Conductor vs Insulator: What Is the Difference?

A conductor is made of material in which charge carriers move easily. An insulator is made of material in which charge carriers cannot move easily. Both can be polarized by a nearby charge, but only a conductor lets charge travel across the object or leave it through a ground wire.

AP Physics: Unit 10 (topics 10.1 Electric Charge and Electric Force, 10.2 Conservation of Electric Charge and the Process of Charging, 10.3 Electric Fields). AP Physics 2 defines this pair in Topic 10.1, essential knowledge 10.1.C.4.ii: 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 definition sits under learning objective 10.1.C, which is about electric permittivity, because 10.1.C.4.i ties permittivity to the ease with which electrons can change configurations within a material. Topic 10.2 supplies the charging statements under a single learning objective 10.2.A, with eight essential knowledge statements and no boundary statement, among them: net charge can change through friction or contact (10.2.A.1.i), induced charge separation polarizes systems including neutral ones (10.2.A.1.ii and 10.2.A.1.iii), any change in net charge is a transfer to or from the surroundings and usually a transfer of electrons (10.2.A.2 and 10.2.A.2.i), and grounding is an electrical connection to a much larger, approximately neutral system such as Earth (10.2.A.3). Topic 10.3 supplies the equilibrium contrast: 10.3.B.1 puts a solid conductor's excess charge on its surface with zero field inside and a perpendicular field at the surface, while 10.3.B.2 spreads an insulator's excess charge through the interior and allows a nonzero field inside. The Topic 10.3 boundary statement limits field calculations to four or fewer charged objects or systems, allows more in situations of high symmetry, and restricts analysis of fields within insulators to qualitative work. Unit 10, Electric Force, Field, and Potential, carries 15 to 18 percent of the multiple-choice section over a suggested 14 to 21 class periods. AP Physics C: Electricity and Magnetism repeats the conductor and insulator sentence word for word at 8.1.C.4.ii.

The distinction, stated once

The AP Physics 2 course description settles this in a single sentence, essential knowledge 10.1.C.4.ii: 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.

That is the whole difference. It is a statement about mobility, not about how much charge a thing has, not about whether it is charged at all, and not about whether it responds to a nearby charge. A neutral copper sphere and a neutral rubber sphere hold the same net charge, zero. Bring a charged rod near either one and both respond. What separates them is what happens next: in the metal, electrons travel across the whole object and can be made to leave it entirely, while in the rubber they shift a little within each molecule and stay put.

Everything else on this page follows from that one sentence. Grounding works on a conductor and does essentially nothing to an insulator, because grounding is a route for charge to travel. Excess charge on a conductor ends up on the surface, because charge that can move keeps moving until it stops pushing itself apart. The electric field inside a charged conductor in electrostatic equilibrium is zero, because any field there would still be moving charge, so the arrangement would not yet be an equilibrium.

Note what the definition does not say. It says nothing about band gaps, valence electrons, or semiconductors. AP Physics 2 defines the pair by observable charge mobility and stops there, and so does this page.

Side by side

	Conductor	Insulator
CED definition	Charge carriers move easily (10.1.C.4.ii)	Charge carriers cannot move easily (10.1.C.4.ii)
Everyday examples	Metals, salt water, the human body	Rubber, glass, dry air, plastic
Can be polarized by a nearby charge	Yes	Yes
Charge can cross the whole object	Yes	No
Where excess charge sits at equilibrium	On the surface only (10.3.B.1)	Throughout the interior as well as at the surface (10.3.B.2)
Field inside at equilibrium	Zero (10.3.B.1)	May have a nonzero value (10.3.B.2)
Field at the surface	Perpendicular to it (10.3.B.1.i)	Not specified by the CED
Effect of grounding	Charge flows to or from Earth, so the object can be left with net charge	No route for charge to travel, so grounding one spot changes little
Charged by rubbing	Yes, but the charge then spreads	Yes, and the charge stays where it was put
Sharing charge on contact	Two identical conducting spheres split the total equally	Touching two insulators transfers almost nothing
AP calculation expectation	Quantitative	Fields within insulators are qualitative only (Topic 10.3 boundary statement)

The row that decides most exam questions is where excess charge sits at equilibrium, because it is the row that turns the mobility definition into something you can draw. On a conductor you draw the charge as a layer on the outside. On an insulator you draw it wherever it was deposited, including inside.

