L3+L4: Measurement Instruments and Resistive Circuits & AC Circuits Flashcards
Where must an ammeter be placed in a circuit and how high are their resistance
- An ammeter must be connected in series with the part of the circuit where the current is to be measured
- Ammeter have a very low resistance, which causes some loading of the circuit
Where must an voltmeter be placed in a circuit
A voltmeter is connected in parallel, i.e across two points to measure their difference in potential
What happens when a voltmeters resistance is not high enough (and what is loading down a circuit)
When voltmeter resistance is not high enough, connecting it across a circuit can reduce the measured voltage. This effect is called loading down the circuit, because the measured voltage decreases due to the additional load current for the meter.
What is an Ohmmeter made up of
An ohmmeter consists of an internal battery in series with ammeter movement, and a current limiting resisitance.
Should the power in a circuit being tested for resistance with an Ohmmeter be powered on or off
The power in the circuit being tested must be shut off for measuring resistance.
What does a Wattmeter do and can it be tested in powered circuits
A wattmeter measures power. It measures voltage and current across a load separartely. Then P = VI gives the power dissipation.
Wattmeters measure voltage, V and current, I, and therefore can be used in powered circuits.
What are Series IR Voltage Drops
- In a series circuit, the IR voltage across each resistance is called an IR drop or voltage drop
- The voltage drop is indicated by - and + signs (+ indicates less negative or ore positive)
What happens with Series-Aiding and Series-Opposing Voltages
What is a Wheatstone Bridge
How can the equation for a Wheatstone Bridge be derived
What happens from Series Voltage Dividers and how is it calculated
The voltage drop across any given resistor R in a series circuit is equal to the ratio of that resistor to the total resistance RT, multiplied by source voltage, i.e VR, = (R/RT) x VT
What happens as a result of Current Dividers with Parallel Resistances and how is current calculated
For any number of resistances in parallel, individual branch currents can be calculated as per below equation where IR and R represent the individual branch current and resistance.
Derivation for a current divider in parallel circuits
What happens as a result of Voltage Dividers with Resistive Load
How is Alternating Voltage made
Alternating voltage is createdby a process called electromagnetic induction, where a conductor (usually a coil of wire) moves through a magnetic field, causing a voltage to be induced in the conductor that periodically reverses direction, resulting in an alternating current due to the changing polarity as the coil rotates within the magnetic field;this is typically done within a device called an alternator or AC generator.
How is the Instantaneous value of a sin-wave voltage calculated
The instantaneous value of a sine-wave voltage is expressed as:
v = Vp sin(Θ±Φ) = Vp sin (2πft±Φ)
- Θ is the angle, f is the frequency of the wave, and t is the time
- Vp = the peak voltage value
- Φ is the phase shift with respect to a reference point
Peak Value in AC circuits definition
Maximum value for either positive or negative peak
Peak to peak in AC circuits definition
From one peak to the opposite
Average Value in AC circuits definition
Arithmetic average of all values in one half-cycle
Root-Mean Square (RMS) or Effective Value in AC circuits definition
Relates the amount of sine wave of voltage or current to the DC value that will produce the same heating effect.
How are the time period and frequency related
Phase Shift Examples - Positive/Negative Shifts
What is a Phasor
What is a Phase Angle
Phase angle (Θ) is the angular difference between the same points on two different waveforms of the same frequency.
What do Phase Angle Diagrams show
How can time for a phase angle be calculated
How are Voltage and Current connected when a sine wave of alternating voltage is connected across a load resistance
- When a sine wave of alternating voltage is connected across a load resistance, the current that flows in the circuit is also a sine wave having the same frequency
- Resistance R has the same effect in reducing current as for either DC or AC current
What types of values should be used for power calculations in AC Circuits
For power calculations in AC Circuits, you must use only rms values of current and voltage, i.e P = Vrms Irms, P=V^2rms/R, P = I^2rms X R.
How can AC and DC Voltages be added
What is an Oscilloscope and what can they do
