Physics Semester Exam

An object with a charge of +5 C is used to charge another object that already has a -3 charge through conduction. The final charge on the first object will be ________.

+1 C

An object with a charge of +5 C is used to charge a neutral object through conduction. The final charge on the second object will be _________.

+2.5 C

An object with a charge of +5 C is used to charge another object that already has a +3 charge through conduction. The final charge on the first object will be ______.

+4 C

An object with a charge of +5 C is used to permanently charge another object through induction. The final charge on the second object will be _________.

-5 C

Which of the following statements comparing the electrostatic force to the gravitational force is/are true? Both forces involve an inverse square relationship. The gravitational force is a field force, but the electrostatic force is a contact force. The gravitational force can only pull, while the electrostatic force can push or pull.

1 and 3 only

Describe two relationships you saw between the variables listed below based on your lab simulation graphs. You must use full sentences and describe observations from the simulation to earn full credit. Describe the relationship of 2 of these: Area vs. Capacitance Area vs. Charge Stored Separation vs. Capacitance Separation vs. Charge Stored

1. Area vs Capacitance has a squared relationship. As the area of the plates increases, the capacitance also increases. 2. Area vs Charge Stored has a squared relationship. As the area of the plates increases, the amount of charge stored also increases. 3. Separation vs Capacitance has an exponential relationship. As the separation of the plates decreases, the capacitance also increases. 4. Separation vs Charge Stored has an exponential relationship. As the separation of the plates decreases, the charge stored also increases.

A 7x10-6 F capacitor stores 4x10-3 C of charge. What is the voltage across the plates? How much energy is stored in the electric field?

1. C = Q / V 7 x 10-6 F = 4 x 10-3 C / V V = 571.43 V 2. PEc = 1/2 * C * V2 PEc = 1/2 * 7 x 10-6 F * 571.432 V PEc = 1.14 J

A charged particle is brought near an electron and the electron accelerates at a rate of 4.8 x 10-3 m/s2 to the right. You must show all work to earn credit. If the same force acted on a proton, what direction would the proton accelerate? What force is applied to the electron? What is the acceleration of the proton?

1. If the same force acted on a proton, the proton would accelerate in the direction opposite to the charged particle (left). 2. F = m * a F = 9.11 x 10^-31 * 4.8 x 10^-3 F = 4.3728 x 10^-33 N 3. F = m * a 4.3728 x 10^-33 N = 1.673 x 10^-27 * a 4.3728×10^-33 N / 1.673 x 10^-27 = a a = 2.61 x 10^-6 m/s^2

A capacitor stores 3.5x10-3 J of energy when connected to a 6 V source. How much charge is stored on the plates of the capacitor?

1. PEc = 1/2 Q * V = 1/2 * C * V2 PEc = 1/2 * Q * V 3.5 x 10-3 J = 1/2 * Q * 6 V 3.5 x 10-3 J = Q * 3 V Q = 1.167 x 10-3 C

An electron moves through a uniform electric field toward a negatively charged plate. If the electron moves through a potential difference of 25 V in 5 cm, Does the electron gain or lose potential energy? How much work is done on the electron? What is the strength of the electric field?

1. The electron and plate are negatively charged, hence they repel one another. It requires work to move them closer, so the electron gains potential energy. 2. Work = PE = V * q Work = 25 V * 1.6 x 10-19 C Work = 4.0 x 10-18 J 3. 5 cm = 0.05 m E = V/m E = 25 V / 0.05 m E = 500 V/m

A charged particle is brought near an electron and the electron accelerates at a rate of 4.8 x 10-3 m/s2 to the right. You must show all work to earn credit. If the same force acted on a proton, what direction would the proton accelerate? What force is applied to the electron? What is the acceleration of the proton?

