Chapter 7 Mastering Physics

The brightness of a light bulb having a specific resistance increases if the current through it increases. For the circuit shown in the figure below, how does the brightness of the light bulb change when the resistance of the 10-ΩΩ resistor is decreased?

The brightness does not change.

How does the brightness of the light bulb change if the magnet is moved through the coil more quickly?

The bulb shines more brightly.

We repeat the experiment from the video, but this time we connect the wires in parallel rather than in series. Which wire will now dissipate the most heat?

The copper wire (resistance 0.1 ΩΩOmega)

For the circuit in the previous part, what happens to the current flowing through the resistor on the right when the switch is closed (allowing current to flow through the resistor on the left)?

The current flowing through the resistor on the right does not change.

What is the voltage 3 mm away from the charge?

3 V

What must be the resistance of a single resistor connected to a 9-VV battery for the current coming out of the battery to be the same as that of the circuit in Part D?

30 Ω

Use the noncontact ammeter to measure the current flowing through the circuit. What is the current?

0.9 A

Two conducting spheres are each given a charge QQ. The radius of the larger sphere is three times greater than that of the smaller sphere. If the electric field just outside of the smaller sphere is E0E0, then the electric field just outside of the larger sphere is

1/9 E0

Consider a point 0.5 mm above the midpoint of the two charges. As you can verify by removing one of the positive charges, the electric field due to only one of the positive charges is about 18 V/mV/m. What is the magnitude of the total electric field due to both charges at this location?

25 v/m

What is it called when it is calibrated to read current?

Ammeter

What is induced by the rapid alternation of a magnetic field?

An electric field is induced in any region of space in which a magnetic field is changing with time.

Distinguish between DC and AC.

DC is an electric current that flows in one direction only; AC is an electric current that repeatedly reverses its direction.

How is Coulomb's law different from Newton's law of gravity?

Electrical forces may be either attractive or repulsive, whereas gravitational forces are only attractive

What condition is necessary for electric charge to flow from one end of a metal bar to another?

Electrical potential difference. The electric charge flows from the end with higher potential to the end with lower potential.

Why do electrons, rather than protons, make up the flow of charge in a metal wire?

Electrons are loosely attached to atomic nuclei; protons are locked in the atomic nuclei.

What is the voltage at the midpoint of the two charges?

Exactly twice the voltage produced by only one of the charges at the same point

Why is iron magnetic and wood is not?

Iron has magnetic domains; wood does not.

How are magnetic poles different from electric charges?

Magnetic poles always come in pairs and cannot be isolated unlike electric charges.

What kind of materials are the best conductors? Why are they so good at conducting electricity?

Metals are the best conductors because their atoms have one or more outer electrons that are loosely bound to their nuclei.

Make a small dipole by bringing the two charges very close to each other, where they are barely touching. The midpoint of the two charges should still be on one of the grid point intersections (see figure below) Measure the strength of the electric field 0.5 mm directly above the midpoint as well as 1 mm directly above. Does the strength of the electric field decrease as 1 over distance squared (1/r21/r2)?

No, it decreases more quickly with distance.

What is the unit of electrical resistance?

Ohm (ΩΩ)

Which type of circuit is favored for operating several electrical devices, each independently of the other −− series or parallel? Defend your answer.

Parallel circuit is better because turning off a device doesn't affect the other devices.

How does the sum of the currents through the branches of a simple parallel circuit compare with the current that flows through the voltage source?

The sum of currents though parallel branches equals the current through the source.

A material with a lot of free charged particles that easily flow through it when an electrical force acts on them is a/an

conductor.

Make an electric dipole by replacing one of the positive charges with a negative charge, so the final configuration looks like the figure shown below. The electric field at the midpoint is ________________.

directed to the right.

For the circuit in the previous part, the current flowing in the wire between the positive terminal of the battery and the resistor is ___________ the current flowing between the resistor and the negative terminal of the battery.

equal to

The magnitude of the electric field 1 mm away from the positive charge is ________________ the magnitude of the electric field 2 mm away.

four times

The current through each resistor in the two-resistor circuit is _________ the current through the resistor in the one-resistor circuit (the circuit in Part A). The voltage across each resistor in the two-resistor circuit is ___________ the voltage across the resistor in the one-resistor circuit.

half / half

A material with very little or no free charged particles and through which current does not easily flow is a/an

insulator.

