Current-Voltage Characteristics

Current-Voltage Characteristics

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Electric circuits are an important part of our lives and we use something called Ohm’s law to study them. This law helps us understand the relationship between three things. When we want to understand more about materials and circuits, we have to study something called current-voltage characteristics. These characteristics help us see how different devices and setups behave.

First off, what is Ohm’s law?

Ohm’s law is a rule that helps us understand electric circuits. This rule says that the amount of electric current flowing through a conductor is related to the potential difference across it. This means that if we increase the potential difference, the current will also increase. We measure potential difference in volts (V), current in amperes (A), and electrical resistance in ohms (Ω). Resistance is the thing that stops the current from moving easily through the material. It depends on many things like the material and temperature. By changing the resistance, we can control the current. That's why we study the behaviour of resistance in materials and circuits. Ohm's law tells us that this relationship is usually constant and linear for most materials.

Non-ohmic materials

Resistance is not always a fixed value that we get by dividing the potential difference by the electric current. It's actually a function that depends on both the,I). Ohm's law is just an approximation for a small area of this function. In materials that aren't ohmic, the resistance won't follow the linear approximation. To calculate resistance, we need to know the relationship between current (I) and voltage (V). If this relationship is linear (V=k·), then the constant of proportionality is the resistance. If the relationship is different, then we need to use a different function. The graph below shows why Ohm’s law only works for a small range of current and potential difference values.

Linear approximation
Linear approximation

Even if the relationship between voltage and electric current isn't a straight line (as shown in the green graph), we can still use Ohm's law by limiting ourselves to a small range where the relationship is approximately linear (as shown by the red line). The smaller the range, the better the approximation. So, if we have a function that describes the relationship between voltage and current (like the green graph), we can use Ohm's law to calculate resistance by approximating the function with a straight line in a small range where voltage and current don't vary much.

Current-voltage characteristics

Current-voltage characteristics are the curves specifying the relationship between the electric current and the potential difference of a device. Let’s study several examples of these curves in different devices and find out what conclusions we can draw from them.

Current-voltage characteristics of an ohmic resistor

The current-voltage characteristics of ohmic resistors are:

The I-V graph for an ohmic resistor is a straight line. The curve passes through the origin, which means that for zero potential difference, we have zero current. The current is directly proportional to the potential difference. The proportionality constant is the resistance. The I-V graph for an ohmic resistor is a straight line.

Current-voltage characteristics of a filament

The current-voltage characteristics of a solar photovoltaic cell are different from those of filaments and diodes because they convert light into electrical energy. In the region of positive potential difference, the current grows arbitrarily and a constant potential difference appears, but this cannot be used efficiently because the material is not receiving light. As the amount of incident light increases, the current becomes more negative, and a negative potential difference appears that can grow arbitrarily depending on the characteristics of the light and the material. The higher the frequency of the light, the more intense the electric current induced.

I-V graphs for a resistor, diode, and battery
I-V graphs for a resistor, diode, and battery

Current-Voltage Characteristics - Key takeaways Ohm’s law states that the relationship between the voltage in a circuit and the current flowing through it is linear and, usually, constant. It is an approximation of the behaviour of most materials. The relationship between voltage and current is not linear. It is determined by the resistance, which measures the obstruction of a medium to the flow of current. It is helpful to study the current-voltage curves or I-V curves of different devices and materials to understand how they work. Diodes, filaments, and photovoltaic cells are good examples of non-ohmic devices that serve different purposes.

Current-Voltage Characteristics

What is the characteristic current voltage of a resistor?

A resistor is a term for a resistance whose value does not vary significantly, which allow us to use Ohm’s law.

Why are the voltage-current characteristics important?

Voltage-current characteristics are important because we can extract valuable information from them about the resistance and other quantities in different regions. With this information, we can build devices that serve different purposes. 

What is the voltage-current graph?

The current-voltage graph or voltage-current graph is the graphical representation of the behaviour of electric current and voltage in a certain circuit or device.

How do you describe current and voltage?

Current is the rate of flow of charge. Voltage is the work done in carrying a unit charge from one point to another. They are not independent and resistance is a quantity that captures their dependence.

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