Resistance, Inductance and Capacitance Elements in Electrical Circuits Flashcards
Define capacitance and the role of a capacitor (true or false):
Capacitance is the ability to store electrical charge
True. Capacitance means the ability to store charge.
Define capacitance and the role of a capacitor (true or false):
A capacitor discharges instantaneously
False. As the time constant for a CR circuit is R.C., discharge does not occur instantaneously.
Define capacitance and the role of a capacitor (true or false):
A capacitor can block alternating current
False. A capacitor functions by inducing equal and opposite charge on the distal plate as the charge on the proximal plate rises, but not when it has reached a steady state level; therefore a capacitor blocks DC, not AC.
Define capacitance and the role of a capacitor (true or false):
With DC, on switch closure, as the current increases, the voltage decreases
False. On switch closure using a battery (DC), the voltage across the capacitor rises, and the electrons flow rapidly initially (high current), then less rapidly (decreasing current) as the capacitor becomes saturated with charge.
Define capacitance and the role of a capacitor (true or false):
A capacitor stores equal and opposite charges on opposite plates
True. A capacitor functions by inducing equal and opposite charge on the distal plate as the charge on the proximal plate.
Define inductance and the role of an inductor (true or false):
Inductance is the ability to induce current from electron flow
False. Inductance refers to the ability of any current-carrying conductor to induce a magnetic field around it. That magnetic field can in turn induce a current in a neighbouring conductor.
Define inductance and the role of an inductor (true or false):
An inductor is a coil of wire carrying current, around which a magnetic field is induced
True
Define inductance and the role of an inductor (true or false):
The magnetic field around the coil induces a voltage which increases the voltage drop across it
False. An inductance coil (an inductor), by virtue of the coils of wire adjacent to each other, amplifies this effect, producing a strong magnetic field through the coil, and inducing a voltage which opposes the prevailing voltage drop and current flow.
Define inductance and the role of an inductor (true or false):
The time constant is inversely proportional to the inductance magnitude
False. The time constant for a circuit with R and L elements is L/R.
Define inductance and the role of an inductor (true or false):
An inductor is a device for inducing current from voltage
False. An inductor produces a strong magnetic field through the coil, and induces a voltage.
Describe what happens to current and voltage on a capacitor when it is connected to a battery through a resistor:
A. Current rises as voltage falls
B. Current falls to zero when voltage reaches maximum
C. Voltage rises to a maximum then falls to zero
D. Voltage and current rise and fall alternately
E. Voltage and current remain constant
B. An R and a C connected to a DC source results in the voltage rising to a maximum value until the proximal capacitor plate is replete with electrons. The rate of flow of electrons (current) is initially high and falls to zero as the capacitor plate becomes saturated with electrons. While electrons are accumulating at the proximal capacitor plate, equal and opposite charges are being induced on the distal plate, and the rate of rise of positive charge on this plate constitutes current flow round the remainder of the circuit. As the rate of rise of positive charge falls, so the current in the circuit falls.
A. Incorrect.
C. Incorrect. There is no reason for the voltage to change once it has risen to a maximum.
D. Incorrect. This may happen in an AC circuit, but not in a DC circuit.
E. Incorrect. This applies to a resistor, not a capacitor.
The following statements refer to current and voltage in an inductor when connected to a battery through a resistance.
Select all those that apply:
A. Voltage rises and current falls
B. Voltage and current follow each other
C. Current rises and voltage falls
D. The rate of rise of current falls as the voltage falls
E. Voltage and current remain constant
C & D. As the induced opposing voltage rises, the rate of rise of current falls.
A. Incorrect. The rising current induces a voltage which opposes the supply EMF so the voltage falls.
B. Incorrect.
E. Incorrect. This would only be true of a resistance. As the switch is closed the voltage drop across the coil causes the current to start to flow. Rising current induces a magnetic field around the coil, which in turn induces a voltage through the coil which opposes the current flow. The maximum value of the current is thus limited.
Which of the following statements are true of RC circuits?
Select all that apply
A. Vout/Vin is inversely proportional to capacitance and frequency
B. The higher the frequency, the higher is Vout/Vin
C. The output waveform is the integral of the input waveform
D. The higher the capacitance, the higher the output voltage
E. For DC circuits, Vout/ Vin = 1
A, C & E.
For an RC circuit:
Vout/Vin = 1 / (1 + 2πRfC)
Therefore Vout/Vin is inversely proportional to both f and C (and R). Therefore the higher the f or C, the lower is Vout/Vin. The circuit acts as an integrator.
Which of the following statements are true for ‘high-pass’, ‘low-pass’ and ‘band-pass’ electronic filters? (true or false)
A high-pass filter is an RC circuit
False. A high-pass filter is a CR circuit.
Which of the following statements are true for ‘high-pass’, ‘low-pass’ and ‘band-pass’ electronic filters? (true or false)
A high-pass filter blocks low frequency signals
True. A high-pass filter ‘passes’ high frequency signals, and thus blocks low frequency signals; a low-pass filter does the reverse.