Consider the situation in the figure below, where two charged rods are placed a distance dd on either side of an aluminum can. What does the can do?

stays still

Now, consider the situation shown in the figure below. What does the can do?

stays still

What is the voltage reading across the resistor, as shown in the figure below?

9.0 V

What is meant by saying that charge is conserved?

Charge cannot be created or destroyed; it can only be transferred.

What is a galvanometer?

A galvanometer is a device that indicates electric current.

How is Coulomb's law similar to Newton's law of gravity?

Coulomb's law and Newton's law of gravity are the inverse-square laws.

You can switch the polarity of the bar magnet by clicking on the button with the bar magnet and two semi-circular arrows on the bottom right. Note how the positions of the north and south poles are switched every time you click on this button. Drag the bar magnet from left to right through the top coil, then repeat the same process after switching the polarity of the magnet. What effect, if any, does switching the polarity have on the brightness of the bulb and the needle of the voltmeter?

Switching the polarity of the magnet has no effect on the bulb brightness, but inverts the direction of the needle deflection in the voltmeter

What condition is necessary for heat energy to flow from one end of a metal bar to another?

Temperature difference. The heat flows from the end with higher temperature to the end with lower temperature.

Now let's repeat the previous experiment with the four-loops coil. Starting from the far left of the screen, move the magnet to the right so it goes through the middle of the four-loops coil at a constant speed and out to the right of the coil. How does the average brightness of the light bulb depend on the number of loops of the pickup coil?

The brightness increases when the number of loops increases.

Now, let's look at how the distance from the charge affects the magnitude of the electric field. Select Values on the menu, and then click and drag one of the yellow E-Field Sensors. You will see the magnitude of the electric field given in units of V/mV/m (volts per meter, which is the same as newtons per coulomb). Place the E-Field Sensor 1 mm away from the positive charge (1 mm is two bold grid lines away if going in a horizontal or vertical direction), and look at the resulting field strength. Consider the locations to the right, left, above, and below the positive charge, all 1 mm away. For these four locations, the magnitude of the electric field is________________.

the same

Now, make an electric dipole by replacing one of the positive charges with a negative charge, so the final configuration looks like the figure shown below. What is the voltage at the midpoint of the dipole? The voltage at the midpoint of the dipole is

zero

If the field strength is EEE = 9 V/mV/m a distance of 1 mm from the charge, what is the field strength EEE a distance of 3 mm from the charge?

1

The electric potential (voltage) at a specific location is equal to the potential energy per unit charge a charged object would have if it were at that location. If the zero point of the voltage is at infinity, the numerical value of the voltage is equal to the numerical value of work done to bring in a unit charge from infinity to that location. Select Values and Grid in the menu, and drag one positive charge to the middle of the screen, right on top of two intersecting bold grid lines. Using the voltage meter, you should find that 1 mm away from the charge, the voltage is 9 V. What is the voltage 2 mm away from the charge?

4.5

For the series circuit in the previous part, change the resistance of the bottom resistor to 20 ΩΩ. What is the voltage across this 20-ΩΩ resistor?

6.0 VV

With the switch open, roughly what must be the resistance of the resistor on the right for the current out of the battery to be the same as when the switch is closed (and the resistances of the two resistors are 20 ΩΩ and 10 ΩΩ)?

7 Ω

Let's start with a simple circuit that you will have seen in the Ohm's Law and Power PhET tutorial, if you completed that one. What is the voltage reading across the resistor, as shown in the figure below? You should see the blue electrons flowing through the circuit, whereas (conventional) current is the flow of positive charge. You can select which current to display in the panel in the upper-right corner.

9.0 V

Considering a kilowatt and a kilowatt-hour, which is a unit of energy and which is a unit of power?

A kilowatt is a unit of power and a kilowatt-hour is a unit of energy.

