Physics Exam I IClickers

Fred the lightening bug has a mass m and a charge +q. Jane has a mass of ¾ m and a charge -2q. Because they have charges of opposite sign, they are attracted to each other. Which is more attracted to the other and by how much?

They are attracted to each other by the same amount.

F= k (Q1Q2)/(r²)

Mass, therefore, does not affect the attraction. The charges only affect the magnitude of the force, not which bug is attracted more.

The electric field lines shown are due to a point charge. What can you say about the field at point 1 compared to the field at point 2?

The field at point 1 is larger because the field lines are closer together in that region.

Closer fields lines to the center are stronger, being directly on a field lied does not affect anything.

A negative point charge is in an electric field created by a positive point charge. Which is true?

The field points away from the positive charge and the force on the negative charge is in the opposite direction to the field

field lines for positive charges point away from the charge and negative charges point toward the charge (therefore the negative charge points opposite of the positive)

Since opposite charges attract, the force points toward the positive charge and is opposite to the field

As an object acquires a positive charge, its mass usually…

decrease

more positive charge means you’re losing electrons therefore losing their mass which lowers the overall mass

If the two charged plates were moved until they are half the distance shown without changing the charge on the plates, the electric field near the center of the plates would…

the electric field would remain almost exactly the same

bringing the plates closer together does not affect magnitude, only the closeness (density) of the red field lines

force would be affected by the plates moving closer but not the electric field itself

We wish to determine the electric field at a point near a positively charged metal sphere (a good conductor). We do so by bringing a small positive test charge, q0, to this point and measure the force F0 on it. F0/q0 will be ______ the electric field E as it was at that point before the test charge was present. 

F0/q0 will be less than the electric field

The test charge will repel + charges on the surface of the conductor → increase their distances from the test charge → they will contribute a smaller amount to the field because they are far

We are usually not aware of the electric force acting between two everyday objects because…

most everyday objects have as many + charges as - charges

same amount of push and pull so we don’t feel any force

To be safe during a lightening storm, it is best to be…

inside a metal car

the electron transfers occur on the surface of the metal car therefore the cavity inside the car has nothing going on (rubber tires ground the car)

What are the worst places to be during a lightening storm?

inside a grassy meadow- humans are taller than grass so we are the higher conductor

next to a tree in the forest- the tree is taller therefore will conduct energy transfers that will affect humans as conductors

inside a wooden building- good insulation until it breaks down

on a metal observation tower- metal is a great conductor therefore will likely attract the lightening, you are also on top of it meaning you’re higher

Which vector best represents the resultant direction of the electric field at the fourth corner of the square due to the three charges as depicted in the picture? 

Vector d

The two positive charges create equal magnitude electric fields pointing in the positive x direction and in the negative y direction → produce an electric field in the direction of d that forces it away from the square

The electric field from the negative charge points in the direction of b since electric fields point toward negative charges → weaker than positive fields

A small metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positively charged rod held near the ball. The charge of the ball must be…

negative or neutral

negative charges works bc it is attracted to a positively charged rod

neutral conductors will become polarized and attract to positive rod

Which of the following statements is valid regarding the relationship between the electric field (E) and electric potential (V)?

If the potential (voltage) throughout the particular region is constant, the field throughout that region must be zero because they are proportional.

A +0.2µC charge is in an electric field. What happens if that charge is replaced by a +0.4µC charge?

The electric potential (voltage) stays the same but the electric potential energy doubles.

Electric potential determined by the electric field → independent of charge placed in the field → doubling the charge will not affect the electric potential

Electric potential energy is the product of the electric potential and the electric charge → doubling the charge will double the electric potential energy

Two identical positive charges are placed near each other. At the point halfway between the two charges…

the electric field is zero and the potential is positive

Electric field is a vector, while the electric potential is a scalar. Electric field at the halfway point → sum of the electric fields from each charge→ magnitudes of the fields are same but point in opposite directions→ sum is zero

The electric potential from each charge is

positive→ the net potential at the midpoint is the sum of the two potentials, which will also be positive.

Four identical point charges are arranged at the corners of a square. The electric field (E) and potential (V) at the center of the square are…

E=0 and V is not 0

The net electric field at the center will have equal magnitudes and point away, so they cancel out. The electric potential from each charge is a nonzero scalar. At the center, the magnitudes of the four potentials are equal and sum to a nonzero value.

If it takes an amount of work (W) to move two +q point charges from infinity to a distance d apart from each other, then how much work should it take to move three +q point charges from infinity to a distance d apart from each other?

3W

it takes W to bring two charges together. When the third charge is brought in, it is repelled by both of the other charges→ takes an additional 2W to bring in the third charge

A proton (Q=+e) and an electron (Q=-e) are in a constant electric field created by oppositely charged plates. You release the proton from near the positive plate and the electron from near the negative plate. Which feels the larger electric force?

the magnitude of the force is the same for both but in opposite directions

the magnitude of the force is the product of the electric field and the charge → same magnitude of charge and same electric field = experience the same magnitude force but in opposite directions

A proton (Q=+e) and an electron (Q=-e) are in a constant electric field created by oppositely charged plates. You release the proton from near the positive plate and the electron from near the negative plate. When the proton and electron strike the opposite plate, which one has more kinetic energy?

both acquire the same kinetic energy

the increase in kinetic energy=the decrease in electric potential energy

The change in potential for each object has the same magnitude just with opposite signs→ experience the same change in electric potential energy→ same final kinetic energy

Which of the following do not affect capacitance?

