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Resistance in Electric Circuits
Science Department
David Ramirez
2008
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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 :
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Short Circuit
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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!).
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Building fire (example of short circuit consequence)
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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.
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Simple Circuit
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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.
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Light Bulb
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Light bulb (incandescent lighting)
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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.
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Simple electric light circuit
Bulb
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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).
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Open circuit
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Open Circuit
Breaker (switch)
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Closed Circuit
• Current flows through the switch.
• Continuity is restablished.
• Light bulb glows.
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KEYWORDS
• CIRCUIT• RESISTANCE• LOAD• BREAKER• SWITCH• INCANDESCENSE• CURRENT• VOLTAGE• SHORT CIRCUIT• CONTINUITY
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To be continued
Units of measure (Amp, Volt, Ohm)
Ohm’s Law
Types of circuits: Series – Parallel
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David RamirezJanuary, 2008
Houston ISD ACP Program