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MCAT Content / Electrostatics / Charge Conductors Charge Conservation

### Charge, conductors, charge conservation

Topic: Electrostatics

Electric charge, like mass and volume, is a physical property of matter. Charges can be positive or negative; a single proton has a charge of 1.602×(10−19) C, while an electron has a charge of -1.602×(10−19) C.

Coulomb (C) is the SI unit for charge, which represents 6.242×(1018)e, where e is the charge of a proton.

Opposite charges will attract, while same charges will repel each other.

Conductors

All conductors contain electric charges that, when exposed to a potential difference, move towards one pole or the other. The positive charges in a conductor will migrate towards the negative end of the potential difference; the negative charges in the material will move towards the positive end of the potential difference. This flow of charge is electric current.

This is related to in the equation Q=It where Q charge, I is current and t is time measured in seconds. 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 inexpensive price. However, gold-plated wires are sometimes used in instances in which especially high conductivity is necessary.

Conservation of Charge

Like mass, electric charge in a closed system is conserved. The net quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved. For a given system

Q(t2)=Q(t1)+Qin−Qout

• Q(t1) is the charge in the system at a given time,
• Q(t2) is the charge in the same system at a later time,
• Qin is the charge that has entered the system between the two times,
• Qout is the amount of charge that has left the system between the two times.

This does not mean that individual positive and negative charges cannot be created or destroyed. Electric charge is carried by subatomic particles such as electrons and protons, which can be created and destroyed. For example, when particles are destroyed, equal numbers of positive and negative charges are destroyed, keeping the net amount of charge unchanged.

Practice Questions

Electrostatics, medicine, and metal spheres

MCAT Official Prep (AAMC)

Practice Exam 2 C/P Section Question 47

Practice Exam 4 C/P Section Passage 10 Question 54

Practice Exam 4 C/P Section Passage 10 Question 56

Key Points

• Charge is measured in Coulombs (C), which represent 6.242×1018 e, where e is the charge of a proton. Charges can be positive or negative, and as such a singular proton has a charge of 1.602×10−19 C, while an electron has a charge of -1.602×10−19 C.

• Conductors contain electric charges that, when exposed to a potential difference, move towards one pole or the other.

• Electric charge, like mass, is conserved.

• Q=It where Q charge, I is current and t is time measured in seconds.

Key Terms

Coulomb: in the International System of Units, the derived unit of electric charge; the amount of electric charge carried by a current of 1 ampere flowing for 1 second. Symbol: C

Conductor: a material which contains movable electric charges

Potential difference: the difference of electrical potential between two points, also called voltage

Current: the rate of flow of electric charge past a point or region

Electric charge: a physical property of matter that causes it to experience a force when placed in an electromagnetic field

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