1. The proton would accelerate to the left. 2. F = m * a F = 9.11 x 10-31 kg * 4.8 x 10-3 m/s2 F = 4.37 x 10-33 N 3. F = m * a 4.37 x 10-33 N = 1.67 x 10-27 * a a = 2.62 x 10-6 m/s2

An electron moving at 6x106 m/s to the right is brought to rest by an electric field in a distance of 5 m. You must show your work to earn any credit. What is the magnitude of the acceleration that the electron experiences? What is the magnitude of the force that the field exerts on the electron? What is the strength of the electric field?

1. v2 = v2i + 2ad a = -v2i / 2d a = -(6 x 106)2 m/s / 2 * 5 m a = -3.6 x 1012 m/s2 2. F = m * a F = 9.11 x 10-31 kg * 3.6 x 1012 m/s2 F = 3.28 x 10-18 N 3. E = FE / q E = 3.28 x 10-18 N / 1.6 x 10-19 C E = 20.5 N/C

As the distance between two charges is doubled, the force exerted by one charge on the other is changed by a factor of ______.

1/4

In the circuit below, R1 = 10 ohms, R2 = 2 ohms, and R3 = 1 ohms. What is the equivalent resistance of the circuit?

13

Two charges are exerting a force on each other. If one of the charges is doubled, the force is changed by a factor of ______.

2

A copper wire of length L has a resistance of R. What is the resistance of a second copper wire identical to the first, but twice as long?

2R

Which of the following statements describes a field force? They require contact between two objects. They can only pull on objects. Their strength decreases with distance.

3 only

A copper wire with a radius of 2 mm has a resistance of R. What is the resistance of a second copper wire of the same length with a radius of 1 mm?

4R

In your own words describe what a capacitor does and why we need them.

A capacitor takes potential energy from a battery, stores it, and releases it in a circuit. We need this to increase the efficiency of power systems.

Electrostatics Lab

As JT shuffles his foot across the floor, he is building up a negative charge (electrons). In my observation, there were 13 electrons. The charge is generating throughout his body, starting from his shuffling leg. Charging is the negative charge created by the action of him shuffling his foot across the floor that creates friction. Discharging is when the electrons pass through his hand as he touches the doorknob. As he shuffles his leg, the electron build-up is gradual and spreads through the entire body. The discharge of electrons is painful because it is fast and in one spot as he touches the doorknob. Discharge occurs quickly and easily when his finger is closest to the doorknob. The electrons did not discharge when the finger was the farthest away from the doorknob. The standard electrical outlet has a constant current that can give an electric shock to someone if they come in contact. Static electricity holds less voltage for a short period of time. The balloon takes electrons from the sweater and it becomes attracted to the sweater. The sweater is left with protons and has no electrons. Yes. As the balloon move towards the sweater, it picks up electrons and becomes negatively charged. Since the sweater is now positively charged, they attract. Static electricity causes sparks of light on the fleece jacket when being removed due to friction.

Series Circuit

As the resistance increases, the current decreases. As the voltage increases, the current increases. Current and resistance have an inverse relationship. As the resistance increases, the current decreases. Voltage has no relationship with resistance.

What is the size (capacitance) of a capacitor that can store 5x10-3 C of charge when connected to a 6 V source?

C = Q/V C = 5 x 10^-3 C / 6 V C = 8.33 x 10^-4 C

How much charge is stored between the plates of a 0.003 F capacitor when it is connected to a 12 V source?

C = Q/V Q = 0.036 C

A 3.2μC and a -2.0μC charge are placed 2.4 cm apart. What is the magnitude of the force of attraction between the charges?

Change cm to m, change μC to C F = k * q1 * q2 / r^2 F = (8.99 * 10^9 N * m2/C2) (3.2 * 10^-6 C) (-2.0 * 10^-6 C) / 0.024^2 m F = -99.89 N The force of attraction is 99.89 N.

Gravitational forces depend on the property called mass. What comparable property underlies electrical forces?

Charge

Two electrons are placed 3 cm apart from each other. What is the magnitude of the electrostatic force of repulsion between the two electrons?