The row worth reading twice is the polarization row, because it is the one that is the same for both. Students who learn "insulators do not respond to charge" get every induced-charge question wrong. Insulators respond. They just cannot pass the response along.

What the CED actually requires here

This pair is unusual in where it lives. The definition is not filed under a topic called conductors and insulators. It sits inside learning objective 10.1.C, which asks you to describe the electric permittivity of a material or medium, as the second sub-point of essential knowledge 10.1.C.4.

The surrounding statements explain why the CED files it there. 10.1.C.1 states that 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 states that 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 states that free space has a constant value of electric permittivity, $\varepsilon_0$ , that appears in physical relationships. 10.1.C.4 states that the permittivity of matter has a value different from that of free space that arises from the matter's composition and arrangement, and 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.

Read in order, the chain is: permittivity measures how much a material polarizes, polarization is electrons rearranging, how much they rearrange depends on how easily they can move, and the two extremes of that ease are called conductors and insulators. The conductor and insulator labels are the endpoints of a mobility scale that the CED introduces through permittivity.

The electrostatic-equilibrium statements are in Topic 10.3 instead, under learning objective 10.3.B, describe the electric field generated by charged conductors or insulators:

10.3.B.1: while in electrostatic equilibrium, the excess charge of a solid conductor is distributed on the surface of the conductor, and the electric field within the conductor is zero. 10.3.B.1.i: at the surface of a charged conductor, the electric field is perpendicular to the surface. 10.3.B.1.ii: the electric field outside an isolated sphere with spherically symmetric charge distribution is the same as the electric field due to a point charge with the same net charge as the sphere located at the center of the sphere.
10.3.B.2: while in electrostatic equilibrium, the excess charge of an insulator is distributed throughout the interior of the insulator as well as at the surface, and the electric field within the insulator may have a nonzero value.

And the Topic 10.3 boundary statement, in full, because its last clause is the one that tells you how far a question can go: AP Physics 2 only expects students to make calculations of the electric field resulting from four or fewer charged objects or systems, analysis of the electric field resulting from more charges is allowed in situations of high symmetry, and students will only be expected to perform qualitative analysis of electric fields within insulators.

That last clause is a gift. You will not be asked to compute a field inside a dielectric slab. You will be asked to say whether it is zero, and for an insulator the answer is that it may not be. AP Physics C: Electricity and Magnetism carries the same conductor and insulator sentence, word for word, at 8.1.C.4.ii, so nothing about the definition changes if you move up to the calculus course. One word does change nearby: the calculus course's 8.3.B.1 says "the excess charge of a conductor", where AP Physics 2's 10.3.B.1 says "the excess charge of a solid conductor".

The case that separates them: one charged rod, two spheres

Hold a negatively charged rod near a neutral metal sphere on an insulating stand, and near a neutral plastic sphere of the same size, without touching either. Then compare.

Both spheres polarize. This is 10.2.A.1.ii and 10.2.A.1.iii: induced charge separation occurs when the electrostatic force between two systems alters the distribution of charges within the systems, resulting in the polarization of one or both systems, and induced charge separation can occur in neutral systems. Both spheres end up with their near side positive and their far side negative. Both are attracted to the rod. Both still have zero net charge.

Only the metal separates charge across the whole object. In the metal, mobile electrons travel to the far side, so the near face is positive and the far face is negative by a measurable amount, and the interior field is zero. In the plastic, each molecule stretches slightly and the separation is per molecule, so the effect appears as a thin surface layer with the bulk still locally neutral. The observable consequence is size, not sign: the metal's induced separation is large and the plastic's is small.

