AP Physics 2 · Topic 10.1

Topic 10.1: Electric Charge and Electric Force

Unit 10: Electric Force, Field, and Potential15-18% of the multiple-choice section

Topic 10.1 defines charge and the force between charges. Charge is a property of all matter, positive or negative, built from the elementary charge. Coulomb's law makes the force proportional to each charge and inversely proportional to the square of the separation. The signs set the direction.

AP Physics: Unit 10 (topics 10.1 Electric Charge and Electric Force). AP Physics 2 Unit 10, Topic 10.1. Three learning objectives. 10.1.A asks students to describe the electric force that results from the interactions between charged objects or systems, supported by 10.1.A.1 (charge is a fundamental property of all matter, with sub-statements on the two signs, the elementary charge e as the smallest indivisible amount of charge, the charges of the electron, proton and neutron, and the point-charge model), 10.1.A.2 (Coulomb's law, with the printed equation for the magnitude of the electrostatic force), 10.1.A.3 (direction depends on the signs of the charges and is parallel to the line of separation, with same signs repelling and opposite signs attracting), and 10.1.A.4 (electric forces underlie macroscopic properties, but the number of particle interactions makes it more convenient to treat everyday forces as nonfundamental contact forces such as normal force, friction and tension). 10.1.B asks students to describe the electric and gravitational forces between charged objects with mass, supported by 10.1.B.1, 10.1.B.2 and 10.1.B.3. 10.1.C asks students to describe electric permittivity, supported by 10.1.C.1 through 10.1.C.4, including the conductor and insulator definition in 10.1.C.4.ii. The topic's boundary statement limits calculations to four or fewer interacting charged objects or systems, and allows analysis of the resulting electric force from more charges in situations of high symmetry. The CED's suggested skills here are 1.A, 2.A, 2.D and 3.B. Unit 10 carries 15 to 18 percent of the multiple-choice section and a suggested 14 to 21 class periods.

What Topic 10.1 actually requires

Topic 10.1 opens Unit 10, Electric Force, Field, and Potential, which the CED weights at 15 to 18 percent of the multiple-choice section and allots roughly 14 to 21 class periods. The unit opener calls it the start of electrostatic phenomena at a fundamental level, and it makes one point worth carrying into every problem in the unit: the electric force is a force, so the Newton's laws you used in AP Physics 1 describe it too.

Three learning objectives sit under this topic.

Objective	What it asks you to do
10.1.A	Describe the electric force that results from the interactions between charged objects or systems.
10.1.B	Describe the electric and gravitational forces that result from interactions between charged objects with mass.
10.1.C	Describe the electric permittivity of a material or medium.

Every verb there is describe. Topic 10.1 is not mainly a calculation topic, which is why its suggested skills lean on representing and reasoning rather than on grinding numbers.

Skill	CED wording
1.A	Create diagrams, tables, charts, or schematics to represent physical situations.
2.A	Derive a symbolic expression from known quantities by selecting and following a logical mathematical pathway.
2.D	Predict new values or factors of change of physical quantities using functional dependence between variables.
3.B	Apply an appropriate law, definition, theoretical relationship, or model to make a claim.

One boundary statement closes the topic, and both of its sentences count. AP Physics 2 only expects students to make calculations of the electric force between four or fewer interacting charged objects or systems. The second sentence then reopens the door: analysis of the resulting electric force from more charges is allowed in situations of high symmetry. Four is the ceiling on brute-force vector addition, not a ceiling on how many charges a question may show you.

Charge, and what the elementary charge means

EK 10.1.A.1 states that charge is a fundamental property of all matter, and four sub-statements pin down what that sentence licenses.

Two signs, no third (10.1.A.1.i). Charge is described as positive or negative. Neutral means the two cancel, not that charge is absent.
The elementary charge is the floor (10.1.A.1.ii). The magnitude of the charge of a single electron or proton, the elementary charge $e$ , can be considered to be the smallest indivisible amount of charge.
The signs are assigned (10.1.A.1.iii). The charge of an electron is $-e$ , the charge of a proton is $+e$ , and a neutron has no electric charge.
A point charge is a model (10.1.A.1.iv). It is a model in which the physical size of a charged object or system is negligible in the context of the situation being analyzed. That makes it a modeling choice you justify, not a property an object owns.

