Electric currentsWhereas static electricity sparks consist of the sudden movement of electrons from a negative to positive surface, Direct current or DC electricity is the continuous movement of the electrons through a wire.

A DC circuit is necessary to allow the current or steam of electrons to flow. Such a circuit consists of a source of electrical energy (such as a battery) and a conducting wire running from the positive end of the source to the negative terminal. Electrical devices may be included in the circuit.

Electrical circuit

An electrical circuit consisting of a source of DC power and a wire making a complete circuit is required for DC electricity to flow.

A flashlight is a good example of a DC circuit

When the switch is closed we say that we have a closed circuit. When the switch is open , there is a break and the current stops flowing. We say that we have an open circuit. If a wire is connected across the switch or across bulb it will act as a bypass for the electric current as it will provide an easier path for it to flow. We say that we have a short circuit.

Current shown opposite

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Although the negative charged electrons move through the wire toward the positive (+) terminal of the source of electricity, the current is indicated as going from positive to negative. This is an unfortunate and confusing convention.

Ben Franklin originally named charges positive (+) and negative (-) when he was studying static electricity. Later, when scientists were experimenting with electrical currents, they said that electricity travels from (+) to (-), and that became the convention.

This was before electrons were discovered. In reality, the negative charged electrons move toward the positive, which is the opposite direction that people show current moving. It is confusing, but once a convention is made, it is difficult to correct it.

Continuous movement of electrons

DC electricity is the continuous movement of electrons through a conducting material such as a metal wire. The electrons move toward a positive (+) potential in the wire.

Voltage, current and resistance

The electricity moving through a wire or other conductor consists of its voltage (V), current (I) and resistance (R). Voltage is potential energy, current is the amount of electrons flowing through the wire, and resistance is the friction force on the electron flow.

A good way to picture DC electricity and to understand the relationship between voltage, current and resistance is to think of the flow of water through a hose, as explained below.

Electrical voltage

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A potential or pressure builds up at one end of the wire, due to an excess of negatively charged electrons. It is like water pressure building up in a hose. The pressure causes the electrons to move through the wire to the area of positive charge. This potential energy is called Voltage, its unit of measurement is the Volt ( V ).

Electrical current

The number of electrons is called current and its unit of measurement is the Ampere or Amp ( A ). Electrical current is like the rate that water flows through a hose.

Resistance

An Ohm ( Ω ) is the unit of measurement of the electrical resistance. A conductor like a piece of metal has its atoms so arranged that electrons can readily pass around the atoms with little friction or resistance. In a nonconductor or poor conductor, the atoms are so arranged as to greatly resist or impede the travel of the electrons. This resistance is similar to the friction of the hose against the water moving through it.

Comparison with hose

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V IR

The following chart compares water running in a hose and DC electricity flowing in a wire:

Water in a Hose DC in a Wire Electrical Unitspressure potential (V) Volts - - Vrate of flow current (I) Amps - Afriction resistance (R) Ohms - Ω

Creating DC electricity

Although static electricity can be discharged through a metal wire, it is not a continuous source of DC electricity. Instead, batteries and DC generators are used to create DC.Batteries rely on chemical reactions to create DC electricity.

The automobile battery consists of lead plates in a sulfuric acid solution. When the plates are given a change from the car's generator or alternator, they change chemically and hold the charge. That source of DC electricity can then be used to power the car's lights .

Current

As a physical quantity, current is the rate at which charges flow past a point in a circuit. As depicted in the diagram below, the current in a circuit can be determined if the quantity of charge Q passing by a cross section of a wire in a time t can be measured. The current is simply the ratio of the quantity of charge and time.

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Charge is defined as the flow of current in a specific time.

Q = I * tElectromotive Force (EMF) Charge can flow in a material under the influence of an external electric field. To maintain a voltage (and flow of charge) requires an external energy source, ie. EMF (battery, power supply ) . EMF is measured in volts.

EMF of a cell is equal to the energy (work done) per unit charge.

V = Work done/ charge

= W / Q or

Work W = Q V

We can substitute Q = It in above and obtain

Work W = VIT5

Circuit Symbols

Circuit symbols are used in circuit diagrams which show how a circuit is connected together.

Wires and connections

 Component   Circuit Symbol  Function of Component

Wire To pass current very easily from one part of a circuit to another.

Wires joined

A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two T-junctions, as shown on the right.