What is induced by the rapid alternation of an electric field?

A magnetic field is induced in any region of space in which an electric field is changing with time.

What produces a magnetic field?

A magnetic field is produced by moving electric charges

What condition is necessary for a sustained flow of electric charge through a conducting medium?

A sustained flow of electric charge requires a device to maintain a difference in electric potential.

The figures below show four circuits, with the resistances of the resistors given. In all cases, the emf of the battery is 10 VV. Rank the circuits in order of descending total current coming out of the battery. (You should be able to answer this question using what you have already learned, but if you want, feel free to build the four circuits and make measurements.)

A. 9.0V,10.0 Ω, 10.0 Ω,50.0 Ω B. 9.0V,10.0 Ω,10.0 Ω C. 9.0V, 10.0 Ω D. 9.0V, 10.0 Ω,10.0 Ω

Place several E-Field Sensors at a few points on different equipotential lines, and look at the relationship between the electric field and the equipotential lines. Which statement is true?

At any point, the electric field is perpendicular to the equipotential line at that point, and it is directed toward lines of lower voltages.

How is the rule for the interaction between magnetic poles similar to the rule for the interaction between electric charges?

In both interactions, opposites attract and likes repel.

Faraday's law of induction deals with how a changing magnetic flux induces an emf in a circuit. Recall that magnetic flux depends on magnetic field strength and the effective area the field is passing through. We'll start our investigation by looking at the field strength around a bar magnet. Position the magnet around the coil so that the region labeled A in the figure below is inside the coil. Move the magnet slowly back and forth and observe the effect on the brightness of the bulb and the needle of the voltmeter. Repeat the same process for the other two regions. For which of the regions shown in the figure is the observed effect the strongest?

Region C

Using the setup from the first question, imagine that you briefly touch the negatively charged rod to the can (assume that this rod is conducting for the sake of effect). You then hold the two rods at equal distances on either side of the can. What does the can do?

Rolls toward the positively charged rod

Double the resistance of the resistor by changing it from 10 Ω to 20 Ω. What happens to the current flowing through the circuit?

The current decreases by a factor of two.

Construct a circuit containing one battery, one resistor, and wire to close the circuit. The order and orientation doesn't matter, but it should look something like the figure below. You can show the values of the components by clicking the Values checkbox in the display at the upper-right corner of the panel. Use the default values of the battery and resistor. You should see the blue electrons flowing through the circuit. In what direction is the (conventional) current flowing through the circuit? Recall that current is the flow of positive charge.

The current flows from the positive terminal, through the wires and resistor, and into the negative terminal.

For the circuit containing one resistor and one battery, what happens to the current if the voltage is tripled and the resistance is doubled?

The current increases

For the circuit containing one resistor and one battery, what happens to the current if the voltage is doubled?

The current increases by a factor of two.

Change the value of the battery emf to 10.0 VV, and make sure the middle resistor is set to 20 ΩΩ. Use extra wire for each "leg" of the circuit so you can measure the current through all legs with the ammeter. How does the current coming out of the battery change when the switch is closed?

The current increases.

Place the crosshairs of the noncontact ammeter just before each resistor to measure the current through the top and bottom resistors. How do these currents compare?

The currents are the same.

Remove the positive charge by dragging it back to the box at the bottom, and drag a negative charge (blue) toward the middle of the screen. Determine how the electric field is different from that of the positive charge. Which statement best describes the differences in the electric field due to a negative charge as compared to a positive charge?

The electric field changes direction (now points radially inward), but the electric field strength does not change.

Select Electric Field and Grid in the green menu. Drag one positive charge and place it near the middle of the screen, right on top of two intersecting bold grid lines. You should see something similar to the figure below. Which of the following describes the electric field produced by the positive charge?

The electric field is directed radially away from the charge at all locations near the charge.

Now, remove the negative charge, and drag two positive charges, placing them 1 mm apart, as shown below. Let's look at the resulting electric field due to both charges. Recall that the electric field is a vector, so the net electric field is the vector sum of the electric fields due to each of the two charges. Where is the magnitude of the electric field roughly equal to zero (other than very far away from the charges)?

