Commons

The setups for measuring current and voltage are different. Whenwe’re measuring current, we’re finding “how much stuff goes through,”so we place the ammeter where all the current is forced to go throughit. Voltage, however, is not “stuff that goes through,” it is a measureof electrical energy. If an ammeter is like the meter that measuresyour water use, a voltmeter is like a measuring stick that tells youhow high a waterfall is, so that you can determine how much energywill be released by each kilogram of falling water. We don’t want toforce the water to go through the measuring stick! The arrangementin figure l/3 is a parallel circuit: one in there are “forks in the road”where some of the current will flow one way and some will flow theother. Figure l/4 is said to be wired in series: all the current willvisit all the circuit elements one after the other.If you inserted a voltmeter incorrectly, in series with the bulb andbattery, its large internal resistance would cut the current down solow that the bulb would go out. You would have severely disturbedthe behavior of the circuit by trying to measure something about it.Incorrectly placing an ammeter in parallel is likely to be evenmore disconcerting. The ammeter has nothing but wire inside it toprovide resistance, so given the choice, most of the current will flowthrough it rather than through the bulb. So much current will flowthrough the ammeter, in fact, that there is a danger of burning outthe battery or the meter or both! For this reason, most ammetershave fuses or circuit breakers inside. Some models will trip theircircuit breakers and make an audible alarm in this situation, whileothers will simply blow a fuse and stop working until you replace it.Discussion questionsA In figure g/4 on page 102, what would happen if you had the ammeteron the left rather than on the right?B Imagine a charged doorknob, as described on page 109. Why isit intuitively reasonable to believe that all the charge will end up on thesurface of the doorknob, rather than on the interior?110 Chapter 5 Electricity