test 2 physics 2

the resistivity of any given metal

a. Depends on temperature.

b. Varies linearly with temperature.

c. is the proportionality constant between the resistance, R, and the ratio of length, L, to the cross-sectional area, A, of a wire made from the metal.

d. Has units of Ohm·meter.

e. Is described by all of the choices above.

e. is described by all the choices above

This is because resistivity is opposing the motion of the charges. This means that temperature influences the motion of charge carriers, and if there is higher temp, there is more motion (linear relationship). R=pL/A, meaning R is the proportionality constant, and is shown in Ohm·meter (Ω⋅m).

The same potential difference is supplied by 2 wires. Wire #1 carries twice the current of wire #2. If the resistance of wire #2 is "R", what is the resistance of wire #1?

a. R

b. 2R

c. R/2

d. 4R

e. R/4

c. R/2

This is because the current is inversely proportional to resistance when the potential difference is constant, so if wire #1 carries twice the current, its resistance must be half that of wire #2. In simple terms, because wire #1 carries twice the current of wire #2 while both are under the same voltage, wire #1 must have less resistance to allow more current to flow through it

using ohms law, wire 1: (2x current than wire 2)

I1 = 2I2

R2=2

I2R=2I2R1

R / 2 = 2R1 / 2

= R/2

The graph shows the potential difference across a resistor as a fnction of the current through the resistor. The slope of this curve represents:

a. The electric field

b. Resistance

c. Charge

d. Emf

e. Work per unit charge

b. Resistance

This is because the slope of the V vs. I graph tells you how much voltage changes for a given change in current, which is essentially what resistance is defined as. Using V = 1 x R (Ohms law), the slope of the curve represents the change in potential difference V divided by the change in current I.

Current flows through a resistor

a. with no direction since it is not a vector

b. from any potential to any different potential

c. from high potential to low potential

d. from low potential to high potential

e. cannot be determined.

c. from high potential to low potential

This is because the electric potential is like a "pressure" that pushes the electric charges (such as electrons) through the circuit. The positive charges would move from high potential to low potential, but in most circuits, we consider the flow of electrons, which actually move from low potential to high potential (electrons flow in the opposite direction of conventional current).

Consider two silver wires. One has one-half of the cross-sectional area and one-half of the length of the other. How do the resistances of these two wires compare?

a. Two wires have the same resistance.

b. The longer wire has four times the resistance of the shorter wire.

c. The longer wire has twice the resistance of the shorter wire.

d. The longer wire has one-half of the resistance of the shorter wire.

e. none of the other answers are correct

a. Two wires have the same resistance.

This is because R is proportional to L/A, and even if it is doubled or halved, it will still be the same.

wire 1:

R1 = p L/A

wire 2:

R2 = p(L/2)/(A/2)

= pL/A = R1

The figure above shows two bar magnets of the same size and strength. Which of the arrows (A - D in the figure) correctly represents the direction of the magnetic field at a point location at the common origin (center) of the arrows? (That point is equidistant from the two magnets)

a. A

b. B

c. C

d. D

e. E

a. A

Both magnets are symmetrically around the origin, and they are both the same distance from the origin point. The magnetic field lines and pull ALWAYS point form the north to the south. So, magnet one field lines start at the N on the left and curve to enter the S on the right, meaning the field lines point from left to right, and in magnet 2 the field lines point from right to left. The field lines from magnet 1 is pointing rightward and magnet 2 is pointing leftward. Since the two fields are in opposite direction horizontally, they cancel out horizontally, so the field will not point in any left or right direction. Vertically, the magnetic field lines curve around the poles and add up vertically, pointing upwards because both magnetic fields curve upward and away from the center because of the symmetry of the setup. The magnetic field does not point downward at the origin, because the field lines are curving in such a way that the vertical components add upward, not downward. Since the horizontal components cancel each other, only the vertical components from both magnets add up. These vertical components from both magnets push the resulting magnetic field upward at the center.

A vertical wire carries a current straight up in a region where the magnetic field vector points due north. What is the direction of the resulting force on this current?

a. Down

b. North

c. South

d. East

e. West

e. West (answer = 0)

The current flows straight up the wire, meaning the current is going upwards. The magnetic field points north (positive x direction). So using the RHR, if you point the thumb upwards (current), your fingers will be in the direction of the magnetic field, and the pointer finger will be going away from you. The answer is 0 because since the magnetic field is pointing north and the current is up, the two vetcors (current and magnetic field) are parallel to each other in a 3D space. the cross product of two parallel vectors is 0. this is because the angle between them is 0, so the magnitude and force are both 0.

The magnetic force on a charge particle

a. Depends on the sign of the charge on the particle.

b. Depends on the velocity of the particle.

c. Depends on the magnetic field at the particle's instantaneous position.

d. is at right angles to both the velocity of the particle and the direction of the magnetic field.

e. is described by all the answers above.

e. is described by all the answers above.

This is because the magnetic force depends on if its negative or positive (F=q(v x B), where q is the charge. If the charge is positive, the force will be in one direction, and if its negative, it will be in the opposite. The force is also proportional to the magnitude of the velocity and the angle between the velocity vector and the magnetic field vector. if the particle is at rest and the velocity is 0, no magnetic force acts on it. The magnetic force depends on the magnetic field at the particles position because the force is directly proportional to the magnetic field strength and the cross product between the velocity and the magnetic field vectors. The direction of the magnetic field on a charged particle is ALWAYS perpendicular to both the velocity of the particle and the magnetic field (VxB)

The image above shows a constant electric current passing through a narrow region of the wire. What happens to the drift velocity of the moving charges as they go from region A to region B and then to region C?

a. The velocity decreases from A to B and then increases B to C

b. the velocity increases from A to B and increases from B to C

c. The velocity remains constant

d. The velocity decreases from A to B and decreases form B to C

e. The velocity increases from A to B and decreases from B to C

e. The velocity increases from A to B and decreases from B to C

In a parallel circuit

a. the current is the same in every branch

b. the potential drop is the sum of those in all branches.

c. the potential drop is the same for each element of the parallel circuit.

d. the heat generated is the same in all branches.

e. the resistance is the sum of the resistances of the branches.

c. the potential drop is the same for each element of the parallel circuit.

The current is not the same in every branch of a PARALLEL circuit because the current SPLITS across the branches depending on the resistance of each branch. The potential drop is not the sum of all those branches since the VOLTAGE (potential drop) is the SAME across all branches, not summed. the potential drop is the same which is why C is correct. The heat generated is not the same in all branches because it depends on the current and resistance in each branch, different resistances will cause different amounts of heat generation. The resistance of the parallel circuit is not the sum of the resistances, because the total resistance in a parallel circuit is found using:

1/Rtot = 1/R1 + 1/R2 ….

If a device is said to be “ohmic” in behavior, what does that mean? What would a graph of voltage (x) vs current (y) look like?

When a device has ohmic behavior, it means that the device follows ohms law, meaning that the current flowing through the conductor is directly proportional to the voltage across it and inversely proportional to R. For an ohmic device, the resistance remains constant, regardless of the voltage applied or the current flowing through it.

A graph of it would have velocity on the x-axis and current on the Y.

Where in a typical circuit do I expect to see a voltage drop and where in the circuit do I expect to see a voltage gain?

There should be a voltage drop in resistors bulbs, or any load, while it should gain across power sources like batteries or power supplies. In a series circuit, the total voltage is divided across each resistor, and in a parallel, the voltage is the same but the current will change.

What happens to the total current as resistors are added to a circuit… a. in series? b. in parallel?

a. in a series, resistors → add up the resistors

b. in a parallel, resistors → 1/Rtotal