Essential idea: When charges move an electric current is created.
Understandings: Electric current; direct current (DC)
Applications and skills: Identifying sign and nature of charge carriers in a metal; Identifying drift speed of charge carriers; Solving problems using the drift speed equation
Data booklet equations: I = ∆ q / ∆ t ; I = nAvq
This video introduces the lattice model of conduction and the concept of drift speed.
An example problem solved for drift speed calculations - and implications.
Charged objects or particles (known as charge carriers) interact with the electric field and experience a force. If there is a regular field in a particular area, and if there is a circular path (a circuit) and / or a consistent source of charge carriers then you can create an electric current, a flow of charge. A substance in which it is possible for charge to flow is called an electrical conductor. Metals are common conductors, using electrons as charge carriers, but non-metals, gases and liquids can also conduct in some cases. A particularly strong electrical field can create charge carriers by creating ions. This happens by pulling apart molecules and atoms, creating ions and free electrons. This phenomenon is known as electrical breakdown and occurs in the atmosphere when we see sparks, on a much larger scale, during a lightning storm.
Electrical conduction and currents occurs whenever it is possible for charge to flow, and conduction in metals is the most significant and commonly encountered case of electrical conduction in the IB DP Physics course. Metals are able to conduct because of metallic bonding, the principle of how atoms in metals are joined together. In metallic bonding a lattice of positively charged ions are surrounded by a sea of electrons, taken from the outermost shells of the metal atoms. Generally in a metal sample the electrons will be spread throughout the sample, repelling each other, vibrating and moving due to the thermal energy of the sample. The ions are also vibrating and ions and electrons do interact. This interaction gives rise to the phenomenon of electrical resistance - a description of how difficult the electrons find it flow through the sample. Since all substances (elements, alloys, etc) have different arrangements of ions and availability of electrons, the resistance of different samples changes. This concept, which we will explore later along a more in-depth examination of resistance, is called resistivity.
If current is a flow of charge, then it can be described as the rate of flow of charge, the amount of charge flowing past a point in a given amount of time. The unit for this quantity is the ampere, generally shortened to amp, and given the symbol A. 1 ampere is a flow of one coulomb per second. The ampere is an S.I. base unit.
The average speed at which charge carriers flow in a conductor as the result of an applied field is known as the drift velocity. The formula for the drift velocity is I = nAvq, where n is the charge carrier density, the number of charge carriers per metre cubed (and therefore a property of the material), A is the cross-sectional area (and therefore a property of the particular object), q is the charge of the charge carrier (in most cases this is e, the elementary charge - i.e. the charge on an electron) and I is the measured current in amperes.
Introduction to the model (known here as the "electron sea model"
Good video on calculations using the drift formula
Oxford Physics: pages 181 - 186 lots of detail including worked examples
Hamper HL (2014): pages 218 - 220 a good summary, but less detail than Oxford
page 230 (the ampere) - 232