Electric Current

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Electric Current

Key Concepts:

Electric Charge

Electric Potential (and Potential Difference)

Electric Current

Resistance

Electric Charge: The electrons found "orbiting" the atoms in the universe have various physical properties. One of the properties is called electric charge. There are two kinds of charge, negative and positive. The electric charge on an electron is very small and it is NEGATIVE. The protons found at the center of atoms have the same amount of charge as the electron except it is a POSITIVE electric charge: +1.6(10)-19 Coulombs.

Charge is measured using the SI unit Coulomb (C).

The electric charge on both electrons and protons is very small and is equal to: +/-1.6(10)-19 Coulombs (minus for electrons and plus for protons)

Two charges of opposite sign attract each other and two charges of similar sign repel each other (similar to magnets). If an object has the same amount of positive and negative charge the net charge on the object is the sum of the two kinds of charge and adds to zero, and we say the object is electrically neutral. So if an object has a number of protons equal to the number of electrons its net charge is zero. All atoms have a net charge of zero, but ions can have a positive or negative charge depending on whether they have more protons or electrons.

Since the electrons are more mobile than the protons (which are buried in the nucleus of the atom) we transfer electrons to a neutral object to make it have a net negative charge. When we do that the neutral object from which we transferred the electrons becomes positively charged. Note that the total electric charge of the two bodies together does not change - this is known as the conservation of charge. We cannot create or destroy electric charge, we simply transfer charges (electrons) from one object to another to change the net charge of objects.

The ancient Greeks discovered that electric charge could be transferred between two objects by rubbing them together. Rubbing plastic and fur together results in electrons from the fur being rubbed off onto the plastic, leaving the fur positively charged and the plastic negatively charged. When glass and silk are rubbed together which one has the electrons rubbed off?

Electric Potential (symbol: V) If two charged conductors are placed in contact with each other the direction of the flow of electrons is determined by a quantity called the electric potential of the conductor. The electric potential depends mainly on the concentration of charges on the conductor (and also it's shape and size). Consider two conductors, one is A with an more electrons than protons and therefore negatively charged, and the second is B with less electrons than protons and therefore positively charged. When A and B are placed in contact with each other (or joined by a metallic wire) electrons will flow from A, which has more electrons, to B, which has less electrons so that eventually both A and B have the same number of excess electrons, or in other words have the same charge.

The conductor A with an excess of electrons is said to be at a negative (or lower) potential while the conductor B with a deficit of electrons is said to be at a positive (or higher) potential.

Electrons flow from the conductor at lower potential to the conductor at the higher potential.

For electrons to flow between two conductors, there has to be a potential difference between them. The SI unit for measuring potential and potential difference is the volt (V). The potential difference between two points is measured using a device called a voltmeter.

Electric Current (symbol: I) An electric current is defined as the rate of flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma.

Electric currents cause Joule heating, which creates light in incandescent light bulbs. They also create magnetic fields, which are used in motors.

The particles that carry the charge in an electric current are called charge carriers. In metals, one or more electrons from each atom are loosely bound to the atom, and can move freely about within the metal. These conduction electrons are the charge carriers in metal conductors.

The direction of the flow of current is by convention taken to be opposite to the direction of the flow of electrons. Thus, current flows from the conductor at higher potential to the conductor at the lower potential. In the example above of two conductors A and B, electrons will flow from A to B, while current will flow from B to A.

The SI unit for measuring an electric current is the ampere (A), which is the flow of electric charge across a surface at the rate of one coulomb per second. Electric current is measured using a device called an ammeter.

Resistance (symbol: R) When an electric current flows through a conductor (for example, a metal wire), free electrons are are moving through the conductor while protons remain in their positions. The electrons move at random, colliding among themselves and colliding with the protons. These collisions reduce the speed of the moving electrons and thus provide an opposition to the flow of electrons.

The electrical resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor. The inverse quantity is electrical conductance, the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with the notion of mechanical friction.

The SI unit of electrical resistance is the ohm (Ω).

The resistance of a conductor depends on the following four factors:

Material: Materials have different concentrations of free electrons. Materials with more free electrons offer less resistance. Materials like silver, copper, lead and aluminium, offer less resistance and are therefore good conductors of electric current, while rubber, glass and wood offer very high resistance and are poor conductors of electric current. Poor conductors of electric current are used as insulators, which are very useful when you want to prevent the flow of electric current. For example, wires that carry current in your home are made of copper which is a very good conductor but are covered with rubber as an insulation to prevent the current from flowing into your body.

Length: In a longer conductor the number of collisions will be more as compared to a shorter one and therefore a longer conductor offers more resistance. The resistance of a conductor is directly proportional to its length.

Thickness: In a thick conductor electrons get a larger area of cross-section to flow through as compared to a thin conductor and therefore a thick conductor offers less resistance. The resistance of a conductor is inversely proportional to the area of its cross-section, perpendicular to the flow of the current.

Temperature: With an increase in the temperature of a conductor the random motion of electrons increases which leads to an increase in collisions and therefore more resistance. Hence, the resistance of a conductor increases with an increase in its temperature.

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