Wires not joined

In complex diagrams it is often necessary to draw wires crossing even though they are not connected. I prefer the 'hump' symbol shown on the right because the simple crossing on the left may be misread as a join where you have forgotten to add a 'blob'!

Power Supplies

 Component   Circuit Symbol  Function of Component

Cell

Supplies electrical energy.The larger terminal (on the left) is positive (+). A single cell is often called a battery, but strictly a battery is two or more cells joined together.

BatterySupplies electrical energy. A battery is more than one cell.The larger terminal (on the left) is positive (+).

DC supplySupplies electrical energy.DC = Direct Current, always flowing in one direction.

AC supplySupplies electrical energy.AC = Alternating Current, continually changing direction.

FuseA safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.

Transformer

Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.

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Earth(Ground)

A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.

Output Devices: Lamps, Heater, Motor, etc.

 Component   Circuit Symbol  Function of Component

Lamp   (lighting)

A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.

Lamp   (indicator)

A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.

Heater A transducer which converts electrical energy to heat.

Motor A transducer which converts electrical energy to kinetic energy (motion).

Switches

 Component   Circuit Symbol  Function of Component

Push Switch(push-to-

make)

A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.

On-Off Switch(SPST)

SPST = Single Pole, Single Throw.An on-off switch allows current to flow only when it is in the closed (on) position.

2-way Switch(SPDT)

SPDT = Single Pole, Double Throw.A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.

Resistors

 Component   Circuit Symbol  Function of Component

Resistor

A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit.

Variable   Resistor (Rheostat)

This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.

Variable   Resistor (Potentiometer)

This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.

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Capacitors

 Component   Circuit Symbol  Function of Component

Capacitor

A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.

Diodes

 Component   Circuit Symbol  Function of Component

Diode A device which only allows current to flow in one direction.

LEDLight   Emitting   Diode

A transducer which converts electrical energy to light.

Meters and Oscilloscope

 Component   Circuit Symbol  Function of Component

VoltmeterA voltmeter is used to measure voltage. The proper name for voltage is 'potential difference', but most people prefer to say voltage!

Ammeter An ammeter is used to measure current.

Galvanometer A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.

Ohmmeter An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.

OscilloscopeAn oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Sensors (input devices)

 Component   Circuit Symbol  Function of Component

LDRA transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor

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Thermistor A transducer which converts temperature (heat) to resistance (an electrical property).

Ohm's Law for Electrical CircuitsThe most fundamental equation in electrical circuits is called Ohm's Law. While doing experiments on how well metals conducted electricity, German physicist George Ohm discovered the law in 1827. Ohm's Law is the equation V = I * R and is used in both AC and DC circuits. Knowing two items in this equation allows you to calculate the third.

Ohm's Law states that in a simple electrical circuit, the voltage (V) equals the electrical current (I) times the resistance (R).

V = I * R

Using equation

The importance of Ohm's Law is that if you know the value of two of the variables in the equation, you can then determine the third.

You can measure any of the parameters with a voltmeter. Most voltmeters or multi-meters measure voltage, current and resistance for both AC and DC.

Find voltage

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If you know current and resistance, you can find voltage from V = I * R. For example, if the current I = 0.2A and the resistance R = 1000 ohms, then

V = 0.2A * 1000 Ω = 200V

Find current

If you know voltage and resistance, you can use algebra to change the equation to I = V / R to find the current. For example, if V = 110V and R = 22000 ohms, then

I = 110V / 22000 Ω = 0.005A

Find resistance

If you know voltage and current, you can use algebra to change the equation to R = V / I to find the resistance. If V = 220V and I = 5A, then

R = 220V / 5A = 44 Ω

Conductors

The wire and electrical devices must be able to conduct electricity. Metal such as copper is a good conductor of electricity and has a low resistance. The tungsten filament in a light bulb conducts electricity, but it has high resistance that causes it to heat up and glow.

SERIES AND PARALLEL CIRCUITSWhat are "series" and "parallel" circuits?

There are two basic ways in which to connect more than two circuit components: series and parallel. First, an example of a series circuit:

Series DC circuit

In an electrical circuit, several electrical devices such as light bulbs can be placed in a line or in series in the circuit

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between the positive and negative poles of the battery. This is called a series circuit.

Two light bulbs in a series circuit with a battery

One problem with such an arrangement is if one light bulb burns out, then it acts like a switch and turns off the whole circuit.

Here, we have three resistors (labeled R1, R2, and R3), connected in a long chain from one terminal of the battery to the other. The defining characteristic of a series circuit is that there is only one path for electrons to flow.

