Experiments with RLC circuitsOrganisation/School Logo here

IPSG

2019

Motivation for this presentation:1. To help H3 students understand the physics of *RLC circuits (DC series) better.2. To utilize the power of demonstration/experimentation in enhancing the learning of physics, especially for phenomena that are not

immediately visible to the eye.3. To help students see the analogy between electrical and mechanical oscillations.4. To deepen students’ joy in learning physics.

The (DC) RC series circuit:

When a constant DC voltage is applied across a capacitor and a resistor connected in series, the p.d. across the capacitor

increases with time according to the equation** The time-constant of the circuit is = RC

Graphically:

Experiment: To investigate the charging of a capacitor Using a breadboard, connect a 10 F capacitor in series with a 1 kresistor. The DC voltage source is a square wave voltage from a signal generator adjusted at 1 V peak-to-peak and 500 Hz. To observe the variation with time of the voltage across the capacitor during charging, connect the probes of a CRO across the capacitor.

By adjusting the time-base, the voltage variation across the capacitor can be displayed on the CRO as shown above.

By reading off the value of (when V = 0.632V0 ) the value of Cused in the circuit can be calculated and compared with the known value. If the values agree, then equation ** above is verified.

*MOE-H3 Physics, Topic B2: Syllabus requirements for RLCcircuits(j) solve problems involving circuits with resistors, capacitors,and sources of constant e.m.f.(k) solve problems involving circuits with resistors, inductors,and sources of constant e.m.f.(l) solve problems involving circuits with inductors andcapacitors only(m) solve problems involving circuits with resistors, inductorsand capacitors only

References:(1) Physics for Scientists and Engineers by Serway and Jewett(2) Electricity and Magnetism by Purcell and Morin.

For detailed theory and derivations of **formulae shown above, pleaserefer to these books and other relevant sources.

The (DC) RL series circuit:

For the RL series circuit with a constant DC voltage supply, the p.d. across the inductor decreases exponentially with time

according to the equation

The time constant of the circuit is = L/RGraphically:

Experiment: To observe the exponential decay of the voltage across an inductor.Connect a 33 mH inductor and a 6 resistor in series. Using the same square wave voltage as before, the voltage across the inductor displayed on the CRO is as shown. The exponential nature of the decay in voltage can further be investigated.

The (DC) RLC series circuit:

When a fully charged capacitor (S1 closed, S2 open) is allowed to discharge through an inductor and a resistor in series (S1 open, S2 closed), the charge on the capacitor varies with time according to the equation

where is the angular frequency of the

electrical oscillations. This situation (or phenomenon) is similar to that for a lightly damped oscillating mass-spring system (simple harmonic oscillator). Experiment: To observe the transient electrical oscillations in a series RLC circuitConnect a 10 F, a 33 mH inductor and a 25 resistor in series, and using the same square wave voltage as before, observe the oscillations in voltage across the capacitor on the CRO.

By using other combinations of R, L and C values, damped oscillations of different frequencies can be observed and studied.

0V

CV

0 1

tRC

CV t V e

resistor

capacitor

To CRO

LV

0

Lt /

RLV =V e

inductor

R

t2L

oQ = Q e cosωt

2

2

1 Rω

LC L

To CRO