Now touch a grounding wire to the far side of each, then remove the wire, then remove the rod.

	Metal sphere	Plastic sphere
Before the rod arrives	Neutral	Neutral
With the rod near, before grounding	Polarized, net charge zero	Polarized, net charge zero
Grounding wire touched to the far side	Electrons are repelled down the wire to Earth	Almost nothing leaves; the electrons cannot travel to the wire
Wire removed, then rod removed	Net positive charge, spread over the surface	Still neutral
Final net charge	Positive	Zero

That table is the whole comparison in one experiment. Same rod, same geometry, same polarization step, and a completely different end state, because one material can pass charge to the wire and the other cannot.

Notice the ordering matters. The wire has to come off before the rod does. Remove the rod first and the electrons that went to Earth come straight back, and you end up with a neutral sphere and nothing to show. The CED does not name this procedure, so no essential knowledge statement will hand you the order. What it gives you is 10.2.A.3, that grounding involves electrically connecting a charged system to a much larger and approximately neutral system, for example Earth, and 10.2.A.2.ii, that the net charge of a system will be constant unless there is a transfer of charge to or from the system. The ordering follows from those two: charge only leaves while there is both a route and a reason.

Grounding, and the language the CED uses for charging

AP Physics 2 describes charging without naming the classroom processes. Searched from front to back, the framework never uses the phrases charging by conduction or charging by induction. Topic 10.2 has exactly one learning objective, 10.2.A, and no boundary statement. Its essential knowledge is eight statements, and here they all are: a system's net charge or charge distribution can change in response to the presence of, or changes in, the net charge or charge distribution of other systems (10.2.A.1), which happens through friction or contact (10.2.A.1.i) or as induced charge separation when the electrostatic force between two systems alters the distribution of charges within them (10.2.A.1.ii), including in neutral systems (10.2.A.1.iii); any change to a system's net charge is a transfer between the system and its surroundings (10.2.A.2), typically a transfer of electrons (10.2.A.2.i), and the net charge stays constant without such a transfer (10.2.A.2.ii); and grounding is electrically connecting a charged system to a much larger and approximately neutral system, for example Earth (10.2.A.3). That is every statement in the topic, counted and read line by line off the page.

So the vocabulary you are guaranteed is friction, contact, induced charge separation, transfer of electrons, and grounding. If a question uses the phrase charging by induction, treat it as shorthand for the grounding sequence above, and answer using 10.2.A.1.ii and 10.2.A.3 rather than by reciting a named recipe.

Why grounding works on a conductor and not on an insulator is now a one-line argument. Grounding supplies a route, and a route is only useful to charge that can travel. Touch a ground wire to one spot on a charged plastic rod and you neutralize roughly that spot. Touch a ground wire to one spot on a charged metal rod and you neutralize the whole rod, because the charge on a conductor is free to find the wire from anywhere on the surface.

Inside each material: zero field, or not

For a conductor in electrostatic equilibrium, the field inside is zero and the excess charge is on the surface. The argument is short enough to write on an exam: a nonzero field inside would exert a force on the mobile charge carriers, that force would move them, and while charge is moving the situation is not in electrostatic equilibrium. So at equilibrium the interior field must be zero, and the charge has nowhere to go but the surface.

This is what makes the sphere calculations in Topic 10.3 work. By 10.3.B.1.ii, outside an isolated sphere with a spherically symmetric charge distribution, the field is the same as that of a point charge with the same net charge sitting at the sphere's center. So for points outside, you use the point-charge relation the AP Physics 2 sheet prints in the electricity block, $\lvert \vec{E} \rvert = k \lvert q \rvert / r^2$ , with $r$ measured from the center. For points inside the conductor, the answer is zero and no calculation is required.

