Resistance in Electric Circuits

Science Department

David Ramirez

2008

Resistance

• Resistance is friction that electricity experiences while flowing thru something.

• Resistance (or the lack of resistance) is used in circuits to control the flow of current.

• Symbol :

Short Circuit

Short circuit

• Very low resistance of conductors.

• This example circuit connects the poles of the voltage source together using a simple piece of wire.

• High current !

• DANGER : High conversion of energy into heat ! Burns, fire ! (don’t mess with it!).

Building fire (example of short circuit consequence)

Short circuit (conclusion)

• In a short circuit the magnitude of the current can be very large.

• The release of energy is dramatic, usually in the form of heat : melts the metal, burns insulators, can start a fire, and injuries (burns) to the man.

Simple Circuit

Simple circuit

• Normally circuits are constructed to make practical use of the released energy in a manner as safe as possible.

• One typical and practical use of a circuit is for the production of electric lighting.

Light Bulb

Light bulb (incandescent lighting)

RESISTANCE

• Electrons encounter opposition when flowing thry the filament, due to friction.

• This phenomenon is known as RESISTANCE.

• Resistance depends on material, cross section, length.

Simple electric light circuit

Bulb

Inside the light bulb

• Conductors have low resistance.• Insulators have high resistance.• Resistance limits the amount of current flowing

thru the circuit for a given voltage of the power supply.

• In the light bulb, a relatively high amount of energy is dissipated due to the resistance of the filament, and manifests as heat – so high that light is emited (filament glows).

Open circuit

Open Circuit

Breaker (switch)

Closed Circuit

• Current flows through the switch.

• Continuity is restablished.

• Light bulb glows.

KEYWORDS

• CIRCUIT• RESISTANCE• LOAD• BREAKER• SWITCH• INCANDESCENSE• CURRENT• VOLTAGE• SHORT CIRCUIT• CONTINUITY

To be continued

Units of measure (Amp, Volt, Ohm)

Ohm’s Law

Types of circuits: Series – Parallel

David RamirezJanuary, 2008

Houston ISD ACP Program