The Goal of This Unit Is to Use the Concept of Electric Charge to Develop an Understanding

Electric Energy – An Overview of the Unit

The goal of this unit is to use the properties of electric charge to develop an understanding of electric force and electrical energy. Charge, electric force and electrical energy will be used to develop a model that enables students to understand the rules used to build simple electric circuits and to analyze the properties of these circuits.

How will we discuss electric charge without talking about electrons, protons and atomic structure?

It is inappropriate to be discussing atoms and molecules at the 6th grade level, much the less the electrons and protons that make up the atoms. Electric charge will be presented as a physical quantity that is present in the tiny particles that make up the objects and substances on Earth and beyond. The important properties of matter and charge presented in this unit are:

Objects (such as rocks, trees, buildings and people) and substances (such as water, air and clay) are made of huge numbers of tiny particles. [The Particle Model is introduced in the 7th grade. We do not need the benefits of this model to develop the concept of charge.]

There are two kinds of electric charges in nature. One kind is called positive (+) charge and the other is named negative (-) charge. Electric charge is found on the tiny particles that make up matter. These particles are much too small to see but each particle contains many electric charges. [We do even mention atoms, molecules, electrons and protons.]

There are huge numbers of electric charges in any object that can be seen with the naked eye, even objects as small as a grain of sand.In general the number of positive charges in an object equals the number of negative electric charges, making the object electrically neutral.

Negative charges can be easily removed from some substances and added to others, but positive charges are extremely difficult to relocate. When an object has an excess or deficiency of negative charges it is called electrically charged.

What will we expect our students to understand about electric force?

Our discussions of electric forces will be qualitative in nature. It will be important that students understand that electric forces are exerted by electric charges on other electric charges. Large collections of electric charge can result in objects exerting large electric forces on each other.

Electric charges exert unique forces, called electric forces, on each other. Positive charges attract negative charges but repel other positive charges. Similarly, negative charges attract positive charges but repel other negative charges.

Objects that have an excess of (+) charge or (-) charge (charged objects) will exert electric forces on each other.

The charges in neutral objects do not cancel each other out. Neutral objects do not exert electric forces on each other because the electric forces exerted by all the charges in neutral objects balance each other.

It is possible for an electrically charged object to attract or repel an electrically neutral object by redistributing the negative charges in the uncharged object.

How is electrical energy introduced in this unit?

Electrical energy is the mechanism that we will use to build a better understanding of electrical circuits. A model will be presented to the students through which they will develop an understanding of the circuitry ‘rules’ learned in the 4th grade. They will also use this model to explain why devices in series circuits behave differently from devices in parallel circuits. The model is based on the transfer of the electrical energy that is stored in the batteries and will be the basis for more sophisticated analysis in high school.

Do we need to discuss the chemistry that takes place in batteries when investigating circuits?

Actually, we need to avoid discussing chemistry and chemical reactions anywhere in this unit. The reactions that take place in a battery release stored chemical energy. This energy is a form of electrical energy, and we avoid the ‘chemistry’ details completely by referring to a battery as a source of stored electrical energy.

Our circuit model is developed to help students understand how electrical energy is transferred throughout the circuit. This model recognizes that electric charges carry electrical energy to the light bulbs and motors in the circuits.

Theelectrical energy is supplied to the charges by the batteries.

The charges flow through the wires carrying electrical energy to the devices in the circuit.

When the charges deliver electrical energy to the light bulbs, the bulbs change the electrical energy into light energy and heat energy.

The electrical energy delivered to the motors is changed into the energy of motion and heat energy.

The charges pass through the bulbs and motors and return through the wires of the circuit to the batteries to ‘pick-up’ more electrical energy.

The charges then leave the battery and carry more electrical energy to the devices in the circuit.

There has been some confusion about which way the charges flow through the circuit. Do they flow from the (+) end of the battery and return through the circuit to the (-) end, or visa versa?

Which way do the charges flow? This question seems to have more than one answer, no wonder it confuses students. At higher levels of instruction, conventions have been adopted that seem to contradict what students are being taught about charges in middle school. High school and college textbooks have positive charges moving through circuits, a physical impossibility. How do we avoid confusing the students or inadvertently prepare them up to be confused in later studies? We can use our model.

It is still important to emphasize that only negative charges are free to move, but as we will see, the sign of the charges moving through the circuit is unimportant.

The battery will be considered to be a source of electrical energy, rather than a source of electric force (EMF).

The charges flow through the battery and while in the battery ‘pick-up’ electrical energy.

The (-) terminal of the battery is the ‘low-energy’ end of the battery. Charges enter this end of the battery with minimal energy and thenpass through the battery. When the charges leave the (+) ‘high-energy’ terminal of the battery they are energized and ready to enter the circuit.

The charges deliver electrical energy to the devices in the circuit, then return to the battery to ‘pick-up’ more energy. Focusing on the energy flow minimizes the importance of which way the charges move through the circuit.

By following the energy flow through the circuit the sign of the charge and the direction of motion through the circuit are not important. This approach also helps to build understanding of energy flow in physical systems and represents the approach that will be used to address the behavior of other physical systems in middle school and high school science.