The number comes off the Constants and Conversion Factors group of the AP Physics 2 equation sheet:

e = 1.60 \times 10^{-19}\ \text{C}

Put 10.1.A.1.ii and that constant together and you get charge quantization without the CED using the word. Every net charge in this course is a whole-number multiple of $e$ , so a measured charge of $-4.8\ \text{nC}$ means exactly $3.0 \times 10^{10}$ surplus electrons, and no experiment in AP Physics 2 will ever hand you two-thirds of an electron's worth.

The point-charge model is easy to read past. It is what lets a whole charged sphere collapse to a single dot with a single distance to everything else. When a question puts two large spheres almost in contact, the model is exactly what breaks, and saying so is a legitimate answer.

Coulomb's law as the CED states it

EK 10.1.A.2 gives the force law in words before it gives it in symbols, and the words are what a description question grades. Coulomb's law describes the electrostatic force between two charged objects as directly proportional to the magnitude of each of the charges and inversely proportional to the square of the distance between the objects.

The relevant equation printed with that statement, identical to the first line of the Electricity column on the AP Physics 2 equation sheet, is:

\left| \vec{F}_E \right| = \frac{1}{4\pi\varepsilon_0} \frac{|q_1 q_2|}{r^2} = k \frac{|q_1 q_2|}{r^2}

Read the bars. There are two sets and neither is decoration. The left side is the magnitude of a vector, and the numerator is the absolute value of the product of the two charges. As printed, the equation cannot return a negative number and cannot return a direction. Direction is the job of the next essential knowledge statement.

The constant sits in the Constants and Conversion Factors group:

k = \frac{1}{4\pi\varepsilon_0} = 9.0 \times 10^9\ \text{N} \cdot \text{m}^2/\text{C}^2

Use $9.0 \times 10^9$ . The sheet does not print $8.99 \times 10^9$ , and a stem may show the constant in either the $k$ form or the $1/(4\pi\varepsilon_0)$ form, because both appear on the Physics 2 sheet.

This page does not repeat the plug-in routine. The Coulomb's law guide owns the step-by-step method for a two-charge force, and the Coulomb's law calculator owns the arithmetic and the microcoulomb conversions. What follows here is the CED framing those pages do not carry.

Direction lives in the signs

EK 10.1.A.3 supplies what the printed equation cannot. The direction of the electrostatic force depends on the signs of the charges of the interacting objects and is parallel to the line of separation between the objects. Two sub-statements make it concrete: two objects with charges of the same sign exert repulsive forces on each other (10.1.A.3.i), and two objects with charges of opposite signs exert attractive forces on each other (10.1.A.3.ii).

$q_1$	$q_2$	Interaction	Direction of the force on each
$+$	$+$	Repulsive	Away from the other charge
$-$	$-$	Repulsive	Away from the other charge
$+$	$-$	Attractive	Toward the other charge
$-$	$+$	Attractive	Toward the other charge

Notice the phrase on each other. The two forces are a Newton's third law pair: equal in magnitude, opposite in direction, and that holds however lopsided the two charges are. A speck carrying $+1\ \text{nC}$ next to a heavily charged sphere pushes on the sphere exactly as hard as the sphere pushes on it. Their accelerations differ enormously because their masses do. The forces do not differ at all.

There is a working method hidden in "parallel to the line of separation". For charges arranged along one line you never need trigonometry. Pick a positive direction, get each force's magnitude from the printed equation, then attach a sign by asking whether that particular pair attracts or repels. Declare the axis before the first calculation and hold it to the end of the problem.

More than two charges, and the four-charge ceiling

Superposition is what the boundary statement is really about. When several charges act on one object, you find the force from each source separately and add those forces as vectors, the same way you add any other forces on a free-body diagram.

The Electricity column of the AP Physics 2 equation sheet runs to twenty lines, and none of them adds electric forces. The column does print summations, five of them: one for electric potential, $V = \frac{1}{4\pi\varepsilon_0} \sum_i \frac{q_i}{r_i}$ , which is a scalar sum and belongs to Topic 10.5, and four for combining resistors and capacitors. For force you supply the vector addition yourself, which is precisely the work suggested skill 1.A is pointing at: draw the arrangement, draw the arrows, then add them.

The boundary statement caps the calculation at four or fewer interacting charged objects or systems, then allows more when the arrangement has high symmetry. Symmetry is the escape hatch. A charge at the center of a square of four identical charges has zero net force on it because the contributions cancel in pairs, and you argue that rather than compute it. A question can put eight charges on a ring and still be fair, because it is asking for the argument.