For an insulator in electrostatic equilibrium, both halves of that picture change. Excess charge sits throughout the interior as well as at the surface, and the interior field may be nonzero. Nothing forces it to zero, because there are no mobile carriers for it to push, so a field inside an insulator can persist indefinitely without violating equilibrium.

The electric field and potential guide works through the point-charge and superposition machinery, and Gauss's law makes the shell results fall out in one line, though the flux formulation itself belongs to the calculus course rather than to AP Physics 2.

When it costs a mark

Saying an insulator cannot be polarized. It can, and the CED says so twice, in 10.1.C.2 and 10.2.A.1.ii. A charged balloon sticks to a wall because the wall polarizes. Answer a why-does-it-stick question with "the wall is an insulator so nothing happens in it" and the point is gone.

Saying a conductor cannot hold net charge. It can. What it cannot do is hold charge in its interior at equilibrium, or sustain an interior field. A metal sphere charged to $+8.0 \ \mathrm{nC}$ keeps that charge on its surface indefinitely if it is insulated from its surroundings.

Grounding an insulator and reporting that it is now neutral. Grounding one point on an insulator neutralizes roughly that point. The rest of the object keeps whatever charge it had, because the charge cannot reach the wire.

Removing the rod before the ground wire. The sequencing error described above. It produces a neutral object and a wrong answer that looks like a reasonable outcome, which is what makes it survive a check.

Treating induced charge as net charge. A polarized neutral object has zero net charge. If a question asks for the net charge of a neutral conductor near a charged rod, the answer is zero no matter how dramatic the separation looks in the diagram. 10.2.A.2.ii is the sentence to cite: the net charge is constant unless charge is transferred to or from the system.

Sharing charge unequally between identical conducting spheres. Two identical conductors brought into contact and separated end up with half the total each, because mobile charge redistributes until the two are equivalent. Two identical insulators brought into contact transfer almost nothing.

Computing a field inside an insulator. The Topic 10.3 boundary statement limits you to qualitative analysis there. A number in that answer box is a sign you have imported a formula the course does not ask for.

Where they behave alike, and why that lulls you

Three situations make the two look interchangeable, and each is a trap for a different reason.

A neutral object of either kind is attracted to a charged rod. Both polarize, both feel a net attraction, and the attraction is toward the rod regardless of the rod's sign. If your only experiment is the attraction test, conductors and insulators are indistinguishable, and that is where the belief that they behave the same way comes from.

Outside a symmetric charged object, the field can look identical. A charged conducting sphere and a uniformly charged insulating sphere of the same radius and the same total charge produce the same external field, by the point-charge equivalence in 10.3.B.1.ii for the conductor. The difference is interior, and an exterior measurement cannot see it.

Both obey conservation of charge exactly. Nothing about being an insulator exempts an object from 10.2.A.2. The mobility difference changes where the charge ends up, never how much of it there is.

The distinction turns on the moment a question asks about the interior of the object, or about a route for charge to travel. Interior charge location, interior field, grounding, sharing on contact, and whether one touch neutralizes the whole object: those five force you to say which material you are holding. Everything else is the special case where the answer is the same either way.

Where this sits on the AP exam

The pair lives in Unit 10, Electric Force, Field, and Potential, which the CED weights at 15 to 18 percent of the multiple-choice section over a suggested 14 to 21 class periods. Three topics carry it. The definition is in Topic 10.1, the charging and grounding statements are in Topic 10.2, and the equilibrium and field statements are in Topic 10.3.

The suggested skills the CED lists for Topic 10.2 tell you the shape of the questions: 1.A, create diagrams, tables, charts, or schematics to represent physical situations; 2.C, compare physical quantities between two or more scenarios or at different times and locations in a single scenario; 3.B, apply an appropriate law, definition, theoretical relationship, or model to make a claim; and 3.C, justify or support a claim using evidence from experimental data, physical representations, or physical principles or laws. Two of those four are justification skills, which is why so many conductor and insulator questions ask you to explain rather than to compute.