Constant = 8.99 * 10^9 N m^2 / C^2 Fe = Ke q1 q2 / r^2 Fe = 8.99 * 10^9 (-1.6 x 10^-19) (-1.6 x 10^-19) / 0.03^2 Fe = 2.56 x 10^-25 N

Describe why increasing the total resistance of a circuit will tend to decrease the current. Explain your answer in terms of ohms. Provide an example using actual numbers to demonstrate this. Use full sentences.

Current and resistance have an inverse relationship as stated in Ohm's Law (I = V / R). For example, if the voltage was 50 V and the resistance was 100 ohms, the current (I) will be 0.5 A. If the resistance increased to 200 ohms, the current will be 0.25 A. Therefore, increasing the total resistance of a circuit will tend to decrease the current.

What is the relationship between the distance and the measured value on the sensor.

Distance and field strength The distance increases when the voltage decreases in an electric field. Inverse squared relationship

An electron gains 2.5x10-7 J of energy by moving a distance of 3 cm through an electric field. What is the strength of the electric field?

E = PE / (q * d) E = 2.5 x 10-7 J / (1.6 x 10-19 C * 0.03 m) E = 5.21 x 10^13 N/C

A proton gains 3.3x10-10 J of energy by moving a distance of 4 cm through an electric field. What is the strength of the electric field?

E = PE / (q * d) E = 3.3 x 10-10 J / (1.6 x 10-19 C * 0.04 m) E = 5.16 x 1010 N/C

On a cold winter day, a spark jumps from your hand to a metal doorknob when your finger is 0.3 cm away from the knob. If there is a potential difference between your hand and the knob of 9000 V, what is the strength of the electric field?

E = V/m E = 9000 V / 0.003 m E = 300,000 V/m

Module 6

Electric Fields & Static Charge

An electric field causes a 20 C charge to lose 3x10^4 J of energy. What potential difference does the charge move through?

Electric potential (V) = PE / Q V = 30,000 / 20 V = 1500 V

A hard plastic rod is rubbed on a piece of felt and becomes negatively charged. How is that charge acquired?

Electrons move from the felt to the rod.

What is "consumed" in an electrical circuit?

Energy

Separation vs. Capacitance

Exponential relationship. As the separation decreases, the capacitance increases.

Separation vs. Charge Stored

Exponential relationship. As the separation decreases, the charge stored increases.

What force will a proton experience in an electric field with a strength of 3x103 N/C?

F = E * q FE = 3 x 10^3 N/C * 1.6 x 10^-19 C FE = 4.8x10-16 N

A test charge of 2 C is placed in a field where the E-field = 7 N/C. What is the force on the test charge?

F = E * q F = 7 N/C * 2 C F = 14 N

Module 6 Equations

F = E * q F = k q1 q2 / r^2 C = Q / V PE = q E d PEc = 1/2 C V^2 = 1/2 Q V E = V/m Vf^2 = Vi^2 + 2ad

A +4x10-6 C charge is placed 4 cm away from a +3x10-6 C charge. What is the force of repulsion that each charge feels?

F = k q1 q2 / r2 F = 8.99 x 109 * 4 x 10-6 C * 3 x 10-6 C / 0.042 cm F = 67.43 N

Two point charges are 4 cm apart. They are moved to a new separation of 2 cm. By what factor does the resulting mutual force between them change?

F = k q1 q2 / r^2 4

Two charges are exerting a force on each other. If one of the charges is tripled and the distance between the two charges is cut in half, the force is changed by a factor of _____?

FE = k * Q1 * Q2 / r^2 FE = (1) (1) (3) / (0.5)^2 FE = 3 / 0.25 FE = 12 N

What is the magnitude of the force exerted on an electron in an electric field with a strength of 6.5 x108 N/C? Remember, the charge on an electron is 1.6 x 10 -19 C.

FE = q * E FE = (1.6 x 10-19 C) (6.5 x 108 N/C) FE = 1.04 x 10-10 N

Explain the relationship of the dielectric constant and stored charge.

