Electric Potential Difference Voltage Electric Potential Difference

Types of energy Nuclear energy Sound energy Mechanical energy Motion energy Thermal energy Electrical energy Chemical energy Radiant energy Gravitational energy

Categories of Energy There are two main categories of energy: POTENTIAL ENERGY; and KINETIC ENERGY.

LAW OF CONSERVATION OF ENERGY Energy can neither be created nor destroyed. It can only change from one form of energy to another. For example, electrical energy can be changed into light energy. All the energy created in the Big Bang is still in the universe and it does not change.

Electric Potential Energy Electric Potential Energy is the energy a charged particle has due to its position in an electric field. Because like charges repel, it takes energy to push two similar charges together. Think of a compressed spring and liken it to two electrons being pushed towards each other.

Electric Potential Energy Electric potential is measured in volts. To get electrons to move through a circuit, we need an electrical potential difference. A charge will not flow between two positions in an electric field with the same electric potential.

Electric Potential Energy Voltage is the potential difference between two points in a circuit. Between the negative and positive terminals. We must have a potential difference in a circuit for current to flow. A battery creates this electric potential difference.

Battery

The Battery Dry cell battery converts stored chemical energy into electrical energy. A chemical reaction produces positive ions (+ cations) at the cathode and negative ions (- anions) at the anode. This creates a potential difference between the negative and positive terminals.

The Battery The electrons want to repel each other; they want to move as far apart as possible. When the switch is open, however, they cannot go any where. The electrolyte acts as a barrier and stops the electrons going to the positive cathode.

The Battery When the switch is closed, we have a complete circuit from the anode to the cathode. The electrons are forced out of the battery by repulsion and move down the wire. Electric energy then flows around the circuit and can be used to do work. This will continue until the chemical reaction is used up.

Voltage Voltage is a measure of how much work is needed to move a charge between two points. Voltage is the amount of energy released as a charge moves between the two points. The higher the voltage, the more energy is released per charge (electron).

Voltage When a charge (electron) moves, some of its potential energy is used to move the charge. The rest of the energy can be used to do other work e.g. light a lamp, create heat Voltage is the push (or force) that drives electrons through a circuit.

Matter or Energy? Remember that electrons are particles of matter. It takes one electron about one hour to move through a meter of wire. It is the electric force that travels near the speed of light. Think of a set of dominos.

Dominos

The Voltmeter What does the Voltmeter measure? The difference in electric potential between the negative and positive terminals or each side of a component. Measure the potential difference between the terminals of your battery. Think where the red and black probes go!

The Voltmeter We measure voltage across a component. The component must be in a circuit with battery. The measurement is called the voltage drop. This is the amount of voltage used by the component. We say that the component drops voltage.

Relationship Summary The sum of the voltage drop across the components EQUALS the voltage at the source (the battery). As the resistance increases, the voltage drop across the resistor increases, and the voltage drop across the lamp decreases. The percentage of resistance contributed by a resistor in a circuit is EQUAL to the percentage of the voltage drop it uses in that same circuit.

The Three Great Truths of Circuitry You have made the same discovery that Gustoff Robert Kirchhoff (1824 – 1887) did.

Gustoff Robert Kirchhoff

The Three Great Truths of Circuitry This is one of Kirchoff’s Laws, but we will call it the First Great Truth about Circuitry : I. THE SUM OF THE VOLTAGE DROPS IN A SERIES CIRCUIT IS EQUAL TO THE VOLTAGE ACROSS THE SOURCE. VOLTAGE AT SOURCE = VD1 + VD2 + VD3 + VDn (Battery)

The Three Great Truths of Circuitry In any series circuit, the larger the resistance imposed by a component, the greater the voltage drop across it. This is the Second Great Truth : II. THE GREATER THE RESISTANCE OF A COMPONENT IN A SERIES CIRCUIT, THE MORE VOLTAGE IT DROPS.

The Three Great Truths of Circuitry You have also discovered the relationship between resistance and voltage drop, which is the … The Third Great Truth : III. THE PERCENTAGE OF RESISTANCE CONTRIBUTED BY A COMPONENT IS EQUAL TO THE PERCENTAGE OF VOLTAGE DROPPED BY THAT COMPONENT IN A SERIES CIRCUIT. Resistance and voltage drop are proportional.

Homework Reading FOSS: p. 9 – 11 Blue Textbook: p. 18 – 21

More about the battery Understanding how a battery works is important to your understanding of circuits. The story of the battery starts with Alessandro Volta, a natural philosopher and professor at the University of Pavia in Italy.

THE BATTERY In 1800, Volta discovered that, when he made an alternating stack of zinc and silver plates, each separated by a sheet of absorbent paper soaked in salt water, an electric current flowed between the top and bottom of the pile. This basic voltaic cell, developed over 200 years ago, with significant refinement, is still a major source of electricity today!

The Voltaic Pile

THE BATTERY The fundamental unit in a battery is a carefully designed chemical reaction contained in a closed system called a cell. The chemical reaction provides the power (voltage) to get electrons moving through a circuit.

THE BATTERY The chemicals in the cell are chosen for their affinity for electrons – one with a greater affinity and one with a lesser affinity. The difference in affinity for electrons between the two chemicals determines how well electrons will flow.

THE BATTERY Zinc metal and the chemical manganese dioxide are found in the familiar primary or D-cell; Zinc gives up electrons and manganese dioxide receives them. The chemical with the potential to give up electrons is called the negative anode. The receiving chemical is called the positive cathode.

THE BATTERY If the two chemicals were simply mixed together, the reaction would take place immediately … … with considerable heat and intensity. We do not want this to happen!

THE BATTERY The trick is to harness the potential in the chemicals by isolating the two chemicals (the electrolyte acts as a barrier), but to connect them with a conductor such as a copper wire. In order for the chemicals to realize their potential to give or receive electrons, they must do it through the wire.

THE BATTERY Chemical reactions in the battery cause a build up of electrons at the anode. This results in an electrical difference between the anode and the cathode. It’s an unstable build up of electrons. The electrons want to rearrange themselves (repel) i.e. go somewhere with fewer electrons.

THE BATTERY The only place they can go is to the cathode. But the electrolyte acts as a barrier. However, when a wire is connected and a complete circuit is formed, the electrons will flow through the wire to the cathode. This build up of electrical potential causes electrons to flow through the circuit.

THE BATTERY When a wire is connected between the anode and the cathode, electrons leave the zinc metal and start their journey toward the manganese dioxide. At the same time, other electrons, already in the wire, leave the wire to be taken up by the manganese dioxide.

THE BATTERY The movement of charge (electrons) through the wire, driven by the potential in the chemical reaction, is electric current. The current is driven by energy stored in the chemicals, and the chemical reaction will proceed until all the chemical energy is used up – and the battery is dead 