Module 2: Current, Voltage and Resistance

Precision Resistor

A resistor with a tolerance of 1% or under

Conductors

Materials like copper that offer low resistance to the passage of electric current

An increase in three gauge numbers has what effect on wire resistance?

The resistance doubles

Describe the three things that must occur in order for work to be done

1. A force must be applied 2. The force must act through a certain distance 3. The force must have a component along the displacement

What four factors affect the resistance of any material with a uniform cross-sectional area?

1. Type of material 2. Length 3. Cross-sectional area 4. Temperature

What is the effective velocity of electrons in a conductor?

186,000 miles per second (speed of light)

What is a chip resistor?

A chip resistor is a small, high-quality resistor manufactured on small, rectangular ceramic chips.

Passive Device

A device that contributes no energy or removes energy

What is the difference between a voltage drop and a voltage rise?

A device that produces voltage, such as a generator, provides a voltage rise for a circuit. A voltage-drop represents the energy consumed by a device such as a load resistor.

Active Device

A device, such as a transistor, capable of controlling voltage or current..

Explain what is meant by the terms "load" and "load resistor"

A load is the name given to any device connected across an energy source. Load resistance represents the amount of opposition to current flow offered by the load.

Voltage Sources: Electric Conversion

A power supply is a device that converts one type of electric potential to current to another. A DC power supply converts an alternating signal into one of a fixed magnitude. In addition to DC power supplies, other examples of electrical conversion sources are rotary converters, and motor-generator sets.

How does a primary cell differ from a secondary cell?

A primary cell provides electrical energy to the limit of its chemical energy. Primary cells are non-renewable voltage sources. A secondary cell is capable of being recharged.

Explain the difference between a thermistor and a varistor

A thermistor is a resistor whose resistance varies with temperature. A varistor is a voltage-dependent resistor with an internal resistance that varies inversely with voltage.

What is the difference between an open-circuit and a short-circuit?

An open-circuit has an infinite value of resistance, while a short-circuit has no resistance.

Open-circuit

An open-circuit is similar to an open switch, in that the flow of current is completely interrupted through the device.

Explain the difference between direct current and alternating current

Direct current always maintains the same direction of flow, while alternating current is constantly changing direction.

A rectifier converts direct current to alternating current

False

One ampmere is equal to one coulomb of electric charge passing a certain point in an electric circuit in one millisecond (True/False?)

False One ampmere is equal to one coulomb of electric charge passing a certain point in an electric circuit in one second

The resistance of a conductor is inversely proportional to its length and directly proportional to its cross-sectional area (True/False?)

False The resistance of a conductor is directly proportional to its length and inversely proportional to its cross-sectional area

Name the four general categories for fixed-type resistors

Fixed type resistors are classified as carbon composition, film, wirewound, and chip resistors.

Semiconductors

Materials like germanium and silicon with intermediate resistivity values

Insulators

Materials such as plastic and rubber that conduct electric charge poorly

If the magnitude of a unidirectional current varies with time, what is it called?

Pulsating current

Resistance

Resistance is the opposition to current flow in a circuit.

Resistivity

Resistivity is a constant of proportionality that is used to compare the inherent resistance characteristics of different materials. Resistivity = resistance x cross-sectional area/length

What causes a resistor to fail?

Resistor failure is often due to excess heat caused by too large a value of current flowing through the resistor.

Describe the two basic types of resistors

Resistors are classified as either fixed or variable. Fixed resistors have permanent ohmic values that cannot be changed. Variable resistors have resistances that can be altered by a rotary dial or other moving mechanism.

Why do some resistors have five colour bands?

The fifth colour band of general purpose resistors represents the reliability factor. This band gives the percentage of failure per 1,000 hours of use. Precision resistors use the fifth band to indicate tolerance.

Voltage sources: Solar and Photovoltaic Cells

The photovoltaic effect, which is the basis of all solar cells, was discovered in 1893 by Edmond Becquerel. Using a pair of electrodes immersed in electrolyte, Becquerel observed a flow of current when the cell was illuminated with sunlight. A solar cell is a semiconductor device consisting of a thing layer of heavily doped P-type silicon on a heavily dope N-type silicon wafer. Incident light passes through the thin layer of P-type silicon to the junction region where is creates a large number of free electron-hole pairs. A strong electric field causes the free electrons to flow to the negative terminal, while free holes move to the positive output terminal. The net result is a potential difference created int he device when it is irradiated with light.

Describe the two types of mechanically variable resistors

The rheostat is a two-terminal variable resistor that consists of a resistance element, wound around a circular insulated form, with a centre terminal called a wiper. A potentiometer is a variable three0terminal resistor, acting as a voltage divider.