Generally linear relationship. As the dielectric constant decreases, the stored charge also decreases.

A 9.38 V battery is connected to an 9.21 ohm resistor. How much current passes through the resistor? Report your answer to two decimal places.

I = V / R I = 1.02 A

A 1.29 V battery is connected to an 2.66 ohm resistor. How much current passes through the resistor? Report your answer to two decimal places.

I = V / R I = 1.29 V / 2.66 ohm I = 0.48 A

A 12 V battery is connected to a 75 ohm resistor. The battery is replaced by a 6 V source and the resistor is replaced with 100 ohms of resistance. The current has changed by a factor of _________.

I = V / R I = 12 V / 75 ohm I = 0.16 A I = V / R I = 6 V / 100 ohm I = 0.06 A New / Old 0.06 / 0.16 = 0.375 A

What relationship did you observe between the charges on the balls and the force on each ball? Answer in full sentences, using data from your data table in your answer.

In the first trial, the charge on the second ball varied but the charge on the first ball remained the same. In the second trial, the charges on both balls changed twice.

Use the left button to play with the resistance and voltage of the battery. Make observations on how this changes the readings on the voltmeter and ammeter. Record your observations below. Be sure to record the changes you made and then the effects.

Increasing the resistance of the battery decreases the circuit voltage and current. Increasing the voltage of the battery increases the circuit voltage and current. Turning the battery around reverses the current and makes the voltage negative.

Click the advanced tab and alter the resistivity of the wire. Record your observations.

Increasing the resistivity of the wire decreases the circuit voltage and current. Decreasing the resistivity increases the circuit voltage and current.

Explain the process of induction. In your explanation, include a discussion of why grounding is often used during induction.

Induction is when a charged object comes near a neutral object but they do not touch each other. This causes a transfer of electrons from the charged object to the neutral object, making the charged object polarized. The neutral object is connected to the ground and electrons are exchanged between the ground and the neutral object. When the connection to the ground is lost and the charged object is pulled away from the now-charged neutral object, the charged object and the now-charged neutral object become opposite charges.

Based on your results, what do you think the relationship is between charge and field strength?

It has a linear relationship as the number of charges increases the voltmeter reading increases.

Explain the relationship of the dielectric constant and capacitance.

Linear relationship. As the dielectric constant decreases, the capacitance also decreases.

A proton moves a distance of 5 cm through an electric field with a strength of 3x103 N/C. What is the strength of the electric field?

PE = Q E d PE = (1.6 x 10^-19) (3 x 10^3) (0.05 m) PE = 2.4x10-17 J

How much energy is stored between the plates of a 6.5x10-6 F capacitor when it is attached to a 24 V source?

PEc = 1/2 C * V^2 PEc = 1/2 (6.5 x 10^-6 F) * (24)^2 PEc = 1.87x10^-3 J

What component in a water system plays the same role as the potential source in an electrical circuit?

Pump

How many electrons are needed to make a charge of 3.2 x10-19 C?

Q = n * e 3.2 x 10^-19 C = n * -1.6 x 10^-19 C n = 2 electrons

How many electrons are needed to make a charge of 6.4x10-19 C?

Q = n * e 6.4 x 10^-19 C = n * -1.6 x 10^-19 C n = 4 electrons

In the circuit below, R1 = 10 ohms, R2 = 15 ohms, and the battery supplies 6 V. If the current through the circuit is 0.2 A, what is the value of R3?

R = V / I R = 5 ohms

Copper has a resistivity of 1.68x10-8 ohm*m. If a piece of copper has a length of 2.5 m and a radius of 0.15 mm, what is the resistance of the wire?

R = p * L / 3.14 * r^2 R = 1.68 x 10-8 ohm*m x 2.5 m / 3.14 * 0.000152 m R = 4.2 x 10-8 / 7.065 x 10-8 m R = 0.5945 ohms

Copper has a resistivity of 1.68x10-8 ohm*m. If a piece of copper has a length of 0.5 m and a radius of 1.5 mm, what is the resistance of the wire?

