Electric Current AP Physics C Montwood High School R.Casao

The Battery The electric battery, invented in 1800 by Volta, represented the basis for a wide range of developments in electrical technology. The wet cloth separating the plates is soaked in a salt solution, producing a potential difference between the two end plates. +- Volta’s original battery Ag Zn wet cloth electrical converter... converts chemical energy to electrical energy

Electrical Description of a Battery A battery does work on positive charges in moving them to higher potential (inside the battery). The EMF (electromotive force) E is the work per unit charge exerted to move the charges “uphill” (to the + terminal, inside), but you can just think of this as an “applied voltage.” Current will flow, in the external circuit outside the battery from the + terminal, to the – terminal of the battery.

Electromotive Force (EMF) Batteries, generators, and solar cells, transform chemical, mechanical, and radiant energy, respectively, into electric energy. These are examples of sources of EMF. EMF is measured in Volts V; The source of EMF provides the energy the charge carriers will conduct through the electric circuit to the resistor.

Potential Difference or Voltage V Current in a circuit moves from an area of high electric potential energy to an area of low potential energy. This difference in electric potential energy is necessary for current to move through a conductor. The positive terminal of a battery is the high electric potential energy terminal and the negative terminal is the low electric potential energy terminal. Potential difference V is also measured in volts.

Potential Difference or Voltage V Within the battery, a chemical reaction occurs that transfers electrons from one terminal to another. Because of the positive and negative charges existing on the battery terminals, a potential difference (voltage) exists between them.

Potential Difference or Voltage V The battery creates an electric field within and parallel to the wire, directed from the positive toward the negative terminal. This field exerts a force on the free electrons, causing them to move. This movement of charge is known as an electric current. The current in the circuit is shown to flow from the positive terminal to the negative terminal.

Potential Difference or Voltage V EMF is the maximum amount of energy per charge the battery can provide to the charge carriers. Voltage is the energy per charge the charge carriers have after moving through the internal resistance r of the battery. –Some of the energy added to the charge carriers has to be used to travel through the battery. –The remaining energy is carried to the resistors outside the battery.

Electric Circuits A simple electric circuit will consist of: –A source of energy (in this case a battery). –Conducting wires. –A resistor R that uses the energy. –A switch to open/close circuit. The source of energy has an internal resistance r.

Two Types of Current DC current (direct current) is a steady flow of current in one direction. AC current (alternating current) - direction of current flow changes many times a second. In the US, the frequency of change is 60 Hz. Therefore, the current changes direction 60 times per second.

Electric Current When charges of like sign move, a current exists. When the charges move perpendicularly to a surface of area A, the current is the rate at which charge flows through this surface.

Electric Current Current I: If the current varies in time, instantaneous current, i: Unit: Ampere Charges flowing through a surface can be positive, negative, or both.

Electric Current The direction of flow of positive charge is used as the direction of the current. In a metallic conductor, the current is due to the motion of electrons, so the direction of the current will the opposite to the direction of flow of the electrons.

Drift Velocity The volume of a conductor of length l is V = A· l. Let n be the number of mobile charge carriers per unit volume, then the number of charge carriers in the volume of the conductor is n·A· l.

Drift Velocity The total charge in the volume of the conductor of length l is : If the charge carriers move with speed v d, the distance they move in time  t is d = v d ·  t. Let d = l. Current:

Drift Velocity If an electric field is present in the conductor, the electrons will start moving in a direction opposite to the field. The motion of the electrons will be disrupted by frequent collisions with the ions. The net result is that the electrons acquire a slow average speed, or drift velocity.

Electron Motion in a Conductor With and Without an Electric Field

Analogy of Electron Motion in a Conductor 12 Volts 0 Volts

Conductor with Current Moving from High Electrical Potential (Volts) to Low Potential

Charges Drifting in a Conductor In a conductor, the electric field that drives the free electrons travels through the conductor with a speed close to that of light. So when you flip a light switch, the electric field reaches the electrons instantly.

Helpful Websites DC Circuit Water Analogy Air Flow Analogy