Circuits from an Experimental Point of View for AP Physics B & C

Practice problems for these concepts can be found at:

Circuits Practice Problems for AP Physics B & C

When a real circuit is set up in the laboratory, it usually consists of more than just resistors—light bulbs and motors are common devices to hook to a battery, for example. For the purposes of computation, though, we can consider pretty much any electronic device to act like a resistor.

But what if your purpose is not computation? Often on the AP exam, as in the laboratory, you are asked about observational and measurable effects. The most common questions involve the brightness of lightbulbs and the measurement (not just computation) of current and voltage.

Brightness of a Bulb

The brightness of a bulb depends solely on the power dissipated by the bulb. (Remember, power is given by any of the equations I ²R, IV, or V ²/R). You can remember that from your own experience—when you go to the store to buy a light bulb, you don't ask for a "400-ohm" bulb, but for a "100-watt" bulb. And a 100-watt bulb is brighter than a 25-watt bulb. But be careful—a bulb's power can change depending on the current and voltage it's hooked up to. Consider this problem.

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.

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 21.8 below, the meter labeled V₂ measures the voltage across the 100 Ω resistor, while the meter labeled V₁ measures the potential difference between points A and B (which is also the voltage across R₁).

Current is the same for any resistors in series with one another. So, if you're going to measure the current through a resistor, the ammeter must be in series with that resistor. In Figure 21.9 below, ammeter A₁ measures the current through resistor R₁, while ammeter A₂ measures the current through resistor R₂.

As an exercise, ask yourself, is there a way to figure out the current in the other three resistors based only on the readings in these two ammeters? Answer is in the footnote.²

Practice problems for these concepts can be found at:

Circuits Practice Problems for AP Physics B & C