R = p L / pi r^2 R = 1.68 x 10^-8 ohm m x 0.5 m / pi 0.0015^2 m R = 1.2 x 10^-3 ohms

In a series circuit below, how will adding another resistor to the circuit change the current (I) moving through the source?

Series Circuit It will decrease I.

In a series circuit below, how will adding another resistor to the circuit change the voltage (V) measured across R1?

Series Circuit It will decrease V

In a series circuit, how will adding another resistor to the circuit change the voltage (V) measured across the battery?

Series Circuit V will stay the same.

Area vs. Capacitance

Squared relationship. As the area increases, the capacitance increases.

Area vs. Charge Stored

Squared relationship. As the area increases, the charge stored increases.

Describe what is happening in real-world terms; explain why the circuit does what it does. What are the blue dots?

The blue dots represent the flow of electrons. The current is clockwise from positive to a negative potential.

What kind of relationship did you obtain?

The current - voltage relationship is linear. As the current increases, the voltage increases.

What relationship did you observe between the distance the balls are apart and the force on each ball? Answer in full sentences, using data from your data table in your answer.

The distance of the balls in the first trial is 10 cm, but the distance it repelled stayed 0 the whole time. In the second trial, the distances before a force was added were 6, 9, and 12 cm.

Predict what will happen to the light when you increase the voltage.

The light will get brighter as the voltage is increased.

A rubber rod with a charge of -2x10-3 C charge is brought near a grounded neutral conductor. The wire to ground is removed while the charged rod is still near the conductor. The rod is then removed. Next the conductor is used to charge a second neutral object using conduction. What is the resulting charge on the conductor? What is the charge on the second neutral object? Briefly explain why in a few sentences.

The negatively charged rod will repel the electrons on the neutral conductor, making the conductor positively charged. The second object will become negatively charged because of more electrons than protons. The second object will become attracted to the conductor. +1x10^-3 C

In a sentence or so, give the relationship between the cross-sectional area of a wire and its resistance.

The relationship is inverse. As the area increases, the resistance decreases.

In a sentence explain the relationship between the length of a wire and its resistance.

The relationship is linear. As the length of the wire increases, its resistance increases.

Please explain how the John Travoltage simulation and the Balloons and Static simulation demonstrate electric charge. Use 3-5 full sentences to explain what you saw in the simulations.

The two simulations demonstrate electric charge by showing the movement of electrons through various systems. The John Travoltage simulation showed electrons transferring from one system (the floor) through the other (his body) via friction when he rubbed his foot against the floor. The electrons then passed through another system (the doorknob) causing static electricity. The Balloons and Static simulation show the reaction of the two balloons when rubbed with the sweater, causing a movement of electrons to the balloons. They become attracted to the sweater. The two balloons also repel each other as they are both the same charge.

Describe the relationship between the number of batteries and the voltage and explain what you think might be happening.

The voltage increases as the number of batteries increases. Each battery is worth 9.00 V and each addition of batteries add the same voltage.

When k increases what happens to: a. Capacitance The capacitance increases as the dielectric constant increases. b. Electric Charge The electric charge increases as the dielectric constant increases. c. Stored Energy The stored energy increases as the dielectric constant increases.

