Charge & Coulomb's Law

What this investigation asks

An electrically neutral object can be attracted by a charged object without already carrying the same kind of net charge as a conductor that has been charged by contact. This investigation separates polarization, electron transfer, and electrostatic force measurements. Begin with a charge account for the rod, cloth, detector, and ground. In the course's solid-material model, electrons move while nuclei remain in place. A separated charge pattern can produce attraction, so observing attraction alone does not uniquely establish the net charge sign of an unknown object.

Design and measure the comparison

Compare charging by rubbing, contact, and induction while retaining the order of grounding and removal steps. Ground connects the apparatus to a large charge reservoir; it does not guarantee that a grounded object is always uncharged under all surrounding conditions. Record which connections are present at each stage and follow the electron-transfer account. For the torsion-balance comparison, use the separation between sphere centers rather than the gap between their surfaces. An inconsistent distance definition can distort the inferred distance dependence even when the angular reading is measured carefully.

Connect the record to the model

Change charge product and distance in separate comparisons. Coulomb's relationship predicts force proportional to the charge product and inversely proportional to squared center separation for the specified point-like geometry. Compare a force proxy or calibrated balance response with those predictions across several settings. Retain whether the force is attractive or repulsive, since magnitude alone omits the directional interaction. Superposition and equal-opposite forces provide further checks: the two objects exert equal force magnitudes on one another even when their charges or masses differ.

Interpret the evidence and its limits

Keep the model's charging and geometry limits visible. Charge leakage, finite sphere size, and the mapping from a torsion angle to force affect the Real comparison differently. A proportionality inferred from one short range is weaker evidence than a controlled series that distinguishes candidate relationships. Ordinary macroscopic charge is represented as a multiple of elementary electron charge in this activity, but a continuous-looking instrument reading does not resolve individual transfer events. Explain which part of the account describes the microscopic transfer and which part is a measured macroscopic force or detector response.

Reference conditions and quantities

At the specified charging voltage of 6.0 kV, the reference sphere charge is 1.2666667 × 10⁻⁸ C. The value follows the course's apparatus relation between voltage and charge. Retain that relation and geometry when using the charge in a force calculation; an applied voltage alone does not specify every object's charge without its capacitance or charging model.

Use the record to test your explanation

Write a prediction before collecting the comparison, naming the quantity that should change and the condition you will alter. Keep a reference run and vary one factor at a time when the question calls for a controlled test. Record the selected apparatus settings, units, and measurement interval. Where an activity supplies Ideal and Real modes or seeded variation, retain that identity with each record rather than mixing incompatible runs in one calculation.

Separate an instrument reading or counted observation from the model relationship used to explain it. Keep raw values alongside derived results so another student can reconstruct the argument. A graph needs labeled axes and a stated comparison range. If a result differs from a reference, inspect the conditions, rounding, and measurement procedure before changing the explanation.

For a classroom discussion, compare claims supported by the same evidence and identify an additional measurement that would distinguish competing accounts. Finish with a claim, the relevant observations, and reasoning tied to this course's model. State a specific limitation instead of assigning a general error label to every disagreement.

Related investigations

Connect force and test charge in Electric Fields & Equipotentials, then inspect stored charge in Capacitors. Distinguish a net-charge transfer from polarization and from the potential difference measured across an electrical arrangement.