The CED's own optional sample activity for Topic 10.1 is the one-dimensional version of this: three known charges at known positions on a line, find the net force on each. The first worked example below is that exercise.

Electric force, gravity, and the contact forces you already know

EK 10.1.A.4 is the statement that connects Unit 10 back to AP Physics 1. Electric forces are responsible for some of the macroscopic properties of objects in everyday experiences. However, the large number of particle interactions that occur make it more convenient to treat everyday forces in terms of nonfundamental forces called contact forces, such as normal force, friction, and tension.

So the normal force and the tension you drew all last year are electric at root. The CED's point is not that they were wrong; it is that tracking every particle interaction in a tabletop would be impossible, so physics names the aggregate and moves on. Contact forces are a convenience, and the CED calls them nonfundamental for that reason.

Objective 10.1.B then compares the electric force with gravity, and its three statements get tested separately.

10.1.B.1, structure. Electrostatic forces can be attractive or repulsive, while gravitational forces are always attractive. That single difference is why electric forces can cancel in bulk matter and gravitational force cannot.
10.1.B.2, scale, with a hedge. For any two objects that have mass and electric charge, the magnitude of the gravitational force is usually much smaller than the magnitude of the electrostatic force. Keep the hedge. The CED wrote usually rather than always, and a question can hand you a massive object with a tiny charge.
10.1.B.3, why gravity still runs the universe. Gravitational forces dominate at larger scales even though they are weaker than electrostatic forces, because systems at large scales tend to be electrically neutral. Gravity wins on the astronomical stage by default. There is no negative mass to cancel it, while a star's positive and negative charges very nearly cancel each other.

Structurally the two laws are twins. Both are inverse-square, both multiply a property of one object by the same property of the other, and both act along the line joining the two. Swap $G$ for $k$ and mass for charge and the algebra is the same. What differs is the sign options and the size of the constant.

Permittivity, the objective students skip

Objective 10.1.C carries no equation of its own. That makes it the easiest third of the topic to leave out of revision and a natural target for a description question, so learn it as four sentences.

10.1.C.1 defines the quantity. Electric permittivity is a measurement of the degree to which a material or medium is polarized in the presence of an electric field.
10.1.C.2 defines the mechanism. Electric polarization can be modeled as the induced rearrangement of electrons by an external electric field, resulting in a separation of positive and negative charges within a material or medium.
10.1.C.3 names the constant. Free space has a constant value of electric permittivity, $\varepsilon_0$ , that appears in physical relationships.
10.1.C.4 says matter is different. The permittivity of matter has a value different from that of free space that arises from the matter's composition and arrangement. Sub-statement 10.1.C.4.i adds that in a given material, electric permittivity is determined by the ease with which electrons can change configurations within the material. Sub-statement 10.1.C.4.ii is the conductor and insulator definition: conductors are made from electrically conducting materials in which charge carriers move easily, and insulators are made from electrically nonconducting materials in which charge carriers cannot move easily.

The permittivity of free space is printed on the sheet:

\varepsilon_0 = 8.85 \times 10^{-12}\ \text{C}^2/(\text{N} \cdot \text{m}^2)

The two printed constants agree with each other, which is worth checking once so you trust the sheet. Evaluate $1/(4\pi\varepsilon_0)$ with that value and you get $8.99 \times 10^9$ , and that rounds to the $9.0 \times 10^9$ the sheet prints for $k$ at the two significant figures it shows.

Note where the conductor and insulator sentence lives. It is filed under Topic 10.1, objective 10.1.C, even though it does most of its work in Topic 10.2, where whether charge carriers can move decides which charging mechanism works. If you are revising charging and cannot find the definition, this is where the CED put it.

How Unit 10 is framed for the exam

The unit opener carries two notes worth reading before you study any topic in Unit 10.

The first is about representations. The CED points at skills 1.A, 1.C, and 2.A across the unit, and lists the representations Unit 10 uses to model ideas: pictures, motion diagrams, graphs, energy bar charts, electric field vector diagrams, and equipotential diagrams. Topic 10.1 mostly draws the first of those plus the free-body diagram you already know; the field and equipotential diagrams belong to Topic 10.3 and beyond.

