According to Ohm’s law, the **voltage** drop, V, across a resistor when a current flows through it is calculated by using the equation V=IR, where I is current in amps (A) and R is the **resistance** in ohms (Ω).

The current that flows through most substances is directly proportional to the voltage *V* applied to it. The German physicist Georg Simon Ohm (1787–1854) was the first to demonstrate experimentally that the current in a metal wire is directly proportional to the voltage applied. The many substances for which Ohm’s law holds are called ohmic. These include good conductors like copper and aluminum and some poor conductors under certain circumstances. Ohmic materials have a resistance R that is independent of voltage V and current I. An object that has simple resistance is called a resistor, even if its resistance is small.

**Resistors in Series**

Resistors are in series whenever the flow of charge, or the **current**, must flow through components sequentially.

The total resistance in the circuit is equal to the sum of the individual resistances.

**Resistors in Parallel**

Resistors are in parallel when each resistor is connected directly to the **voltage** source by connecting wires having negligible resistance. Each resistor thus has the full voltage of the source applied to it.

Each resistor in the circuit has the full voltage. According to Ohm’s law, the currents flowing through the individual resistors are I1=VR1

The total resistance in a parallel circuit is equal to the sum of the inverse of each individual resistance.

**Resistivity** is the property of a material that quantifies how strongly it resists or conducts electric current. A low resistivity indicates a material that readily allows electric current and vice versa. It is calculated as:

ρ=R•A/L

R is the electrical resistance of a uniform specimen of the material

l is the length of the specimen

A is the cross-sectional area of the specimen

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Key Points

• Voltage drop V across a resistor when a current flows through it is calculated by using the equation V=IR

• The total resistance in the circuit is equal to the sum of the individual resistances.

• The total resistance in a parallel circuit is equal to the sum of the inverse of each individual resistance.

• Resistivity measures how strongly a material resists or conducts electric current.

Key Terms

**current**: is the amount of charge moving through a cross-section, over a period of time.

**voltage**: Electrical potential difference, expressed in Volts

**resistance**: Resistance is a measure of the opposition to current flow in an electrical circuit.

**resistivity:** the property of a material that quantifies how strongly it resists or conducts electric current