Module 3

1. Module 3

1.48. Module Summary/Assessment

Module Summary and Assessment

Module 3—Electrical Phenomena

Module Summary

In Lesson 1 you looked at key concepts that are essential to understanding electrostatics, including the attraction and repulsion of charges, the law of conservation of charge, and the three methods of transferring charge: by friction, by conduction, and by induction.

In Lesson 2 you worked with Coulomb’s law:

This law says, if the charges are held constant and the distance of separation increases by some factor, then the effect on the resulting electrostatic force can be predicted. For example, if the distance increases by a factor of four, then the electrostatic force would be 1/16th its original value.

In Lesson 3 you learned more about the enormous amounts of charge that a lightning strike can deliver from a cloud to Earth. You also saw that Coulomb’s law can be used to calculate the net force on one point charge due to other point charges. You looked at solutions to these sorts of problems, which required you to consider the vector nature of electrostatic forces.

In Lesson 4 you learned that an electric field is the property of space surrounding a source charge that enables the source charge to exert forces on other charges that enter this region. One way to describe electric fields is to use equations:

The direction of an electric field is determined by the direction of the electrostatic force experienced by a small positive test charge. This can be used to describe electric fields in terms of patterns of electric field lines around source charges.

In Lesson 5 you saw that both electric potential energy and gravitational potential energy result when work is done to move an object in a field. In both cases, work is done by applying an external force on the object that is opposite to the force that the field would exert on the object. Gravitational potential energy can be increased when a mass is moved up, away from Earth’s surface, and parallel to the gravitational field. Electric potential energy can be gained by moving a positive charge toward a positive source, or by moving a negative charge away from a positive source. In both cases the distance moved is parallel to the electric field.

While potential energy is measured in joules, electric potential is measured in volts. A volt is a joule per coulomb. Many people prefer to use the term voltage for electric potential because it reminds them how this quantity is measured.

In Lesson 6 you learned that charged particles in uniform electric fields move according to Newton’s laws of motion. This makes the motion of a charged particle very similar to the motion of a mass in a gravitational field. In both cases the test body accelerates in the direction of an unbalanced force. If there is no unbalanced force, then the test body maintains its velocity. The equations used to analyze projectile motion in previous courses can be applied to the motion of a charged particle in a uniform electric field.

For example, the equation Δd =½aΔt2 can be applied not just to projectile motion in a gravitational field, but also to the motion of a charged particle in a uniform electric field. The formula for projectile motion can be used to calculate the velocity, displacement, and acceleration of the motion of charged particles.

Module Assessment

As your module assessment do the following two questions. The first is a graphing question, and the second is a holistic question.

Use the following information to answer this graphing question.

A student performed an experiment that verified Coulomb’s Law of Electrostatics by measuring the repulsion between two charged spheres, A and B, as a function of the separation of the spheres. The spheres were identical in size and mass. The measurements are shown in the table of values and plotted on the graph below.

Separation (m)

0.10

0.13

0.20

0.40

0.60

Force (N)

0.790

0.480

0.200

0.050

0.022

Show that the results verify Coulomb’s law by manipulating the data and providing a new table of values that, when plotted, will produce a straight-line graph.
Plot the new data with the responding variable on the vertical axis.
Calculate the slope of your graph.
Using the slope value or another suitable averaging technique, determine the charge on sphere B if the charge on sphere A is 3.08 × 10–7 C
Determine the magnitude of the force between spheres A and B when they are at a distance of 2.00 m apart. Use the hypothetical value of 4.00 × 10–6 C for the charge on sphere B if you were unable to determine the actual value.

Graphing Question Marking Guide

Graphing

(5 marks)

Scoring Guides for Graphing Skill-Based Questions––Mathematical Treatment

Score	Description
5	All formulas are present. All substitutions are given and are consistent with the graphed data. The relationship between the slope, area, or intercept, and the appropriate physics is explicitly communicated. The final answer is stated with appropriate significant digits and with appropriate units. Unit analysis is explicitly provided, if required. Note: one minor error may be present.*
4	The response contains implicit treatment. or The response contains explicit treatment with up to three minor errors or one major error.*
3	The response is incomplete but contains some valid progress toward answering the question; i.e., coordinates of relevant points are read correctly, including powers of 10 and units, and a valid substitution is shown.
2	The coordinates of one relevant point are read. The reason for requiring a point is addressed or implied.
1	A valid start is present.
0	Nothing appropriate to the mathematical treatment required is present.

*Minor errors include:

Misreading a data value while interpolating or extrapolating up to one-half grid off.
Stating the final answer with incorrect (but not disrespectful) units.
Stating the final answer with incorrect (but not disrespectful) significant digits.
Missing one of several different formulas.

