Physics 2 Exam

ch 24.18 A parallel plate capacitor is connected to a power supply that maintains a fixed potential difference between the plates. A. If a sheet of dielectric is then slid between the plates, what happens to (i) the electric field between the plates, (ii) the magnitude of charge on each plate,and (iii) the energy stored in the capacitor? B. Now suppose that before the dielectric is inserted , the charged capacitor is disconnected from the power supply. In this case, what happens to (i) the electric field between the plates (ii) the magnitude of charge on each plate (iii) the energy stored in the capacitor? Explain differences between the situations.

(i) When the dielectric sheet is inserted between the capacitor plates, due to polarization of charges on either side of the dielectric, it produces an electric field of its own which acts in a direction opposite to that of the field due to the source. This makes the net electric field zero between the plates. (ii) The magnitude of the charge on each capacitor will increase. (iii) As both the charge on the capacitor and capacitance increase, and we know, that the energy stored in a parallel plate capacitor is U= 1/2*C(V^2). So, the energy stored in parallel plate capacitors will also increase. B: (i) In this case the system will remain the same E'= E/εr. (ii) the charge will remain the same. (ii) U'= U/εr

ch 24.14 Supposed you bring a slab of dielectric close to the gap between the plates of a charged capacitor, preparing to slide it between the plates. what force will you feel? what does this force tell you about the energy stored between the plates once the dielectric is in place, compared to before the dielectric is in place?

-The force you feel is the induction of charges on the dielectric material which will cause it to get attracted to the plates. -Since there is an attractive force and hence positive work, the overall electrical energy stored within the plates, the net energy stored in the capacitor will decrease. - This tells that the energy stored after the insertion of dielectric will be smaller than that without insertion.

Ch 26.4 In the circuit shown in Fig. Q26.4, three identical light bulbs are connected to a flashlight battery. How do the brightness's of the bulbs compare? Which light bulb has the greatest current passing through it? Which light bulb has the greatest potential difference between its terminals? What happens if bulb A is unscrewed? Bulb B? Bulb C? Explain your reasoning.

A) For the parallel circuits the voltage drop across each branch is same. Voltage across A is E and that across B and C is E/2. Bulb A is the greatest and B and C are half that of A. B) For the bulbs to be identical the resistance of each of them is equal, means A has the greatest current passing through them. C) If A is unscrewed, the circuit becomes a two bulb circuit connected in series. The voltage drop across each is E/2 the brightness remains the same. D) If B is unscrewed the path between B and C becomes open, and the circuit becomes one with a single bulb A connected to the battery. The potential difference across A is E, the brightness does not change.

Ch 28.10 What are the relative advantages and disadvantages of Ampere's law and the law of Biot and Savart for practical calculations of magnetic fields?

Ampere's law is useful to find the magneticfields in those cases the magnetic fields are highly symmetric. butBiot savart's law is used to calculate the magnetic field due to acurrent carrying wire of any shape and the process iscomplicated. Both laws can be derived from each other

Ch 24.2 Suppose several different parallel-plate capacitors are charged up by a constant-voltage source. Thinking of the actual movement and position of the charges on an atomic level, why does it make sense that the capacitances are proportional to the surface areas of the plates? Why does it make sense that the capacitances are inversely proportional to the distance between the plates?

Between two plates the field lines are parallel, there is no spreading, and the force is the same for any value of r (provided it's small compared with the width of the plate) since the force is the same, the charge must be increased when the distance between the capacitor plates is decreased, to maintain a "balancing" voltage equal to the applied voltage. If the applied voltage stays the same, and the plates are brought closer, then there are more electrons per area but the potential does not increase, rather the force increases, proportional to 1/distance electric potential = potential energy per test charge = work done per test charge = force per test charge times distance so, if we change the distance, the force per test charge changes, and is proportional to 1/distance, so the electric potential (= applied voltage) stays the same.

ch 24.19 Liquid dielectrics that have polar molecules (such as water) always have dielectric constants that decrease with increasing temperature. Why?

Dielectric constant of a mole is defined as follows: K=e/e0. Here, K is dielectric constant, e is permeability of material and e0 is permeability of free space. Now, as temperature increases electron move randomly hence, magnitude of induced electric field decreases this will increase the denominator of above expression and over quantity that is dielectric constant decreases.

Ch 25.10 Electrons in an electric circuit pass through a resistor. The wire on either side of the resistor has the same diameter. (a) How does the drift speed of the electrons before entering the resistor compare to the speed after leaving the resistor? Explain your reasoning. (b) How does the potential energy for an electron before entering the resistor compare to the potential energy after leaving the resistor? Explain your reasoning.

