Tru Physics

Chapter 9: Current

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9.1 Introduction to Electric Current

Electric current is the flow of electric charge through a conducting medium, such as a wire or a circuit. The movement of charged particles, typically electrons, results in the transfer of energy and information through the medium. In this chapter, we will explore the properties and characteristics of electric current, as well as the factors that influence it.

Pylons (like the one pictured above) are so tall because they conduct electricity at high voltages and generally very high electric current.
Pylons (like the one pictured above) are so tall because they conduct electricity at high voltages. However, high voltages alone do not cause death. Generally, these high voltages correspond to currents much greater than 0.2 Amps. It is this high flow of electric charge which can cause shock and/or death. As they say: current kills, not voltage.

9.2 Definition of Current and Current Direction

The electric current (I) is defined as the rate at which charge flows through a given cross-sectional area. Mathematically, it is represented as:

I = \dfrac{\Delta Q }{\Delta t}

where

  • I is the current in amperes (A),
  • \Delta Q is the charge that flows through the area in coulombs (C),
  • \Delta t is the time interval in seconds (s).

In a conventional current flow, the current direction is considered to be in the direction of positive charge flow, from the positive terminal to the negative terminal of a voltage source.

Of course, the equation above can also be applied in the calculus limit as \Delta t goes to zero:

I = \displaystyle\lim_{\Delta t \to 0}\dfrac{\Delta Q}{\Delta t}=\dfrac{dQ}{dt}

where \frac{d}{dt} is the derivative with respect to time.

9.3 Ohm’s Law

Ohm’s Law is a fundamental relationship in electrical circuits that relates the voltage (V), current (I), and resistance (R) of a conductor. It is expressed as:

V = I R

where

  • V is the voltage in volts (V),
  • I is the current in amperes (A),
  • R is the resistance in ohms (\Omega).

Ohm’s Law is a critical tool for understanding the behavior of electric current in circuits and determining the values of voltage, current, and resistance when given two of the three quantities.

9.4 Types of Current

There are two types of electric current: direct current (DC) and alternating current (AC). In direct current, the charge flow is constant and in one direction. DC is typically produced by batteries and is used in many electronic devices. In alternating current, the charge flow changes direction periodically, usually following a sinusoidal pattern. AC is generated by power plants and is used in power transmission and distribution systems.

9.5 Factors Affecting Current Flow

Several factors affect the flow of electric current in a circuit. These include:

  1. Voltage: A higher voltage applied to a conductor will cause a higher current flow, as indicated by Ohm’s Law.
  2. Resistance: The current flow in a conductor is inversely proportional to its resistance, also based on Ohm’s Law.
  3. Temperature: As temperature increases, the resistance of a conductor typically increases, leading to a decrease in current flow.
  4. Material: Different materials have different conductivities, which affect their ability to carry electric current.

Chapter Summary

In this chapter, we explored the concept of electric current, its definition, and direction. We discussed Ohm’s Law and its importance in understanding electrical circuits, as well as the two types of current: direct and alternating. Additionally, we examined the factors that influence the flow of electric current in a circuit. Understanding these concepts is essential for studying electricity and its applications in physics and engineering.

Continue to Chapter 10: Resistance and Resistivity

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