Clickers & Homework Questions for Test One
A solid, conducting sphere of radius R is positively charged. Of the following distances from the center of the sphere, which location will have the greatest electric potential? (Take V = 0 at r = infinity.)
r=0 (center of the sphere)
Make a prediction! When we connect the power supply, the magnet will
turn (north pole towards coil) and be attracted
What do you know about the electric potential?
voltage is the common word for potential difference voltages are always measured between two points All points on an equipotential surface have the same electric potential
Which of the following are true of static charges on everyday objects? (a) Like charges repel. (b) Like charges attract. (c) Opposite charges repel. (d) Opposite charges attract. (e) A positively charged object has lost electrons. (f) A positively charged object has gained protons. (g) A negatively charged object has lost protons. (h) A negatively charged object has gained electrons.
(a) Like charges repel. (d) Opposite charges attract. (e) A positively charged object has lost electrons. (h) A negatively charged object has gained electrons.
A and B represent objects with -2 and +1 unit of charge respectively. Choose the pair of force vectors that correctly compare the electric force on A (caused by B) with the electric force on B (caused by A).
(b)
What is the direction of the Coulomb force on the charge pointed at by the arrow? (Charge is given in arbitrary units.)
Towards the right
Suppose that a picture of electric field lines is drawn following the convention that 4 field lines emerge from a small sphere with 4 pC of charge. In this picture there is an irregular closed surface, the interior of which is hidden, as shown below. The net amount of charge inside the closed surface must be
+4 pC
A capacitor with a charge of 0.7 C has its terminals shorted by a metal wire so that the charge flows off within 2.6 s. What is the average current flowing during that time in Ampere (A)? Enter a numnber with two digits behind the decimal point.
0.27 I = delta q / delta t
In the early universe and in stars, deuterium nuclei are produced from the combination of one proton and one neutron, with the release of a gamma ray. What is the charge on a deuterium nucleus in units of qe = 1.6*10-19 C?
1
Which of the points shown in the figure below are at the same potential?
1 and 4
Consider the 5 arrangements of charges below. (Electric Field Hockey Simulation) Which of the following statements is true? 1) All of the pucks feel a force to the right. 2) The puck in C feels a greater force to the right than the puck in D. 3) The puck in E feels a force to the right that is four times greater than that felt by the puck in B. 4) The net force on the puck in A is zero. 5) The puck in E feels a force to the right that is four times greater than that felt by the puck in D.
1) All of the pucks feel a force to the right. 3) The puck in E feels a force to the right that is four times greater than that felt by the puck in B.
A cube with sides of area 97 cm2 contains a 13.2 nanoCoulomb charge. Find the flux of the electric field through the surface of the cube in unis of Nm2/C. Enter a number with one digit behind the decimal point.
1,491.5
What do you think will happen when a negatively charged balloon is moved towards the neutral wall?
1. The balloon will be attracted to the wall.
The resistivity of aluminum is 58% higher than that of copper. A copper high voltage line has diameter 1 cm. If is replaced by an aluminum line of the same resistance and length, the aluminum line has diameter
1.26 cm
A series series RC circuit has a resistance of 106 Ω and a time constant of 10-5 s. What is the capacitance of this circuit?
10 picoFarad
In a series RC circuit a charged capacitor with capacitance C is being discharged though a resistor of resistance R. Plots of voltage and current versus time are shown below. What is the resistance of this series RC circuit?
10000 Ω
Two positive point charges, each with charge q, separated by a distance d, repel each other with a force of magnitude 20 N. What is the magnitude of the force between two positive point charges of magnitude 9.02 q, separated by a distance 3.9 d in units of N? Enter a number with two digits behind the decimal point.
106.98
Determine the equivalent resistance between points A and B in the following circuit.
15 kΩ
A current of 3.8 A flows through the heating element of heater converting 615 J of electrical energy into thermal energy every second. What is the voltage (in V) across the ends of the heating element? Enter a number with one digit behind the decimal point.
161.8 P = I x V
A wire carries a steady current of 18.2 A. What total charge (in C) passes through a cross-sectional area of the wire in a 9.7 s time interval? Enter a number with one digit behind the decimal point.
176.5 I = delta Q / delta T
The potential difference between the two plates of a capacitor is 8.7 V. If the separation between the plates is 4 mm, what is the strength of the electric field between the plates N/C? Enter an integer.
2,175 (8.7 V / .004)
When a positive charge is released from rest in a uniform electric field, it will
2. move with constant acceleration.
