# Voltage, Current

#### Air

Can I have some information about these topics?

• Voltage
• Current
• Resistance
• Internal Resistance

#### leright

You need to be a little more specific.

#### wxrocks

Last edited by a moderator:

#### russ_watters

Mentor
Questions that general/vague will at best get you a reference to google or a wik link. You'll need to be more specific if you want some actual answers.

#### Mindscrape

Voltage = potential difference between two points
Current = rate of change of charge with time
Resistance = proportional to the voltage and inversely proportional to current
Internal Resistance = Thevenin resistance seen by outside source

#### Air

Mindscrape said:
Voltage = potential difference between two points
Current = rate of change of charge with time
Resistance = proportional to the voltage and inversely proportional to current
Internal Resistance = Thevenin resistance seen by outside source
Thank You. I needed information like this.

#### Air

What is Internal Resistance in relation to EMF?

#### Agnostic

Think of it analogous to water flowing from an elevated tank through a small hose.

Voltage is the pressure behind the water which corresponds to amount of water in the tank pushing down.

Current is the speed of the water.

Resistance is something in the hose obstructing the path of water flow.

Internal resistance is the resistance of the water having friction with pipe walls.

#### Air

Agnostic said:
Think of it analogous to water flowing from an elevated tank through a small hose.

Voltage is the pressure behind the water which corresponds to amount of water in the tank pushing down.

Current is the speed of the water.

Resistance is something in the hose obstructing the path of water flow.

Internal resistance is the resistance of the water having friction with pipe walls.
Thats a very good analogy. Thanks.

______________________________________________________

What is Internal Resistance in relation to EMF?

#### NoTime

Homework Helper
What do you mean by internal resistance?
Do you disagree with Mindscrape's definition?

E=IR

#### Gza

Anived said:
What is Internal Resistance in relation to EMF?
Well, if you have a given EMF, generated by a magnetic field changing in time, and are near some circuit or wire, you can just use good ol' ohms law to find the resistance:

V=IR, or using different notation for your EMF, EMF=IR, or R=EMF/I, so the reistance in the circuit is proportional to the strength of the EMF, and inversely proportional to current.

#### Air

NoTime said:
What do you mean by internal resistance?
Do you disagree with Mindscrape's definition?

E=IR
The Voltage that is lost in the Power supply.

#### NoTime

Homework Helper
Usually, for most things, it's negligable.
For actively regulated power supplys it can be 0 or even negative.

If it is a factor, then the power supply resistance gets added to the rest of the resistance in the circuit.

#### Air

NoTime said:
Usually, for most things, it's negligable.
For actively regulated power supplys it can be 0 or even negative.

If it is a factor, then the power supply resistance gets added to the rest of the resistance in the circuit.
Does this affect the circuit in any way?

#### Hootenanny

Staff Emeritus
Gold Member
Does this affect the circuit in any way?
Yes, in exactly the same way a normal resistor would. Suppose you have a cell with a 12V emf and 1$\Omega$ internal resistance. This would be equivalent to a circuit containing a 12V cell with no internal resistance, in series with a 1$\Omega$ resistor.

#### Air

Hootenanny said:
Yes, in exactly the same way a normal resistor would. Suppose you have a cell with a 12V emf and 1$\Omega$ internal resistance. This would be equivalent to a circuit containing a 12V cell with no internal resistance, in series with a 1$\Omega$ resistor.
I see.

Is it dangerous if the resistance gets high? If so why and what happens?

#### Hootenanny

Staff Emeritus
Gold Member
I see.

Is it dangerous if the resistance gets high? If so why and what happens?
Resistors dissipate energy, how do you suggest they do this?

#### Air

Hootenanny said:
Resistors dissipate energy, how do you suggest they do this?
Resistance serves to limit the amount of Current through the circuit with a given amount of Voltage supplied by the battery.

#### Hootenanny

Staff Emeritus
Gold Member
Resistance serves to limit the amount of Current through the circuit with a given amount of Voltage supplied by the battery.
Yes, it does this by dissipating (removing) energy (from the circuit). This energy is dissipated as heat. Therefore, if you have a high resistance together with a high current flow, then large amounts of heat will be dissipated.

#### Air

Yes, it does this by dissipating (removing) energy (from the circuit). This energy is dissipated as heat. Therefore, if you have a high resistance together with a high current flow, then large amounts of heat will be dissipated.
After lots of heat is dissipated, does the Resistance, Voltage or Current change (i.e go lower or higher)?

I thank all the people who have helped me so far. You have increased my knowledge.

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