Question

 A piece of pencil lead is connected in series with an ammeter and a power supply. The power supply is turned on. After a few minutes, although the potential difference across the pencil lead does not change, the current in the circuit increases significantly. Explain why the current increases

Answer 1

The pencil lead is a mixture of clay and graphite (an allotrope of carbon) which is a good conductor of electric current. From ohms law, it happens that for the electric current to increase while the potential difference is constant, there must be a drop in the total resistance in the circuit. The conductivity of the free mobile electrons in graphite to free flow of electric current causes a drop in the resistance offered initially by the pencil lead, thus, giving rise to an increase in the current in the circuit.