Wheatstone bridge

Wheatstone bridge.

DC current is the unidirectional flow of electric charge. Traditionally, in undergraduate physics courses, DC current problem solving involves the application of formulas to simple electrical circuits where the flow of charge takes place within metallic conductors. The new MCAT places less emphasis on this type of problem solving, but you still need to master it. Not only because many devices from the medical and life science research context rely on DC current technology, such as electrophoresis apparatus, but also because, like so much with physics, problem solving with the model system is the first step before applying concepts to real world situations. On the new MCAT you are more likely to find situations where charge carriers are ions moving through a solution or across a membrane than to be faced with electrons moving through a metal wire, but to interpret current in the life science context you need to first master the language of voltage, current, resistance, and power to describe circuits composed of wires, resistors, and voltage sources.

WikiPremed Resources

DC Current Cards
Chapter from the Wisebridge Learning System for Physics

DC Current Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for DC Current
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Be able to conceptualize how various types of charge carriers in an array of materials or environments may constitute an electric current.

Have a good, clear sense of what it means to say that a volt is a joule per coulomb and an ampere is a coulomb per second.

Understand Ohm's Law conceptually and be able to solve basic problems involving simple circuits.

Understand the basis for the trends of resistivity with temperature in metallic conductors and semiconductors.

Be prepared to discuss the molar conductivity of electrolyte solutions.

Possess a good facility for solving for unknowns in circuits involving series and parallel resistors.

Understand Kirchoff's Current Law and the Voltage Law and know how to apply these to solving for unknowns in complex circuits.

Be familiar with the concept of electric power in an intuitive, conceptual way and in quantitative problem solving.

Be able to describe what happens over time when an RC circuit (resistor and capacitor) is switched on or off.

Know how to solve problems involving the Wheatstone Bridge.

Suggested Assignments

Review the basic terms for DC current using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Study the physics cards for DC current.

In ExamKrackers Physics, read pp. 92-101. Perform practice items 81-88 on pp. 102-103.

Take a review tour of DC current web resources.

Conceptual Vocabulary for DC Current

Direct current
Direct current is the constant flow of electric charge.
Electric current
Electric current is the flow of electric charge.
Ohm's law
Ohm's law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference across the two points, and inversely proportional to the resistance between them.
Electrical conductor
Conductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons.
Voltage drop
Voltage drop is the reduction in voltage in an electrical circuit between the source and load.
Voltage source
A voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary potential from a primary source of the electromotive force.
The ampere is the SI unit of electric current.
Kirchhoff's circuit laws
Kirchhoff's circuit laws are a pair of laws that deal with the conservation of charge and energy in electrical circuits.
Electrical conduction
Conduction is the movement of electrically charged particles through a transmission medium which can form an electric current in response to an electric field.
Electrical conductivity
Electrical or specific conductivity is a measure of a material's ability to conduct an electric current.
Electrical resistivity is a measure of how strongly a type of material opposes the flow of electric current.
A voltmeter is a very high resistance ammeter used for measuring the electrical potential difference between two points in an electric circuit.
An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to the flow of an electric current.
A semiconductor is a solid that has electrical conductivity in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically.
Free electron model
In solid-state physics, the free electron model is a simple model for the behaviour of valence electrons in a crystal structure of a metallic solid.
Current source
A current source is an electrical or electronic device that delivers or absorbs flow of electric charge.
Wheatstone bridge
A Wheatstone bridge is a device used to measure an unknown electrical resistance which works by balancing two legs of a bridge circuit, one leg of which includes the unknown component.
Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field.
Bridge circuit
A bridge circuit is a type of electrical circuit in which the current in a conductor splits into two parallel paths and then recombines into a single conductor, thereby enclosing a loop.
Current density
Current density is a vector whose magnitude is the electric current per cross-sectional area measured in the SI system as amperes per square meter.
The siemens is the SI derived unit of electric conductance.
A potentiometer is a variable resistor that can be used as a voltage divider.
Advanced terms that may appear in context in MCAT passages
Drift velocity
The drift velocity is the average velocity, not speed, that a particle, such as an electron, attains due to an electric field.
Voltage divider
A voltage divider is a simple device designed to create a voltage which is proportional to another voltage.
Electronic band structure
The electronic band structure of a solid describes ranges of energy that an electron is forbidden or allowed to have.
N-type semiconductor
An N-type semiconductor is obtained by carrying out a process of doping with valence-five elements to a valence-four semiconductor in order to increase the number of free, negative charge carriers.
P-type semiconductor
A P-type semiconductor is obtained by carrying out a process of doping the semiconductor to increase the number of free, positive charge carriers.
An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor.