IB DP Physics: HL复习笔记11.2.2 Root-Mean-Square Current & Voltage

• Direct current sources provide a constant voltage and current over time, making it easy to measure
• In situations involving alternating voltage and current, the average values of voltage and current will always be zero
  • This can make it difficult to measure

The mean value for alternating current and voltage is always zero

• The use of root mean square values gets around this problem
  • First remove all the negative signs by simply squaring the peak current, or voltage
  • Find the average of the squared value
  • And finally, take the square root

• Root-mean-square (rms) values of current, or voltage, are a useful way of comparing a.c current, or voltage, to its equivalent direct current (d.c), or voltage
  • The rms values represent the direct current, or voltage, values that will produce the same heating effect, or power dissipation, as the alternating current, or voltage

• The rms value of an alternating current is defined as:

The square root of the mean of squares of all the values of the current in one cycle

• An alternate definition is:

The equivalent direct current that produces the same power

• A resistive load is any electrical component with resistance eg. a lamp

Vrms and peak voltage. The rms voltage is about 70% of the peak voltage

Step 1: Write down the Vrms equation

• The average power of a supply is the product of the rms current and voltage:

Average power = Irms × Vrms