Using the table of dielectric constants and this equation, what substance would be the best at storing a large charge? Pure water would be the best at storing a large charge because it has the highest dielectric constant. When the constant is large, the capacitance will also be large. Using the table of dielectric constants, what substance would be the worst at storing a large charge? The worst would be the vacuum because of its low dielectric constant. 13. Use the formula to find the capacitance for a capacitor with a separation of 1mm of porcelain between the 10cm x 10cm square plates. Check your answer below. You must show work or no credit will be given for this problem C = K * Eo * A / d C = 6 * 8.56x10-12 F/m * 0.01 m / 0.001 m C = 5.13 x 10−10 F 14. A 9x10-11 F capacitor is made up of two circular plates (r=5cm) separated by 1.0mm. Find the necessary dielectric constant for this capacitor. Check your answer below. You must show work or no credit will be given for this problem. A = π r2 A = 7.85 x 10-3 m C = K * Eo * A / d 9 x 10-11 F = K * 8.56 x 10-12 F/m * 7.85 x 10-3 cm / 0.001 m 9 x 10-11 F = K * 6.72 x 10-11 K = 1.34

A 100 ohm resistor has 3 A of current passing through it. If the resistor is replaced by one with a resistance of 200 ohms, how much current passes through the new resistor?

V = I * R V = 300 ohms 300 ohms = I * 200 ohms I = 1.5 A

What component of an electrical circuit plays the same role as pipes in a water system?

Wires

Using observations from the lab work. You must use full sentences and describe observations from the simulation. a. Explain the relationship of the dielectric constant and capacitance. b. Explain the relationship of the dielectric constant and stored charge.

a. Dielectric Constant vs Capacitance has a linear relationship. As the dielectric constant decreases, the capacitance also decreases. b. Dielectric Constant vs Stored Charge has a linear relationship. As the dielectric constant decreases, the amount of charge stored also decreases.

Answer these questions in 3-5 sentences. Use lab observations and ideas to support your answers. a. What is the relationship between the distance and the measured value on the sensor. b.What is the relationship between charge and field strength?

a. The sensor measured the highest voltage where the charge is. From that point, any distance from the left, to the right, upwards, or downwards would have lesser voltage. As the sensor is further away from the charge, the voltage decreases. b. The field lines are stronger near the charge and become weaker when it's further from the charge. As the charge becomes greater, the force increases, causing the field strength to also increase. Field lines that are densely concentrated contain a larger force as per the simulation.

How would placing a material that is a better insulator (than air) affect the a capacitor when it is stays connected to the same voltage source?

capacitance increases; stores more charge

An object is placed in a circuit and electrons flow freely through the circuit. This object is a ___________.

conductor

The SI unit of charge is the ____________.

coulomb

In an electrical circuit, increasing the total resistance will ______________ the current.

decrease

In non-ohmic resistors, such as light bulbs, resistance is highly dependent on temperature. As the temperature increases, so does the resistance. This will tend to _____________ the current in the circuit.

decrease

Passing current through a resistor is a way to _______________ the energy of the charges.

decrease

Which of the following is a vector quantity?

electric field strength (E)

An electron moves through a uniform electric field toward a negatively charged plate. If the electron moves through a potential difference of 25 V in 5 cm. Does the electron gain or lose potential energy?

gain

A 6 V battery is replaced by a 12 V battery in a circuit. The current will _______________.

increase by a factor of 2

As the electric field strength at a location increases, the force felt by a test charge at this location __________.

increases

When a charge is created by an object brought near, but not touching another object, ______________ has occurred.

induction

An object is considered to be positively charged because it _______________.

is missing electrons

A proton moves through a uniform electric field toward a negatively charged plate. If the electron moves through a potential difference of 25 V in 5 cm. Does the electron gain or lose potential energy?

lose

To increase its potential energy, an electron should be _______________.

moved further from a positive charge

Which of the following motions represent a positive charge moving from high potential energy to low potential energy?

moving away from a positive charge

A 6 V battery is replaced by a 12 V battery in a circuit. The resistance will _________________.

not change

When an electroscope is charged, the leaves repel each other and remain at an angle. What balances the electric force of repulsion so that the leaves don't separate further?

the force of gravity

A charged particle is brought near an electron and the electron accelerates at a rate of 5x10^-3 m/s^2 to the right. If the same force acted on a proton, what direction would the proton accelerate in?

to the left

A proton experiences a force pushing it to the right in an electric field. In the same field, an electron would feel a force _____________.

to the left