The electric field is roughly zero near the midpoint of the two charges.

Try to have the equipotential lines equally spaced in voltage. Then, use an E-Field Sensor to measure the electric field at a few points while looking at the relationship between the electric field and the equipotential lines. Which of the following statements is true?

The electric field strength is greatest where the equipotential lines are very close to each other.

Equipotential lines are usually shown in a manner similar to topographical contour lines, in which the difference in the value of consecutive lines is constant. Clear the equipotential lines using the Erase button on the voltage tool. Place the first equipotential line 1 mm away from the charge. It should have a value of roughly 9 VV. Now, produce several additional equipotential lines, increasing and decreasing by an interval of 3 VV (e.g., one with 12 VV, one with 15 VV, and one with 6 VV). Don't worry about getting these exact values. You can be off by a few tenths of a volt. Which statement best describes the distribution of the equipotential lines?

The equipotential lines are closer together in regions where the electric field is stronger.

A light bulb is basically a resistor that gets so hot that it glows, emitting light. For this tutorial we will assume the resistor in the light bulb is ohmic (that means Ohm's law applies to the resistor). The rate of energy emitted by the light bulb is its output power, commonly referred to as luminosity (brighter means more luminous). Hook up a light bulb to a 5-VV battery. Right-click (or control-click) on the light bulb, and change its resistance. How does the brightness of the light bulb depend on its resistance?

The light bulb gets dimmer as the resistance is increased.

Roughly where is the magnet when the light bulb is the brightest? (The brightness of the light bulb correlates with how much the needle of the voltmeter gets deflected away from the middle.)

The light bulb is brightest when either end of the magnet is in the middle of the coil.

In Part H, you discovered that the luminosity of a light bulb increases if the current increases. The rate at which electric potential energy is converted into heat depends on the current flowing through the bulb and the voltage across the bulb. This energy is supplied by the battery. Mathematically, the luminosity PP of the light bulb is given by P=ΔVIP=ΔVI, where ΔVΔV is the voltage across the bulb and II is the current.What happens to the luminosity of the light bulb if the voltage of the battery is doubled? (Note that the PhET simulation does not display a numerical value for the luminosity, so you should use the relationship between the luminosity, the voltage across the bulb, and the current.)

The luminosity increases by a factor of four.

Which part of an atom is positively charged, and which part is negatively charged?

The proton has a positive charge and the electron has a negative charge.

With the switch closed, how does the voltage across the 20-Ω resistor compare to the voltage across the 10-Ω resistor?

The voltage across the 20-Ω resistor is equal to the voltage across the 10-Ω resistor.

Now, remove the positive charge by dragging it back to the basket, and drag one negative charge toward the middle of the screen. Determine how the voltage is different from that of the positive charge. How does the voltage differ from that of the positive charge?

The voltages become negative instead of positive and keep the same magnitudes.

Click on the button with two coils in it in the lower part of the window. The circuit should now have two sets of coils. Place the bar magnet inside the coil containing two loops. Try to find a location where the stationary magnet induces a current in the coil and causes the light bulb to shine. Which of the following is correct?

There is no induced current in the coil, so the light bulb does not shine, if the magnet is stationary (for any location of the magnet).

What is it called when it is calibrated to read voltage?

Voltmeter

For the parallel circuit in the previous part (with the switch closed), the current through the 20-ΩΩ resistor is _________ the current through 10-ΩΩ resistor.

less than

Which has more resistance −− a short wire or a long wire of the same thickness and made from the same material?

long wire

The figure shows the path of a charged particle moving in a magnetic field directed into the screen. What is the particle's charge?

negative

A material that falls into the midrange of conductivity, since it possess few electrons that are free to move is a/an

semiconductor.

Drag a battery into the construction panel, and use the voltmeter to determine which end of the battery is the positive terminal. The positive terminal has a higher potential than the negative terminal (recall that the voltmeter measures the potential difference between the red probe and the black probe). Which end of the battery is the positive terminal?

the orange end

Which has more resistance −− a thick wire or a thin wire of the same length and made from the same material?

thin wire