A light bulb is rated at 100 W in the United States, where the standard wall outlet voltage is 120 V. If
this bulb were plugged in in Europe, where the standard wall outlet voltage is 240 V, which of the
following would be true?(A) The bulb would be one-quarter as bright.
(B) The bulb would be one-half as bright.
(C) The bulb’s brightness would be the same.
(D) The bulb would be twice as bright.
(E) The bulb would be four times as bright.Your first instinct might be to say that because brightness depends on power, the bulb is exactly as bright.
But that’s not right! The power of a bulb can change.
The resistance of a light bulb is a property of the bulb itself, and so will not change no matter what the
bulb is hooked to.Since the resistance of the bulb stays the same while the voltage changes, by V 2 /R , the power goes up,
and the bulb will be brighter. How much brighter? Since the voltage in Europe is doubled, and because
voltage is squared in the equation, the power is multiplied by 4—choice E.
Ammeters and Voltmeters
Ammeters measure current, and voltmeters measure voltage. This is pretty obvious, because current is
measured in amps, voltage in volts. It is not necessarily obvious, though, how to connect these meters into
a circuit.
Remind yourself of the properties of series and parallel resistors—voltage is the same for any
resistors in parallel with each other. So if you’re going to measure the voltage across a resistor, you must
put the voltmeter in parallel with the resistor. In Figure 19.7 , the meter labeled V 2 measures the voltage
across the 100 Ω resistor, while the meter labeled V 1 measures the potential difference between points A
and B (which is also the voltage across R 1 ).