Understanding Voltage: Common Questions and Answers in Basic Physics

[Kyoma]

I think I have a wrong definition of the word 'voltage'. For example, a bird perching on a live wire with its 2 legs will not be electrocuted, but a bird perching with one of its legs on the ground and another on the wire will be electrocuted. Why? Wouldn't the current pass through the bird regardless of the bird's feets' positions? Why wouldn't it die when it has two of its feets on the wire?

Furthermore, in practical electricity, there are the live and neutral wires. The live wire records a voltage reading but the neutral doesn't. Why?

Lastly, voltage or current or both of them would affect the brightness of a light bulb?

Thanks. I'm relatively new to physics, so I need some simple answers. :P

 

[Andrew Mason]

I think I have a wrong definition of the word 'voltage'. For example, a bird perching on a live wire with its 2 legs will not be electrocuted, but a bird perching with one of its legs on the ground and another on the wire will be electrocuted. Why? Wouldn't the current pass through the bird regardless of the bird's feets' positions? Why wouldn't it die when it has two of its feets on the wire?

To be electrocuted, a large number of charges in the bird's body have to move (electrical current). The charges will move only if they are pushed. In order to be pushed, a potential difference is required. If there is no potential difference between the contact points (feet) there will be no electric field (electric force) created in the bird's body to push the electrons.

If both feet are on the wire, the feet are at the same potential. There is no potential difference if both feet are on the wire (eg. +15000V to +150000V) so no current flows. If one foot is on the wire (eg. +15000V) and the other is at ground (0V), the potential difference is huge (+150000V) and current flows through bird.

Furthermore, in practical electricity, there are the live and neutral wires. The live wire records a voltage reading but the neutral doesn't. Why?

In typical household AC there are 4 wires, a red, black, white and copper (ground) wire. The copper wire is connected to ground so there is no potential difference between it and ground. The others are connected to a voltage source. There is a potential difference between those wires and from the wires to the ground.

Lastly, voltage or current or both of them would affect the brightness of a light bulb?

The power in a light bulb is the product of voltage x current. Since voltage is supplied from a constant voltage source, the current is determined by the resistance of the light bulb according to Ohm's law: V = IR; I = V/R

AM

 

[Kyoma]

To be electrocuted, a large number of charges in the bird's body have to move (electrical current). The charges will move only if they are pushed. In order to be pushed, a potential difference is required. If there is no potential difference between the contact points (feet) there will be no electric field (electric force) created in the bird's body to push the electrons.

If both feet are on the wire, the feet are at the same potential. There is no potential difference if both feet are on the wire (eg. +15000V to +150000V) so no current flows. If one foot is on the wire (eg. +15000V) and the other is at ground (0V), the potential difference is huge (+150000V) and current flows through bird.

But isn't the current still flowing in the wire? Aren't the electrons still being pushed in the wire?

In typical household AC there are 4 wires, a red, black, white and copper (ground) wire. The copper wire is connected to ground so there is no potential difference between it and ground. The others are connected to a voltage source. There is a potential difference between those wires and from the wires to the ground.

I know. But, why wouldn't the live wire have a zero voltage while the neutral voltage have a recorded voltage? Why does it have to be the other way?

 

[Andrew Mason]

But isn't the current still flowing in the wire? Aren't the electrons still being pushed in the wire?

Yes. There is very small potential difference from one end of a transmission wire to the other. That is because copper is a very good conductor. So there is almost no potential difference between the bird's two feet. A significant potential difference is needed in order to drive electrons through a bird. You will understand this better when you study electricity in greater depth.

I know. But, why wouldn't the live wire have a zero voltage while the neutral voltage have a recorded voltage? Why does it have to be the other way?

Voltage is potential DIFFERENCE. The convention is to give the earth, which is essentially electrically neutral (no net charge) a potential of 0. All other voltages are measured relative to that.

I would suggest you read a good book on electricity and see if you can answer some of these questions yourself. That is a good way to learn.

AM

 

[homology]

Just to add a thought: Kyoma, have you tried using a voltmeter? In order to measure a voltage you have two leads. That's important, because as Andrew Mason discusses, you need to compare the potential at one point with the potential at another.