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More on simple battery circuits

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Review A simple voltaic cell diagram shown at right What direction do the electrons flow? Do they change at all? If the electrons only flow in one direction, this is a called a Direct Current

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Series circuits Batteries can be connected in series to increase voltage For any circuit, the total voltage is the sum voltage of the batteries connected in the series

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More about Direct Current The current flowing from a battery through a circuit only flows in ONE direction The current is described as Direct Current (or DC) power Voltage of a battery diminishes as the chemical reactions driving the battery run out of reagents (run out of chemical potential)

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What is happening in an electrical circuit? First – what does “circuit” mean? – What is a circuit workout? – What is a racing circuit – In an electrical circuit, “charge” flows in a circular pathway The energy of electrons may be transformed multiple ways throughout their journey Anode – where electrons are generated This switch is closed, meaning it is connected and charge is flowing through it Here, kinetic energy of electrons is put to work, in this case powering a light The reason electrons are flowing – there is a positive charge at the cathode they are attracted to

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Things that affect flow of charge Voltage – symbol is V (sometimes written as E) Amperage – symbol is I Ohms (resistance) – symbol is R (sometimes shown as Ω)

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Ohm’s law Ohm’s law relates how Voltage, current and resistance relate to each other For any electrical circuit, ΔV = I R Put into words, the current flow depends on the Voltage divided by the resistance Given constant resistance, to get more flow what must be changed? The voltage must be increased! Given constant voltage, to get more flow what must be changed? The resistance must be lowered!

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What about Watts? Simply put, Wattage (power) is Voltage times current, or W = I x V So electrical devices can be described in terms of either the current they like to take (because Voltage in American electrical outlets is always the same), or – They may be described in terms of their wattage In American homes, electrical outlets are standard at 120 V European systems are standard at 220 V

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Parallel circuits and current

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Capacity of a battery is how long it can generate a current Current (measured in Amperes or “Amps”), uses symbol I A measure of the amount of electrical charge passing through a point in a circuit in a given time Sometimes this is described in terms of “Amp-hours” Gift wrap available!

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Increasing capacity The capacity of a battery (amount of current it generate in a unit time) is fixed But when batteries are joined in parallel, the capacities add up A parallel connection links the battery’s anodes to each other, and the battery’s cathodes to each other Voltage is unchanged, capacity increases

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Series versus Parallel In a series connection, voltages add, but capacity is not changed In parallel connections, voltages remain the same, but capacity increases Series connection – only Voltage increases Parallel connection – only capacity increases

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Series versus parallel

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Sometimes a circuit can be both

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What is Voltage/Capacity of this system? Series connection Parallel connection

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Resistance… …is futile

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Resistors A resistor is a device to increase the resistance at a particular point in a circuit by reducing the current that can flow through a point Think of a valve in pipe, in our “water analogy” Unit is the Ohm Resistors are used to generate heat, lower voltage, produce light, etc. Essentially, a light bulb can be thought of as a resistor

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Resistors are color-coded

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Resistors in series When resistors are in series – the total resistance is the sum of the individual resistor’s value in Ohms So it works just like batteries connected in series

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Resistors in parallel

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Practice problems Three resistors are connected in a series. The value of each resistor is 10 Ω. What is the resistance of the circuit? 30 Ω The same three resistors are now assembled in parallel. What is the resistance of the circuit? 3.33 Ω

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Conclusion - resistance All electrical circuits will have some resistance Even though Copper is a good conductor, good is not “perfect” So all circuits will lose some of their electrical energy as heat (electronics get warm when you use them because of resistance in the circuits) With heat – resistance increases! A superconductor is a material that can carry current with little – or even no – resistance! But the material often has to be cooled to very low temperatures to become superconductive

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Super-cooled substances show strange effects

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Unit 7: Electricity and Magnetism

Unit 7: Electricity and Magnetism

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