Charge on the plates and energy stored in the capacitor

C = εA/d

C →capacitance

ε→ permittivity of the dielectric material

A → plate area

d→ plate separation

A battery establishes a voltage V on a parallel-plate capacitor. After the battery is disconnected, the distance between the plates is doubled without loss of charge. Accordingly, the capacitance ___ and the voltage between the plates ____.

capacitance decreases and voltage between the plates increases

When the battery is disconnected, the charge remains constant on the plates. The capacitance decreases as the plates are pulled apart, since the capacitance is inversely proportional to the separation distance. For the charge to remain constant with smaller capacitance, the voltage between the plates increases. 

V=Q/C

Which of the following is a vector?

electric field

A +0.2µC charge is in an electric field. What happens if that charge is replaced by a -0.2µC charge?

the electric potential stays the same, but the electric potential energy changes sign

The electric potential is determined by the electric field and is independent of the charge placed in the field. Therefore, changing the charge will not affect the electric potential. The electric potential energy is the product of the electric potential and the electric charge. Changing the sign of the charge will change the sign of the electric potential energy.

When connected to a battery, a light bulb glows brightly. If the battery is reversed and reconnected to the light bulb, the bulb will glow…

with the same brightness

it will glow equally bright regardless of the direction in which current flows through it since the direct current will go through anyway

When a battery is connected to a light bulb properly, current flows through the light bulb and makes it glow. How much current flows through the battery compared with the light bulb

the same amount

the # of electrons going in is the same going out therefore the current is the same going in as going out

Which of the following statements of Ohm’s Law is true?

Ohm’s Law relates the current through a wire to the voltage across the wire

V=IR

Voltage=Current*Resistance

Where in the circuit is the current the largest?

It is the same at all points

The current is a measure of the rate that electrons pass a given point. If the current were different at two points in the circuit, then electrons would be changing between those two points. The buildup of electrons would cause the circuit to be time dependent and not a steady state system. The flow of electrons (current) must be the same at all points in a loop

Electrons carry energy from a battery to a light bulb. What happens to electrons when they reach the light bulb?

they get excited but do not get used up, stay in the light bulb, or are emitted as light

Electrons at higher potential energy enter the lightbulb and give off that energy as they pass through the bulb

When you double the voltage across a certain material or device, you observe that the current increases by a factor of 3. What can you conclude?

Ohm’s Law is not obeyed in this case.

Ohm’s law requires that the ratio of voltage to current be constant. Since it is not constant in this case, the material does not obey Ohm’s law.

When current flows through a resistor…

charge nor the current are used up

The resistor removes energy from the system, such that the electrons exiting the resistor have less potential energy than the electrons entering, but the number of electrons (charge carriers) entering and exiting the resistor is the same. The rate of electrons entering and exiting is equal to the constant current in the circuit.

The unit kilowatt-hour is a measure of…

an amount of energy

the kilowatt-hour is the product of the unit of power (kilowatt) and a unit of time (hour), resulting in a unit of energy.

Why might a circuit breaker open if you plug too many electrical devices into a single circuit?

the current becomes too high

Each device added to the circuit is added in parallel. The voltage across the circuit does not change as the devices are added. Each new device, however, creates a path for additional current to flow, causing the current in the circuit breaker to increase. When the current becomes too high, the circuit breaker will open.

Nothing appears to happen when birds land on a powerline, yet we are warned not to touch the powerline with a ladder. What is the difference?

there is little to no voltage drop between a bird’s feet but there is a significant voltage drop between the top of the ladder on the powerline and the bottom on the ground

When a ladder is placed between the ground and the wire, it creates a path for the current to flow from the high-voltage wire to ground (zero voltage). This large potential difference will enable a large current to flow through the ladder.

When a light switch is turned on, the light comes on immediately because…

the electrons already in the wire are instantly pushed by a voltage difference

When the switch is turned on, the electric potential across the circuit creates an electric field in the wire that causes all of the conduction electrons in the wire to move.

Which resistors are connected in parallel?

R1 and R3

Following a path from the positive terminal of the battery, all of the current from the battery passes through R2 before reaching the junction at the end of that branch.

The current then splits and part of the current passes through R1 while the remainder of the current passes through R3 before meeting at the right junction

A 10,000 Ohm resistor is placed in series with a 100 Ohm resistor. The current in the 10,000 Ohm resistor is 10Amps. If the resistors are swapped, how much current flows through the 100 Ohm resistor?

10A

The two resistors are in series, so they must have the same current flowing through them. It does not matter which resistor is first.

In which circuit(s) shows are the resistors in series?

