Voltage, Current & Resistancemserdelyiscience.weebly.com/.../5.6_voltage_current__resistance.pdf ·...

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Understanding Circuits

Transcript of Voltage, Current & Resistancemserdelyiscience.weebly.com/.../5.6_voltage_current__resistance.pdf ·...

Page 1: Voltage, Current & Resistancemserdelyiscience.weebly.com/.../5.6_voltage_current__resistance.pdf · 4 Factors affecting internal resistance: 1. Type of Material How freely electrons

Understanding Circuits

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Electric current is the rate of electron flow past a specific point in a circuit.

Unit: ampere (A)

Symbol: I

Instrument: ammeter Andre‐Marie Ampere(1775‐1836)

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Do not copy this page Recall: an ammeter must be connected in series

This ensures all the electrons that flow through the lamp also flow through the ammeter.

A reading of 1 A through a circuitrepresents the flow of 6.2 x 1018

electrons flowing through the circuit each second.

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Safety with current: Homes Circuit breakers & fuses used to protect homes from excess current

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Safety with current: Human Body Your brain controls your body with electrical signals 0.001 A through body  tingling 0.050 ‐ 0.15 A muscle contractions / convulsions▪ Aka “let‐go” threshold

1.0 – 4.3 A  stops heart Wall outlets deliver 15 A

CURRENT KILLS!

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Potential difference (voltage) is the difference in electrical potential energy per unit charge measured at 2 points.

aka – electrical potential

Unit: volt (V)

Symbol: V

Instrument: voltmeter

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Potential difference between 2 ends of a battery/dry cell

Electrons leave negative terminal with electrical potential energy that can operate the motor

When electrons return to positive terminal they have less electric potential energy because some was used to run the motor

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Do not copy this page Recall: a voltmeter must be connected in parallel

This is because voltage is relative to 2 points A drop of voltage occurs across a load / energy source

Voltage across battery would be the same as the voltage across the load (because it is the only load)

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Electrical resistance is the opposition to the movement of electrons as they flow through a circuit

Unit: ohm (Ω)

Symbol: R

Instrument: ohmmeter

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All materials have some internal resistance

↑resistance  leads to↓ current & the warmer it becomes

b/c as electrons bump into atoms in material electrical energy is converted to thermal energy

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4 Factors affecting internal resistance: 1. Type of Material▪ How freely electrons can move within a material▪ Copper & Silver very low resistance

2. Cross‐Sectional Area▪ Thicker = less resistance b/c there is more room

3. Length▪ Longer = increased resistance b/c electrons travel through more material

4. Temperature▪ Resistance increases with temperature b/c more collisions

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An ohmmeter must be connected in parallel

Circuit must be turned off to use ohmmeter. It provides own electric current through the load

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Resistors in Circuits Resistors reduce electrical current in circuits Ceramic & lightweight carbon

Ex. volume controls, dimmer switches

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Read lab on page 567 Construct the observation table

Be ready to complete lab tomorrow