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Back to Exercise: Electrical Charges, Conservation of Charge, and Transfer of Charge

Exercises: Electrical Charges, Conservation of Charge, and Transfer of Charge

Work through each section in order. Show your work on calculation problems.

Grade 11·21 problems·~30 min·OpenStax Physics (High School)·section·sec-18-1
Work through problems with immediate feedback
A

Warm-Up: Prerequisite Review

These questions review prior knowledge needed for this section.

1.

An atom has 6 protons and 6 electrons. What is its net electric charge?

2.

A proton has charge +1.6×10−19+1.6 \times 10^{-19} C and an electron has charge −1.6×10−19-1.6 \times 10^{-19} C. What is the total charge of a system containing one proton and one electron? Express your answer in coulombs.

3.

Two charged objects are brought near each other and repel. What can you conclude about their charges?

B

Fluency Practice

Apply core concepts from this section. Show your work on calculation problems.

1.

A positively charged rod is brought near a neutral conducting sphere. Which statement correctly describes the resulting interaction?

2.

A glass rod is rubbed with silk and becomes positively charged. Which particle transferred and in which direction?

3.

Two identical conducting spheres touch. Sphere A initially has a charge of +8+8 nC and Sphere B has a charge of −2-2 nC. After they touch and are separated, what is the charge on each sphere? Express your answer in nC.

Diagram showing two spheres before and after contact, with initial charges of −12 nC and 0 nC
4.

A charged sphere with −12-12 nC touches a neutral sphere of identical size. After separation, what is the charge on each sphere? Express your answer in nC.

5.

Which of the following best explains why metals are good conductors of electricity?

6.

A negatively charged rod is brought near a grounded neutral conductor (but does not touch it). The ground wire is then removed while the rod is still nearby. The rod is then moved away. What is the sign of the charge remaining on the conductor?

C

Mixed Practice

These problems apply the concepts in varied formats.

1.

An atom has 8 protons and 10 electrons. Calculate the net charge of this atom in elementary charges and state whether it is a positive ion, negative ion, or neutral. Explain your reasoning.

2.

A plastic rod is charged by rubbing and then touched to one end of a wooden dowel. A few seconds later, the far end of the dowel shows little to no charge. Why?

Diagram showing electron transfer from hair to balloon during friction charging, leaving the hair positively charged and the balloon negatively charged
3.

A rubber balloon is rubbed on dry hair and becomes negatively charged. Which statement about the hair is correct?

4.

A negatively charged rubber rod touches a neutral metal sphere. After the rod is removed, what is the charge on the sphere?

5.

A positive charge is brought near (but not touching) a neutral conductor that is then grounded, and then the ground is disconnected before the positive charge is removed. What sign of charge does the conductor have after the positive charge is moved away?

D

Application Problems

Read each scenario carefully. Show your work on calculation problems.

1.

Three conducting spheres are brought into contact simultaneously. Sphere X has charge +10+10 nC, Sphere Y has charge −4-4 nC, and Sphere Z has charge −3-3 nC. After contact, the spheres are separated.

What is the charge on each sphere after separation? (All three spheres are identical.) Express your answer in nC.

2.

A glass rod rubbed with silk becomes positively charged with +5+5 nC. The rod is then touched to a small neutral metal sphere (Sphere A). After the rod is lifted away, Sphere A is brought into contact with a second identical neutral metal sphere (Sphere B), then the two spheres are separated.

1.

After the glass rod touches Sphere A and is removed, what charge does Sphere A carry? (Assume the rod and sphere share charge equally after contact.) Express your answer in nC.

2.

After Sphere A (+2.5+2.5 nC) touches neutral Sphere B and the two are separated, what charge does each sphere carry? Express your answer in nC.

3.

In a factory, a rubber belt runs over metal rollers. As the belt moves, friction causes the belt to accumulate charge. A worker touches the metal roller frame and feels an electric shock.

Which combination of physics principles explains what happened?

E

Find the Mistake

Each problem shows incorrect reasoning. Identify the error.

1.

A student explains charging by conduction: "When the positive rod touches the sphere, positive charges flow from the rod into the sphere, making the sphere positive."

What is the error in this student's explanation?

2.

After a lab, a student writes: "Rubbing the balloon on the wool sweater created negative charge on the balloon. Before rubbing, neither object had any charge."

What is the scientific error in this conclusion?

F

Challenge Problem

This bonus problem requires multi-step reasoning.

1.

A student performs induction charging using a negatively charged balloon. The student holds the balloon near (but not touching) a metal sphere, then touches the sphere briefly with a finger (grounding it), then removes the finger, and finally moves the balloon away.
Describe step by step what happens to the electrons in the sphere at each stage. Explain the sign and origin of the final charge on the sphere.

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