The second is about the free-response section. The second free-response question on the AP Physics 2 Exam is the Translation Between Representations question, which asks students to create graphical and verbal models of a scenario and then compare those models with a mathematical representation of the same situation. The CED sketches an example built on Unit 10: sketch equipotential lines around a small positively charged sphere, then build energy bar charts for a small point charge released from rest near it, then explain how the two representations are consistent with each other. The CED immediately adds the caveat that matters. Unit 10 content gives especially good practice for that question, but content from any unit may be included in it. Unit 10 is not a guaranteed subject for question two.

The unit's own essential questions make a decent self-test once you have read Topic 10.1. How could you suspend a charged water droplet in the air? Since balloons are made of rubber, how can they be charged so that they stick to the wall? Where is the safest place to be during a lightning storm? How can you protect your electronics from an EMP? The first is Topic 10.1 plus a field, and the middle two run straight into Topic 10.2.

Where Topic 10.1 goes wrong

Signs inside the printed equation. The bars mean magnitude. Feed in $|q_1|$ and $|q_2|$ , get a positive number, then decide attraction or repulsion from 10.1.A.3. Carrying a minus sign through leaves you with a negative force and nothing to interpret it against.
Believing the larger charge feels the larger force. It does not, ever. The two forces are a third-law pair whatever the ratio of the charges. Only the accelerations differ.
Dropping the hedge in 10.1.B.2. The CED says the gravitational force is usually much smaller. A question built around a massive, weakly charged object is testing exactly that word.
Treating the point-charge model as automatic. 10.1.A.1.iv makes it conditional on the object's size being negligible in the situation being analyzed. Two spheres nearly touching fail that test.
Skipping 10.1.C. No equation means no practice problems, which means students meet permittivity for the first time in a description question. Read 10.1.C.1 through 10.1.C.4 as sentences you could write out.
Adding magnitudes when the forces oppose. Three charges in a line routinely give two forces pointing opposite ways. Set the axis first, then add signed numbers.
Assuming the boundary statement forbids five charges. It caps calculation at four, and then explicitly allows more in high-symmetry arrangements, where the answer is an argument rather than a sum.

Net force on the middle charge in a line of three

Three point charges lie on the $x$ -axis: $q_A = +3.0\ \mu\text{C}$ at $x = 0$ , $q_B = -2.0\ \mu\text{C}$ at $x = 0.40\ \text{m}$ , and $q_C = +5.0\ \mu\text{C}$ at $x = 0.70\ \text{m}$ . Find the net electric force on $q_B$ .

Set the convention before anything else: positive $x$ points to the right, from $A$ toward $C$ . Three interacting charges is inside the Topic 10.1 boundary statement's limit of four or fewer, so a direct calculation is what the CED expects here.
Separations. $A$ to $B$ is $r_{AB} = 0.40\ \text{m}$ . $B$ to $C$ is $r_{BC} = 0.70 - 0.40 = 0.30\ \text{m}$ .
Magnitude of the force $A$ exerts on $B$ , using absolute values only: $|\vec{F}_{AB}| = k\dfrac{|q_A q_B|}{r_{AB}^2} = (9.0 \times 10^9)\dfrac{(3.0 \times 10^{-6})(2.0 \times 10^{-6})}{(0.40)^2} = (9.0 \times 10^9)\dfrac{6.0 \times 10^{-12}}{0.16} = 0.3375\ \text{N}$ .
Direction of that one. $q_A$ is positive and $q_B$ is negative, so by 10.1.A.3.ii they attract: $B$ is pulled toward $A$ , which is the $-x$ direction. Signed value $-0.3375\ \text{N}$ .
Magnitude of the force $C$ exerts on $B$ : $|\vec{F}_{CB}| = (9.0 \times 10^9)\dfrac{(5.0 \times 10^{-6})(2.0 \times 10^{-6})}{(0.30)^2} = (9.0 \times 10^9)\dfrac{1.0 \times 10^{-11}}{0.090} = 1.0\ \text{N}$ .
Direction. $q_C$ is positive and $q_B$ is negative, so they attract as well: $B$ is pulled toward $C$ , the $+x$ direction. Signed value $+1.0\ \text{N}$ .
Add along the axis, because both forces are parallel to the line of separation: $F_{\text{net}} = +1.0 - 0.3375 = +0.6625\ \text{N}$ .
Round to the two significant figures the data carry: $0.66\ \text{N}$ . Sanity check on the sign: $q_C$ is both larger and closer than $q_A$ , so it should win, and it does.

