Understanding Ohm’s Law

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This material was developed with funding from the
National Science Foundation under Grant # DUE 1601612

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Click the +/– buttons to see how current changes when resistance changes.

Voltage

Current

Ohm’s Law describes states that electrical voltage and electrical current are directly proportional.

Ohm’s Law is the most fundamental law in electricity. The law describes the relationship between three of the most important electrical properties. These electrical principles are voltage, current and resistance.

Short circuit damage

Ohm’s Law also states that electrical resistance and electrical current are inversely proportional.

Resistance

Click the +/– buttons to see how current changes when voltage changes.

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Decrease Force

+

–

Current flow can be thought of as the movement of a cart. Voltage is the applied force to move the cart. As the force increases the cart movement increases.
If the force decreases the movement decreases. The force applied and the movement is directly proportional.

Increase Force

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On the other hand, Electrical resistance is like the weight and friction opposing the cart movement. If we increase the weight of the cart the movement will decrease. If we decrease the weight the movement will increase. These principles are inversely proportional.

Decrease
Resistance

Increase
Resistance

Click each item for more information.

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Voltage is what makes electric charges (electrons) move through an electrical circuit. Voltage is the push that causes charges to move in a wire or other electrical conductors. Voltage is also called electromotive force (EMF). Voltage is represented by a Capital letter “E” and is measured in Volts represented by a small “v”. The electric force is referred to as electrical potential. To create electron flow or electricity, you must have a difference in electrical potential within an electrical circuit. One way to produce voltage is to connect an electrical battery to the circuit.

The unit to measure electron volume is the coulomb. One coulomb of electrical charge (6.24 x 1018 electrons). One ampere of current represents one coulomb of electrons moving past a specific point in one second.

Current is represented by a Capital letter “I” and is measured in Amperes represented by a capital “A”. The letter I represents intensity, a term used early on. An ampere is a unit of measure of the rate of electron flow or current in an electrical conductor. When measuring flow of something like water, there must be a unit of volume past a specific point over a period of time.

Electric Current

The charge of an electron
e = 1.6 x 10-19 Coulomb

How many electrons are
needed to make 1 Coulomb?
6 x 1018

Voltage is the electrical force that would drive an electric current (electrons) between two points.
Current is the rate at which electric charge (electrons) flows past a point in a circuit.

6,000,000,000,000,000,000 electrons

6,000,000,000,000,000,000 electrons

electron

proton

neutron

Electrical resistance is the opposition that a substance offers to the flow of electric current. Resistance is caused by the attraction between electrons and protons in the atoms of the material used to carry electricity.
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Resistance is represented by a capital letter “R” and is measured in Ohms represented by the Greek letter omega“Ω”.

Conductor

Insulator

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The smoother top path shows a good conductor. The bottom path is an example of a poor conductor or insulator.

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Gold

Resistance is weaker in electrical conductors like copper, silver or gold. Therefore, these materials are used to carry electrical current in most electrical circuits.

Electrical Conductors

Copper

Steel

Silver

half the
length

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half the
resistance

twice the
length

half the diameter

twice the diameter

twice the
resistance

The size of a material also impacts its electrical resistance. The resistance of a long copper wire is greater than the resistance of a short wire.

double the
resistance

R

E

Ohm’s law describes the relationship between electrical current, voltage and resistance. Ohm’s law state that current is directly proportional to circuit voltage and inversely proposition to circuit resistance. This means that if the voltage connect to a circuit increases the current will increase at the same rate. The same is true is the voltage decreases, the current will decrease at the same rate.

Click each part of the diagram
to see the equation

I

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Glass

Resistance is strong in electrical insulators like rubber, wood, and ceramic. These materials contain electrical current within a circuit.

Rubber

Wood

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Electrical Insulators

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R will

decrease
proportionally

increase
proportionally

If E is increased:

Electrical Voltage and Current are Directly Proportional

I will

remain state constant

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If E is decreased:

If R is increased:

E will

Electrical Resistance and Current are Inversely Proportional

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If R is decreased:

R may

If I is increased:

E may

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