Resistance, Capacitance and Inductance Elements in a Circuit Flashcards
What is capacitance?
CAPACITANCE (C): Is the ability of an object to store electrical charge.
It is equal to the charge per unit voltage. It is measured in farads (F) but as this is large, mostly in microfarads (μF).
C = Q/V
What is a capacitor?
A capacitor comprises of a pair of conducting plates divided by a space filled with an insulator
How do you measure the capacitance of a capicitor?
The Capacitance (C) of a capacitor is equal to the permittivity* (E) of the insulating material multiplied by the surface area of the plate (A), over the distance between the conducting plates (d)
C = EA/d
*capacity of the material to allow an electric field to exist between the plates
What happens to current and voltage in an RC circuit containing a battery?
A battery or cell provides direct current (DC)
As the direct current is applied the potential on the proximal plate of the capacitor rises until it reaches its maximum.
As the proximal plates potential rises the rate of rise of its potential (voltage) decreases.
Remember current can be described as rate of change of charge. Therefore as the voltage increases the current decreases.
Once the potential (voltage) has reached its maximum, there is no further change in potential therefore the current = 0.
The rate of change or current or voltage in a capacitor occurs in an exponential fashion in response to a step (DC) change of input voltage or current,
with an associated time constant, τ.
The time constant in an RC circuit is T=R.C (therefore electrical discharge does not occur instantaneously.
What is inductance?
INDUCTANCE (L): is the tendency of a conductor (carrying an AC current) to induce an electromotive force (EMF) that opposes the voltage drop due to the circuit resistance.
This is measured by henry (H).
What is an inductor and how does it work?
An inductor is a component in an electrical circuit that can store energy in its magnetic field.
Any wire carrying a current has an associated magnetic field around it that induces voltage (electromotive force/EMF) and current in nearby conductors. Coiling of the wire results in an EMF to oppose the main voltage drop.
The rate of change of current or voltage in an inductor (L) occurs in an exponential fashion in response to a step (DC) change of input voltage or current, with
an associated time constant (τ). (For a circuit
containing R and L elements, τ = L/R.)
Simplified you can think of an inductor as a water wheel, wires as pipes, and the water pump as the battery.
The water pump will pump water around the pipes, when the water gets to the water wheel there will be lots of resistance to flow so the majority of water will flow down an alternative pipe (low current/high voltage). As more water starts to flow through the water wheel it will begin to spin and the resistance will drop until it is freely flowing (high current/low voltage). If you were to turn off the water pump due to the water wheel still initially turning for a short period water would continue to flow.
The opposition to the voltage drop is that the water won’t initially flow readily due to the resistance of the wheel.
How does a capacitor behave in AC current?
There is changing charge on the plates so the current is induced to flow round the circuit beyond the capacitor proportional to the rate of change of voltage.
The current waveform leads the voltage waveform i.e initially there will be high current and no voltage and the current will drop and the voltage will increase, the voltage will then drop and the current will increase and this will repeat.
How does a inductor behave in AC current?
The electromotive force on the inductor is constantly changed and the current is induced to flow through and beyond the inductor. The voltage is proportional to the rate of change of current and leads the current waveform.
Describe the characteristics of an RC circuit?
RC circuit = a resistor capacitance circuit.
It acts as a voltage divider. If the waveform frequency is high then voltage out/voltage in ratio tends to be low.
The circuit therefore tends to block high frequency signals and allows low frequency signals to pass.
It could also be described as a low-pass electronic filter and a mathematical integrator.
Describe the characteristics of a CR circuit?
CR circuit = capacitance resistor circuit.
If the waveform frequency is low then the voltage out/voltage in ratio is small.
The circuit therefore tends to block low frequency signals.
It acts as a high pass electron filter or a mathematical differentiator.
It can be used to block out excess noisy signal.
What are the characteristics of an LR circuit?
It is an inducer resistor circuit. It acts as a low pass filter.
*Memory tip LR (lower) therefore low pass filter.
As a capacitor is the opposite as an inductor CR will be a high pass filter.
What is a band pass filter?
A band-pass filter allows through signals
within a specified bandwidth, blocking
high and low frequency components
What is reactance?
Reactance is frequency dependent resistance.
Inductor element resistance is directly proportional to f (waveform frequency)
Capacitor element resistance is inversely proportional to f (waveform frequency)
What is the ‘resonant’ frequency?
There will be a frequency when the resistance of both the capacitor and inductor elements will be equal.
At this point the circuit resistance will be at a minimum.
What is impedance?
Impedance (Z) Is the measure of a circuit’s
resistance to flow of current taking into account the effects of inductance and capacitance. Therefore, unlike resistance, it takes into account the current frequency.
It is a combination of the reactance and resistance.
An LRC circuit i.e. a circuit containing an inductor (L) a resistor (R) and a capacitor (C) has both resistance and reactance therefore can be described as having impedance.