Charge flows together through the external circuit at a rate which is everywhere the same. The current is no greater at one location as it is at another location. There is a clear relationship between the resistance of the individual resistors and the overall resistance of the collection of resistors.

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Total Resistance of a circuit is the amount of resistance which a single resistor would need in order to equal the overall effect of the collection of resistors which are present in the circuit.

RT = R1 + R2 + R3 + ...

where R1, R2, and R3 are the resistance values of the individual resistors which are connected in series.

1. The current in a series circuit is everywhere the same.

2. Charge does NOT accumulate at any given location.

3. Charge does NOT become used up by resistors.

The charges can be thought of as marching together through the wires of an electric circuit, everywhere marching at the same rate. Current - the rate at which charge flows - is everywhere the same. It is the same at the first resistor as it is at the last resistor as it is in the battery. Mathematically, one might write

Ibattery = I1 = I2 = I3 = ...

where I1, I2, and I3 are the current values at the individual resistor locations.

These current values are easily calculated if the battery voltage is known and the individual resistance values are known. Using the individual resistor values and the equation above, the equivalent resistance can be calculated. And using Ohm's law (V = I • R), the current

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in the battery and thus through every resistor can be determined.

In series circuits, the resistor with the greatest resistance has the greatest voltage drop.Since the current is everywhere the same within a series circuit, the I value of V = I • R is the same in each of the resistors of a series circuit. So the voltage drop (V) will vary with varying resistance. Wherever the resistance is greatest, the voltage drop will be greatest about that resistor.

V1 = I • R1 V2 = I • R2 V3 = I • R3

Parallel DC circuit

Devices can also arranged in a parallel configuration, such that if any bulbs go out, the circuit is still intact. Not only is a parallel circuit useful for holiday lighting, the electrical wiring in homes is also in parallel. In this way lights and appliances can be turned on and off at will. Otherwise if you turned one light off--or one burned out--all the other lights in the house would go off too.

Two light bulbs in a parallel circuit

If either light bulb would go out, the other would still shine. You could add other bulbs or even appliances such as electric motors in parallel to this circuit, and they would remain independent of each other.

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Again, we have three resistors, but this time they form more than one continuous path for electrons to flow. Each individual path (through R1, R2, and R3) is called a branch.

The defining characteristic of a parallel circuit is that all components are connected between the same set of electrically common points.

In a parallel circuit, charge divides up into separate branches . Nonetheless, when taken as a whole, the total amount of current in all the branches when added together is the same as the amount of current at locations outside the branches. The rule that current is everywhere the same still works. The current outside the branches is the same as the sum of the current in the individual branches. It is still the same amount of current, only split up into more than one pathway.

In equation form, this principle can be written as

Itotal = I1 + I2 + I3 + ...

where Itotal is the total amount of current outside the branches (and through the battery) and I1, I2, and I3

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represent the current in the individual branches of the circuit.

Total Resistance There is a clear relationship between the resistance of the individual resistors and the overall resistance of the collection of resistors. Let's begin with the simplest case of two resistors placed in parallel branches, each having the same resistance value of 4 . Since the circuit offers two equal pathways for charge flow, only one-half the charge will choose to pass through a given branch. While each individual branch offers 4 of resistance to any charge that flows through it, only one-half of all the charge flowing through the circuit will encounter the 4 resistance of that individual branch. Thus, as far as any given charge is concerned, the presence of two 4- resistors in parallel would be equivalent to having one 2- resistor in the circuit. In the same manner, the presence of two 6- resistors in parallel would be equivalent to having one 3- resistor in the circuit. And the presence of two 12- resistors in parallel would be equivalent to having one 6- resistor in the circuit.

This method is consistent with the formula

1 / R T = 1 / R1 + 1 / R2 + 1 / R3 + ...

Voltage Drops for Parallel Branches

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In a parallel circuit, a charge does not pass through every resistor; rather, it passes through a single resistor. Thus, the entire voltage drop across that resistor must match the battery voltage. It matters not whether the charge passes through resistor 1, resistor 2, or resistor 3, the voltage drop across the resistor which it chooses to pass through must equal the voltage of the battery. Put in equation form, this principle would be expressed as

Vbattery = V1 = V2 = V3 = ...

If three resistors are placed in parallel branches and powered by a 12-volt battery, then the voltage drop across each one of the three resistors is 12 volts. A charge flowing through the circuit would only encounter one of these three resistors and thus encounter a single voltage drop of 12 volts.

