# Air Electric Breakdown: Understanding Voltage/Distance Limits

• ctech4285
In summary: A spark gap is used in internal combustion engines to ignite the fuel/air mixture.In summary, the professor states that a ball with a radius of 1m cannot exceed 3MV due to the electric breakdown of air. This means that the strength of the electric field, not the potential of the ball, determines when the air will break down. This can be calculated using Gauss' law and it is important to note that for a given voltage, the field strength varies as 1/r. This phenomenon is also observed in lightning and in spark gaps used in internal combustion engines.

#### ctech4285

i been watching some lectures from OCW and the professor said something like a ball with a radius of 1m can not exceed 3MV because of the electric breakdown of air.

to me it sounds like you can not exceed a potential difference 3MV per meter. so it does not matter the physical size of an object just the voltage difference over a distance is less.

ctech4285 said:
i been watching some lectures from OCW and the professor said something like a ball with a radius of 1m can not exceed 3MV because of the electric breakdown of air.

to me it sounds like you can not exceed a potential difference 3MV per meter. so it does not matter the physical size of an object just the voltage difference over a distance is less.
It is the strength of the electric field, not the potential of the ball that determines when the air breaks down.

You can use Gauss' law to determine the electric field:

$$\int E\cdot dA = \frac{q}{\epsilon_0}$$

$$E = \frac{q}{4\pi r^2\epsilon_0}$$

The electric potential outside the sphere is the same as if you were dealing with a point charge:

$$V = \frac{q}{4\pi r\epsilon_0}$$

which means that E = V/r

So you can see that for a given voltage of a conducting sphere, the field strength varies as 1/r.

AM

http://en.wikipedia.org/wiki/Electrical_breakdown

Electrical breakdown occurs within a gas (or mixture of gases, such as air) when the dielectric strength of the gas(es) is exceeded. Regions of high electrical stress can cause nearby gas to partially ionize and begin conducting. This is done deliberately in low pressure discharges such as in fluorescent lights (see also Electrostatic Discharge) or in an electrostatic precipitator.

Partial electrical breakdown of the air causes the "fresh air" smell of ozone during thunderstorms or around high-voltage equipment. Although air is normally an excellent insulator, when stressed by a sufficiently high voltage (an electric field strength of about 3 x 106V/m[1]), air can begin to break down, becoming partially conductive. If the voltage is sufficiently high, complete electrical breakdown of the air will culminate in an electrical spark or arc that bridges the entire gap. While the small sparks generated by static electricity may barely be audible, larger sparks are often accompanied by a loud snap or bang. Lightning is an example of an immense spark that can be many miles long. The color of the spark depends upon the gases that make up the gaseous media.

## 1. What is air electric breakdown and how does it occur?

Air electric breakdown is the phenomenon where a material, in this case air, that is normally an insulator becomes conductive and allows electric current to flow through it. This occurs when the voltage between two points exceeds a certain threshold and causes the air molecules to ionize, creating a path for the current to flow through.

## 2. What is the importance of understanding voltage/distance limits in air electric breakdown?

Understanding the voltage/distance limits in air electric breakdown is crucial in preventing electrical accidents and ensuring the safety of both people and equipment. It also helps in determining the maximum voltage that can be applied to a particular distance without causing air electric breakdown.

## 3. How do you calculate the voltage/distance limits for air electric breakdown?

The voltage/distance limits for air electric breakdown can be calculated by using the Paschen's law, which relates the breakdown voltage to the product of pressure and distance between the two points. There are also online calculators and tables available for this purpose.

## 4. What factors can affect the voltage/distance limits for air electric breakdown?

The voltage/distance limits for air electric breakdown can be affected by various factors such as air pressure, humidity, temperature, and the presence of impurities or contaminants in the air. These factors can alter the breakdown voltage and distance, so they must be taken into consideration when calculating the limits.

## 5. Can the voltage/distance limits for air electric breakdown be increased?

Yes, the voltage/distance limits for air electric breakdown can be increased by using insulating materials or increasing the distance between the two points. However, it is important to note that exceeding the limits can still result in air electric breakdown and should be avoided to ensure safety and prevent damage to equipment.