Two 2.8 microCoulomb charges are 1.1 m apart. How much energy (in milliJoule) went into assembling these two charges? Enter a number with one digit behind the decimal point
64.1
Two light bulbs, one rated at 50 W and a second rated at 100 W, are both supposed to be connected to a 110 V source of emf. Which one of the following is true?
The 50 W bulb has twice the resistance as the 100 W bulb.
A 30 microCoulomb charge is located at the origin and a -87 microCoulomb charge is located on the y-axis at y = 30.3 cm. What is the magnitude of the force with which the charges attract each other in units of N? Enter a number with two digits behind the decimal point.
255.86
Two 1 microCoulomb charges are 3 m apart. What is the potential at each charge due to the other in V? Enter an integer.
3,000
The red charge on the left is a positive 1 microCoulomb charge. What is the magnitude and sign of the blue charge on the right?
3. -2.6 microCoulomb
A high voltage transmission line has an aluminum cable of diameter 4.5 cm and is 178 km long. The resistivity of Aluminum is 2.8*10-8 Ωm. What is the resistance of this cable in Ω? Enter a number with one digit behind the decimal point.
3.1 R = (pI)/(A)
Find the magnitude of the electric field produced by a point charge q = 2.47 nanoCoulomb a distance r = 2.6 meters from the charge in units of N/C. Enter a number with two digits behind the decimal point.
3.28
What is the time constant for the discharge of this capacitor?
4 s
What is the equivalent resistance between the points A and B?
4.3Ω
The immediate cause of many deaths is ventricular fibrillation, an uncoordinated quivering of the heart as opposed to proper beating. An electric current discharged to the chest can cause momentary paralysis of the heart muscle, after which the heart will sometimes start organized beating again. A defibrillator is a device that applies a strong electric shock to the chest over a time interval of a few milliseconds. Assume that an energy of 300 J is to be delivered from the defibrillator, having a 30.0 microFarad capacitance. To what potential difference must the defibrillator be charged?
4472 V
Two small objects each with a net charge of Q exert a force of magnitude F on each other. We replace one of these with a charge 4Q. What is the magnitude of the force after the replacement?
4F
The figure below shows different ways to connect a light bulb to a battery with wires. Which of the bulbs will light up?
5
Two uniformly charged spheres are firmly fastened to and electrically insulated from frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces?
5
How much energy (in milliJ) is stored in a 29 microFarad capacitor if the voltage across the capacitor is 19 V? Enter a number with two digitsbehind the decimal point.
5.23 v= 1/2 x C x V^2 1/2 x .029 z 19^2
560 J of work are required to carry a 93 C charge from point 1 to point 2. What is the magnitude if the potential difference between point 1 and point 2 in units of V? Enter a number with one digit behind the decimal point.
6 560/93 = 6.0215
Which graph can represent the gravitational vector field at large distances from the surface of Earth?
A
The charges in the groups A and B below are all given in units of Q. Which group of charges took less work by an external force to bring together from infinity?
A +2 connected horizontally with a +1 charge
Which of the following statements are true about the electric potential or the electric potential difference? (a) The SI unit of the electric potential is volts (V). (b) The electric potential can also be expressed in units of Joules. (c) The electric potential produced by a charge at a given location provides a measure of the rate at which charge flows past that point. (d) As a positive charge moves in the direction of an electric field, it gains electric potential. (f) The electric potential difference between two points is the difference in potential energy possessed by another charge at those two points, divided by this charge.
A and F only (a) The SI unit of the electric potential is volts (V). (f) The electric potential difference between two points is the difference in potential energy possessed by another charge at those two points, divided by this charge.
What do you know about atoms, elementary particles, and charges? Select multiple correct answers!
A. Neutral objects become negatively charged by acquiring additional electrons from other objects. B. Protons and electrons have charges qe and -qe, respectively, where qe is the smallest quantum of charge. D. Electrically neutral materials contain the same number of protons and electrons.
What do you know about steady currents?
A. Steady currents flow in continuous loops. C. A wire can carry a current and still be neutral. E. Two conducting paths from a voltage source to an electrical device are needed to operate the device.
Equipotential lines around two point charges are shown. What is the direction of the electric field at points A and B?
A: to the right B: up
Rank in order, from brightest to dimmest, the identical bulbs A to D.
A>B>C=D
Which of the following statements about a conductor in electrostatic equilibrium is false? (a) The electric field inside the conductor is zero. (b) Just outside the conductor, the electrostatic field is perpendicular to its surface. (c) The net charge inside the conductor is zero, all excess charge resides on the surface. (d) A charge located within a hole in a conductor at equilibrium feels no force from charges outside the conductor.
