Animated Solution for Physics - Current Electricity: Which of the following will not be observed when a multimeter (operating in resistance measuring mode) probes connected across a component, are just reversed?

Imagine you are a detective, and your multimeter is your magnifying glass.

When you switch a multimeter to resistance mode, it doesn't just passively listen. It actively interrogates the component by sending out a tiny search party of electrons.

This is powered by its internal battery. The multimeter applies a known DC voltage and measures the resulting current.

The deflection of the needle on an analog multimeter is directly proportional to this current. Maximum current means maximum deflection (zero resistance), while zero current means no deflection (infinite resistance).

Let's test our first suspects: the metal wire and the standard resistor.

These are ohmic components. They don't care which way the current flows.

Whether you connect the red probe to the left and black to the right, or vice versa, the resistance remains exactly the same. Therefore, the current remains the same, and the multimeter shows an equal deflection in both cases.

Next up is the Light Emitting Diode (LED).

An LED is a p-n junction diode, which acts like a strict one-way valve for electricity. When you connect the probes such that the LED is forward-biased, it allows current to flow freely.

This results in a visible splash of light and a clear deflection on the multimeter.

But the moment you reverse the probes, the LED becomes reverse-biased. The depletion region widens, blocking the current entirely. The current drops to zero, resulting in no deflection.

Now we arrive at the real star of this problem: the capacitor.

A capacitor is essentially two conductive plates separated by an insulator. It is like an empty bucket for electrical charge.

When you first connect the multimeter probes to an uncharged capacitor, the internal battery immediately starts pushing electrons onto one plate and pulling them from the other.

This initial rush of charge is called a transient current.

Because of this sudden surge of current, the multimeter needle will experience a sharp, momentary deflection!

However, as the capacitor fills up with charge, the voltage across its plates builds up to match the multimeter's battery. Once fully charged, it acts as an open circuit for DC. The current drops to zero, and the needle falls back to rest.

What happens when you reverse the probes on this fully charged capacitor?

You are essentially flipping the applied voltage. The capacitor must now violently discharge its stored energy and then recharge in the opposite polarity.

This massive shift in charge creates another powerful surge of transient current. Consequently, the needle will show another momentary deflection!

Option (c) claims that a capacitor will show no deflection in both cases. As we have just proven, this is completely false. The transient charging and discharging currents guarantee a deflection every time the probes are connected or reversed.

This makes statement (c) the incorrect observation, and thus, the correct answer to our question!