PSAA Curriculum Unit Physical Science Systems

Problem Area Energy and Power Systems

Generating an Electromotive Force Lesson

What is needed to produce energy? Can electricity be produced with two nails and a piece of fruit?

Multimeter What electrical measurements can be taken with this meter? What are some practical uses for these measurements?

Learning Objectives Generate an electromotive force by means of chemicals, magnetism, heat, and solar radiation. Construct a primary cell and understand its function.

Learning Objectives Explain the function of a voltmeter and construct a voltmeter circuit, which is the same type as used in commercial voltmeters. Properly connect a voltmeter to a circuit and measure voltage in a circuit.

Terms Ammeters Cell Conductor D’Arsonval meter movement Electricity Electromotive force Electrons Linear scale Magnetic field Multimeters Nonlinear scale Solar Cell Square-law scale Voltmeter

What different methods can be used to generate an electromotive force? Electricity is a form of energy present when electrons move through a complete path. Electrons are negatively charged atomic particles that normally revolve around the nucleus of an atom. The force that causes the electrons to move through the path is voltage or electromotive force. The path the electrons follow through easily is a conductor.

What different methods can be used to generate an electromotive force? The most popular commercial method of generating an electromotive force is by means of magnetism. A coil of wire has a moving magnetic field passed through it and thus produces an electromotive force. This type of generation occurs in an alternator in an automobile or in the large generators used to produce electricity for the home.

Magnetism In 1820, Danish physicist, Hans Oersted discovered that a compass needle could be deflected by an electric current. Until that time, it was generally thought electricity and magnetism were completely different.

Magnetism (cont.) In order for there to be a reading on a voltmeter, for example, a force must act upon the charges that are moving along the magnetic field lines. If the charge is not moving or is moving in the same direction as the magnetic fields, then there is no force. But, if the charge is moving across the field lines, then the charge has force acting upon it and an electrical current can be measured.

Magnetism (cont.) Hans Oersted showed that electricity can have an effect on magnetic fields, but it wasn’t until 1832 that Michael Faraday showed that magnetic fields could have an effect on electrical currents. This is called the complementary effect and is demonstrated in this experiment.

What different methods can be used to generate an electromotive force? Emf (electromotive force) by chemical reaction was discovered by Anastasio Volta who invented the first battery by discovering that certain combinations of metals produced an electromotive force.

Electromotive Force (Emf) Emf by heat is demonstrated by the use of a thermocouple, a device that consists of the junction of two dissimilar metallic conductors, such as, copper and iron. An emf is induced when the conductors are maintained at different temperatures.

Electromotive Force (Emf) Emf by solar radiation is demonstrated by the photovoltaic cell. This solar cell has the capacity to convert light directly into electricity without having a heat engine (defined as any device that changes thermal energy into mechanical work) involved. Emf by magnetism was discussed earlier in this lesson.

Solar Cell

What is a primary cell? How does a cell generate electromotive force? Chemical reaction is one of the six basic forms of energy (Others include friction, magnetism, pressure, heat, and light). Chemical energy is converted into electrical energy by a device known as a cell. Two or more cells connected form a battery.

Primary Cells Primary cells can be recharged using only new materials while secondary cells are rechargeable without replacing materials used to make the cell. The lead-acid storage battery is a type of secondary cell.

How does a cell generate electromotive force? Each cell in the battery has two groups of coated lead plates known as electrodes. The positive plate is made of lead peroxide and the negative plate is made of lead. The electrodes are separated from each other and immersed in a liquid solution called an electrolyte.

How does a voltmeter function and how can we construct a voltmeter circuit? A voltmeter circuit contains a d’Arsonval meter (similar to a galvanometer) and an extremely high ohm resistor in series with the meter.

How does a voltmeter function? A linear scale on the meter has readings divided into equal portions. A nonlinear scale on the meter is marked off by uneven spaces. Voltmeters indicate the amount of voltage present in the circuit, ammeters measure current flow through the conductor, and ohmmeters indicate electrical resistance.

How does a voltmeter function and how can we construct a voltmeter circuit? Multimeters are capable of measuring more than one characteristic of a circuit, i.e. voltage, amperage, and resistance. Multimeters usually show both linear and nonlinear scales. A square-law scale is a type of nonlinear scale which increases by squares. Each larger scale marking indicates that the measured value is multiplied by itself.

How does a voltmeter function and how can we construct a voltmeter circuit? d’Arsonval meter movement is the portion of the meter consisting of a fixed permanent magnet and a moving electromagnetic coil. The voltmeter is calibrated to show the amount of voltage which would cause an amount of current to flow through the carefully selected resistance of the voltmeter. The voltmeter is current sensitive. The extent of the movement of the meter pointer is directly proportional to the amount of current flowing through the meter.

How is a voltmeter used to properly measure voltage in a circuit? A voltmeter is always connected in parallel in a circuit. It is put across a resistor or voltage source. The multiplier resistor allows for varying degrees of accuracy when using a multimeter for ampere, ohms, and volt readings.

Connecting a Voltmeter in parallel to a test circuit.

What is a voltage drop? Voltage drops occur in power lines running from the transformer to the circuit box and in the wiring inside a building. A small amount of power is always lost as electricity flows through the wires. The energy loss is measured as voltage drop. The voltage drop is calculated by multiplying the current times the resistance.

Review/Summary What different methods can be used to generate an electromotive force? What is a primary cell? How does a cell generate electromotive force?

Review/Summary How does a voltmeter function and how can we construct a voltmeter circuit? How is a voltmeter used to properly measure voltage in a circuit? What is a voltage drop?