ALL of the statements are TRUE
How should the field lines inside this bar magnet be drawn?
Approximately parallel from the South pole to the North pole.
Earth and Moon attract each other via the gravitational force. Which force diagram correctly represents the magnitude and direction of the force on each of these objects?
B
A diagram of an irregularly shaped charged conductor is shown below. Four locations along the surface are labeled A, B, C, and D. Rank these locations in increasing order of the strength of the electric field just outside the surface, beginning with the smallest electric field
B < C < D < A
Four arrangements of charge are shown. Each charge has the same magnitude, but some are positive (+) and some are negative (-). All distances are to the same scale. In which arrangement would the magnitude of the force felt by a positive test charge placed at P be the largest?
B and D
What do you know about the electrostatic force that can exist between two interacting objects? Pick the correct statements from the choices below.
B. It depends on the net charge of each of the interacting objects. C. It varies inversely as 1/r2, where r is the distance between the interacting objects. D. It can be attractive. E. It can be repulsive.
Which is the best explanation of why the electric field at A is zero?
B. Point A is midway between two positive charges, so the field vectors cancel.
Two point charges with charge ±q and a point P are located as shown, at the vertices of an equilateral triangle. What is the direction of the electric field due to the two point charges at point P?
B. The -y direction
Select two true statements about the electric field.
B. The direction of the electric field at a point in space is the same as the direction of the electrostatic force on a positive charge at that location. D. The electric force F can be calculated using F = qE, where E is the electric field and q is the charge placed in the field.
To maintain a negative charge on sphere A and a positive charge on sphere B once the rod is removed, what must be done?
B. The spheres are separated and then the charged rod is removed.
Which graph can represent the gravitational vector field near the surface of Earth?
C
Two balls rest on a frictionless surface. One ball is given a charge of +Q. The other ball is given a charge of +3Q. Which diagram best represents the magnitude and direction of the force vectors on the balls?
C (for every action, there is an equal and opposite reaction)
What do you know about circuits with resistors and capacitors?
C. The time constant in heart defibrillation is crucial to limiting the time the current flows. The time constant of a RC circuit is the product of the capacitance and the resistance of the circuit. D. For capacitors in parallel, the capacitances add.
The flux of water through this 3D spherical volume is
C. Zero
Two capacitors have an equivalent capacitance of 8 pF if connected in parallel, and 2 pF, if connected in series. Find C1 and C2.
C1 = 4 pF, C2 = 4 pF
What do you know about conductors in electrostatics?
Charges can move freely in conductors. The electric field is zero inside a conductor. The entire conductor is at the same potential.
A negatively charged balloon is attracted to a neutral wooden cabinet due to polarization. Which one of the following diagrams best depict why this occurs?
D
Equipotential lines are shown. For which region is the magnitude of the electric field the greatest?
D
Are all the diagrams below valid field line diagrams?
D. Only diagram d is not valid.
Two neutral conducting metal spheres on insulating stands are in contact with each other. A positively charged rod is brought near to sphere A. Which of the following statements must be true about the spheres when the charged rod is in the position shown?
D. Sphere B is positive and sphere A is negative.
Two parallel long wires each have a large current I flowing out of the page, as shown. In what direction does the magnetic field point at position P?
Down
Which of the charge configurations depicted would give rise to the potential shown below?
E
Which diagram does not represent an electrical current?
E positive and negative charges going in the same direction
A parallel combination of resistors has an equivalent (or effective) resistance of 2 Ω. Which one of the following is true?
Each of the individual resistances is greater than 2 Ω.
Through rubbing an object has acquired a charge of -1 nanoCoulomb.
Electrons have been transferred to the object
A positive charge is placed at one of 3 locations in a region where the electric field is uniform. Compare the magnitude F of the electric force on the charge at the different locations.
F is the same at all locations, but not zero
If a resistor of resistance R is connected to a battery of voltage V, the equation for the power dissipated in the resistor, P = I2R, implies that a larger resistance R will dissipate more energy and get hotter than a smaller resistance.
False
Assume that the potential difference between the positive and negative plate is 100 V. A proton travels from the positive to the negative plate
Its kinetic energy increases by 1.6*10-17 J = 100 eV.
Charges Q1 and Q2 are situated as shown below. The electric field is zero at point P. What conclusions can be drawn about the charges Q1 and Q2? I. The magnitudes of Q1 and Q2 are equal. II. The magnitude of Q1 is greater than the magnitude of Q2. III. The magnitude of Q1 is less than the magnitude of Q2. IV. Q1 and Q2 have the same sign. V. Q1 and Q2 have different signs
II and IV are correct.
