physics exam 1 2019
What is the emf of a battery that does 0.60 J of work to transfer 6.0×10−2 C of charge from the negative to the positive terminal?
.6/.06 =10 V
Suppose two parallel-plate capacitors have the same charge Q, but the area of capacitor 1 is A and the area of capacitor 2 is 2A. If the spacing between the plates in capacitor 1 is d, what should the spacing between the plates in capacitor 2 be to make the capacitance of the two capacitors equal?
2d
the junction rule describes the conservation of which quantity? Note that this rule applies only to circuits that are in a steady state.
Charge
What is the SI unit of charge?
Coulomb
Two glass marbles (1 and 2), each supported by a nylon thread, are rubbed against a piece of silk and then are placed near a third glass marble (3), also supported by a similar thread. Assuming that marble 3 has not been in contact with the piece of fabric, which of the following statements best describes the situation when the three marbles are brought together?
Marbles 1 and 2 repel each other, but no interaction occurs with marble 3.
Two identical metal spheres A and B are connected by a plastic rod. Both are initially neutral. 6.0×1012 electrons are added to sphere A, then the connecting rod is removed. Afterward, what is the charge of A?
Qa=(N)(-E) Qa=(6.0x10^12)(-1.6x10^-19) -960
A glass marble is rubbed against a piece of silk. As a result the piece of fabric acquires extra electrons. What happens to the glass marble?
The marble has lost the same number of electrons acquired by the piece of silk. The marble acquires a positive charge and attracts the piece of silk.
What is the strength of an electric field that will balance the weight of a 1.0 g plastic sphere that has been charged to −3.0nC?
mg=QE (.001)(9.8)/(3x10^-9) 3.2666666
potential difference between the positive and negative ends of the jumper cables, which are a short distance apart. An electron at the negative end ready to jump to the positive end has a certain amount of potential energy. On what quantities does this electrical potential energy depend?
only the potential difference and the charge
An uncharged capacitor is connected to the terminals of a 4.0 V battery, and 9.0 μC flows to the positive plate. The 4.0 V battery is then disconnected and replaced with a 8.0 V battery, with the positive and negative terminals connected in the same manner as before. How much additional charge flows to the positive plate?
q=v q2=(9)(8)/4=18 18-9=9
What property of objects is best measured by their capacitance?
the ability to store charge
What is the electric potential energy of an electron at the negative end of the cable, relative to the positive end of the cable? In other words, assume that the electric potential of the positive terminal is 0 V and that of the negative terminal is −12V. Recall that e=1.60×10−19C.
(-12-0)(-1.6x10^-19)=1.92x10^-18
Two small spheres spaced 20.0 centimeters apart have equal charge.How many excess electrons must be present on each sphere if the magnitude of the force of repulsion between them is 4.57×10−21 newtons?
(.2)sqr(4.57x10^-21/9x10^9)= 1.425x10^-16 1.425x10^-16/1.6x10^-19 + 891 electrons
A 3.0 cm × 3.0 cm parallel-plate capacitor has a 3.0 mm spacing. The electric field strength inside the capacitor is 1.1×105 V/m . How much charge is on each plate?
(1.1x10^5)(8.85x10^-12)(9x10^-4)= 8.76x10^-10
A 3.0 cm × 3.0 cm parallel-plate capacitor has a 3.0 mm spacing. The electric field strength inside the capacitor is 1.1×105 V/m .What is the potential difference across the capacitor?
(1.1x10^5)(8.85x10^-12)(9x10^-4)= 8.76x10^-10 (8.76x10^-10)(3x10^-3)/ (8.85x10^-12)(9x10^-4) = 330v
A particle with charge 8.00×10−19 C is placed on the x axis in a region where the electric potential due to other charges increases in the +x direction but does not change in the y or z direction. The particle, initially at rest, is acted upon only by the electric force and moves from point a to point b along the x axis, increasing its kinetic energy by 1.44×10−18 J . In what direction and through what potential difference Vb−Va does the particle move?
(1.44x10^-18)/(8x10^-19) -1.80
Now find the magnitude of the force on an electron placed at this same point. Recall that the charge on an electron has magnitude e=1.60×10−19C.