**Implicit treatment means:

Substituting appropriate values into a formula from the data sheets without stating the formula.
Starting with memorized, derived formulas not given on the equations sheet.
Substituting the value from one calculation into a second formula without communicating that the physics quantity in the two formulas is the same.

***Major errors include:

Using off-line points (most often, this is calculating the slope using data points that are not on a linear line of best fit).
Using a single data point ratio as the slope.
Missing powers of 10 in interpolating or extrapolating.

Use the following information to answer this holistic question.

A relatively new type of painting procedure is called powder coating. This new process is very different than traditional liquid paints. Liquid paints dissolve the coloured paint particles in a solvent. When the solvent evaporates, the paint particles are left behind and form the paint coating. Powder coating is much more like a photocopier or laser printer. The dry powdered plastic paint particles are given an electrostatic charge as they are blown out of the paint gun. The particles are then deposited on the charged or grounded target. Once the target is coated, it is baked which melts the powder to create a very even durable paint covering when cooled.

There are two types of powder painting paint guns: the corona and the tribo.

A corona paint gun works by using a negatively charged electrode with a potential difference of 30 kV to 100 kV at the nozzle where the powder particles come out. The target is given a positive charge. Corona paint guns are often used because they can paint large areas quickly with little wasted paint and good “wrap around’ effect. The one large disadvantage is that the corona guns experience faraday cage effect.

A tribo gun often uses multiple smaller pipes with a Teflon liner. As the nylon paint powder goes through the Teflon lined pipes it picks up a positive charge before leaving the gun. The target is grounded. Tribo guns require specific tribo paint powders made from nylon. They are not as fast as corona guns but they don’t suffer from faraday cage effect.

Faraday cage effect occurs when powder paint particles will not enter small openings in a surface due to the electrostatic forces. Corona guns suffer this effect whereas tribo guns do not.

Compare and contrast how each gun charges the paint powder.
Why are corona guns affected by the Faraday cage effect whereas tribo guns are not?
Explain why electrostatic spraying of powdered paint is far more efficient in terms of the amount of paint that attaches to the target than sprayed liquid paint.

Holistic Scoring Guide

Holistic Scoring Guide

Major Concepts: Conservation of charge, electric fields, electric forces
Score	Description
5	The nature of a response that will receive a score of 5 has the following characteristics: The response addresses, with appropriate knowledge, all the major concepts in the question (all bullets must be attempted). The student applies major physics principles in the response (appropriate physics principles are stated). The relationships between ideas contained in the response are explicit* (physics principles are clearly linked to the application). The reader has no difficulty following the strategy or solution presented by the student. Statements made in the response are supported explicitly.* Note: the response may contain minor errors or have minor omissions.
4	The nature of a response that will receive a score of 4 has the following characteristics: The response addresses, with appropriate knowledge, all the major concepts in the question (all bullets must be attempted). The student applies major physics principles in the response (appropriate physics principles are stated). The relationships between the ideas contained in the response are implied(physics principles are stated but not properly linked to the application). The reader has some difficulty following the strategy or solution presented by the student. Statements made in the response are supported implicitly. Note: the response is mostly complete and mostly correct, although it may contain errors or have omissions, and contains some application of physics principles.
3	The nature of a response that will receive a score of 3 has the following characteristics: The response addresses, with some appropriate knowledge, all the major concepts in the question (all bullets must be attempted). The student does not apply major physics principles in the response (all appropriate physics principles are not stated). There are no relationships between the ideas contained in the response (physics principles are stated but not applied). The reader may have difficulty following the strategy or solution presented by the student.
2	The nature of a response that will receive a score of 2 has the following characteristic: The response addresses, with some appropriate knowledge, two of the major concepts in the question (only two bullets are attempted).
1	The nature of a response that will receive a score of 1 has the following characteristic: The response addresses, with some appropriate knowledge, one of the major concepts in the question (only one bullet is attempted).
0	The student provides a solution that is invalid for the question.

* Explicit means the response is clearly stated; the marker does not have to interpret.

** Implicit (implied) means the response is not clearly stated; the marker must interpret.

For example:

Explicit: An electron has a negative charge while a proton has a positive charge.

The answer is clear with no possible misinterpretation.

Implicit: An electron has a negative charge while a proton does not.

The answer is not clear because the marker does not know if a proton is neutral or positively charged. There is more than one possible way to interpret the answer.

Your answers must clearly communicate your understanding of the physics principles you used to solve these questions. You may communicate this understanding mathematically, graphically, and/or with written statements.