Combination circuits

In this configuration, we'd say that R2 and R3 are in parallel with each other, while R1 is in series with the parallel combination of R2 and R3. By applying the total resistance of parallel branches to a combination circuit, the combination circuit can be transformed into a series circuit. Then an understanding of the equivalent resistance of a series circuit can be used to determine the total resistance of the circuit.

Series and parallel resistor configurations have very different electrical properties.

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In a series circuit, all components are connected end-to-end, forming a single path for electrons to flow. Current I is the same in all resistors while voltage is different across each resistor.(depends on the resistance)

In a parallel circuit, all components are connected across each other, forming exactly two sets of electrically common points. The voltage drop is the same across all resistors while the current is different in each resistor (depends on resistance)

A "branch" in a parallel circuit is a path for electric current formed by one of the load components (such as a resistor).

Alternating Current (AC) Electricity

Alternating current or AC electricity is the type of electricity commonly used in homes and businesses throughout the world. While the flow of electrons through a wire in direct current (DC) electricity is continuous in one direction, the current in AC electricity alternates in direction. The back-and-forth motion occurs between 50 and 60 times per second, depending on the electrical system of the country. AC is created by an AC electric generator, which determines the frequency. What is special about AC electricity is that the voltage in can be readily changed, thus making it more suitable for long-distance transmission than DC electricity. But also, AC can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications.

With an AC generator, a slightly different configuration alternates the push and pull of each generator terminal. Thus the electricity in the wire moves in one direction for a while and then reverses its direction when the generator armature is in a different position.

The charge at the ends of the wire alternates between negative (-) and positive (+). If the charge is negative (-), that pushes the negatively charged electrons away from

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that terminal. If the charge is positive (+), the electrons are attracted in that direction.

Rate of change

AC electricity alternates back-and-forth in direction 50 or 60 times per second, according to the electrical system in the country. The frequency is designated as either 50 Hertz (50Hz) or 60 Hertz (60Hz).

Light bulbs

Many electrical devices--like light bulbs--only require that the electrons move. They don't care if the electrons flow through the wire or simply move back-and-forth. Thus a light bulb can be used with either AC or DC electricity.

Transformer

The major advantage that AC electricity has over DC is that AC voltages can be transformed to higher or lower voltages. This means that the high voltages used to send electricity over great distances from the power station could be reduced to a safer voltage for use in the house.

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Changing voltages is done by the use of a transformer. This device uses properties of AC electromagnets to change the voltages.

Tuning circuits

AC electricity also allows for the use of a capacitor and inductor within an electrical or electronic circuit. These devices can affect the way the alternating current passes through a circuit. They are only effective with AC electricity.A combination of a capacitor, inductor and resistor is used as a tuner in radios and televisions. Without those devices, tuning to different stations would be very difficult.

Electrical PowerThe electrical power used in operate an electrical device is defined as the potential energy or voltage times the current passing through the device. This could also apply to a whole electrical system, such as the the power used in running your household appliances. This is compared to the mechanical definition of power as the work done over a period of time. The electric company uses the power used over a period of time to calculate the energy used and thus your electric bill.

It is compared with the power required to do some work over a period of time.

Determining electric power

The electrical power required to operate a device is the input voltage times the current required.

P = V * I

where P = power, V = voltage and I = current in amperes.

Electrical power is measured in watts. If the amount of watts is large, kilowatts are used. 1 kilowatt = 1000 watts, just like 1 kilometer = 1000 meters. The abbreviation for kilowatt is usually kW.

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Current

If you look at the top of a light bulb, you will see its power rating. One example is a 100 watt light bulb. You can use the equation for electrical power to determine the amount of current passing through that light bulb.

If your house voltage is 240 volts, then you can see that:

100W = 240V * I

Thus I = 100 / 240 = ______ amps.

Resistance

You can also find the resistance of the light bulb, using Ohm's Law V = I * R.

240V = _______A * R

Thus R = 240 / = __________ohms.

Comparing with mechanical power

The standard or mechanical definition of power is the work per unit time. In other words, power equals work divided by time.

P = W / T

where P = power in watts, W = the work done in joules and T = the time of measurement. Since energy is often defined as the ability to do work, let's substitute energy E for work and rearrange the equation:

E = P * T

Equation 2:

P = V • I

P = (I • R) • I

P = I2 • R

Equation 3:

P = V • I

P = V • (V / R)

P = V2 / R

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Thus, the electrical energy used is the electrical power times the time. If we measure the electrical power as kilowatts and the time as hours, we get the energy used by an electrical system in terms of kilowatt-hours. That is the unit of measurement the electric company uses when determining your bill.

