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## Physics - Resistors

Resistors resist the flow of current. All conductors are also resistors since all conductors resist the flow of current to some extent. Resistance is expressed in ohms. A resistor is composed of substances which enable it to resist the flow of current.

The ability of a given material to resist current flow is called the resistivity. Materials with the greatest resistivities are called insulators and include glass, rubber and wood. Semiconductors are a group of materials with moderate resistivities, like silicon. Conductors have very low resistivities. Metals are good conductors.

Remember that the resistivity of a material is affected by the temperature. In general, resistivity increases with increasing temperature. Ohm’s Law states that voltage is linearly proportional to current.

Rule Number 35: Ohm’s Law

Voltage equals current times resistance, or V=IR.

Power is defined as the energy used per unit of time. Electrical power is proportional to the square of the current flowing through a resistor and equal to the current times the voltage.

Rule Number 36:
Power equals current squared times resistance, or P=I2R.
Power is also current times voltage, or P=IV.
Finally, power equals voltage squared divided by resistance, or P=V2/R.

The unit of electrical power is the watt, which is I joule per second.

Resistors can be connected to one another, and you may have to calculate their total resistivity.

For resistors connected in series, RTotal = R1+R2+R3 and etc.

For resistors connected in parallel, 1/RTotal = 1/R1+1/R2+1/R3 and etc.