Current-Voltage Relationships and Ohm’s Law - Learn

Current-Voltage Relationships and Ohm’s Law – Learn

Current, Voltage and Resistance are common terms that are used to describe electricity in circuits.

Current is the amount of charge that passes through a point in a conductor each second. The symbol for current is I and the unit is amps (A).

Voltage (potential difference) is the work done to move a charge against an electric field between two points. The symbol for voltage is V and the unit for voltage is volts (V)

Resistance is a measure of how difficult it is for current to flow through a material. The symbol for resistance is R and resistance is measured in ohms (Ω). There are several factors that effect the resistance of a conductor:

Length – If the material has a longer length, this will result in a higher number of collisions with other ions within the conductor, increasing the resistance and making it harder for the current to flow.
Cross-sectional area – With a large cross-sectional area, there is less chance of collisions and the resistance will decrease.
Temperature – Higher temperatures will result in an increased vibration of the ions, making the chances of electrons colliding higher and the resistance will increase.

Ohm’s Law:

Ohm’s law describes the relationship between current, voltage and resistance:

$V=IR$

where:

$V$ = the voltage (in V)

$I$ = the current (in A)

$R$ = the resistance (in Ω).

Ohmic and non-ohmic resistors:

Ohmic conductors obey Ohm’s law and have a constant resistance. The resistance of non-ohmic conductors varies for different voltages. Light bulbs are an example of a non-ohmic conductor. The resistance of light bulb increases as the temperature of the filament increases.

Ohmic and non-ohmic conductors can be explored by plotting a graph of current vs voltage. The graph for an ohmic conductor is a straight line. The resistance can be found from the gradient of the graph. The current vs voltage graph for a non-ohmic conductor is not a straight line.

Example 1:

What voltage will flow through a wire with a resistance of 1.5Ω to produce a current of 4A?

Answer:

Using $V=IR$

where:

$I$ = 4A

$R$ = 1.5Ω

$V=4\times 1.5$

$V$ = 6V

Example 2:

What is the resistance of a wire that produces a current of 8A when a voltage of 2V is applied?

Answer:

Using $V=IR$ and rearranging to give: $R=\cfrac { V }{ I }$

where:

$V$ = 2V

$I$ = 8A

$R=\cfrac { 2 }{ 8 }$

$R$ =0.25Ω