What r.m.s. Means
What r.m.s. Means
- The mean current over one complete symmetric a.c. cycle is zero, but the resistor still becomes warm. A different average is needed.
Compare equal heating in the same resistor
Definition
2 marksWhat is meant by the root-mean-square value of an alternating current?
Model answer: The value of the steady direct current that produces the same average power in a given resistor.
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- The r.m.s. current is an effective d.c. value. It compares heating or average power in the same resistor.
- The r.m.s. voltage is the steady d.c. voltage that would produce the same average power in the same resistor.
- Do not average positive and negative current values. Heating depends on the square of current, so both directions heat the resistor.
Your turnmeaning check
A 4.0 A r.m.s. alternating current and a steady 4.0 A direct current pass separately through the same resistor. Compare their average heating powers.
Show worked answer
They produce the same average heating power. Equal r.m.s. and steady d.c. values are defined by equal average power in the same resistor.