# Physics

Name: Lab Day/Time:

Homework 1 Electrostatics

Homework is due at the beginning of the Wednesday lecture. It must be handwritten, not typeset. The multiple- choice answers must be circled. In the space after the problem, a short justification of each multiple-choice the answer must be included. The open-response answers must be worked out clearly using good physics presentation and will be graded on correctness and how carefully the work is explained. The problems should be worked in the space after the problem on the assignment printout; additional paper may be used if needed. No credit will be given for answers without appropriate supporting work. Minimum good presentation requires the following: (1) Symbolic expression for any formula, (2) Manipulation of symbolic expressions, not numeric expressions, (3) Substitution of numbers with units, (4) Reporting final answers with correct units and vector expressions, (5) Enough English description to allow the reader to have some idea what you are doing without looking at the math.

Early Questions

The questions in this section are over material that will be covered by Friday. These may be worked before the other questions.

Homework Problem 1.1 A 10cm sphere charged with a car battery (12V), picks up a positive charge of +100pC. How many excess or deficient electrons are on the sphere?

Select One of the Following:

(a) 3.2× 109 deficient electrons

(b) 1.1× 1014 excess electrons

(c) 6.7× 1011 deficient electrons

(d) 9.5× 105 excess electrons

(e) 6.3× 108 deficient electrons

Homework Problem 1.2 Your standard number 2 mechanical pencil has a graphite lead with mass 0.05g. How many protons are in this quantity of graphite? The atomic mass of carbon is 12g/mole and the atomic number is 6.

Select One of the Following:

(a) 3.1× 1010protons

(b) 1.5× 1022protons

(c) 5.1× 1024protons

(d) 2.3× 1021protons

(e) 6.5× 1023protons

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Homework Problem 1.3 An insulating cylinder has height 5cm and radius 10cm. If it carries a uniform surface charge density σ = −1µC/m2 including the ends, compute the total charge of the cylinder.

Select One of the Following:

(a) Q = −2× 10−2C

(b) Q = −3× 10−4C

(c) Q = −5× 10−6C

(d) Q = −9× 10−8C

Homework Problem 1.4 A problem you should have easily been able to do in Phys 111, but I get asked it all the time when I ask for a good question on electricity and magnetism the first day of class. Suppose a human can confortably live in a spaceship accelerating at 1g = 9.81m

s2 , ignoring relativistic effects, how long does it take

the spaceship to reach the speed of light? Report your answer in years. You may use the approximate conversion 1yr = π × 107s.

Select One of the Following:

(a) 15.7yr

(b) 0.19yr

(c) 0.97yr

(d) 15, 000yr

Homework Problem 1.5 How does the mass of the electron compare with the mass of the proton?

Select One of the Following:

(a) The two masses are equal.

(b) The electron is slightly more massive than the proton.

(c) The electron is slightly less massive than the proton.

(d) The electron is greatly more massive than the proton.

(e) The electron is greatly less massive than the proton.

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Homework Problem 1.6 In the nuclear process “beta decay”, a neutron is converted into some other charged particles, one of which is a proton. Some neutral particles are also emitted. What other charged particle must be emitted in beta decay?

Select One of the Following:

(a) an electron or something with the charge of an electron

(b) two electrons or particles with total charge equal to two electrons

(c) a proton or something with the charge of a proton

(d) an electron and a proton

(e) No other charged particles are required.

Multiple-Choice Questions

The questions in this section are over material that will be covered by the Monday before the assignment is due.

Homework Problem 1.7 A patch of positive charge is placed on a conducting sphere. Where will the positive charge be at a later time? Assume no charge is lost to the environment.

Select One of the Following:

(a) The charge will remain in the same place.

(b) The charge will stay bunched together but will move around the surface of the conductor.

(c) The charge will separate as much as possible spreading over the surface of the conductor.

(d) The charge will transform into neutral charge and disappear.

(e) The charge will spread apart, but will eventually come back together.

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Homework Problem 1.8 Which of the following describes an experiment that demonstrates that there are at least two different types of electric charge?

Select One of the Following:

(a) Rub one pair of rods made of the same material – for example, glass – with felt. Observe that the glass rods repel one another.

(b) Charge a pair each of glass and rubber rods by rubbing them with felt. Observe that that (1) the glass rods repel each other, (2) the glass rods attract the rubber rods, and (3) the rubber rods repel each other.

