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More on simple battery circuits. 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. Series 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
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
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)
What is happening in an electrical circuit? The reason electrons are flowing – there is a positive charge at the cathode they are attracted to Here, kinetic energy of electrons is put to work, in this case powering a light • 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 This switch is closed, meaning it is connected and charge is flowing through it Anode – where electrons are generated
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)
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!
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
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!
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
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
What is Voltage/Capacity of this system? Parallel connection Parallel connection Series connection Series connection
Resistance… …is futile
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
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
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
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