(1.6x10^-19)(6220) 9.95×10−16N
A 75 W electric blanket runs at 18 V . What is the resistance of the wire in the blanket?
(18)^2 / 75 = 4.3
If the total positive charge is Q = 1.62×10−6 C , what is the magnitude of the electric field caused by this charge at point P, a distance d = 1.53 m from the charge?
(8.99x10^9)(1.62x10^-6)/(1.53)^2 6220 N/C
If the particle moves from point b to point c in the y direction, what is the change in its potential energy, Uc−Ub?
0 Every time a charged particle moves along a line of constant potential, its potential energy remains constant and the electric field does no work on the particle.
A dielectric-filled parallel-plate capacitor has plate area A = 15.0 cm2 , plate separation d = 8.00 mm and dielectric constant k = 5.00. The capacitor is connected to a battery that creates a constant voltage V = 5.00 V . Throughout the problem, use ϵ0 = 8.85×10−12 C2/N⋅m2 . Find the energy U1 of the dielectric-filled capacitor.
5(8.85x10^-12)(15/10000)/ 8/1000 = 8.3x10^-12 .5(8.3x10^-12)(5)^2=103x10^-10
If a 1.5 V battery stores 5.9 kJ of energy (a reasonable value for an inexpensive C cell), for how many minutes could it sustain a current of 1.4 A ?
5900/1.5 = 3933.33 3933.33/1.4 = 2809.52 2809.52/60 =47
A 6.5 V battery supplies a 4.0 mA current to a circuit for 6.2 hr . How much work has been done on the charges that passed through the battery?
6.5(4x10^-3)(6.2)(60)^2
Consider an air-filled charged capacitor. How can its capacitance be increased?
Decrease the spacing between the plates of the capacitor.
Consider a charged parallel-plate capacitor. How can its capacitance be halved?
Double the plate separation. Halve the plate area.
Two batteries supply current to the circuit in the figure. The figure shows the potential difference across two of the resistors and the value of the third resistor. 3v 4.5v
Find the voltage of the resistance 150 ohms V_150=V(batteries)-V(other resistors) V_150=7.5V-5.5V=2V Then we can get the current by the formula I=V/R I=2/150 I=0.01333A I=13.33mA
Consider a charged parallel-plate capacitor. Which combination of changes would quadruple its capacitance?
Halve the plate separation and double the plate area.
A 60-cm-long heating wire is connected to a 120 V outlet. If the wire dissipates 45 W, what are (a) the current in and (b) the resistance of the wire?
I= 45/120 = .375 A R= 120/.375 = 320 Ohms
Assume that two of the electrons at the negative terminal have attached themselves to a nearby neutral atom. There is now a negative ion with a charge −2e at this terminal. What are the electric potential and electric potential energy of the negative ion relative to the electron?
The electric potential is the same and the electric potential energy is twice as much.
What is the motion of the negative electrons and positive atomic nuclei caused by the external field?
The electrons move to the left and the nuclei are almost stationary.
What is the force felt by the electrons and the nuclei in the rod when the external field described in the problem introduction is applied?
The nuclei experience a force to the right and the electrons experience a force to the left
Light bulbs are typically rated by their power dissipation when operated at a given voltage. Which of the following light bulbs has the largest current through it when operated at the voltage for which it's rated?
divide W by V
There is a single electron at a distance from the point charge. On which of the following quantities does the force on the electron depend?
the distance between the positive charge and the electron. the charge on the electron. the charge of the positive charge
Imagine that the rightward current flows in the rod for a short time. As a result, what will the net charge on the right and left ends of the rod become?
left end negative and right end positive
What is the magnitude of an electric field in which the electric force on a proton is equal in magnitude to its weight? Use 1.67×10−27 kg for the mass of a proton, 1.60×10−19 C for the magnitude of the charge on an electron, and 9.81 m/s2 for the magnitude of the acceleration due to gravity.
(1.67) 10^-3/1 (9.8)/ (690) = 1.02x10^-7
A wire carries a 1.4 A current for 10 s .How many electrons move past a point in the wire?