For the force between the charges you end up with, the Coulomb's law guide covers the arithmetic, using the sheet value $k = 9.0 \times 10^9 \ \mathrm{N \cdot m^2/C^2}$ . For the energy bookkeeping once charge has moved, see electric potential vs electric potential energy. Once the charge is moving steadily rather than sitting still you are in circuits, where the same mobility idea reappears as a material property with a number attached: see resistance vs resistivity, and note that 11.3.A.2.i defines resistivity as the property that quantifies how strongly a material opposes the motion of electric charge, which is the conductor and insulator scale made quantitative.

Two conducting spheres share charge; two insulating spheres do not

An isolated metal sphere carries a charge of $+8.0 \ \mathrm{nC}$ . It is touched to an identical, initially neutral metal sphere and then separated, and the two are held with their centers $0.20 \ \mathrm{m}$ apart. (a) Find the charge on each sphere afterward. (b) Find the magnitude of the electrostatic force between them. (c) Say what the answers would be if both spheres were identical insulators instead.

(a) Charge on a conductor is mobile, and the two spheres are identical, so after contact there is no reason for one to hold more than the other. Conservation of charge (10.2.A.2) fixes the total at $+8.0 \ \mathrm{nC}$ , so each carries $+4.0 \ \mathrm{nC}$ .
(b) Use Coulomb's law with the sheet value $k = 9.0 \times 10^9 \ \mathrm{N \cdot m^2/C^2}$ , and treat each sphere as a point charge at its center, which 10.3.B.1.ii licenses for a spherically symmetric distribution.
$\lvert \vec{F}_E \rvert = k \lvert q_1 q_2 \rvert / r^2 = (9.0 \times 10^9)(4.0 \times 10^{-9})(4.0 \times 10^{-9}) / (0.20)^2$ .
Numerator: $(4.0 \times 10^{-9})(4.0 \times 10^{-9}) = 1.6 \times 10^{-17}$ , and $(9.0 \times 10^9)(1.6 \times 10^{-17}) = 1.44 \times 10^{-7}$ .
Denominator: $(0.20)^2 = 0.040$ . So $\lvert \vec{F}_E \rvert = 1.44 \times 10^{-7} / 0.040 = 3.6 \times 10^{-6} \ \mathrm{N}$ , repulsive because both charges are positive (10.1.A.3.i).
(c) For identical insulators, the contact transfers almost nothing, because the charge carriers cannot move to the contact point. The charged sphere keeps essentially all of its $+8.0 \ \mathrm{nC}$ and the second stays essentially neutral, so the force between them is essentially zero: with $q_2 \approx 0$ , the product $q_1 q_2$ is approximately zero whatever $q_1$ is.
Sanity check on part (b): halving each charge from $8.0$ to $4.0 \ \mathrm{nC}$ would cut the product by four. Had the full $8.0 \ \mathrm{nC}$ somehow faced another $8.0 \ \mathrm{nC}$ at the same separation, the force would have been $1.44 \times 10^{-5} \ \mathrm{N}$ , four times larger. Sharing is what reduced it.

Each conducting sphere carries $+4.0 \ \mathrm{nC}$ and they repel with $3.6 \times 10^{-6} \ \mathrm{N}$ . Identical insulators would not share: one keeps $+8.0 \ \mathrm{nC}$ , the other stays neutral, and the force between them is essentially zero. Same geometry, same total charge, and the material decides.

Inside and outside a charged conducting sphere

A solid metal sphere of radius $0.10 \ \mathrm{m}$ carries a net charge of $+5.0 \ \mathrm{nC}$ and is in electrostatic equilibrium. Find the magnitude of the electric field (a) at a point $0.050 \ \mathrm{m}$ from the center, inside the metal, (b) at the surface, and (c) at a point $0.20 \ \mathrm{m}$ from the center. (d) Say which of the three answers would change if the sphere were a uniformly charged insulator instead.

