Princeton Ch 10 - Electricity and Magnetism

Question 1

Define current.

Answer 1

The net movement of charge. I = Q/t [A = C/s];
Amount of charge that moves past a certain point per unit time.

A = ampere.

Question 2

Value of an elementary charge?

Answer 2

1.6*10^-19C

Question 3

What causes a current?

Answer 3

1) If there’s an E-field induced in a metal, negative charges move in the direction opposite to the electric field lines, e- would be induced to drift to the right if the electric field pointed to the left.
2) But what makes the e- drift right? Negative charges naturally move toward regions of higher EP, e- would be induced to drift to the right if the right end of the wire were maintained at a higher potential than the left.

ie. Voltages create currents. Voltage = diff in potential.

Question 4

Resistance.

Answer 4

R = V/I [V/A] = [Ω] = pL/A –> p = resistivity of material, A = surface area; L is length

V = voltage applied to the ends of an object
I = the resulting current

Insulators like rubber have very few free e-, so no current is produced when with an applied voltage

Question 5

Ohm’s law.

Answer 5

V = IR

Question 6

Resister in series and how to deal with them.

Answer 6

Two or more resisters are said to be in series if each follows the others along a single connection in a circuit.

1) R(equivalent) = R1 + R2
2) Resistor in series always have the same current (I).

Question 7

Resisters in parallel and how to deal with them.

Answer 7

Two or more resistors are said to be in parellel if they provide alternative routes from one point in a circuit to another. We go through one or the other, but not both resistors.

1) R(equivalent) = R1*R2/R1+R2 or 1/R = 1/R1 + 1/R2
2) Resistors in parallel always have the same voltage.

Question 8

True or false. For parallel combination, the equivalent resistance is always less than the smallest resistance in the combination.

Answer 8

True.

Question 9

On a circuit, the terminal that’s at a higher potential is denoted by the longer line and called the ___. The terminal that’s at the lower potential is denoted by the shorter line and called the ___ terminal.

Answer 9

On a circuit, the terminal that’s at a higher potential is denoted by the longer line and called the POSITIVE TERMINAL. The terminal that’s at the lower potential is denoted by the shorter line and called the NEGATIVE terminal.

Question 10

What is the conventional direction of current flow?

Answer 10

The direction of current is taken to bet eh direction that positive charge carriers would flow, even though the actual charge carriers that do flow might be negatively charged. The convention is counterclockwise from the positive terminal around to the negative terminal.

Question 11

When working backward with a circuit equation, going back to a series combination, use ___. When going back to parallel, use ___.

Answer 11

When working backwards with a circuit equation, going back to a series combination, use I. When going back to parallel, use V.

Question 12

Kirchoff’s laws.

Answer 12

1) For a circuit containing one battery as the voltage source, the sum of the voltage drops across the resistors in any complete path starting at the + terminal and ending at the - terminal matches the voltage of the battery.
2) The amount of current entering the parallel combination is equal to the sum of the currents that pass through all the individual resistors in the combination.

Question 13

Resistors dissipate heat. That rate at which the resistor dissipate heat energy is the power dissipated by the resistor. To calculate this, what is the Joule Heating Law?

Answer 13

P = I²R

Question 14

The total power dissipated by the resistors is equal to what?

Answer 14

The power supplied by the battery, given by I²R or P = IV

If you want to find the power dissipated by a resistor using PIV, remember to only use V for that resistor, not the V for the entire circuit.

Question 15

The total power dissipated by the resistors and absorbed by other voltage sources (ie the total power used by the circuit) is equal to the power supplied to the circuit by the ____.

Answer 15

Highest voltage power source. Some circuits contain more than battery/power source, and in these cases the lower voltage absorbs power from the higher voltage.

Question 16

What is energy in terms of power and time?

Answer 16

Power = energy/time

Energy = power * time

Question 17

The total voltage drop across all the resistors must match the voltage of the battery. But remember the voltage drop for parallel resistors are___ (same/doubled).

Answer 17

Same.

Question 18

When something is grounded, what is the potential?

Answer 18

The potential is considered to be 0.

Question 19

If three identical light bulbs are connected to a battery in parallel and the middle one goes out, what happens to the other two in terms of light intensity.

Answer 19

The current can still flow through the top and bottom bulbs, and the current through each will still be I = VR. Because the intensity of the light is directly related to the power each one dissipates, the fact that the current doesn’t change means that P = I²R won’t change, so the light intensity of the other two bulbs will remain the same.

