Circuits Async-4

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Apr 3, 2024

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Department of Physics & Astronomy Undergraduate Labs, Distance Learning 1/18/2023 1 Circuits and Ohm’s Law (Asynchronous) Introductory Video: https://youtu.be/Djp3YXqA0Sw In equilibrium, the electric field inside a conductor is zero. However, the flow of electrons through a circuit is not an equilibrium case; electrons are continuously pushed from one terminal of a battery or power supply to the other terminal. An electric field, that is directed parallel to the conductor, provides a force that pushes electrons through the circuit. The flow of charge through a circuit obeys Kirchoff’s circuit laws. Kirchoff’s junction rule is a statement of charge conservation. It states that the algebraic sum of currents into any junction is zero. In other words, whatever charge flows into a junction must also flow out. Kirchoff’s loop rule is a statement about energy conservation. It states that the algebraic sum of potential differences in any loop, including those associated with emfs and resistive elements, must equal zero. A resistor is a circuit component that opposes the passage of electrons, much more so than other parts of the circuit. Resistors can be composed of all sorts of materials and geometries. Below is a figure of a copper wire connected to a carbon resistor (gray) that just happens to be the same thickness of the wire. Carbon is a much weaker conductor than copper because electrons within carbon are much less mobile. As a result, when connected to a voltage source, charge accumulates on the copper-carbon interface until a sufficient electric field builds up to make the current through the carbon the same as that through the copper. The unit of resistance is the Ohm (Ω) with 1 Ω ≡ 1 V/A . For many materials, Δ࠵? and ࠵? are directly proportional to one another, and therefore ࠵? is constant. ∆࠵? = ࠵?࠵? This proportionality is called Ohm’s Law and materials that satisfy it are called “Ohmic” materials. A fluid analogy for a circuit is shown below. Electrical resistance ࠵? is analogous to a narrow constriction in a pipe. The voltage Δ࠵? across a power supply is analogous to the pressure difference Δ࠵? across a pump. Electrical ground is analogous to a fluid reservoir. ࠵? ! ࠵? " ࠵? # = ࠵? ! + ࠵? " Junction Current: ࠵? ࠵? Power Supply ࠵? High Pressure Low Pressure Flow Rate: ࠵? High Voltage Low Voltage Ground

Department of Physics & Astronomy Undergraduate Labs, Distance Learning 1/18/2023 2 Prelab Exercise You are given a battery, a resistor, a light bulb, and all the connecting wire that you need. 1. Using all components, draw a schematic circuit diagram that will produce the brightest bulb and explain your reasoning. 2. Using all components, draw a schematic circuit diagram that will produce the dimmest bulb and explain your reasoning. Goals of this Lab 1. Understand how charge flows through a circuit 2. Learn to measure current and voltage at various points in a circuit 3. Understand the behavior of Ohmic circuit elements Lab Materials Variable Voltage Source This voltage source will act as a battery that can provide a variable (i.e. user defined) voltage. Multimeter The multimeter will be used to measure voltage differences and current throughout circuits. Differential Voltage Probe These can be interfaced to Logger Pro for automatic data collection of voltage differences in a circuit. Current Probe These can be interfaced to Logger Pro for automatic data collection of currents in a circuit. Logger Pro Logger Pro will be used to run the differential voltage and current probes and used for data analysis. Logger Pro Light Bulbs and resistors mounted on banana plug connectors Resistors and light bulbs have been connected to each terminal of a banana plug connector to facilitate connection. Battery Resistor Light Bulb

Department of Physics & Astronomy Undergraduate Labs, Distance Learning 1/18/2023 3 Measure Voltage Differences and Current using a Multimeter Measuring DC voltage differences: • Voltage differences are measured between two points on a circuit. • Connect the probes of the multimeter to two points on the circuit. This is equivalent to connecting the multimeter in parallel to the element you want to measure. Measuring DC current: • Current is measured along a wire in a circuit. You can think of the ammeter replacing a wire. • Disconnect a part of your circuit and connect the multimeter in series . 8.2 COM V W 10A 400mA V V OFF ~ _ mV _ W ) ) ) ) ) mA ~ _ _ A ~ V DC 0.22 COM V W 10A 400mA V V OFF ~ _ mV _ W ) ) ) ) ) mA ~ _ _ A ~ A DC Measuring the voltage across a light bulb Measuring the current through a light bulb

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