10/2/2018 Electrical Instruments Electrical Instruments Galvanometer – measures current Galvanometer – Current through coil creates a magnetic field that interacts with field due to other magnets. Needle deflection is proportional to Voltmeter – measures potential difference current. Ammeter – measures current Ohmmeter – measures resistance Electrical Instruments Voltmeter Adding an instrument changes the circuit. The • Voltmeter connected in parallel to measure measurement is of the circuit with the instrument potential difference between two points. and not of the original circuit. • must be large to minimize change from original circuit. Example: A galvanometer has an internal resistance of Voltmeter 12Ω and experiences full deflection with a current of 0.4A. Design a voltmeter using the galvanometer that Creating a voltmeter from a galvanometer will read full scale at 20V. Series G V Series G Given that max creates a full-scale deflection, select Series such that max produces max through the galvanometer. 1 10/2/2018 Voltmeter Voltmeter What happens if a voltmeter is connected in What happens if a voltmeter is connected in series? series? • Circuit is drastically changed. • Very little current flows. Ammeter Ammeter • Ammeter connected in series to measure Creating an ammeter from a galvanometer current through resistor. • must be small to minimize change from G original circuit. 1 1 1 A Shunt G Shunt Given that Gmax creates a full-scale deflection, select Shunt such that Amax produces Gmax through the galvanometer. Example: A galvanometer has an internal resistance of 12Ω and experiences full deflection with a current of Ammeter 0.4A. Design an ammeter using the galvanometer that will read full scale at 10A. What happens if an ammeter is connected in parallel? 2 10/2/2018 Ammeter Ohmmeter What happens if an ammeter is connected in Creating an ohmmeter from a galvanometer. parallel? Connected to a resistor in isolation, the ohmmeter forms a circuit with the unknown resistor. • Circuit is drastically changed. • Massive current flows. O Series G •Flash •Noise •Smoke •Smell T Series G • Loss of ammeter • Angry instructor Solve O T to determine . RC Circuits RC Circuits What happens when capacitor is connected to a Apply Kirchhoff’s loop rule. resistor in a circuit? + - + - + - + - 0 RC Circuits RC Circuits 0 0 Positive because current through the resistor is associated with an increase of charge on the capacitor. 0 3 10/2/2018 RC Circuits RC Circuits ln ln 1 ln RC Circuits RC Circuits Charging a capacitor ln • Charge on capacitor 1 • Voltage across capacitor 1 1 • Current through resistor 1 ≡ RC Circuits RC Circuits What happens when charged capacitor is Apply Kirchhoff’s loop rule. connected to a resistor in a circuit without a battery? + - + - 0 4 10/2/2018 RC Circuits RC Circuits 0 0 Negative because current through the resistor is associated with a decrease of charge on the ln capacitor. 0 Example: An 8F capacitor is connected in series with a RC Circuits 50Ω resistor and a 60V battery. (a) When does the capacitor become fully charged? (b) What is the charge Discharging a capacitor on the capacitor when it is fully charged? (c) When does • Charge on capacitor the capacitor hold half of its ultimate full charge? (d) What is the current in the resistor when the capacitor holds half of its ultimate full charge? • Voltage across capacitor • Current through resistor ≡ 5.