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

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