# AS AQA Phys Unit 1- Current, Potential Difference and Resistance

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Current

The current in a wire is similar to water which flows in a pipe. The amount of water which flows is dependent upon flow rate and time. The same applies for electricity; current is the rate of flow of charge.

(change in)Q = I*(change in)t  or  I = (change in)Q / (change in)t

Where Q is the charge in coulombs (C), I is the current (A) and t is time taken (s).

The coulomb is the unit of charge; one coulomb (C) is defined as the amount of charge that passes in 1 second when the current is 1 ampere. Current flowing through part of a circuit can be measured using an ammeter -an ammeter should always be attached in series circuits.

Current is due to the passage of charged particles called charge carriers;

1) In metals, the charge carriers are conduction electrons. They move about in the metal repeatedly colliding with each other and with fixed positive ions in the metal.

2) In comparison, when an electric current is passed through a salt solution, the charge is carried by ions, which are charged atoms or molecules.

In electrical terms, a material can be classified as an electrical conductor, insulator or semiconductor.

-In an insulator, each electron is attached to an atom and cannot move away from the atom. When a voltage is applied across an insulator, no current passes through the insulator, because no electrons can move through.

-In a metallic conductor, most electrons are attached to atoms but some are not. These are the charge carriers in the metal. When a voltage is applied, these conduction electrons are attracted to the positive terminal of the metal, so a current flows.

-In a semiconductor, the number of charge carriers increases with an increase of temperature. The resistance of a semiconductor therefore decreases as its temperature is raised. A pure semiconducting material is reffered to as an intrinsic semiconductor…