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MCAT Content / Circuit Elements / Conductivity

### Conductivity

Topic: Circuit Elements

When exposing an insulator to an electric field, the electric charges do not flow freely—it has a high resistivity. A conductor permits the flow of electric charges; its resistivity is low.

Conductors

All conductors contain electric charges. When exposed to potential difference, the positive charges in a conductor will migrate towards the negative end, the negative charges in the material will move towards the positive end of the potential difference. This flow of charge is electric current.

Ionic substances and solutions can conduct electricity, but the most common and effective conductors are metals. Copper is commonly used in wires due to its high conductivity and relatively low price. However, gold-plated wires are sometimes used in instances in which especially high conductivity is necessary.

Metallic conduction

In metals, delocalized electrons can move freely around the positively charged metal ions. The movement of these electrons when a voltage is applied means metals are good conductors of electricity.

Electrolytic conduction

Electrolytic conduction occurs in an ionic solution when electrodes are placed in an electrolyte solution and a voltage is applied, the electrolyte will conduct electricity. Lone electrons cannot usually pass through the electrolyte; instead, a chemical reaction occurs at the cathode that consumes electrons from the anode. Another reaction occurs at the anode, producing electrons that are eventually transferred to the cathode. As a result, a negative charge cloud develops in the electrolyte around the cathode, and a positive charge develops around the anode. The ions in the electrolyte neutralize these charges, enabling the electrons to keep flowing and the reactions to continue. If there is no electrolyte there is no conductivity. The concentration of the electrolyte can also affect conductivity; a higher concentration does not always mean more conductive as it can hinder the mobility of the ions, reducing conductivity.

Every conductor has a limit to its ampacity, or amount of current it can carry. This usually is the current at which the heat released due to resistance melts the material.

Insulators

Insulators are materials in which the internal charge cannot flow freely and thus cannot conduct electric current to an appreciable degree when exposed to an electric field.

While there is no perfect insulator with infinite resistivity, materials like glass, paper and Teflon have very high resistivity and can effectively serve as insulators in most instances. Just as conductors are used to carrying electrical current through wires, insulators are commonly used as a coating for the wires.

Insulators, like conductors, have their physical limits. When exposed to enough voltage, an insulator will experience what is known as an electrical breakdown, in which current suddenly spikes through the material as it becomes a conductor.

Practice Questions

MCAT Official Prep (AAMC)

Practice Exam 4 C/P Section Passage 10 Question 56

Key Points

• Resistivity measures how strongly a material resists or conducts electric current. It is the main factor in determining whether a substance is a conductor or an insulator.

• Conductors contain electric charges that, when exposed to a potential difference, move towards one pole or the other. This flow of charge is electric current.

• Insulators are materials in which the internal charge cannot flow freely and thus cannot conduct electric current to an appreciable degree when exposed to an electric field.

Key Terms

conductor: a material which contains movable electric charges

insulator: a substance that does not transmit heat (thermal insulator), sound (acoustic insulator) or electricity (electrical insulator)

delocalized: not held in electron shells

electrolytic: conduction of current in an ionic solution

cathode: the negative electrode

anode: a positive electrode

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