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Current Electricity & Ohm's Law

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**Circuit Basics All electrical circuits require three elements.**

A source voltage, that is, an electron pump usually a battery or power supply. [ ENERGY IN] (2) A conductor to carry electrons from and to the voltage source (pump). The conductor is often a wire. [ENERGY TRANSFER] (3) A load or resistance. A point where energy is extracted form the circuit in the form of heat, light, motion, etc. [ENERGY OUT]

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**THE MOST BASIC ELECTRICAL CIRCUIT**

LOAD (RESISTANCE) [ENERGY OUT] ELECTRONS INTO LOAD ELECTRONS OUT OF LOAD CONDUCTOR CONDUCTOR ELECTRONS OUT OF SOURCE ELECTRON PUMP (SOURCE VOLTAGE) [ENERGY IN] ELECTRONS BACK TO SOURCE HIGHER ENERGY ELECTRONS LOWER ENERGY ELECTRONS

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**Potential Changes of Current in a Circuit**

Resistance (Potential Drop) Low Energy current High Energy current Voltage Source (Potential Rise) Low Energy current High Energy current

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**Potential Rise Across a Power Source**

volts current current Battery Electrons have More Energy Electrons have Less Energy Electrons get An energy boost

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**Potential Drop Across a Resistor**

volts current Resistor Electrons have Less Energy Electrons have More Energy Energy is lost In the resistor

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**MEASUREABLE QUANTITIES THAT CAN BE OBTAINED FROM AN ELECTRICAL CIRCUIT**

(1) VOLTAGE RISE – MEASURES THE ENERGY GIVEN TO ELECTRONS AS THEY LEAVE A VOLTAGE SOURCE. IT IS MEASURED IN VOLTS (+) (2) VOLTAGE DROP – MEASURES THE ENERGY LOST BY TO ELECTRONS WHEN THEY LEAVE A RESISTANCE. IT IS MEASURED IN VOLTS (-) (3) CURRENT – MEASURES THE FLOW RATE THROUGH A CONDUCTOR. IT IS MEASURED IN AMPERES (AMPS) (4)RESISTANCE – MEASURES THE OPPOSITION TO CURRENT FLOW THROUGH A CONDUCTOR OR RESISTOR IT IS MEASURED IN OHMS (ITS SYMBOL IS OMEGA )

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**ELECTRICAL QUANTITIES**

6.25 x 10 18electrons 1 coulomb 1 coulomb 1 second 1 amp = 1 coul / sec 1 volt = 1 joule / coul 1 joule 1 coulomb of charge

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**Electrical Meters volts Ammeters measure current in amperes**

and are always wired in series in the circuit. AMPS Voltmeters measure potential in volts and are always wired in parallel in the circuit. volts

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An actual Voltmeter

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An Actual Ammeter

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**Electrical Symbols A V battery + - junction wiring terminal voltmeter**

AC generator ammeter A Variable resistance resistance Variable capacitor capacitor

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**Relationships Among Electrical Quantities in a Circuit**

Measure electrical quantities in an electrical Circuit are related to eachother by: OHM’S LAW OHM’S LAW says, if the source voltage remains constant, increasing the resistance in a circuit will cause a decrease in current flow in that circuit. In mathematical terms it tells us that current flow is inversely proportional to resistance. In equation form it says: Voltage (V) = Current (I) x Resistance (R) or V(in volts) = I (in amps) x R (in ohms)

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**Ohm's Law V = I R Potential In volts (joules / coul) Current**

In amperes (coul / second) Resistance In ohms (volts / amp) Drop across a resistance Current passing Through the resistor

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**Voltage vs Current for a Constant Resistance**

(volts) Current (I) in amps I v The slope of the Line gives the resistance R = v /I or rearranged v = I x R

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**Voltage Sources and Internal Resistance**

All voltage sources contain internal resistance, that is resistance that is part of the voltage producing device itself which cannot be eliminated. The voltage that the device (battery for example) could produce if no internal resistance was present is called its EMF. EMF stands for electromotive force – the force that moves the electrons. The useable voltage which is available to the circuit after the internal resistance consumes its share of the EMF is called The terminal voltage.

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**Electromotive Force (EMF) and Terminal Voltage**

rise Voltage drop conventional current flow EMF of Source Internal resistance + - Terminal voltage

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**terminal voltage = EMF (+) + Internal resistance loss (-)**

Since voltage rise across a source or Voltage drop across a resistance can be Calculated by Ohm’s Law V= IR And the above equation becomes V terminal = EMF – I x R internal Note that if R internal is very small then a large Percentage of the EMF is available to the circuit. Also note that if I, the current is very large then a large percentage of the EMF is consumed within the battery itself which can cause overheating and failure. This is generally the result of a short circuit.

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**Internal Resistance and EMF**

Net Voltage Rise of cell (Terminal Voltage volts current current Battery Ri EMF -V +V Voltage drop Due to internal resistance Voltage rise Due to EMF

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THE END

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Unit 7: Electricity and Magnetism

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