When a glass rod is rubbed with silk, it becomes positively charged. Which statement best explains why it attracts a negatively charged piece of ebonite?

Opposite charges attract, causing the glass rod to pull the ebonite toward it

The friction creates heat that expands the air, pulling the objects together

The glass rod gains electrons from the ebonite, creating a magnetic field

Both objects have the same type of charge, leading to mutual attraction

In a metallic conductor, which particles constitute the mobile charge carriers that constitute electric current?

Free electrons moving from the negative to the positive terminal

Positive ions drifting toward the negative terminal

Neutral atoms vibrating in place

Protons moving through the lattice toward the positive terminal

A circuit contains a resistor and an ammeter in series. If the ammeter is set to a higher range than necessary, what is the most likely effect on the measured current reading?

The reading will be less precise due to a larger absolute uncertainty

The measured current will be higher than the actual current

The ammeter will display a negative current value

The circuit will become open and no current will flow

According to Kirchhoff's current law (node rule), what must be true at a junction where three branches meet?

The sum of currents entering equals the sum of currents leaving the node

The total voltage across the node is zero

The resistance of each branch must be identical

The product of currents entering equals the product of currents leaving

If a solution electrolyte contains both cations and anions, how do the ions move when a voltage is applied across two electrodes?

Cations move toward the positive electrode, anions toward the negative electrode

Cations move toward the negative electrode, anions toward the positive electrode

Ions remain stationary while electrons flow through the external circuit

All ions move toward the positive electrode regardless of charge

Electric Charge and Current Fundamentals | Free quiz

Understanding Electric Charge and Current Fundamentals

Electric charge and current are foundational concepts in physics that explain how objects interact and how electricity flows through circuits. This course breaks down the key ideas tested in a typical quiz, providing clear explanations, real‑world examples, and important formulas. By the end of the lesson, you will be able to answer questions about static electricity, charge carriers in metals, measurement accuracy, Kirchhoff’s current law, and ion movement in electrolytes.

1. Static Electricity: Why Opposite Charges Attract

When a glass rod is rubbed with silk, electrons are transferred from the glass to the silk. The glass loses electrons and becomes positively charged, while the silk gains electrons and becomes negatively charged. The attraction between the positively charged glass rod and a negatively charged piece of ebonite can be explained by a simple rule:

Opposite charges attract. The electric field produced by the positive charge on the glass rod pulls the negative charge on the ebonite toward it.

This principle is central to many electrostatic phenomena, such as the operation of a Van de Graaff generator or the way a balloon sticks to a wall after being rubbed.

2. Mobile Charge Carriers in Metallic Conductors

In metals, the atoms are arranged in a lattice of positively charged ions surrounded by a "sea" of free electrons. These electrons are not bound to any particular atom and can move freely throughout the material. When a potential difference is applied across a conductor, the following occurs:

Free electrons drift from the negative terminal toward the positive terminal. This drift constitutes the electric current.
The ions remain essentially stationary, providing the structural framework of the metal.

Understanding that free electrons are the mobile charge carriers helps explain why metals are excellent conductors and why current direction is conventionally defined opposite to electron flow (from positive to negative).

3. Measuring Current Accurately: Ammeter Range Selection

An ammeter measures the flow of electric charge per unit time (current). Most analog and digital ammeters have multiple ranges to accommodate different current magnitudes. Selecting a range that is too high for the expected current has a specific effect:

Reduced precision. While the reading will still be close to the true value, the absolute uncertainty (e.g., ±0.01 A) becomes a larger fraction of the measured current, making the result less precise.
There is no systematic error that makes the reading consistently higher or lower; the main issue is the loss of resolution.

For accurate measurements, always choose the lowest range that safely exceeds the anticipated current.

4. Kirchhoff’s Current Law (KCL) – The Node Rule

KCL is a fundamental principle for analyzing complex circuits. It states:

The sum of currents entering a junction equals the sum of currents leaving the junction.

This law reflects the conservation of electric charge: no charge can accumulate at a node in a steady‑state circuit. It is often written mathematically as:

∑ I_in = ∑ I_out

Applying KCL allows you to set up equations for unknown currents, simplify circuit analysis, and verify that your solutions obey charge conservation.

5. Ion Movement in Electrolytic Solutions

When a voltage is applied across electrodes immersed in an electrolyte, both cations (positive ions) and anions (negative ions) respond to the electric field:

Cations migrate toward the positive electrode (the anode).
Anions migrate toward the negative electrode (the cathode).

This movement constitutes the electric current within the solution and is the basis for processes such as electroplating, batteries, and corrosion.

Key Takeaways

Opposite electric charges attract; this explains static interactions like a charged glass rod pulling a negatively charged object.
In metals, free electrons are the mobile charge carriers that move from negative to positive terminals.
Choosing an ammeter range that is too high reduces measurement precision due to larger absolute uncertainty.
KCL requires that the total incoming current equals the total outgoing current at any circuit node.
In electrolytes, cations move toward the positive electrode and anions toward the negative electrode when a voltage is applied.

Electric Charge and Current Fundamentals