1.4x10=14 14/1.6x10^19 = 8.8x10^-19
A 75 W electric blanket runs at 18 V . How much current does the wire carry? r= 4.3 o
18v / 4.3 ohms = 4.2A
A science-fair radio uses a homemade capacitor made of two 30 cm × 30 cm sheets of aluminum foil separated by a 0.20-mm-thick sheet of paper. Assume dielectric constant of paper is 3.0. What is its capacitance?
3 (8.85x10^-12)(.09)/.20(10^-3/1) = 1.2x10^-8
A 8.0 V battery powers a flashlight bulb that has a resistance of 9.0 Ω . How much charge moves through the battery in 17 min ?
8/9=.9 .9(17)(60)
Two identical metal spheres A and B are connected by a plastic rod. Both are initially neutral. 6.0×1012 electrons are added to sphere A, then the connecting rod is removed.Afterward, what is the charge of B?
The charge acquired by sphere B is zero. The spheres are connected by plastic rod which is not a conductor. So, no charge is transferred through the plastic rod.
If you wanted to move an electron from the positive to the negative terminal of the battery, how much work W would you need to do on the electron?
(-12-0)(-1.6x10^-19)=1.92x10^-18
What must the charge (sign and magnitude) of a particle of mass 1.43 g be for it to remain stationary when placed in a downward-directed electric field of magnitude 690 N/C ?
-(1.43x10^3)(9.8)/690 = 2.03x10^-5
A small object A, electrically charged, creates an electric field. At a point P located 0.250 m directly north of A, the field has a value of 40.0 N/C directed to the south.What is the charge of object A?
40(.250)^2/8.99x10^9 =2.78x10^-10
A 6.5 V battery supplies a 4.0 mA current to a circuit for 6.2 hr. How much charge has been transferred from the negative to the positive terminal?
4x10^-3 (6.2)(60)^2 =89 c
Large electric fields in cell membranes cause ions to move through the cell wall. The field strength in a typical membrane is 1.0×107N/C. What is the magnitude of the force on a calcium ion with charge +e?
F=(1.6×10^−10C)(1.0×10^7N/C) =1.6×10−12N
At the negative terminal of the battery the electron has electric potential energy. What happens to this energy as the electron jumps from the negative to the positive terminal?
It is converted to kinetic energy.
A small metal ball is given a negative charge, then brought near (i.e., within about 1/10 the length of the rod) to end A of the rod (Figure 1). What happens to end A of the rod when the ball approaches it closely this first time?
It is strongly attracted
How does end B of the rod react when the charged ball approaches it after a great many previous contacts with end A?
It is strongly repelled
How does end A of the rod react when the charged ball approaches it after a great many previous contacts with end A? Assume that the phrase "a great many" means that the total charge on the rod dominates any charge movement induced by the near presence of the charged ball.
It is strongly repelled.
The charge imbalance that results from this movement of charge will generate an additional electric field in the region within the rod. In what direction will this field point?
It will point to the left and oppose the initial applied field.
An electron has been accelerated from rest through a potential difference of 600 V. What is the electron's kinetic energy, in joules?
KE = (1.6x10^-19)(600) = 9.60x10^-17 j
An electron has been accelerated from rest through a potential difference of 600 V.What is the electron's kinetic energy, in electron volts?
KE=(1.6x10^-19)(600)= 9.6x10^-17 9.6x10^-17(1/1.6x10^-19)= 600eV
After a great many contacts with the charged ball, how is the charge on the rod arranged (when the charged ball is far away)?
There is negative charge spread evenly on both ends.
Suppose two parallel-plate capacitors have the same charge Q, but the area of capacitor 1 is A and the area of capacitor 2 is 2A.If the spacing between the plates, d, is the same in both capacitors, and the voltage across capacitor 1 is V, what is the voltage across capacitor 2?
V/2
An electron has been accelerated from rest through a potential difference of 600V. What is the electron's speed?
V= sqr (2)(9.60x10^-17) / 9.11 x10^-31)= 1.47x10^7
For the same situation as in Part A, on which of the following quantities does the electric field at the electron's position depend?
the distance between the positive charge and the electron. the charge of the positive charge.