(c) Charge a pair each of glass and quartz rods by rubbing them with felt. Observe that that (1) the glass rods repel each other, (2) the quartz rods repel the glass rods, and (3) the quartz rods repel each other.

Homework Problem 1.9 Can a charged object exert a force on an uncharged insulator? If yes, why; if no, why not?

Select One of the Following:

(a) Yes, by inducing an electrical polarization in the insulator; the insulator is then attracted to the charged object.

(b) Yes, by inducing an electrical polarization in the insulator; the insulator is then repelled by the charged object.

(c) No, because uncharged objects do not feel electrical forces.

(d) No, because only conductors are attracted to or repelled by charged objects.

(e) No, because an insulator does not permit the motion of electric charge.

Homework Problem 1.10 It used to be that one could count on a water pipe as a good “ground”. A “ground” is a continuous conducting path to a long conductor buried in the Earth. Today, plumbing often involves plastic pipes and valves. Explain why this makes a water pipe something we now have to be careful of as a ground.

Select One of the Following:

(a) Plastic is only a conductor part of the time.

(b) The plastic is an insulator and may prevent there from being a continuous path to ground.

(c) Water pipes do not go into the ground anymore and thus won’t be grounded.

(d) The plastic will prevent the water from absorbing the charge.

(e) The plastic will divert any charge going to ground into the water which will shock anyone who uses the water.

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Homework Problem 1.11 Which of the following describes an experiment that demonstrates that there are at least two different types of electric charge?

Select One of the Following:

(a) Rub one pair of rods made of the same material – for example, glass – with felt. Observe that the glass rods repel one another.

(b) Charge a pair each of glass and rubber rods by rubbing them with felt. Observe that that (1) the glass rods repel each other, (2) the glass rods attract the rubber rods, and (3) the rubber rods repel each other.

(c) Charge a pair each of glass and quartz rods by rubbing them with felt. Observe that that (1) the glass rods repel each other, (2) the quartz rods repel the glass rods, and (3) the quartz rods repel each other.

Homework Problem 1.12 After sliding down a plastic slide at the park, your hair stands on end. It continues standing on end even after you get off the slide. What does this imply?

Select One of the Following:

(a) It implies that you have picked up a net charge from the slide.

(b) It implies that your hair has become polarized.

(c) It implies that your hair is covered in water and has become a better conductor.

(d) It implies that your hair has become conducting.

(e) It implies that your hair has become insulating.

Homework Problem 1.13 When an object is grounded, its net charge is reduced to approximately zero. Is this consistent with the Law of Conservation of Charge, if so how?

Select One of the Following:

(a) It is not consistent with conservation of charge; charge is actually destroyed when an object is grounded. Conservation of Charge only applies in some cases.

(b) It is consistent with Conservation of Charge; charge is actually destroyed on the object, but an equivalent charge will be created somewhere else in the universe.

(c) It is consistent with Conservation of Charge; charge is not destroyed but is transferred to another object.

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Homework Problem 1.14 A negatively charged golf tube (plastic rod) is brought near an uncharged conducting bucket. A positively charged pith ball is suspended within the bucket. Select the figure that accurately shows how the pith ball will react.

Select One of the Following:

(a) Figure (a) (b) Figure (b) (c) Figure (c)

GT

conductor

+

_ _

pith

Figure (a)

GT

conductor

+

_ _

pith

Figure (b)

GT

conductor

+

_ _

pith

Figure (c)

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Open Response Questions

All questions in this section must be worked. One of the questions will be graded.

Homework Problem 1.15 A hula-hoop (a circular hoop) has radius 0.6m and linear charge density around its edge of 0.3µC/m. What is the total charge of the hoop in Coulombs?

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Homework Problem 1.16 Since 1982, a penny has contained 2.5grams of copper. The atomic weight of copper is 63.546g/mole and the atomic number is 29. How many electrons are in a penny?

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Homework Problem 1.17 In the following, explain each step carefully and provide a drawing. A Coke can is brought into the electric field of a negatively charged golf tube.

(a)What is the direction of the force on the Coke can and why?

(b)The Coke can is then grounded in the presence of the golf tube. The golf tube had negative charge then what is the sign of the charge on the Coke can?

(c)What is the direction of the force on the Coke can after grounding and why?

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