Question 20

If three identical light bulbs are connected to a battery in series and the middle one goes out, what happens to the other two in terms of light intensity.

Answer 20

If anyone of those bulbs burn out, they’d all go out because the circuit would be broken.

Question 21

Voltmeter.

Answer 21

A device used to measure the difference between two points in a circuit.

Question 22

Ammeter.

Answer 22

A device used to measure the current through a particular point in the circuit.

Question 23

Where should we connect an ammeter if we are interested in finding the current flowing through a resistor?

Answer 23

Connect the ammeter in series with the resistor of interest.

Question 24

Where should we connect a voltmeter when trying to measure the voltage of a resistor?

Answer 24

The voltmeter should be placed in parallel with the resistor.

Question 25

Capacitor.

Answer 25

A pair of conductors that hold equal but opposite charges. Conductors can be of any shape but most common are parallel metal plate capacitors.

Question 26

Capacitance.

Answer 26

Q = CV

Q = charge on the capacitor
V = potential difference between he plates of a charged capacitor
C = capacitance [C/V] = Farad = [F]; this is a constant dependent on the size of the plates and how far apart they are.

Question 27

Capacitance of a parallel-plate capacitor.

Answer 27

C = ε0(A/d)
d = separation
A = area of each plant
ε0 = constant known as permittivity of free space = 1/4πk = 8.85*10-12 F/m

Question 28

Give an intuitive definition of capacitance.

Answer 28

Capacitance measures the plates capacity for holding a charge at a certain voltage. If we had two capacitors with different capacitances, and we want to store as much charge as possible, we’d choose the capacitor with the greater capacitance.

Question 29

A charged capacitor has charge Q, and the voltage between the plates is V. What will happen to C if Q is doubled?

Answer 29

Nothing. C is a constant. Since Q = CV, V will be doubled.

Question 30

Why are capacitors used?

Answer 30

1) Create a uniform electric field.

2) To store electrical potential energy.

Question 31

Why isn’t it possible to create a uniform electric field using source charges.

Answer 31

The E field created bt one or more point source charges varied, depending on location. For ex, as we move farther from the source charge, the E field gets weaker. We could never obtain an E field that was constant in both magnitude and dir throughout some region of space from point source charges.

Question 32

Ed’s formula.

Answer 32

V = Ed

d = separation between capacitors.

Question 33

If a particle is accelerated through a parallel plate capacitor, what happens to the electron’s horizontal velocity.

Answer 33

Because the acceleration of the e- is vertical, the e-

horizontal velocity will not change.

Question 34

How can you store electrical potential energy with parallel plate capacitors?

Answer 34

The process of charging a capacitor involves transferring e- from one plate to the other. During the charging process, the voltage source (battery) must do work against the E field that’s created between the plates of the capacitor. Once you begin the process of transferring e- from one plate to another, it becomes increasingly difficult to transfer more. It takes the effort to remove e- from a plate that is left positively charged an effort to place them on a negatively charged plate.

Question 35

Higher Voltage = (increase/decrease) in stored potential energy.

Answer 35

Increase.

Question 36

Formula for electrical PE stored in a capacitor.

Answer 36

PE = 1/2QV

Don’t forget Q = CV

Question 37

As you transfer e from one plate of a capacitor to another, you do work against the E field of the capacitor, storing PE. To recapture the stored E, you let the e- go back to their original plate, a process called ___.

Answer 37

Discharging the capacitor.

Question 38

When we connect the charged capacitor plates by a wire with some resistors along it, the charge drains off rapidly at first, but the rate at which the charge leaves gradually decrease as time goes on. Can the same be said of the current.

Answer 38

Yes. The current too starts off high and then gradually drops to zero as the capacitor discharges.

Question 39

What happens to C, Q, V, E, and PE if you charge a capacitor to voltage V and then disconnect the battery and insert dielectric.

Answer 39

C increases by a factor of K.
Q stays the same; the charge that exists on the plates is trapped.
V decreases by a factor of K.
E decreases by a factor of K. V=Ed
E = E-Einduced. The insulator is a dielectric and an induced dipole decreases the E field.
PE: stored electrical PE decreases by a factor of K. PE = 1/2QV

Question 40

What happens to C, Q, V, E, and PE if you charge a capacitor to voltage V and then insert a dielectric while keeping the battery connected.