A and C

Resistors are connected in series when there are no junctions between the resistors. With no junctions between the resistors, any current flowing through one of the resistors must also flow through the other resistor

two identical 10 volt batteries and two identical 10 ohm resistors are placed in series. if a 10 ohm light bulb is connected with one end connected between the batteries and the other end between the resistors, how much current will flow through the light bulb?

0 A

Current will only flow through the lightbulb if there is a potential difference across the bulb. If we consider the bottom branch of the circuit to be at ground, then the left end of the lightbulb will be at a potential of 10 V. The top branch will be at 20 V. Since the two resistors are identical, the voltage drop across each will be half the voltage from the top to bottom, or 10 V each. This makes the right side of the lightbulb also at 10 V. Since both sides of the lightbulb are at the same potential, no current will flow through the lightbulb.

which resistor has the greatest current going through it? Assume all resistors are equal.

R5

Resistors R1 and R2 are in parallel, so each has half of the current from the battery. R3 and R4 are in series and add to produce twice as much resistance as R5. Since they are in parallel with R5, 1/3 of the current from the battery goes through them, while 2/3 goes through R5. The greatest current therefore goes through R5.

Three identical light bulbs are in a circuit. What happens to the brightness of bulb A if you replace bulb B with a short circuit?

bulb A goes out

Current takes the path of least resistance. Since bulb A is in parallel with the short circuit, all of the current will pass through the short circuit, causing bulb A to go out.

When the switch is closed, what will happen to the voltage across resistor 4?

decrease

when the switch is closed, the additional parallel resistor makes the effective resistance of the parallel resistors smaller, and therefore the resistance of the entire circuit gets smaller

With a smaller effective resistance, a greater current flows through the battery and through R1, resulting in a greater voltage drop across R1.

Since the voltage from the battery is equal to the sum of the voltages across R1 and R4,  increasing the voltage across R1 results in a decrease in voltage across R4. 

When the switch is closed, what will happen to the voltage across resistor 1?

increase

When the switch is closed, it adds an additional resistor in parallel to R3 and R4, making the effective resistance of the entire circuit smaller. With less effective resistance, a greater current flows through the circuit, increasing the potential difference across R1.

As a capacitor is being charged in an RC circuit, the current flowing through the resistor is…

decreasing

As the capacitor charges, the voltage drop across the capacitor increases, thereby diminishing the voltage drop across the resistor. As the voltage drop across the resistor decreases, the current decreases

For the circuit shown, what happens when the switch S is closed?

the capacitor eventually charges up to the full battery emf at a rate determined by r and c

Even though steady current cannot flow through a capacitor, charge can build up on the capacitor, allowing current to initially flow in the circuit. As the charge builds on the capacitor, the voltage drop across the capacitor increases, and the current decreases. The rate of charging is determined by the time constant, which is the product of R and C.

A resistor and a capacitor are used in series to control the timing in the circuit of a heart pacemaker. To design a pacemaker that can double the heart rate when the patient is exercising, what must happen to the capacitor?

it needs to discharge faster so the resistance should be decreased

To double the heart rate, the time of discharging must be shorter, so the discharge rate must be faster. The resistance limits the rate at which the current can flow through the circuit. Decreasing the resistance will increase the current flow, causing the capacitor to discharge faster.

The capacitor in a circuit is charged to an initial value Q. When the switch is closed, it discharges through the resistor. It takes 2.0 seconds for the charge to drop to 0.5 Q. How long does it take to drop to 0.25 Q.

4.0 seconds

every 2.0 seconds, half of the remaining charge on the capacitor will discharge. After 2.0 seconds, half of the charge remains. After 4.0 seconds, half of the half, or one-fourth, of the charge remains. After 6.0 seconds, one-eighth of the charge remains.

When capacitors are connected in series, the effective capacitance is _____ the smallest capacitance

When capacitors are connected in parallel, the effective capacitance is _____ the largest capacitance

less than; greater than

Connecting capacitors in series effectively increases the plate separation distance, decreasing the net capacitance. Connecting capacitors in parallel effectively increases the plate area, increasing the net capacitance. Therefore, when capacitors are connected in series, the effective capacitance will be less than the capacitance of the smallest capacitor, and when connected in parallel, the effective capacitance will be greater than the capacitance of the largest capacitance.

Why is an appliance cord with a three-prong plug safer than one with two prongs?

the third prong grounds the case, so the case cannot reach a high voltage

In a two-prong cord, one prong is at high voltage and the other is grounded. Electricity flows through the appliance between these two wires. However, if there is a short between the high- voltage wire and the casing, then the casing can become charged and electrocute a person touching the case. The third prong connects the external case to ground, so that the case cannot become charged.

If ammeters and voltmeters are not to significantly alter the quantities they are measuring…

the resistance of an ammeter should be much lower and the resistance of a voltmeter should be much higher than those of the circuit being measured

The ammeter is placed in series with the circuit and therefore should have a small resistance, so there is minimal voltage drop across the ammeter. The voltmeter is placed in parallel with the circuit. It should have a large resistance so that minimal current from the circuit passes through the voltmeter instead of passing through the circuit.