version H28986 2014
An oscilloscope records the peak value of a p.d., Vo
Further applications
The circuits can be used to determine r.m.s. and half cycle average values of nonsinusoidal
waveforms. For frequencies other than 50 - 60Hz, the frequency response of the voltmeter may
be significant.
The ‘form factor’ of the waveform can be obtained from
F = r.m.s. value/half cycle average value
Maximum Power and Internal Resistance
Apparatus:
Joulemeter/wattmeter
10V d.c. meter instead of 1mA d.c. meter. Resistor,
4.7kΩ
Resistance, 0 to 10kΩ
Power supply, 10V 2mA d.c. or a.c.
Procedure:
Record the power, P
L
, dissipated in the load when the load resistance, R
L
, is
varied from 1 to 10kΩ
The 4.7kΩ resistor acts as the internal resistance, R
s
, of the source.
P
L
should have a maximum value when R
L
= R
S
(In this condition, the power dissipated in the load
is equal to that dissipated in the source.)
Power in A.C. Circuits
Apparatus:
Joulemeter/wattmeter
10V d.c. meter instead of 1mA d.c. meter.
Audio signal generator capable of 5V r.m.s. output Meters
50mA a.c. and 5V a.c.
Resistor, 470Ω
Capacitor, 470nF
Inductor, 2400 turns ≈150mH
Procedure:
Set the signal generator frequency to 100Hz and adjust the output p.d. to give a current of about
10mA in the resistive load. Ensuring that the p.d. remains constant, vary the frequency, f, from
100Hz to 1kHz and record the current, I, and the power, P, dissipated in the load.
Maintaining the same p.d., repeat the procedure with the capacitor and the inductor, separately, in
place of the resistor. In the case of the inductor, the frequency must be kept above about 250Hz
to avoid current overload.
The joulemeter provides evidence that
(a)
the resistance of a pure resistor is independent of frequency
(b)
no power is dissipated in a pure capacitor (in practice, resistive leakage may become
apparent at high frequencies, as the reactance of the capacitor decreases.