Drift speed Vd = I/neA i is current n is density of free electrons e is charge of eelctron A is area so for given i, n , a and A Drfit speed remains same ------------------------------------------------ b. Potential eenrgy U = Vq so for a given charge, higher the potential V, higher the energy so as charge enters at high potential and levaes at low potential PE is greater while entering and lesser while leaving

Ch 25.6 Can the potential difference between the terminals of a battery ever be opposite in direction to the emf ? If it can, give an example. If it cannot, explain why not.

From the above circuit diagram the emf ε direction is from positive point a to low potential b as show in the figure. Hence the potential difference between the terminals of the battery is opposite of the emf on the battery.

Ch 27.10 A loose, floppy loop of wire is carrying current I. The loop of wire is placed on a horizontal table in a uniform magnetic field B perpendicular to the plane of the table. This causes the loop of wire to expand into a circular shape while still lying on the table. In a diagram, show all possible orientations of the current I and magnetic field B that could cause this to occur. Explain your reasoning.

If current I is clockwise and the magnetic field is outwards, or if the current I is clockwise and the electric field is inwards to the plane of figure, then the force on the loop will act radically outwards and tend to expand the loop.

Ch 28.11 Magnetic field lines never have a beginning or an end. Use this to explain why it is reasonable for the field of an ideal toroidal solenoid to be confined entirely to its interior, while a straight solenoid must have some field outside.

In a toroidal solendoid the turns are not precisely circular loops but rather, segments of a bent helix( it is like a tightly wound solenoid bend into a circle.) The field lines make complete circles totally within a the toroid. For a tightly wounded toroid, the magnetic field outside the toroid is almost zero. So we can say the magnetic field lines never have a beginning or an end. A simple solenoid consists of a helical winding of a wire on a cylinder. The field lines near the center are almost uniformly parallel outside the solenoid. The field lines spread out from the solenoid and weak field is produced.

ch 24.4 To store the maximum amount of energy in a parallel-plate capacitor with a given battery (voltage source), would it be better to have the plates far apart or close together?

In order to have more energy stored in the capacitor when a battery is connected, it is best to have plates closely.

Ch 27.6 If the magnetic force does no work on a charged particle, how can it have any effect on the particle's motion? Are there other examples of forces that do no work but have a significant effect on a particle's motion?

Magnetic force (F) on a moving charged particle always remains perpendicular to its velocity (v), and therefore it does no work on s charged particle. Magnetic force only changes the direction of the motion of the charged particle, but not the magnitude of its velocity. In the circular motion of particle, the centripetal force does no work, but has a significant effect on the particles motion

Ch 25.18 Eight flashlight batteries in series have an emf of about 12 V, similar to that of a car battery. Could they be used to start a car with a dead battery? Why or why not?

No as they would not have enough amps to do the job. Depending on the brand and style they would provide about 2 a/hr of capacity. They typical small car battery has closser to 40+ a/hr so you would have to put 20+ rows in parallel od 8 batteries in series (160 batteries)

Ch 27.4 The magnetic force on a moving charged particle is always perpendicular to the magnetic field B. Is the trajectory of a moving charged particle always perpendicular to the magnetic field lines? Explain your reasoning.

Not necessarily always perpendicular. If the particle has two components namely: (a) Component of velocity along the direction of magnetic field. (b) Component of velocity perpendicular to magnetic field, then the magnetic field affects the perpendicular component of velocity, but not the parallel component. In this case, the charged particle can move in a direction other than the path of field lines. If the particle moves in a direction parallel to the magnetic field, then it experiences no force

Ch 25.17 The energy that can be extracted from a storage battery is always less than the energy that goes into it while it is being charged. Why?

Q25.17 In both the charging process and when the battery delivers energy to a circuit some energy is lost to thermal energy as the current flows through the internal resistance of the battery.

Ch 25.9 We have seen that a coulomb is an enormous amount of charge; it is virtually impossible to place a charge of 1 C on an object. Yet, a current of 10 A, 10 C>s, is quite reasonable. Explain this apparent discrepancy.

Q25.9 A wire carrying a current of 10 A remains electrically neutral. But a huge amount of charge (10 C) passes a cross section of the wire each second.

Ch 28.1 A topic of current interest in physics research is the search (thus far unsuccessful) for an isolated magnetic pole, or magnetic monopole. If such an entity were found, how could it be recognized? What would its properties be?