$|\vec{F}_{\text{net}}| \approx 0.66\ \text{N}$ on $q_B$ , pointing along $+x$ , toward $q_C$ . Both individual forces are attractive; they oppose only because $q_A$ and $q_C$ sit on opposite sides of $q_B$ .

Predicting a factor of change without any numbers

Two point charges exert forces of magnitude $F$ on each other. One charge is then tripled, the other is halved, and the separation is reduced to two-thirds of its original value. By what factor does the magnitude of the force change?

Suggested skill 2.D asks for functional dependence, so work in ratios and never substitute a number. Start from the printed relationship, $|\vec{F}_E| = k\dfrac{|q_1 q_2|}{r^2}$ .
Write each new quantity as a multiple of the old one: $q_1' = 3q_1$ , $q_2' = \tfrac{1}{2}q_2$ , and $r' = \tfrac{2}{3}r$ .
Substitute: $|\vec{F}_E'| = k\dfrac{\left|3q_1 \cdot \tfrac{1}{2}q_2\right|}{\left(\tfrac{2}{3}r\right)^2} = k\dfrac{\tfrac{3}{2}|q_1 q_2|}{\tfrac{4}{9}r^2}$ .
Collect the numerical factors: $\tfrac{3}{2} \div \tfrac{4}{9} = \tfrac{3}{2} \times \tfrac{9}{4} = \tfrac{27}{8}$ .
So $|\vec{F}_E'| = \tfrac{27}{8}F$ . Read it as two separate effects: the charge changes contribute a factor of $1.5$ , and the distance change contributes $\left(\tfrac{3}{2}\right)^2 = 2.25$ .
The distance term is the bigger of the two even though the distance barely moved, which is the whole lesson of an inverse-square law.

The force grows by a factor of $\tfrac{27}{8} = 3.375$ , so $F' \approx 3.4F$ . No value of $k$ , $q$ , or $r$ was ever needed.

Two protons: electric force against gravity

Two protons are held a distance $r$ apart. Each carries charge $+e = 1.60 \times 10^{-19}\ \text{C}$ and has mass $m_p = 1.67 \times 10^{-27}\ \text{kg}$ . Find the ratio of the electric force magnitude to the gravitational force magnitude, then evaluate both forces at $r = 1.0 \times 10^{-15}\ \text{m}$ .

Write both magnitudes. Electric, from the Electricity column: $|\vec{F}_E| = k\dfrac{e^2}{r^2}$ . Gravitational, from the Mechanics and Fluids table that the Physics 2 sheet also prints: $|\vec{F}_g| = G\dfrac{m_p^2}{r^2}$ .
Both constants come off the sheet: $k = 9.0 \times 10^9\ \text{N} \cdot \text{m}^2/\text{C}^2$ and $G = 6.67 \times 10^{-11}\ \text{N} \cdot \text{m}^2/\text{kg}^2$ .
Take the ratio and watch $r^2$ cancel: $\dfrac{|\vec{F}_E|}{|\vec{F}_g|} = \dfrac{k e^2}{G m_p^2}$ . The answer does not depend on how far apart the protons are, which is the useful part.
Numerator: $k e^2 = (9.0 \times 10^9)(1.60 \times 10^{-19})^2 = (9.0 \times 10^9)(2.56 \times 10^{-38}) = 2.304 \times 10^{-28}$ .
Denominator: $G m_p^2 = (6.67 \times 10^{-11})(1.67 \times 10^{-27})^2 = (6.67 \times 10^{-11})(2.7889 \times 10^{-54}) = 1.860 \times 10^{-64}$ .
Divide: $\dfrac{2.304 \times 10^{-28}}{1.860 \times 10^{-64}} = 1.2 \times 10^{36}$ to two significant figures.
Now the absolute sizes at $r = 1.0 \times 10^{-15}\ \text{m}$ , so $r^2 = 1.0 \times 10^{-30}\ \text{m}^2$ . Electric repulsion: $|\vec{F}_E| = \dfrac{2.304 \times 10^{-28}}{1.0 \times 10^{-30}} = 2.3 \times 10^2\ \text{N}$ . Gravitational attraction: $|\vec{F}_g| = \dfrac{1.860 \times 10^{-64}}{1.0 \times 10^{-30}} = 1.9 \times 10^{-34}\ \text{N}$ .
That is EK 10.1.B.2 in numbers. EK 10.1.B.3 is why the universe does not behave like this at large scales: bulk matter is very nearly neutral, so almost all of that enormous electric force cancels, and the tiny gravitational one has nothing to cancel it.

