Resistance is the opposition of a conductor to a current. When a voltage is applied, electrons tend to move more easily in some conductors than others.
Here is where the term ‘opposition’ comes. Certain conductors oppose the flow of electrons and thus have a higher resistivity.
A good conductor has low electrical resistance whereas a poor conductor has a high electrical resistance.
The resistance of a wire of a certain material depends upon three factors:
- It increases as the length of a wire increases,
- It increases as the cross-sectional area of a wire decreases,
- It depends on the material through which it is passing the material.
The Ohm and Ohm’s law
If the current in a wire is I when the voltage across it is V, its resistance R is defined by the following equation:
This equation is what ohm’s law and resistance is all about, so it is extremely important that you note that down!
This is a reasonable way to measure resistance since the smaller I is for a given V, the greater is R.
Resistance is measured in Ohms.
A resistor can be used to control the amount of current flowing around a circuit. A resistor has two terminals for the current to flow in one and get out from another.
As non-conductors tend to be bad conductors of electricity, high-resistance resistors tend to be made from graphite, particularly as they have high melting points.
A variable resistor (sometimes also called as a potentiometer), can be used to alter the current flowing through a circuit.
A variable circuit has three terminals; as the control is turned, the contact slides over the resistive track. The current enters at one end and flows through the resistive track until it reaches the contact, where it leaves through the terminal.
The resistance depends on the length of the resistive track and the position of the contact.
Uses of variable resistors include:
- Used to control volume of radio and video players.
- Used to control fan speed.
- Used in laboratories as a rheostat.
Resistance in series circuits
If several resistors are connected together in a series circuit, then it means that the current must flow through all of them.
The combined resistance R in the circuit is simply the sum of all the separate resistances.
If there are three resistors in a series circuit, their combined resistance will be:
Hence for the resistors in a series circuit:
- The combined resistance is equal to the sum of all resistances.
- The current is the same at all points around the circuit.
Resistance in parallel circuits
The effective resistance of several resistors connected in a parallel circuit is less than that of any of the individual resistors.
This is due to the fact that it is easier for the current to flow.
Hence to calculate the effective resistance R for three resistors in a parallel circuit, the equation will be as follows:
Thus, to summarise the resistance in parallel circuits:
- The effective resistance is less than the resistance of individual resistors.
- The current from the source is greater than the current through individual resistors.
It has been proven that the resistance R of a wire of a given material is:
- Directly proportional to its length: This means that the greater the length of the wire, the greater its resistance would be.
- Inversely proportional to its cross-sectional area: This means that the shorter the diameter (thickness) of a wire, the greater its resistance shall be.
Potential divider circuits
A potential divider is a part of a circuit consisting of two resistors connected in series.
Many a times batteries used in circuits have a fixed voltage (say 5V or 10V). To obtain a smaller or a variable voltage (potential difference), the fixed voltage is split up using a potential divider.
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