannot gain or lose protons

false

inside an electric conductor the electric field can be non zero

true

the electric field produced by a positive point charge points toward the point charge

false

the electric force by one point charge on another point charge always points along the line connecting the two charges

true

When there is current in a metal wire electrons move through the wire and this causes the electric potential energy of the system to increase

false (like every conductor, metal wire has some resistance R and when current goes through it, there is always loss of electric potential energy because there are collisions of the moving electrons with atoms inside the conductor)

If the same potential difference is applied across two different conductors the absolute value of the current will be lower in the one that has lower resistance

false (R=V/I so I = V/R, so lower R implies higher absolute value of current)

Two resistors in parallel always have a lower equivalent resistance than the resistance of either resistor alone

true (resistors in parallel are the 1/Req equation)

When current is flowing into a junction in a circuit charge can build up at the junction

false (charge does not build up at junctions)

t=RC is the amount of time it takes to fully charge a capacitor that has capacitance C in a circuit with resistance R

false (it is the time it takes for a capacitor to be charged to about 63%)

The resistivity of a conductor depends only on the type of material it is made from, but the resistance depends on both resistivity and the dimensions of the conductor

true (R=p(L/A) where p - resistivity and resistivity depends on the type of material)

the voltage of a battery refers to the electric potential energy difference going from its negative terminal to its positive terminal

false

if two negative point charges are brought closer together this decreases the potential energy for the system

false (increases U in equation which implies increased potential energy)

two electric dipoles that are free to rotate will rotate such that they attract each other via electric force

true (electric forces cause them to rotate so that + and - are closer to each other)

if a uniform electric field acts on an electric dipole it may experience torque and a net force

false (there is no net force on a dipole in a uniform field, only in non-uniform fields)

a uncharged capacitor may be charged by connecting a wire across its two conductors

false (charging a capacitor requires applying a potential difference by connecting a battery)

if two capacitors are in parallel the equivalent capacitance of the two is greater than that of either one

true (for parallel capacitors Ceq=C1+C2)

if an object is made from an ohmic material this means that its resistance is constant

true (this is indeed what is meant by the term ohmic material, R = constant)

the average current going through a conductor is inversely proportional to the average velocity at which charged particles are moving inside it

false (the average current is given by I = nqvA where v is the average velocity of the moving charged particles, so its directly proportional)

if two resistors are in parallel the current through each must be the same

false (part of the current goes through one resistor and part goes through the other and these currents will be different if the two resistors have different resistances)

if you start at any point in a circuit, trace a path around any loop, and come back to the same point, the sum of all the potential differences around the loop must be negative

false (Kirchoff’s loop rule states that the sum of all potential differences must be equal to 0)

During both charging and discharging of a capacitor the absolute value of the current is initially high and decreasing

true (I =I0exp(-t/RC) where I0=Q/RC is the initial maximum current)

the resistance of a certain type of wire is inversely proportional to its length

false (R=p(L/A), where p is the resistivity of the material, L the length of the wire, and A the cross sectional area of the wire. So R is directly proportional to the length)

electric fields point in a direction of increasing electric potential

false

if an object is made from an ohmic material this means that its resistance is constant

true

if two negative point charges are brought closer this increases the potential energy of the system

true

when charged a parallel plate capacitor stores energy and an approximately uniform electric field is produced between the two plates

true

an electric dipole is an object consisting of two positive charges separated by some distance

false

regardless of the direction that charges move through a resistor electric potential energy is converted into heat

true

the resistance of a certain type of wire is inversely proportional to its cross-sectional area

true

current characterizes movement of positive and/or negative charged particles through a conductor

true

t=RC is the amount of time it takes to discharge a capacitor, that has capacitance C in a circuit with resistance R, from a full charge down to about 37% of full charge

true

a negative point charge tends to move towards a position where the electric potential acting on it is higher

true

if two negative point charges are brought closer together this increases the potential energy of the system

true

Two wires with current going through them them exert no magnetic forces on each other if both are uniformly uncharged

false (they can exert magnetic force on each other if current is going through each, regardless of whether they are uniformly uncharged. due to the currents each one produces a magnetic field that acts on the other wire and this causes magnetic force on each)

The only way to produce a magnetic field is with a magnet

false (can also be produced by moving charged particles AKA current)

The direction of the magnetic force on a point charge in a magnetic field depends on the direction the point charge is moving, the direction of the magnetic field, and the sign of the charge

true

If current is going through a straight wire that is placed in a magnetic field no magnetic force will be exerted on the wire if the wire is perpendicular to the magnetic field

false

Two parallel wires with current going through them always exert attractive force on each other

false

The north pole of a magnet repels the north pole of another magnet

true

Paramagnetic atoms produce a magnetic field

true

There are two types of magnets: those that have a north pole and those that have a south pole

false

Outside a magnet, the magnetic field it produces points away from the north pole of the magnet and toward the south pole of the magnet

true

When a bar magnet is placed in a uniform magnetic field, it can experience both a net force and torque

false

If a magnetic field is applied to a ferromagnetic material, it will produce a field that points in a direction opposite to that of the applied field

false

The magnetic flux through a surface is maximum when a magnetic field is parallel to the surface

false

The absolute value of the potential induced across a wire loop in a magnetic field is directly proportional to the rate of change of the magnetic flux through the loop

true

AC electricity is current that periodically changes direction

true

A circuit with higher inductance will produce a lower absolute value of induced potential difference in response to a changing current

true

If two inductors are connected in series, the potential difference across each must be the same

false

Two inductors connected in series have an equivalent inductance that is higher than the inductance of either one alone

true

A magnet can exert only attractive force on another magnet

false

If a point charge is moving in a direction parellel to a uniform magnetic field it will have magnetic force exerted on it

false

Magnetic field and magnetic force are both vector quantities but they have different units

true

A point charge can have either electric force or magnetic force exerted on it, but not both at the same time

false

Many atoms have a magnetic dipole moment and this causes them to experience torque when placed in an electric field

false

The magnitude of the magnetic field produced by a moving point charge is directly proportional to the absolute value of its charge and directly proportional to the magnitude of its velocity

true

Diamagnetic atoms do not produce a magnetic field

true

The north pole of a bar magnet is pulled toward the southern direction by the Earth’s magnetic field

false

When a magnet is acted upon by a magnetic field, the north pole of the magnet experiences a force in a direction perpendicular to the magnetic field

false

When a magnetic field is applied to a paramagnetic material, a fraction of the atoms in the material produce fields that are largely aligned with the applied field

true

The magnetic flux through a surface is maximum when a magnetic field is perpendicular to the vector normal to the surface

false

A changing magnetic field at the surface of a wire loop always induces a current through the wire

false

When a current that changes in time is made to flow through a wire loop by a battery, this current produces a magnetic field that changes in time, and this causes a changing magnetic flux through the loop, and this also causes an induced potential difference across the wire loop

true

The inductance of an inductor depends on the amount of current going through it

false

If two inductors are connected in series the overall potential difference across both must equal the sum of the potential differences across each

true

when a current that changes in time is made to flow through a wire loop by a battery, this current produces a magnetic field that changes in time, and this causes an induced potential difference across the wire loop

true