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**Science 9 : Introduction to Current Electricity**

Electric Current Part 2 Science 9 : Introduction to Current Electricity Page

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**What is an Electrical Current?**

Science 9 : Introduction to Current Electricity What is an Electrical Current? An electrical current is defined as a flow of positive charges from a positive electrode to a negative electrode. Page

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**What is an Electrical Circuit?**

Science 9 : Introduction to Current Electricity What is an Electrical Circuit? An electrical circuit is a complete pathway that allows positive charges (actually electrons) to flow. Page

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**Parts of Electrical Circuits**

Science 9 : Introduction to Current Electricity Parts of Electrical Circuits Most electrical circuits have four main components: a power source to make a current flow, a path(s) for current to move through, a device (load) that runs off the energy that the current gives it and a switch to start and stop current flow. Page

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**Symbols for Electrical Circuits**

Science 9 : Introduction to Current Electricity When drawing electrical circuits, symbols are used to represent parts of the circuit. These symbols need to be memorized. Page

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**Science 9 : Introduction to Current Electricity**

Sketch a Circuit Science 9 : Introduction to Current Electricity From the picture make a circuit diagram. Page

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**Science 9 : Introduction to Current Electricity**

Sketch a Circuit Science 9 : Introduction to Current Electricity From the picture make a circuit diagram. Page

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**Science 9 : Introduction to Current Electricity**

Sketch a Circuit Science 9 : Introduction to Current Electricity From the picture make a circuit diagram. Page

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**Measuring an Electrical Current**

Science 9 : Introduction to Current Electricity An electrical current is measured with a device called an ammeter. This device records the number of charges passing through it per second. Specifically, a current of 1 ampere (amp) is a flow past a point of 1 coulomb of charge per second (1 A = 1C/s). The ampere is named in honor of Andre-Marie Ampere who investigated electrical current. Page

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**What is a Coulomb of Charge?**

Science 9 : Introduction to Current Electricity A coulomb is the amount of charge carried by x electrons or protons. It is a – x 1018 electrons or a x 1018 protons. The unit of coulomb is named in honor of Charles-Augustin de Coulomb who investigated many laws of electrical charges. Page

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**Electrical Current Formula**

Science 9 : Introduction to Current Electricity I = Q/t (I is current, Q is charge and t is time) 1 A = 1 C/s (A is amps of current, C is coulombs of charge and s seconds of time) Page

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**Electrical Current is Measured by an Ammeter**

Science 9 : Introduction to Current Electricity An ammeter measure electrical current. It is placed in line so that all the electrical current passes through it. Page

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**Science 9 : Introduction to Current Electricity**

Voltage, The “Push” Behind or Energy Carried by the Charge (Current) Moving Science 9 : Introduction to Current Electricity Voltage is defined as the energy carried per unit of charge. It is the energy in Joules (J) per Coulomb (C) of charge. V = E/Q, 1 Volt = 1 Joule/Coulomb Page

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**Measuring Voltage Rise and Drop (Voltmeter)**

Science 9 : Introduction to Current Electricity Since voltage is a potential difference between two regions, voltage is measured with a voltmeter at two points in an electrical circuit to see the difference in voltage at these two points. If the voltage is measured across the power source, it will be a voltage (energy) rise. If the voltage is measured across a load, it will be a voltage (energy) drop. Page

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**Measuring Voltage Rise and Drop (Voltmeter)**

Science 9 : Introduction to Current Electricity Since voltage is a potential difference between two regions, voltage is measured with a voltmeter at two points in an electrical circuit to see the difference in voltage at these two points. If the voltage is measured across the power source, it will be a voltage (energy) rise. If the voltage is measured across a load, it will be a voltage (energy) drop. Page

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**Ammeter and Voltmeter : Different Connections**

Science 9 : Introduction to Current Electricity An ammeter is placed directly in the current flow to measure it while the voltmeter is placed with connections at two different locations to measure the voltage difference at these two points. Page

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**Electrical Resistance : Ohms**

