Current Electric Potential Difference VOLTAGE Lets first revisit

  • Slides: 25
Download presentation
Current

Current

Electric Potential Difference. VOLTAGE Let’s first revisit gravitational potential energy… What is gravitational potential

Electric Potential Difference. VOLTAGE Let’s first revisit gravitational potential energy… What is gravitational potential energy dependent on? PE = mgh Work (energy) is required to lift these rocks against the force of gravity. m 2 m 2 h h

Work is also required to move charges in an electric field. If the direction

Work is also required to move charges in an electric field. If the direction of an electric field is such that it opposes the motion of a charged particle, work must be done to move the particle in that direction. Work must be done to keep 2 like charges together or two opposite charges apart. Doing work on a system transfers energy. This influences the electric potential (think potential energy) of the charge. The electric potential of this positive test charge changes as it is moved closer to the positive metal sphere. W = Fd + + ++ +

A + B - + A - B Electric Field Lines + + +

A + B - + A - B Electric Field Lines + + + + For each situation below determine if work is done on the test charge to move it from point A to point B. NO work is done. The + charge is moving with nature; work is not required when it moves with the electric field. YES work is done. The + charge is moving against nature; work is required when it moves against the electric field.

A + - B The positive test charge will naturally move in the direction

A + - B The positive test charge will naturally move in the direction of the electric field. No work is done in moving it. The potential energy will decrease (just like an object falling with gravity). + + + + Electric Field Lines B + A - + + + + Now let’s look at the electric potential of the charges. Are the charges losing or gaining potential energy when they are moved from A to B? The positive test charge does not want to naturally move against the electric field (same charges repel) so work needs to be done to move it to point B. The potential energy will increase (just like raising an object up against gravity).

We can now conclude that the high energy location for a positive test charge

We can now conclude that the high energy location for a positive test charge is a location nearest the positive source charge; and the low energy location is furthest away. Electric Field Lines + Lower PE + + Higher PE

Electric Potential Difference (V) The electric potential difference between two points in an electric

Electric Potential Difference (V) The electric potential difference between two points in an electric field is the work done per unit charge. V=W q 1 J/C = 1 volt

If the electric potential difference between two locations is 1 volt, then 1 C

If the electric potential difference between two locations is 1 volt, then 1 C of charge will gain 1 J of potential energy when moved between those two locations. Electric Field Lines + + +

If 8 Joules of work are required to move 2 Coulombs of charge through

If 8 Joules of work are required to move 2 Coulombs of charge through a 3 -ohm resistor, what is the potential difference across the resistor? V=W q V = 8 J 2 C V = 4 V

Units- Joules vs. e. V How much work is done to move an elementary

Units- Joules vs. e. V How much work is done to move an elementary charge (+/- e) against an electric field through a potential difference of 1 volt? V=W q q. V = Wq q W = Vq W = (1 V)(1. 6 x 10 -19 C) = 1. 6 x 10 -19 J = 1 e. V (electronvolt) Electronvolts are like inches and Joules are like miles.

How much energy in e. V is needed to move one electron through a

How much energy in e. V is needed to move one electron through a potential difference of 1. 0 x 102 V? W = energy W = Vq W = (1. 0 x 102 V)(-1. 6 x 10 -19 C) W = 1. 6 x 10 -17 J = 100 e. V 1 e. V = 1. 6 x 10 -19 J

How many electronvolts are in 320 x 10 -19 J of energy? 1 e.

How many electronvolts are in 320 x 10 -19 J of energy? 1 e. V = 1. 6 x 10 -19 J 200 e. V

e. V and Joules are both units of ENERGY!

e. V and Joules are both units of ENERGY!

Static Electricity vs. Electric Current Static electricity is a one time event. Electric current

Static Electricity vs. Electric Current Static electricity is a one time event. Electric current is a constant flow.

Electric current (I) is the rate at which charge passes a given point in

Electric current (I) is the rate at which charge passes a given point in a electric circuit. An electric circuit is a closed path along which charged particles move.

Current I = Δq t I = amps (A) An ammeter is a device

Current I = Δq t I = amps (A) An ammeter is a device used to measure current.

Determine the current for these situations: 1. A cross section of wire is isolated

Determine the current for these situations: 1. A cross section of wire is isolated and 20 C of charge are determined to pass through it in 40 s. 20 C 2. A cross section of wire is isolated and 2 C of charge are determined to pass through it in. 5 s. 2 C 40 s I = Δq t = 20 C 40 s . 5 s =. 5 A I = Δq t =2 C. 5 s =4 A

Requirements necessary for an electric current: 1. There must be a closed conducting path

Requirements necessary for an electric current: 1. There must be a closed conducting path which extends from the 2. positive terminal to the negative terminal of a cell or battery 3. (combination of cells).

Requirements necessary for an electric current: 2. There must be a difference in electric

Requirements necessary for an electric current: 2. There must be a difference in electric potential between the two end points in the circuit. The potential difference may be supplied by a cell or battery. In other words you need an energy source. The potential difference or voltage of a circuit can be measured using a voltmeter.

Light Bulbs • Using only a light bulb, a wire, and a battery come

Light Bulbs • Using only a light bulb, a wire, and a battery come up with 4 ways to create a complete circuit so that the light bulb lights up.

You need to create a closed circuit.

You need to create a closed circuit.

Conventional flow vs. THE TRUTH Ben Franklin envisioned positive charges as the carriers of

Conventional flow vs. THE TRUTH Ben Franklin envisioned positive charges as the carriers of charge. Because of this an early convention for the direction of an electric current was established to be in the direction which positive charges would move. The direction of an electric current is by convention the direction in which a positive charge would move. However, we now know that the positive charges (protons) are NOT the charged particles that are moving. IT’S THE ELECTRONS. And the negatively charged electrons would be moving in the opposite direction.

Alternating Current AC - The electrons in alternating current flow in one direction, then

Alternating Current AC - The electrons in alternating current flow in one direction, then in the opposite direction—over and over again. Electricity from a power plant is alternating current. Direct Current DC - The electrons in direct current flow in one direction. The current produced by a battery is direct current.

In the United States, the current flow alternates 120 times per second. (In Europe

In the United States, the current flow alternates 120 times per second. (In Europe it alternates 100 times per second. ) The current supplied to your home by the local utility is alternating current.

War of Currents DC vs. AC EDISON WESTINGHOUSE Edison carried out a campaign to

War of Currents DC vs. AC EDISON WESTINGHOUSE Edison carried out a campaign to discourage the use of alternating current, including spreading disinformation on fatal AC accidents, publicly killing animals, and lobbying against the use of AC in state legislatures. Edison directed his technicians to preside over several AC-driven killings of animals, primarily stray cats and dogs but also unwanted cattle and horses. Acting on these directives, they were to demonstrate to the press that alternating current was more dangerous than Edison's system of direct current. TESLA