(a) Zero. Essential knowledge 10.3.B.1 states that while in electrostatic equilibrium, the electric field within a solid conductor is zero. No calculation is needed and none is possible from the given data, because the result does not depend on the charge or the radius.
(b) At the surface, $r = 0.10 \ \mathrm{m}$ . By 10.3.B.1.ii the external field is that of a point charge at the center, so $\lvert \vec{E} \rvert = k \lvert q \rvert / r^2 = (9.0 \times 10^9)(5.0 \times 10^{-9}) / (0.10)^2$ .
Numerator: $(9.0 \times 10^9)(5.0 \times 10^{-9}) = 45$ , in units of $\mathrm{N \cdot m^2/C}$ . Denominator: $(0.10)^2 = 0.010 \ \mathrm{m^2}$ . So $\lvert \vec{E} \rvert = 45 / 0.010 = 4.5 \times 10^3 \ \mathrm{N/C}$ , directed radially outward and, by 10.3.B.1.i, perpendicular to the surface.
(c) At $r = 0.20 \ \mathrm{m}$ : $\lvert \vec{E} \rvert = 45 / (0.20)^2 = 45 / 0.040 = 1.1 \times 10^3 \ \mathrm{N/C}$ , keeping two significant figures.
Check the inverse square: doubling $r$ from $0.10$ to $0.20 \ \mathrm{m}$ should quarter the field. $4.5 \times 10^3 / 4 = 1.125 \times 10^3$ , which rounds to the $1.1 \times 10^3 \ \mathrm{N/C}$ just computed. The two routes agree.
(d) Only part (a). Outside the sphere, a spherically symmetric charge distribution gives the same field whatever the material, so (b) and (c) are unchanged. Inside, 10.3.B.2 says an insulator's excess charge is spread through the interior and the interior field may be nonzero, so the zero in (a) no longer follows. The Topic 10.3 boundary statement limits that interior analysis to qualitative work, so the honest answer for the insulator is "not necessarily zero", not a number.

Inside the conductor the field is zero; at the surface it is $4.5 \times 10^3 \ \mathrm{N/C}$ ; at $0.20 \ \mathrm{m}$ from the center it is $1.1 \times 10^3 \ \mathrm{N/C}$ . Swapping to a uniformly charged insulator leaves both exterior answers alone and destroys only the interior zero.

Grounding a conductor, and grounding an insulator

A negatively charged rod is held near, but not touching, the left face of a neutral object mounted on an insulating stand. While the rod is in place, a grounding wire is touched to the object's right face, then the wire is removed, then the rod is removed. Describe the object's final net charge and its distribution when the object is (a) a metal block and (b) a block of dry plastic. (c) State what would have happened if the rod had been removed before the wire.

Set up the polarization step first, because it is the same for both. The rod's negative charge repels electrons in the object toward the right face, leaving the left face positive. This is induced charge separation, 10.2.A.1.ii, and 10.2.A.1.iii confirms it happens in neutral systems. At this stage the net charge is still zero for both blocks, by 10.2.A.2.ii.
(a) Metal block. The electrons that gathered on the right face are mobile, and the ground is a much larger, approximately neutral system (10.2.A.3), so they are repelled down the wire into it. Removing the wire strands the block with a deficit of electrons, which is a net positive charge. Removing the rod then lets that positive charge redistribute, and 10.3.B.1 puts it on the surface, spread so that the field inside the metal is zero.
(b) Plastic block. The polarization is molecular and the electrons cannot travel to the right face or into the wire. Touching the wire neutralizes at most a small amount of charge in the immediate contact region. Removing the wire and then the rod leaves the block with a net charge of essentially zero.
(c) Removing the rod first removes the reason the electrons were crowded on the right face. They flow back through the still-connected wire until the block is neutral again. Then removing the wire leaves a neutral block. Order matters because charge only leaves while there is both a route and a push, and the rod supplies the push.
Quantitative note for the metal case: nothing in the problem fixes how much charge leaves, so no number can be produced. The magnitude of the induced charge depends on the rod's charge and its distance, neither of which is given. A question wanting a number would have to supply them.