Three resistors connected in series carry currents labeled I1, I2, I3, respectively. Which of the following expresses the value of the total current IT in the system made up of the three resistors?
IT = I1 = I2 = I3
Two cylindrical copper wires have the same mass. Wire A is 2 times as long as wire B. How are their resistances are related? Hint: same masssame volume volume of a cylinder = area*length What happens to the area when the length is doubled?
RA = 4RB
Knowing that the severity of a shock depends on the magnitude of the current through your body, would you receive a bigger shock being in series or in parallel with a resistance, such as the heating element of a toaster, if shocked?
In parallel
An electron moves perpendicular to a magnetic field. A magnetic force acts on it in the direction shown. What is the direction of B?
Into the page
Compare the electric flux through surfaces (a) and (b). All charges have magnitude q
NONE OF THE ABOVE
Choose the force vector that correctly shows the direction o f t h e net force on charge B due to the other two charges. Charge is given in arbitrary units.
None of these, the vector should point up and left
Which requires the most positive work done by an external force, to move a positive point charge from point P to point 1, 2, 3, or 4? All those points are the same distance from P.
P --> 1 arrow going horizontal in the direction opposite of the field lines
Which requires zero work, to move a positive point charge from point P to point 1, 2, 3, or 4? All those points are the same distance from P.
P --> 3 Perpendicular to P... therefore zero work!
Using the image below as your only evidence, what MUST be true about charged objects 1 and 2 below?
The charge on both objects has the same sign.
Two parallel wires are carrying currents in opposite directions as shown. Wire #1 has a current I1 that is out of the page, whereas wire #2 has a current I2 that is into the page. What effect does the magnetic field from wire #1 have on wire #2?
The field exerts a force on wire #2 (directed right)
What do you know about the magnetic force?
The magnetic force F on a moving charge is perpendicular to the direction of the magnetic field B. In a uniform magnetic field the net force on a current loop is always zero.
Which statement is false? 1. There is no net force acting on either loop. 2. There is no torque acting on loop A but a torque is acting on loop B. 3. Loop A has lower potential energy than loop B. 4. Loop A is in stable equilibrium and loop B is in unstable equilibrium.
There is no torque acting on loop A but a torque is acting on loop B
A bulb (i.e., a resistor) is connected in series to a switch, a battery, and an uncharged capacitor. At t = 0, the switch is closed. Which of the following best describes the brightness of the bulb as a function of time?
c brightness decreases as time goes on
Make a prediction. When we connect the power supply, the wire will move
forward
An electron travels through free space from point A, which is at 100 V, to point B, which is at 200 V. The kinetic energy of the electron during this trip
increases by 1.6*10-17 J.
Consider a uniform electric field, for example the field inside a parallel plate capacitor, as shown. If an electron is taken from location A to location B to location C, how does its potential energy change?
it increases arrows going from + to - plate
When a positive charge is released from rest in a uniform electric field, it will
move with constant acceleration
A certain physics textbook shows a region of space in which two electric field lines cross each other. We should conclude that
the author of the textbook made a mistake.
Equipotential lines around several point charges are shown. What are the signs of the charges at points A and B?
the charge at B is positive and the charge at A is negative
A parallel-plate capacitor initially has a voltage of 12 V and stays connected to the battery. If the plate spacing is now doubled, what happens?
the charge decreases **The battery is a charge pump. It can pump charge from one plate to the other to maintain a constant potential difference.
A parallel-plate capacitor initially is connected to a battery and the plates hold charge ±Q. The battery is then disconnected. If the plate spacing is now doubled, what happens?
the voltage increases **No battery <-- --> no charge pump. Charge cannot move from one plate to the other.
A bird sits on a high-voltage line. Why does the bird not get hurt?
the whole bird is at approximately the same potential, so there is no electric field across it
A circular loop of wire carries a constant current. If the loop is placed in a region of uniform magnetic field, the net magnetic force on the loop is
zero
Location P is equidistant from the two charges of an electric dipole. The voltage at P is
zero
If electrical energy costs $0.10 for each kWh, what would it cost to raise a house of mass 5*105 kg (500 metric tons) a height of 1 m above the ground, using an electric motor of 50% efficiency? Use g = 10 m/s2.
~$0.28
The graph below shows a contour map of the equipotential surfaces due to 3 point charges. Estimate the magnitude and direction of the electric field at point P
~800 V/m down
In a series RC circuit a charged capacitor with capacitance C is being discharged though a resistor of resistance R. Plots of voltage and current versus time are shown below. Find the time constant τ of the circuit.
τ = 1 s