Q28.1 It would be analogous to a point charge. Magnetic field lines would terminate on it. The magnetic flux through a closed surface would be proportional to the net number of magnetic monopoles in the volume enclosed by the surface.

Ch 28.2 Streams of charged particles emitted from the sun during periods of solar activity create a disturbance in the earth's magnetic field. How does this happen?

Q28.2 The moving charges produce their own magnetic field. The net field is then the vector sum of the field of the moving charges and the earth's field.

Ch 28.3 The text discussed the magnetic field of an infinitely long, straight conductor carrying a current. Of course, there is no such thing as an infinitely long anything. How do you decide whether a particular wire is long enough to be considered infinite?

Q28.3 The wire can be considered infinitely long when calculating the magnetic field at points whose distance from the wire is much less than their distance from either end of the wire.

Ch 28.5 Pairs of conductors carrying current into or out of the power-supply components of electronic equipment are sometimes twisted together to reduce magnetic-field effects. Why does this help?

Q28.5 The wires carrying currents in opposite directions produce magnetic fields that cancel because they are in opposite directions

Ch 28.6 Suppose you have three long, parallel wires arranged so that in cross section they are at the corners of an equilateral triangle. Is there any way to arrange the currents so that all three wires attract each other? So that all three wires repel each other? Explain.

Q28.6 Currents in the same direction attract and currents in opposite directions repel. If all three wires carry currents in the same direction they will all three attract each other. There is no way to have all pairs with opposite currents, so it is not possible to have all three wires repel each other.

Ch 28.9 A current was sent through a helical coil spring. The spring contracted, as though it had been compressed. Why?

Q28.9 Adjacent turns of wire carry currents in the same direction and therefore attract each other.

Ch 25.2 A cylindrical rod has resistance R. If we triple its length and diameter, what is its resistance, in terms of R?

R=(pL)/A final resistance will be one-third of the intial value. Hence, the required resistance is R/3

Ch 26.16 Identical light bulbs A, B, and C are connected as shown in Fig. Q26.16. When the switch S is closed, bulb C goes out. Explain why. What happens to the brightness of bulbs A and B? Explain

Since bulb C is removed then the equivalent resistance of the circuit is reduced. As the power is inversely proportional to the resistance of the circuit and also the applied voltage is constant, this means that the power supplied between the bulb increases. So, the brightness of the bulb A and B will also increase.

Ch 25.3 a cylindrical rod has resistivity p. If we triple its length and diameter, what is its resistivitiy, in terms of p?

Specific resistance of a material is dependent upon the nature of the material and its raise of temperature. It is independent on the dimensions of the material. Therefore, if we triple its length and diameter of the cylinder, its resistivity does not change.

Ch 27.12 Each of the lettered points at the corners of the cube in Fig. Q27.12 represents a positive charge q moving with a velocity of magnitude v in the direction indicated. The region in the figure is in a uniform magnetic field B, parallel to the x axis and directed toward the right. Which charges experience a force due to B? What is the direction of the force on each charge?

The magnitude of force acting on the particle at point a is qvB, and will be directed midway between negative y-axis and negative z-axis

Ch 26.21 When a capacitor, battery, and resistor are connected in series, does the resistor affect the maximum charge stored on the capacitor? Why or why not? What purpose does the resistor serve?

The maximum charge stored on the capacitor will be affected only by the capacitance of the capacitor and emf of the battery. Due to this reason, the resistor does not affect the maximum charge stored on the capacitor. Hence, resistor plays no role when the capacitor is fully charged in a RC series circuit. Resistor affects the rate at which the capacitor charges

Ch 26.10 A real battery, having non-negligible internal resistance, is connected across a light bulb as shown in Fig. Q26.10. When the switch S is closed, what happens to the brightness of the bulb? Why?

The power consumed by the bulb remains the same as the voltage drop across the bulb does not change. Therefore, the brightness of the bulb does not change

Ch 27.13 A student claims that if lightning strikes a metal flagpole, the force exerted by the earth's magnetic field on the current in the pole can be large enough to bend it. Typical lightning currents are of the order of 10^4 to 10^5 A. Is the student's opinion justified? Explain your reasoning.

This claim is not justified, because although the current from the lightning is large, the force from Earth's weak electric field is not strong enough to bend the pole

Ch 25.4 Two copper wires with different diameters are joined end to end. If a current flows in the wire combination, what happens to electrons when they move from the larger-diameter wire into the smaller-diameter wire? Does their drift speed increase, decrease, or stay the same? If the drift speed changes, what is the force that causes the change? Explain your reasoning.

