Conservation of electric charge and the process of charging

Changing Net Charge by Friction or Contact

Conservation of charge means the total charge in an isolated system never changes — it can only move from one object to another. When two objects touch or rub together, electrons transfer from one surface to the other.

Friction charging works because different materials hold electrons with different strengths. Rubbing a glass rod with silk strips electrons from the glass. The glass becomes positive; the silk becomes equally negative. No charge was created — it simply moved.

Contact charging [charging by touching a charged object to a neutral one] transfers charge directly. A negatively charged rod touched to a neutral metal sphere pushes excess electrons onto the sphere. Both objects end up with the same sign of charge. The total charge before contact equals the total charge after contact.

MisconceptionStudents sometimes say friction "creates" charge. Friction only transfers electrons between surfaces. The total charge of the two-object system remains zero.

Exam TipAlways state that charge is transferred, never created or destroyed.

Induced Charge Separation and Polarization

Even a neutral object can respond to a nearby charge. Induced charge separation occurs when an external charged object exerts electrostatic forces that rearrange the charge distribution inside a neutral system, producing polarization [a state where positive and negative charges are displaced to opposite sides].

In a conductor, free electrons are pushed or pulled by the external field. Bring a negative rod near a metal sphere: electrons in the sphere are repelled to the far side. The near side becomes positive; the far side becomes negative. The sphere's net charge is still zero.

In an insulator, electrons cannot flow freely. Instead, individual molecules distort slightly. The electron cloud in each molecule shifts toward or away from the external charge. This creates a weak surface charge on the insulator.

Because the near side of the polarized object has the opposite sign to the external charge, there is always a net attractive force. This is why a charged balloon sticks to a neutral wall.

Examiner InsightFRQs often ask why a charged rod attracts a neutral object. The scoring requires you to describe the charge separation and then explain that the closer opposite charges produce a stronger force than the farther like charges.

Exam TipState both steps — separation of charge, then net attraction due to distance difference.

Charge Transfer Between System and Surroundings

Any change in a system's net charge results from charge crossing the system boundary — not from charge appearing or disappearing inside the system. Charging typically involves the transfer of electrons, because protons are locked inside nuclei and cannot move between objects in ordinary processes.

When a negatively charged rod touches a neutral sphere, electrons flow from rod to sphere. The rod loses negative charge (becomes less negative). The sphere gains negative charge. The total charge of the rod-plus-sphere system stays constant throughout.

Net charge is constant unless there is a transfer of charge across the system boundary. If no electrons enter or leave a system, its net charge cannot change — even if the distribution of charge within it shifts.

MisconceptionStudents sometimes say "protons move to the other object." In solid materials, only electrons transfer. Protons remain fixed in atomic nuclei.

Exam TipAlways say "electrons transfer from A to B," never "positive charge moves from A to B."

Grounding a Charged Object

Grounding means electrically connecting a charged object to a much larger, approximately neutral system — typically the Earth. Earth is so large that any charge transferred to or from it produces no measurable change in Earth's charge.

When a negatively charged sphere is grounded, excess electrons flow from the sphere into Earth. The sphere becomes neutral because Earth absorbs the excess without noticeably changing its own charge. When a positively charged sphere is grounded, electrons flow from Earth into the sphere until the sphere is neutral.

Grounding is especially powerful when combined with induction. To charge a sphere by induction:

1. Bring a positive rod near a grounded metal sphere: electrons flow up from Earth into the sphere, attracted by the rod.
2. Remove the ground connection while the rod is still nearby.
3. Then remove the rod.

The sphere retains a net negative charge — it has been charged by induction.

Examiner InsightThe order of steps in charging by induction matters. Removing the ground before removing the rod traps the induced charge. If the rod is removed first, electrons return to Earth and the sphere stays neutral.

Exam TipOn FRQs, state the sequence explicitly — ground while rod is near, disconnect ground, then remove rod.