# Current in a conductor

In electrostatic shielding, we have read that the charged particles in a conductor experience no force. In the current carrying conductor, electric field is applied across its ends and the electrons experience force due to this field and constitute electric current. However the two ideas oppose each other. If there is no electric field inside conductor, then how there will be flow of charge?

## 2 Answers

The electric field is zero in a conductor only in electrostatic situations. And this happens after a (usually extremely short) time comparable to the so-called dielectric relaxation time $\tau=\epsilon/\sigma$ for the redistribution of charges. If you apply a steady voltage difference to a conductor, there will be an electric field inside the conductor which produces a stationary current.

• I have a doubt, we know that when conductors are placed in electric field they adjust their charges in such a way that the net field inside it becomes zero. In case we are applying a constant potential, isn't it analogous to the constant Electric field situation? Then why do we still observe flow of charges. – Ankur Singh Feb 25 '18 at 6:23
• Yes. Me asking the same thing. – Gurbir Singh Feb 25 '18 at 7:54
• @a.b - You apply a potential difference (electromotoric force) with electrical contacts which maintains a steady electrical current. Charge carriers which are moved to one contact from the inside or removed from the other by the electric field are transported into the contact or replenished there from the outside by the voltage source which supplies the current. In the electrostatic situation the charges carriers don't enter or leave the conductor. There is no current after the initial movement of the charges to the conductor surface which produces the shielding of the field penetration. – freecharly Feb 25 '18 at 13:34

The electric field inside an ideal conductor will be zero when under static conditions. In this case, a current can flow without an electric field present inside the conductor. But this is only true for an ideal conductor, like a superconductor. Most people, when talking about electromagnetics, will make the assumption that there is no electric field inside a conductor, however people talking about circuits will say that there is a (small) electric field inside the conductor.

In a non-ideal conductor, such as a real world wire or metal plate, a small electric field is required to maintain a constant flow of current under constant potential. This is only true because the electric field, which is directly related to the electric force on the charges in the wire, must overcome the non-zero resistance of the wire.

The mechanism at play in a typical circuit that allows for a constant potential is the existence of a power source which can maintain a difference in charge between two points, thus providing a constant voltage. In order for this to work, charges traveling into one end must be transferred to the other end continuously, or else the wire would "succeed" in rearranging all of its charges. This is done in any number of ways, e.g. electrochemically with a battery.

Side note: Power sources can also supply constant current, generating whatever potential difference is required to maintain a constant current. These are called current sources, but they are only common when you work with circuit design; they aren't as familiar so they are often left out.

It's worth noting that applying a potential difference to a wire will be effectively "shorting" the power supply, allowing very high currents. In the case of an ideal conductor, it would require infinite current to maintain a constant potential. This is why we need current-limiting elements such as resistors. Charges inside an ideal conductor will immediately rearrange, and the only way to maintain a potential is by allowing infinite charges through the power source, which is, of course, impossible.