Answer 40

C increases by a factor of K
Q increases by a factor of K
V stays the same; since the capacitors are still connected to the battery, the V b/t the plates must match the voltage of the battery.
E stays the same
PE: increases, as more charge, is transferred to the plates more PE is stored.

Question 41

Dielectric Breakdown.

Answer 41

For certain capacitors, the maximum E field strength is a certain amount (air is 3 million volts per meter). If the value of E were to exceed this max value, the insulator (or like of one like air) is no longer functional. e- would be pulled out of the space, ionizing the air, and the e- of the negative plate would have a conducting pathway through the air; we’d see a spark as the capacitor would discharge quickly; suffering a dielectric breakdown.

Question 42

Capacitors in parallel all have the same ___. To get the equivalent capacitor, you should:

Answer 42

Capacitors in parallel all have the same voltage, just like resistors in parallel. The equivalent capacitor is the sum of individual capacitances = Ceq = C1+C2+C3…

Question 43

Capacitors in series have the same ___. To get the equivalent capacitor you should:

Answer 43

Capacitors in series have the same charge, just like resistors in series having the same CURRENT. To get the equivalent capacitor: 1/Ceq = 1/C1 + 1/C2 + …

Question 44

How are magnetic fields created?

Answer 44

Magnetic fields are created by moving electric charges.l since a charge in motion constitutes a current, we can also say the B fields are produced by electric currents.

Question 45

The formula for magnetic force.

Answer 45

Fb = q(v * B) ; Fb=|q|vBsinθ -> B = [N/Cm/s] = N/(C/s)m) = N/A*m

Question 46

The direction of Fb is always (parallel/perpendicular) to both v and B.

Answer 46

The direction of Fb is always perpendicular to both v and B.

Question 47

If B exerts a force in a particular direction on a charge q moving with velocity v, then it would exert a force in the (opposite/same) direction on -q with velocity v.

Answer 47

If B exerts a force in a particular direction on a charge q moving with velocity v, then it would exert a force in the opposite direction on -q with velocity v.

Question 48

What is the work for magnetic force?

Answer 48

Because the magnetic force a charge feels is always perpendicular to the velocity of the charge, magnetic forces do no work. Work = Fdcosθ

Question 49

What can we say about the KE of a moving particle? What about work in relation to KE?

Answer 49

Since Fb never do work, they can never change the KE of a particle, meaning that KE is constant. Since Fb cannot change the KE of a particle, they can’t change the speed of a particle. All Fb can do is make charge particles change their direction; they can’t make them speed up or slow down.

Question 50

The centripetal force caused by a magnetic field.

Answer 50

F = mv²/r = qvB

Question 51

The magnetic field created by a straight wire carrying a current I is proportional to I and (inversely proportional/proportional) to the distance r from the wire.

Answer 51

B∝ I/r

Question 52

For a counter-clockwise current: B fields points (out/in) of the page inside the loop and points (out/in) the page outside the loop.

Answer 52

For a counter-clockwise current: B fields points out of the page inside the loop and points into the page outside the loop.

Question 53

For a clockwise current: B fields points (out/in) of the page inside the loop and points (out/in) the page outside the loop.

Answer 53

B field points into the page inside the loop and points out the page outside the loop.

Question 54

Solenoid.

Answer 54

Helical coil of wire in which a magnetic field produced inside can result from a flow o current. The B field produced is parallel to the central axis and acheives its max magnitude n the central axis, getting weaker as we move away from the center of the coils.

Question 55

Where is the B field strongest through a coil?

Answer 55

The B field will be stronger if the current is increased or if the solenoid wire loops are tightly packed.

Question 56

The B field is proportional to the ___ and ___ when it comes to solenoids.

Answer 56

Current and number of turns per unit length.

Question 57

By convention, magnetic field lines emanate from the ____ pole when it comes to magnetic.

Answer 57

By convention, B field lines emanate from the end of the magnet designated the north pole and then curl around and re-enter the magnet at the end designated the south pole.

Question 58

If a piece of iron is placed in an external magnetic field ( ex. one created by a current solenoid), the individual magnetic dipole moments of the e- will…

Answer 58

They will be forced to more or less line up. Because iron is ferromagnetic, these now-aligned magnetic dipole moments tend to retain this configuration, thus permanently magnetizing the bar and causing it to produce its own magnetic field.