Electric Charge Examples: 46 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Electric Charge.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

A fundamental property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).

Some particles carry an invisible 'label' — positive or negative — that makes them push or pull on each other.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Electric Charge asks students to follow the circuit path and identify what quantity changes at each component.

Common stuck point: Students often know a formula related to electric charge but skip the recognition step: Can I identify the circuit path, what quantity is flowing or changing, and which electrical rule links the quantities? That leads to a correct-looking substitution attached to the wrong physical model.

Sense of Study hint: Ask: Can I identify the circuit path, what quantity is flowing or changing, and which electrical rule links the quantities?

Common Mistakes to Watch For

Before you work through the examples, skim the mistake guide so you know which shortcuts and sign errors to avoid.

Electric Current Mistakes Coulomb's Law Mistakes

Worked Examples

Example 1

easy

An object has an excess of

3.0 \times 10^{13}

electrons. What is the total electric charge on the object? Use

e = 1.6 \times 10^{-19} \text{ C}

Answer

Q = - 4.8 μ C

First step

Each electron carries a charge of

-1.6 \times 10^{-19} \text{ C}

Full solution

2
Total charge: $Q = n e = 3.0 \times 10^{13} \times (- 1.6 \times 10^{- 19})$
3
$Q = - 4.8 \times 10^{- 6} C = - 4.8 μ C$

Electric charge is a fundamental property of matter. It is quantized — it always comes in multiples of the elementary charge

e = 1.6 \times 10^{-19} \text{ C}

. Excess electrons give a negative charge.

Example 2

medium

Two charged spheres carry charges of

+5 \text{ } \mu\text{C}

and

-3 \text{ } \mu\text{C}

. They are brought into contact and then separated. What is the charge on each sphere afterward?

Example 3

medium

A sphere carries

-1.6\ \mu\text{C}

. How many excess electrons does it carry?

Example 4

medium

A current of

0.75\ \text{A}

flows for

4\ \text{minutes}

. How much charge passes?

Example 5

medium

An object loses

5\times10^{15}

electrons. What is its net charge?

Example 6

hard

A wire passes

0.5\ \text{C}

in the first second and

0.3\ \text{C}

in each of the next

4\ \text{s}

. What is the average current over

5\ \text{s}

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium

A current of

2 \text{ A}

flows through a wire for

30 \text{ s}

. How many electrons pass through the wire? Use

e = 1.6 \times 10^{-19} \text{ C}

Example 2

hard

A Van de Graaff generator accumulates charge on a metal sphere of radius

0.15 \text{ m}

. If the electric field at the surface reaches

3 \times 10^6 \text{ V/m}

(air breakdown), what is the maximum charge on the sphere? Use

k = 9 \times 10^9 \text{ N m}^2/\text{C}^2

Example 3

easy

What is the SI unit of electric charge?

Example 4

easy

How many electrons make up approximately one coulomb of charge?

Example 5

easy

An object gains

5

extra electrons. What is the sign of its net charge?

Example 6

easy

Two objects both carry positive charge. Do they attract or repel?

Example 7

easy

2

C of charge passes a point in

4

s, is this asking about charge, current, or voltage?

Example 8

easy

Charging a balloon by rubbing it transfers electrons. Is new charge created?

Example 9

easy

What is the charge of a single electron, including sign?

Example 10

easy

A neutral atom loses one electron. What is its net charge?

Example 11

medium

A wire carries

3

C past a point in the first

2

s and

5

C in the next

4

s. What total charge passed in

6

Example 12

medium

A current of

2

A flows for

10

s. How much charge passes?

Example 13

medium

How many electrons flow when

4.8

C of charge passes through a wire?

Example 14

medium

Sphere A has

+6

C and touches identical sphere B with

-2

C. After contact, what charge does each carry?

Example 15

medium

A device draws

0.5

A. How long must it run to move

30

C of charge?

Example 16

medium

A current of

250

mA flows for

8

s. How much charge passes, in coulombs?

Example 17

medium

Object X gains

4\times10^{18}

electrons. What is its net charge in coulombs (magnitude)?

Example 18

medium

Why does a flow of electrons to the left correspond to conventional current to the right?

Example 19

challenge

Two identical spheres carry

+8

C and

+2

C. They touch and separate, then the

+5

C sphere (now equalized) touches a third identical sphere that was neutral. What charge ends on the third sphere?

Example 20

challenge

A wire carries a steady current that delivers

1.6

C every

0.8

s. How many electrons pass per second?

Example 21

challenge

A capacitor plate must hold

-0.32

C. Starting neutral, how many electrons must be added, and what does conservation imply about the other plate?

Example 22

medium

An object has a net charge of

-3.2\times10^{-19}

C. How many excess electrons does it carry?

Example 23

easy

A plastic rod is rubbed with wool and gains

10

extra electrons. What is the sign of its net charge?

Example 24

easy

A positive charge and a negative charge are brought near each other. Do they attract or repel?

Example 25

easy

How many coulombs of charge are carried by

1.25\times10^{19}

electrons?

Example 26

easy

A current of

0.1\ \text{A}

flows for

50\ \text{s}

. How much charge passes?

Example 27

easy

A proton and an electron are released from rest near each other. What kind of force do they feel?

Example 28

medium

Two identical metal spheres carry charges

+10\ \mu\text{C}

and

-4\ \mu\text{C}

. They touch then separate. What is the final charge on each?

Example 29

medium

How many coulombs are needed to deposit

6.24\times10^{20}

electrons onto a plate?

Example 30

medium

Sphere A (

+9\ \mu\text{C}

) touches identical sphere B (

+1\ \mu\text{C}

), then is separated. A is then touched to identical neutral sphere C. What is the final charge on C?

Example 31

medium

A point passes

0.6\ \text{C}

every

5\ \text{s}

. What current does this correspond to?

Example 32

medium

A defibrillator delivers

0.20\ \text{C}

of charge in

4.0\ \text{ms}

. Find the average current.

Example 33

medium

5\times10^{18}

electrons flow past a point each second, what current does this represent?

Example 34

medium

Two charges

+3\ \mu\text{C}

and

+7\ \mu\text{C}

are placed in an isolated system. After a spark equalizes them on identical conductors, what total charge remains?

Example 35

hard

A wire delivers a current that rises linearly from

0

4\ \text{A}

over

10\ \text{s}

. How much charge passes in that interval?

Example 36

hard

A copper wire carries a steady

3.2\ \text{A}

. How many electrons pass a cross-section in

1\ \text{minute}

Example 37

hard

A balloon rubbed on hair gains

5.0\times10^{10}

electrons. What charge does the hair carry as a result?

Example 38

hard

Two identical spheres carry

-12\ \mu\text{C}

and

+4\ \mu\text{C}

. After they touch and separate, sphere A then touches a third identical neutral sphere. What is the final charge on each of A and the third sphere?

Example 39

challenge

A capacitor plate holds

-2.0\ \mu\text{C}

. A student claims the opposite plate must hold exactly

+2.0\ \mu\text{C}

by conservation. Is this strictly required by conservation of charge alone? Explain.

Example 40

challenge

Four identical metal spheres carry charges

+8\ \mu\text{C}

+8\ \mu\text{C}

+8\ \mu\text{C}

, and

0

. Sphere 4 (neutral) is touched in turn to sphere 1, then 2, then 3. What is the final charge on sphere 4?

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Related Concepts

Electric Current Electric Field Coulomb's Law