$|\vec{F}_E| / |\vec{F}_g| \approx 1.2 \times 10^{36}$ , and the ratio is the same at every separation because $r^2$ cancels. At $1.0 \times 10^{-15}\ \text{m}$ that is roughly $2.3 \times 10^2\ \text{N}$ of repulsion set against $1.9 \times 10^{-34}\ \text{N}$ of attraction.

Frequently asked questions

Do like charges attract or repel?

Like charges repel and opposite charges attract. AP Physics 2 states this as two separate pieces of essential knowledge under Topic 10.1: two objects with charges of the same sign exert repulsive forces on each other (10.1.A.3.i), and two objects with charges of opposite signs exert attractive forces on each other (10.1.A.3.ii). Either way the force on each object points along the line joining the two objects, and the two forces are equal in magnitude and opposite in direction, because they are a Newton's third law pair.

What is a point charge in AP Physics 2?

A point charge is a model, not a kind of object. Essential knowledge 10.1.A.1.iv defines it as a model in which the physical size of a charged object or system is negligible in the context of the situation being analyzed. So it is conditional, and you should be ready to say when it fails. Two small beads a meter apart can be treated as point charges; two large spheres nearly touching cannot, because their size is no longer negligible next to their separation. The single distance r in Coulomb's law only has an unambiguous meaning once the point-charge model applies.

How many charges can an AP Physics 2 question make you combine?

Four, for a calculation. The Topic 10.1 boundary statement says AP Physics 2 only expects students to make calculations of the electric force between four or fewer interacting charged objects or systems. It then adds a second clause that students often drop: the analysis of the resulting electric force from more charges is allowed in situations of high symmetry. So a question can legitimately show you six or eight charges in a symmetric arrangement and ask which way the net force points, because that is an argument from cancellation rather than a calculation.

Is the elementary charge on the AP Physics 2 equation sheet?

Yes. The elementary charge, e = 1.60 x 10^-19 C, is printed in the Constants and Conversion Factors group of the AP Physics 2 equation sheet, alongside the Coulomb constant k = 9.0 x 10^9 N m^2/C^2 and the vacuum permittivity, 8.85 x 10^-12 C^2/(N m^2). You do not have to memorize any of them. What you do have to remember is what 10.1.A.1.ii says about e: it is the smallest indivisible amount of charge, so every net charge you meet is a whole-number multiple of it.

If the electric force is so much stronger, why does gravity control planets and stars?

Because large systems are almost exactly neutral. Essential knowledge 10.1.B.3 states it directly: gravitational forces dominate at larger scales even though they are weaker than electrostatic forces, because systems at large scales tend to be electrically neutral. A star contains a colossal amount of positive and negative charge, and the two cancel to a fantastic precision, so almost none of that electric force survives at a distance. Mass has no negative counterpart, so every kilogram in the star adds to the gravitational pull and nothing subtracts from it.

What is electric permittivity, and why is it in Coulomb's law?

Electric permittivity is a measurement of the degree to which a material or medium is polarized in the presence of an electric field (essential knowledge 10.1.C.1). Polarization here means the induced rearrangement of electrons by an external field, which separates positive and negative charge inside the material (10.1.C.2). It reaches Coulomb's law through the permittivity of free space: the AP Physics 2 equation sheet prints the Coulomb constant in exactly that form, as one divided by the quantity four pi times the permittivity of free space. Matter has a different permittivity from free space, set by how easily its electrons can change configuration (10.1.C.4 and 10.1.C.4.i).

What is the difference between Topic 10.1 and Topic 10.2?

Topic 10.1 is about the interaction. It defines charge, defines the elementary charge and the point-charge model, gives Coulomb's law for the force between two charges, sets the rule for attraction and repulsion, and compares the electric force with gravity. Topic 10.2 is about how objects come to be charged in the first place: conservation of charge, charging by friction or contact, induced charge separation and polarization, and grounding. One tells you what the force does; the other tells you where the charge came from. The conductor and insulator definition is filed under 10.1, in essential knowledge 10.1.C.4.ii.