How do objects become charged, and why is charge never created or destroyed?
Topic 10.2 Conservation of Charge and the Process of Charging: apply conservation of charge to charging by friction, conduction and induction.
A focused answer to AP Physics 2 Topic 10.2, covering the conservation of electric charge, the difference between conductors and insulators, and the three charging processes (friction, conduction and induction with grounding), with full worked examples.
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What this topic is asking
The College Board (Topic 10.2) wants you to apply the conservation of electric charge to the three ways objects become charged, friction, conduction and induction, and to use the distinction between conductors and insulators to explain how charge moves.
Conservation of charge and the two material types
Every charging process is a redistribution of existing charge, governed by conservation. Whether charge can move easily depends on the material: in a conductor, free electrons flow in response to forces, so charge spreads over the surface; in an insulator, charges are locked in place and only shift slightly within atoms. This split is the key to predicting what each charging method does.
The three ways to charge an object
The two contact methods are easy to tell apart: friction and conduction both involve touching, but friction separates electrons between two insulators (opposite charges), while conduction shares charge across a contact (same sign). Induction is the subtle one because the inducing charge never touches the object. The strategy for an induction problem is to track the steps: bring the charge near (charges separate), ground the far side (charge flows away), remove the ground, then remove the inducing charge, leaving the object oppositely charged.
Polarization and attraction of neutral objects
A charged object can attract a neutral one without charging it, through polarization: the charged object pulls the neutral object's opposite charges slightly closer and pushes its like charges away, so the nearer (attracting) charges feel a stronger force than the farther (repelling) ones, giving a net attraction. This is why a charged comb picks up small paper bits and why a charged balloon sticks to a wall. The strategic thread of this topic is that all of electrostatics rests on conservation of charge plus the mobility of charge in conductors. Knowing whether charge can move, and tracking it through each step, lets you predict the final sign in any charging scenario, which feeds directly into the fields (Topic 10.3) and potentials (Topic 10.5) those charges produce, and into the currents of Unit 11.
Try this
Q1. State the sign of an object charged by conduction with a negative rod. [1 point]
- Cue. Negative (the same sign as the rod).
Q2. State why a charged balloon attracts a neutral wall. [1 point]
- Cue. By polarization: the balloon induces opposite charge nearer its surface, giving a net attraction.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2024 (style)6 marksSection II (short FRQ). A negatively charged rod is brought near (but not touching) one end of an isolated neutral metal sphere on an insulating stand. (a) Describe the distribution of charge induced on the sphere while the rod is held nearby. (b) The far side of the sphere is briefly grounded while the rod is still present, then the ground is removed and finally the rod is taken away. State the final net charge on the sphere and justify it. (c) State how this process compares with charging by conduction.Show worked answer →
A 6-point FRQ on charging by induction.
(a) Induced distribution (2 points): the negative rod repels the sphere's free electrons to the far side, leaving the near side positive and the far side negative; the sphere is still neutral overall.
(b) Final charge (3 points): grounding the far side lets the repelled electrons flow to ground. Removing the ground, then the rod, leaves the sphere with a net positive charge (it lost electrons).
(c) Comparison (1 point): in charging by induction the sphere ends up with the opposite sign to the rod and the rod never touches it; in charging by conduction they touch and end with the same sign.
Markers reward the induced separation, the loss of electrons to ground giving a net positive charge, and the opposite-sign result of induction.
AP 2023 (style)1 marksSection I (multiple choice). A positively charged rod touches a neutral metal sphere and is removed. What is the sign of the sphere's charge afterward? (A) positive (B) negative (C) neutral (D) it cannot be determined. Justify your reasoning.Show worked answer →
A 1-point MCQ on charging by conduction. The answer is (A).
In charging by conduction the rod and sphere touch, so charge flows between them until they share the same sign. A positive rod draws electrons from the sphere, leaving the sphere positive (the same sign as the rod). The trap is (B): conduction gives the same sign, not the opposite.
Related dot points
- Topic 10.1 Electric Charge and Coulomb's Law: describe electric charge and apply Coulomb's law to the force between point charges.
A focused answer to AP Physics 2 Topic 10.1, covering the two kinds of electric charge, the attraction and repulsion rule, the quantisation and conservation of charge, and Coulomb's law for the inverse-square force between point charges, with full worked examples.
- Topic 10.3 Electric Fields: define the electric field, calculate the field of a point charge, and represent fields with field lines and superposition.
A focused answer to AP Physics 2 Topic 10.3, covering the electric field as force per unit charge, the field of a point charge, field-line diagrams and their rules, superposition of fields, the uniform field between parallel plates, and fields in conductors, with full worked examples.
- Topic 10.4 Electric Potential Energy: calculate the electric potential energy of a system of point charges and relate it to work done.
A focused answer to AP Physics 2 Topic 10.4, covering electric potential energy as the work stored in assembling charges, the formula U = k q1 q2 / r for a pair of point charges, the role of sign, the work-energy connection, and superposition over multiple pairs, with full worked examples.
- Topic 10.5 Electric Potential and its Relation to the Electric Field: define electric potential, relate potential difference to field and to potential energy, and use equipotentials.
A focused answer to AP Physics 2 Topic 10.5, covering electric potential as energy per unit charge, the potential of a point charge, the relation between potential difference and the field, equipotential surfaces, and the work done moving a charge through a potential difference, with full worked examples.
- Topic 10.6 Capacitors: relate charge, voltage and capacitance, find the capacitance of a parallel-plate capacitor, and calculate the energy stored.
A focused answer to AP Physics 2 Topic 10.6, covering capacitance as charge per volt, the parallel-plate capacitor and what sets its capacitance, the role of a dielectric, the uniform field between the plates, and the energy stored, with full worked examples.
Sources & how we know this
- AP Physics 2: Algebra-Based Course and Exam Description — College Board (2024)