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 G max creates a full-scale deflection, select Shunt such that A max produces G max 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 8 F 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