Resistance in Circuits
Electrical resistance is the opposition to the movement of electrons as they flow through
a circuit.
Example: insulators then to minimize the amount of electron flow, so the internal
resistance of an insulator is quite high. Conversely, a conductor like copper has a very
low internal resistance. This is why electrons flow so easily in a copper wire.
Factors that affect resistance:
All materials have some internal resistance. The greater the resistance, the low the
current, and the warmer the material becomes when current flows through it. This
happens because, as the electrons move through the material, they bump into atoms
that make up the material. In the process, electrical energy is converted to thermal
energy.
There are four main factors that determine the amount of resistance:
1. Type of material
The ability of a material to conduct electricity is determined by how freely
electrons can move within the material. Copper is used in circuits because it is
an excellent conductor. It has low electrical resistance. Silver is an ever better
conductor, but is too expensive to use in low-cost electrical devices
2. Cross-sectional area
Thicker wires have less internal resistance than thinner ones. Electrons flowing
through a thicker wire have more room to move freely.
3. Length
As you increase the length of a wire, its internal resistance increases. This
happens because electrons have to travel though more material.
4. Temperature
When a wire gets warmer, the atoms that make up the wire gain more energy
and vibrate faster. The increased vibration results in more collisions between the
atoms and the free-flowing electrons in the current. Since greater vibrations
causes more collisions, resistance increases with temperature.
Calculating resistance:
German physicist Georg Ohm discovered a mathematical relationship between
potential difference and current. Ohm’s findings applied only to certain types of
materials, but for those materials you can plot a graph of the voltage versus the
current and you will get a straight line relationship.
The slope of the straight line represents the resistance of the material. The
steeper the slop of the straight line, the greater the resistance.
The relationship among voltage, current and resistance can be written
mathematically as an equation
R= V
I
This relationship is called Ohm’s law. It states that as the potential difference
across a load increases, so does the current
Example problem:
A load has 1.2 A of current flowing through it. The voltage across the load is
6.0V. Calculate the resistance of the load.