Vd = i/(n*e*A)...... larger is the diameter then larger is Area of cross section A...... more is the A then smaller is the Vd........ when electrons moves from larger to smaller..... Vd will increases Electrosttic force among electrons causes increase the drift speed

Ch 27.2 At any point in space, the electric field E is defined to be in the direction of the electric force on a positively charged particle at that point. Why don't we similarly define the magnetic field B to be in the direction of the magnetic force on a moving, positively charged particle?

We do not define the electric field (B) to be in the direction of the magnetic force on a positively charged moving particle, because the magnetic force on a moving charged particle is perpendicular to both the electric field (B) and the particles velocity (V)

Ch 27.3 Section 27.2 describes a procedure for finding the direction of the magnetic force using your right hand. If you use the same procedure, but with your left hand, will you get the correct direction for the force? Explain

We will get the direction of the force to be exactly opposite of the correct one. This is because our two hands are mirror images of each other.

Ch 26.9 A light bulb is connected in the circuit shown in Fig. Q26.9. If we close the switch S, does the bulb's brightness increase, decrease, or remain the same?Explain why.

When Switch is not closed then the parallel combination of two resistors are in series with another resistor. When switch is closed, then the parallel combination of three resistors in series with another resistor. Thus effective resistance when switch is closed is less than when switch is opened. So power consumption of bulb is more when switch is closed. Thus brightness of the bulb increases.

Ch 27.7 A charged particle moves through a region of space with constant velocity (magnitude and direction). If the external magnetic field is zero in this region, can you conclude that the external electric field in the region is also zero? Explain. (By "external" we mean fields other than those produced by the charged particle.) If the external electric field is zero in the region, can you conclude that the external magnetic field in the region is also zero?

Yes in the first case we can conclude electric field is zero, if there exists a net electric field it will always try to accelerate a charge particle as a force of qE where q is charge of the body and E is electric field acts on the particle. But when E is zero we cannot conclude that B is zero, since When B is parallel to the direction of velocity of the moving particle as the body is moving in the same direction, net force of B on the particle is zero. A parallel B field to the velocity can exist.

Ch 27.17 In a Hall-effect experiment, is it possible that no transverse potential difference will be observed? Under what circumstances might this happen?

Yes, if the conductor placed in a magnetic field is a semiconductor, then the Hall voltage produced because of free electrons is opposite in polarity o that which was produced because of holes. From this, it is possible to observe an absence of a transverse potential difference.

Ch 26.13 Is it possible to connect resistors together in a way that cannot be reduced to some combination of series and parallel combinations? If so, give examples. If not, state why not.

Yes, it is possible to connect resistors together. example:https://word-to-html-images.s3.amazonaws.com/9780805386844/279-26-9dq-i1.png

Ch 28.12 Two very long, parallel wires carry equal currents in opposite directions. (a) Is there any place that their magnetic fields completely cancel? If so, where? If not, why not? (b) How would the answer to part (a) change if the currents were in the same direction?a

a) If two long parallel wires carry equal currents in opposite directions then their magnetic fields will not cancel at any place. B) If two long parallel wires carry equal currents in same direction then their magnetic fields will cancel in between the wires and the point where the magnetic field has to be zero should be equidistant from both the wires.

Ch 26.6 If two resistors R1 and R2 1R2 7 R12 are connected in parallel as shown in Fig. Q26.6, which of the following must be true? In each case justify your answer. (a) I1 = I2. (b) I3 = I4. (c) The current is greater in R1 than in R2. (d) The rate of electrical energy consumption is the same for both resistors. (e) The rate of electrical energy consumption is greater in R2 than in R1. (f) Vcd = Vef = Vab. (g) Point c is at higher potential than point d. (h) Point f is at higher potential than point e. (i) Point c is at higher potential than point e.

a)false because as potential difference across R1 and R2 should be same I1>I2 b)true. Inward current = outward current c)true explained in part(a) d)false Electrical consumption is VI and since V is same directly proportional to I which gives electrical consumption on R1 > R2 e)false as explaineed above f)true. since there is no additional resistance to cause potential drop in between g)true since current flow sfrom higher to lowerpotential h)false same reason as explained above i)false eqipotential points

Ch 26.19 Verify that the time constant RC has units of time.

tau = RC. tau = VtC/q. = ((V)(s)(C/V))/(C) = seconds (s)