State that charge is measured in coulombs

4.2.1 Electric Charge

Learning Objectives

• State that electric charge is measured in coulombs (C).
• Identify the two types of charge and describe the forces between them.
• Explain how static charge can be produced and detected using simple experiments.
• Understand that frictional charging involves the transfer of electrons.
• Define an electric field and state its direction.
• Relate macroscopic charge (C) to the elementary charge \(e\).
• Use the formula \(Q = I t\) to calculate charge transferred.

What is Electric Charge?

Electric charge is a fundamental property of matter that determines how a body interacts with electric and magnetic fields.

Types of Charge

• Positive charge – carried by protons.
• Negative charge – carried by electrons.

Force Between Charges

• Like charges ( + /+ or – /– ) repel each other.
• Opposite charges ( + /– ) attract each other.

Producing and Detecting Static Charge

Charging by Friction

When two different materials are rubbed together, electrons are transferred from one surface to the other. The body that loses electrons becomes positively charged; the body that gains electrons becomes negatively charged.

• Classic examples:
  • Rub a glass rod with silk → glass becomes positive, silk negative.
  • Rub a plastic rod with wool → plastic becomes negative, wool positive.

Detecting Static Charge

• Electroscope: The metal leaves diverge when the device is charged.
• Balloon‑paper demo: A charged balloon attracts small pieces of paper, showing the presence of static charge.

Safety Tip

Static discharges can give a mild shock. Avoid touching metal objects with both hands while working with highly charged bodies, and keep flammable materials away from strong static fields.

Unit of Charge

The SI unit of electric charge is the coulomb, symbol C. One coulomb is the amount of charge transferred by a constant current of one ampere flowing for one second.

\( Q = I \times t \)

• \(Q\) – charge (coulombs, C)
• \(I\) – current (amperes, A)
• \(t\) – time (seconds, s)

Relation to the Elementary Charge

The charge of a single electron (or proton) is the elementary charge \(e\):

\( e = 1.602 \times 10^{-19}\ \text{C} \)

Thus one coulomb contains approximately

\( 1\ \text{C} = \dfrac{1}{e} \approx 6.242 \times 10^{18}\) elementary charges.

Electric Field (Supplementary)

An electric field is a region of space in which a charge experiences a force. It is represented by the symbol **E** and is defined by the relationship

\( \mathbf{F} = q\,\mathbf{E} \)

where \( \mathbf{F} \) is the force on a charge \( q \). The direction of the electric field is the direction of the force that would act on a **positive** test charge.

Key Points

• Charge is expressed in coulombs; multiples such as mC, µC, kC are also used.
• Electrons carry negative charge (‑), protons carry positive charge (+).
• Frictional charging involves the transfer of electrons.
• Static charge can be created by friction and detected with simple apparatus.
• The quantitative link between charge, current and time is given by \(Q = I t\).
• An electric field is a region where a charge feels a force; its direction is that of the force on a positive charge.

Summary Table

Worked Example

Problem: Calculate the charge transferred when a current of 2.5 A flows for 8 s.

1. Write the formula: \( Q = I \times t \).
2. Insert the values: \( Q = 2.5\ \text{A} \times 8\ \text{s} \).
3. Calculate: \( Q = 20\ \text{C} \).

Therefore, 20 coulombs of charge have been transferred.

Suggested Diagram for the Lesson