http://www.ck12.org Chapter 17. Circuits
17.2 Ohm’s Law
Objectives
The student will:
- Understand how conventional current is defined.
- Understand what is meant by electrical resistance.
- Understand how to solve problems using Ohm’s law.
Vocabulary
- Ohm’s law:R=VI, meaning that resistance is equal to the proportion of velocity to current.
- resistance: The effect of the movement of electrons through a wire, which is not smooth.
Introduction
We discussed voltage and electrical current earlier. At that time, we took for granted that a wire connected to the
terminals of a battery should conduct an electrical current. As we will see, there is a very important relationship
between currentIand potential differenceVwhich we shall shortly discuss.
We begin with a few brief statements concerning batteries and current flow.
Batteries and Current
We often use the flow of water to model some of the properties of electrical current, although they are two very
different things. Water may be stored in a tank just as electrical charge (and electrical energy) may be stored in a
battery (or a capacitor).
The first battery was constructed in 1800 by the Italian physicist Allessandro Volta (1745-1827), Figure17.4. This
was the first device that provided a continuous flow of electricity. Prior to this, the only type of electrical current
created in the laboratory was the rapid discharge of an electrical spark. Volta’s invention sparked the beginning of a
new age.
Volta’s basic battery construction has not changed in the last two hundred years. Batteries are made of two different
metals which react with a liquid called an electrolyte, and cause one of the metals to become positively charged and
the other to become negatively charged. We refer to the positive and negative parts of the battery as theterminals
of the battery. If one end of a conducting wire is connected to the positive terminal of a battery and the other end
is connected to the negative terminal of the battery, a direct current will exist in the wire because of the potential
difference between the terminals. The symbol DC is often used when referring to direct current.