Science 9 : Introduction to Current Electricity Electrical Resistance is the property of any material that slows the flow of current through it. There is resistance to current flow because current bumps up against atoms and bounces back before resuming its flow. Page

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**Electrical Resistance : Different for Different Substances**

Science 9 : Introduction to Current Electricity Some substances have high resistance (insulators). Some metals have higher resistance than others (nichrome is higher than aluminum which is higher than copper) Copper is commonly used in electrical circuits because it has very low electrical resistance. Page

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**Electrical Resistance : Affected by a Conductor’s Thickness**

Science 9 : Introduction to Current Electricity The thicker a wire, the lower its electrical resistance, just like it is easier to suck up a milkshake through a thicker straw than a thinner straw. Page

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**Electrical Resistance : Greater With Longer Wires**

Science 9 : Introduction to Current Electricity Electrical Resistance : Greater With Longer Wires The longer a wire, the greater its electrical resistance since the current bumps into more atoms along its way, slowing its progress. Page

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**Resistance of Wires and Loads**

Science 9 : Introduction to Current Electricity In a circuit, the electrical resistance of wires is usually very low (so that energy is not lost in the wires) but the electrical resistance of the load(s) is high because they use up the most of the energy that the charge is carrying. Page

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**Electrical Resistance Produces Heat**

Science 9 : Introduction to Current Electricity The more electrical resistance a wire has, the more heat it produces. Devices that produce heat (like toaster elements or stove elements) use conductors with high electrical resistance. Resistance takes energy from the current and turns it into heat. Page

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**Measuring Electrical Resistance : Ohms**

Science 9 : Introduction to Current Electricity Electrical resistance is measured in ohms (Ω), a unit named in honor for Georg Ohm who made many discoveries about electrical circuits. Page

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**Science 9 : Introduction to Current Electricity**

Resistors Science 9 : Introduction to Current Electricity A resistor is a device placed in an electrical circuit to control the flow (amperes) of current. Its symbol is shown below right. Page

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**Science 9 : Introduction to Current Electricity**

Resistor Colour Code Resistors come with colour bands and these indicate what resistance the manufacturer made these with. Page

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**Find The Resistor’s Resistance**

Science 9 : Introduction to Current Electricity Page

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**Science 9 : Introduction to Current Electricity**

What is its Resistance? Science 9 : Introduction to Current Electricity Page

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**Science 9 : Introduction to Current Electricity**

What is its Resistance? Science 9 : Introduction to Current Electricity Page

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**Ohm’s Law : Relation Between Volts Amps and Resistance**

Science 9 : Introduction to Current Electricity In an electrical circuit, the more voltage (“push”) applied to a current, the more current will flow and the more resistance in the conductor, the less current will flow. R = V / I Page

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**Science 9 : Introduction to Current Electricity**

Ohm’s Law : R = V / I Science 9 : Introduction to Current Electricity In an electrical circuit, the ratio of the voltage to the current is a constant (the resistance). The more voltage (“push”) applied to a current, the more current will flow. R = V / I Page

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**Science 9 : Introduction to Current Electricity**

Ohm’s Law : R = V / I Science 9 : Introduction to Current Electricity In the voltage (“push”) applied to a current doubles (like the circuit to the right), the current will also double if the resistance is the same. R = 2V / 2I or R = V / I Page

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**Using the Ohm’s Law Equation, R = V / I**

Science 9 : Introduction to Current Electricity Using the Ohm’s Law Equation, R = V / I Put the letters of the equation into a triangle as shown with V over top of I (V/I). To find a resistance, divide V by I. To find current I, divide V by R. To find voltage V, multiply I times R. In the pyramid below, the horizontal lines (---) stand for division and the vertical line (|) stands for multiplication. Page

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**Using the Ohm’s Law Formula for Problems**

Science 9 : Introduction to Current Electricity In problems, determine what is given and what is to be determined (the unknown). Rewrite Ohm’s Law to determine what operation (multiplication or division) needs to be performed. If I is unknown, I = V/R If R is unknown, R = V/I If V is unknown, V = I x R Page

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