The metal block ends up with a net positive charge spread over its surface, with zero field inside it. The plastic block ends up essentially neutral. Reversing the order, rod first and wire second, leaves both blocks neutral. Grounding is a route for charge to travel, so it only changes the object whose charge can travel.

Frequently asked questions

What is the difference between a conductor and an insulator?

A conductor is made from electrically conducting material in which charge carriers move easily, and an insulator is made from electrically nonconducting material in which charge carriers cannot move easily. That is the AP Physics 2 definition, essential knowledge 10.1.C.4.ii. The consequences are that excess charge on a conductor sits on its surface and the field inside it is zero at equilibrium, while excess charge on an insulator can sit throughout the interior and the field inside may be nonzero. Both materials can still be polarized by a nearby charge.

Can an insulator be charged?

Yes. Rubbing is the usual way, and the CED covers it in 10.2.A.1.i, which says the net charge of a system can change due to friction or contact between systems. The difference from a conductor is what happens after: charge deposited on an insulator stays roughly where it was put, including inside the material, while charge given to a conductor spreads over the surface. This is why a rubbed balloon holds a patchy charge and a charged metal sphere holds an even one.

Why is the electric field zero inside a conductor?

Because the situation described is electrostatic equilibrium, and a nonzero field inside a conductor would not be one. A field inside would exert a force on the mobile charge carriers, that force would move them, and charge in motion means the arrangement has not settled. So at equilibrium the interior field is zero and any excess charge has been pushed to the surface. Essential knowledge 10.3.B.1 states both halves of that result. The argument does not apply to an insulator, because an insulator has no mobile carriers for the field to push, so 10.3.B.2 says the field inside an insulator may have a nonzero value.

Why does a charged balloon stick to a wall if the wall is an insulator?

Because insulators polarize. The balloon's charge rearranges the electrons in the wall material, which the CED calls induced charge separation in 10.2.A.1.ii and electric polarization in 10.1.C.2. The wall's surface nearest the balloon ends up with the opposite sign, so the two attract, even though the wall's net charge is still zero. Nothing about being an insulator prevents polarization; it only prevents charge from travelling across the object.

Does grounding work on an insulator?

Barely. Grounding means electrically connecting a system to a much larger, approximately neutral system such as Earth, which is essential knowledge 10.2.A.3, and it works by giving charge a route to travel. An insulator's charge carriers cannot travel easily, so a ground wire touched to one spot on a charged insulator neutralizes roughly that spot and leaves the rest of the object charged. The same wire touched anywhere on a charged conductor neutralizes the whole conductor, because its surface charge is free to find the wire.

Does the AP Physics 2 course use the terms charging by conduction and charging by induction?

No. Searching the AP Physics 2 course and exam description turns up neither phrase. Topic 10.2 instead gives you friction or contact as ways the net charge of a system can change (10.2.A.1.i), induced charge separation for the effect of a nearby charge (10.2.A.1.ii), transfer of electrons as the usual mechanism (10.2.A.2.i), and grounding as a connection to a much larger neutral system (10.2.A.3). If a textbook question uses the named processes, answer it with those statements rather than by reciting a recipe the framework does not print.

Do conductors and insulators ever produce the same electric field?

Outside the object, often yes. A charged conducting sphere and a uniformly charged insulating sphere with the same radius and the same total charge produce the same field at every external point, because for a spherically symmetric distribution the exterior field is that of a point charge at the center. The two differ inside: the conductor's interior field is zero at equilibrium while the insulator's may be nonzero. An exterior measurement cannot tell them apart, and an interior question separates them completely.