List the three types of electrical current flow and briefly describe each

The three types of electrical current flow are hole flow, electron flow, and conventional current flow. The current consisting of electrons jumping from one covalent bond position to another is called hole flow. Electron flow is based on the flow of electrons through a circuit from the negative terminal of a power supply to the positive terminal. Conventional current flow is the direction in which positive charge carriers flow in a circuit.

Voltage Sources: Thermoelectric generation

Thermoelectric generation is based on the principle that if a metal rod is heated at one end, negatively charged electrons flow from the hot end to the cooler end to reduce their energy. Examples of such sources are the thermocouples and semiconductor thermoelectric engines.

Voltage sources: Chemical

These sources convert chemical energy into electrical energy. Examples of such sources are primary cells, secondary cells, and fuel cells. Fuel cells operate on the principle that when hydrogen and oxygen are combined, water is produced and electrical energy is released. Hydrogen is available from hydrocarbon sources such as ammonia, petroleum, propane, and natural gas.

Voltage Sources: Electromagnetic Generation

This type of device is capable of converting mechanical energy to electrical energy. A generator, or dynamo, is a device in which mechanical energy is used to rotate conductors in a magnetic field to produce an emf. Examples of such sources are DC generators.

Current Flow

When most of the electron movement in a material takes place in the same direction, so that one part of the material loses electrons while another part gains electrons, the net electron movement or flow is called current flow.

What is a zero-ohm resistor used for?

Zero ohm resistors are used as jumpers in printed circuit boards

General purpose resistors have tolerances a) between 5% and 20% b) below 5% c) between 2% and 20% d) between 5% and 25%

a) between 5% and 20%

Resistors are devices that a) conduct electricity and dissipate energy as heat b) completely block the flow of current c) are not designed to dissipate heat d) act as short-circuits

a) conduct electricity and dissipate energy as heat

The unit of measurement of resistance is the a) ohm b) volt c) ampere d) watt

a) ohm

Hole flow is: a) opposite to electron flow b) opposite to conventional flow c) the same as electron flow d) none of the above

a) opposite to electron flow

The relationship between angle and resistance of a variable resistor is called a) taper b) wiper c) tolerance d) trimmer

a) taper

When no external field is present, a) electrons move in a fixed pattern b) electrons move in a random pattern c) electrons flow from the positive to the negative terminal d) there is no electron movement

b) electrons move in a random pattern

If a resistor is open-circuited, its internal resistance would be a) 0 ohms b) infinite c) equal to its rated value d) low

b) infinite

In the SI system, the unit of work is the a) Watt b) Volt c) Joule d) Coulomb

c) Joule

The units of electric potential are expressed in a) Joules per second b) Coulombs per second c) Joules per coulomb d) Volts per second

c) Joules per coulomb

Current flow is defined as a) The random motion of electric charge b) The motion of charged particles through a conductor in a random direction c) The flow of charged particles through a conductor in a fixed direction d) none of the above

c) The flow of charged particles through a conductor in a fixed direction

When the temperature of a conductor increases a) its resistance decreases b) its resistance remains constant c) its resistance increases d) non of the above

c) its resistance increases

Conductance is a) measured in ohms b) the reciprocal of voltage c) the reciprocal of resistance d) the opposition to current flow

c) the reciprocal of resistance

Zero-ohm resistors are a) not practical b) poor conductors c) used as jumpers on printed circuit boards d) made by accident

c) used as jumpers on printed circuit boards

A thermistor is a resistor whose resistance varies a) with light intensity b) mechanically c) with temperature d) with voltage

c) with temperature

List three functions of resistors in electronic circuits

current limiting resistors, bleeder resistors and load resistors

To prevent resistors from being damaged, the power loss should be a) 1/4 of its rated value b) equal to its rated value c) double its rated value d) 1/2 its rated value

d) 1/2 its rated value

When a unidirectional current is unchanging, it is called a) Pulsating current b) Alternating current c) Electron flow d) Direct current

d) Direct current

Electron flow is: a) From the positive terminal of a power supply to the negative b) Arbitrary and cannot be determined c) The direction positive charge carriers flow in a circuit d) From the negative terminal of a power supply to the positive

d) From the negative terminal of a power supply to the positive

A material with low resistivity will be a) a poor conductor b) a semiconductor c) an insulator d) a good conductor

d) a good conductor

A source of emf is called a a) voltage drop b) passive device c) current source d) voltage rise

d) voltage rise