current passes through the filament, electrons lose energy in collisions with the atoms

of the filament, and the filament’s temperature increases. It becomes hot (up to

3000°C), and energy leaves the filament in the forms of heat and light.

Although tungsten is an effective filament, when it is hot the tungsten vaporizes and the

filament becomes thinner. This increases its resistance, which means it shines less

brightly. Eventually, when it “burns out”, it becomes so thin and brittle that it ruptures.

The flashlight contains all the essential elements of a circuit. It has a source of energy,

the batteries. They create a potential difference that causes electrons to move. As the

electrons move through the light bulb, they encounter resistance, which causes this

resistor to dissipate energy.

The current flows only in one direction in this circuit, which makes it a direct current

circuit. Direct current circuits are the topic of this chapter. We study these circuits

primarily when the current has reached a steady state í a constant flow í not in the

brief moments when the current changes, such as immediately after the flashlight is

switched on or off.

Scientists and engineers use symbols to represent components in circuit diagrams, as

shown in Concept 3. The thin black lines represent wires. The battery and resistance

symbols are labeled in the circuit. The switch is “on”, so it is in its closed position in this

diagram. The red arrow indicates the direction of flow of the conventional current.

Circuits usually contain:

An energy source

A load resistance

Wires connecting it all in a loop

A switch

Drawing electric circuits

Use circuit diagrams with symbols

27.2 - Electromotive force

Electromotive force (emf,Ǜ):

Maximum potential

difference from an energy

source such as a battery.

A potential difference applied across a conductor

causes a current to flow. Common devices such as a

flashlight need a continuing source of potential

difference í an emfí so that current will keep flowing

and the flashlight’s bulb will stay illuminated.

A battery often supplies the emf. The symbol for emf

isǛ. Like any potential difference, an emf is

measured in volts. There are many sources of emf:

electric generators, solar photovoltaic cells and so on. Even living creatures can be an

emf source. Humans rely on emfs generated in the body to cause electric currents in

nerves.

The term “electromotive force” is misleading because an emf is not a force. It is a

potential difference and its unit is the volt. It is a well-established term in physics,

however, and we will use it too. In any case, since we typically write it in its abbreviated

form as emf, you should not too often be confused by seeing the word “force”.

A battery is the typical emf source for direct current circuits. Chemical reactions within

the battery cause one terminal of the battery to be positively charged, and the other to

be negatively charged. These terminals are marked with plus (+) and minus (í) signs.

Batteries are classified by their emf. A typical battery used in a flashlight has an emf of

1.5 volts, while a car battery has an emf of 12 volts. If a battery has an emf of 1.5 volts,

this means that the electric potential of its positive terminal is 1.5 volts higher than that

of its negative terminal.

Sometimes batteries are referred to as “charge pumps.” They increase the potential energy of the charge flowing through the circuit. The unit of

Batteries are a common source of emf.

emf (Ǜ)

A potential difference

(^490) Copyright 2000-2007 Kinetic Books Co. Chapter 27