Whenever there is movement of charge carriers in a substance, there is an electric current. Current is measured in terms of the number of electrons or holes passing a single point in 1 second.

A great many charge carriers go past any given point in 1 second, even if the current is small. In a household electric circuit, a 100-watt light bulb draws a current of about six quintillion (6 followed by 18 zeros) charge carriers per second. Even the smallest bulb carries quadrillions (numbers followed by 15 zeros) of charge carriers every second. It is impractical to speak of a current in terms of charge carriers per second, so it is measured in coulombs per second instead. A coulomb is equal to approximately 6,240,000,000,000,000,000 electrons or holes. A current of 1 coulomb per second is called an ampere, and this is the standard unit of electric current. A 100-watt bulb in your desk lamp draws about 1 ampere of current.

When a current flows through a resistance—and this is always the case because even the best conductors have resistance—heat is generated. Sometimes light and other forms of energy are emitted as well. A light bulb is deliberately designed so that the resistance causes visible light to be generated. Electric current flows at high speed through any conductor, resistor, or semiconductor. Nevertheless, it is considerably less than the speed of light.

What you are expected to learn

At the end of this module, you should be able to:

• State the two laws of electrostatic charges.

• Define coulomb.

• Identify the unit used to measure current flow.

• Define the relationship of amperes, coulombs, and time through a formula.

• Describe how current flows in a circuit.

• Describe how electrons travel in a conductor.

How to learn from this module

Going through this module can be both a fun and a meaningful learning experience. All you need to do is make use of your time and resources efficiently. To do this, here are some

tips for you:

1. Take time in reading and understanding each lesson. It is better to be slow but sure than to hurry finishing the module only to find out that you missed the concepts you are supposed to learn.

2. Do not jump from one chapter to another. Usually, the lessons are arranged such that one is built upon another, hence an understanding of the first is essential in comprehending the succeeding lessons.

3. Be honest. When answering the test items, do not turn to the key to correction page unless you are done. Likewise, when performing experiments, record only what you have really observed.

4. Safety first. Perform the experiments with extra precaution. Wear safety gears whenever necessary.

5. Don’t hesitate to ask. If you need to clarify something, approach your teacher or any knowledgeable person.

What to do before (Pretest)

Lesson 3: Current

• Laws of electrostatic charges: like charges repel, unlike charges attract.

• Electrical charge (Q) is measured in coulombs (C).

• One coulomb is equal to 6.24 x 10^18 electrons.

• An electric current is the slow drift of electrons from an area of negative charge to an area of positive charge.

• Current flow is measured in amperes.

• One ampere (A) is the amount of current that flows in a conductor when one coulomb of charge moves past a point in one second.

• The relationship between current, electrical charge, and time is represented by the formula: I = Q/T

• Electrons (negative charge) represent the charge carrier in an electrical circuit.

• Hole movement (positive charge) occurs in the opposite direction to electron movement.

• Current flow in a circuit is from negative to positive.

• Electrons travel very slowly through a conductor, but individual electrons move at the speed of light.

ELECTRIC CHARGES

Charge carriers, particularly electrons, can build up, or become deficient, on things without flowing anywhere. You’ve experienced this when walking on a carpeted floor during the winter, or in a place where the humidity was low. An excess or shortage of electrons is created on and in your body.

You acquire a charge of static electricity. It’s called “static” because it doesn’t go anywhere. You don’t feel this until you touch some metallic object that is connected to earth ground or to some large fixture; but then there is a discharge, accompanied by a spark.

Current in Circuits

Current in a Series Circuit

Current in a Parallel Circuit

EFFECTS OF CURRENT FLOW

Current flowing through a material will have some effects on the material. Two of the most common effects are the following:

Heating effect - Heat is produced when free electrons move through a material in a current flow. This is because the speeding electrons would inadvertently bump into other free electrons in the material. Such collisions release heat energy.

Electromagnetic effect - Free electrons are charged particles and thus their interaction with other charges produce electric fields. When a free electron is in motion it creates a changing electric field. Whenever an electric field is produced, it creates a magnetic field. Thus we say that current flow always creates magnetic lines of force or magnetic flux.

CURRENT FLOW CONVENTIONS

There are two conventions currently being used to describe the direction of current flow. The two conventions of current flow are:

Electron flow convention - The flow of electrons around the circuit is opposite to the direction of the conventional current flow being negative to positive..

Conventional current flow - The flow of positive charge around a circuit, being positive to negative.

Virtual Laboratory

Self Check