Calculating your electric bill

The electric company sends you a bill determined by the amount of work the electricity has done or amount of energy expended in kilowatt-hours. Most homes have an electric meter outside that measures the amount of electrical energy used by the house over a period of time.

Many electric companies charges about 3.5 c per kilowatt-hour. Thus, you multiply the number of kilowatts of electricity you use times the amount of time you use it and multiply that by 3.5 c to get your electric bill.

For example, if you used a 1500-watt hair dryer for 100 hours in a month at a cost of 3.5 c per kilowatt-hour, the electric company would bill you for:

1500 watt * 100 hours = 150,000 watt-hours = 150 kilowatt-hours.

Thus your bill would amount to:

150 kilowatt-hours * 3.5 c / kW-hr = Lm 5.25

Alternating Current (AC) Home Wiring

Home wiring

Typically, homes receive 240 volts of AC electricity. Certain high-power devices, such as an electric heater, use the full 240 volts.

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Wires into home - the Flex

A length of flex will usually consist of three insulated conductors, encased in an insulating sheath. Each of the conductors will have a different colour insulation, according to the terminal it should be connected to:

Brown - Live Blue - Neutral Yellow and green - Earth

If you have any old style appliances, they may well have different colours:

Red - Live Black - Neutral Green – Earth

The reason for the colour change has to do with red-green colour-blindness. Under the old system, red-green colour-blind people were unable to distinguish between the Live wire and the Earth wire. The colours were changed to avoid the potentially deadly consequences of this situation.

Note that appliances which are double-insulated may not have an Earth wire, and some appliances (such as doorbells or fairy lights) may have flex consisting of two uncoloured wires.

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Copper wire is used because it is a good conductor of electricity. Materials that do not conduct electricity as good usually have a higher resistance. This results in wasted energy and the tendency to get hot, which could be a safety hazard.

Wall outlets

(These types are mainly used in the United Kingdom, Ireland, Cyprus, Malta, Malaysia and Singapore).

Power outlets incorporate shutters on line and neutral contacts to prevent someone from pushing a foreign object into the socket.

The domestic electrical system uses a ring circuit in the building which is rated for 32 amps (6 amps for lighting circuits). Moreover, there is also a fusing in the plug; a cartridge fuse, usually of 3 amps for small appliances like radios etc. and 13 amps for heavy duty appliances such as heaters. Almost everywhere else in the world a spur main system is used. In this system each wall socket, or group of sockets, has a fuse at the main switchboard whereas the plug has none

Safety

Proper grounding and the use of fuses are important for protection against shock, as well as to prevent electrical overheating and fire hazards. Correct grounding is very important. Often ground wires are connected to water pipes that normally go into the ground or to an earth electrode.

Another consideration in using water pipes to ground the circuit is that plastic piping is often being used in plumbing. You must make sure there are no plastic pipes between your connection and the outside earth or ground.

Fuses

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Fuses and circuit breakers are used as a safety measure in case of short circuits. A fuse or circuit breaker will break the connection if more current is passing through the wire than is considered safe. This will prevent the house wiring to overheat and start a fire.

Most homes now use circuit breakers instead of fuses. One reason is because people with bad wiring in their homes that constantly blow out fuses, would then force pennies in the fuse receptacles, thus bypassing the requirement for a fuse. This removed the aggravation, as well as the expense of buying new fuses, but it also often resulted in serious electrical fires in the house.

Measurement of Voltage, Current, and Resistance.

You will be familiar with the use of voltmeters and ammeters in circuits, in that the ammeter is wired in series with the component, while the voltmeter is wired in parallel . We have always treated ammeters and voltmeters as perfect.

        A perfect voltmeter has an infinite resistance so takes no current.

        A perfect ammeter has zero resistance. Therefore there is no voltage drop across it.

However, real voltmeters and ammeters are not perfect by any means.

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 Voltmeters

Voltmeters should have a high value of resistance, but this is not always the case. Some moving iron meters (the kind with a needle and a scale) have quite a low resistance, about 10 000 . This can lead to serious reading errors when we measure high resistance circuits.

Digital voltmeters have a very high resistance, about 107 , which makes them almost perfect.

Ammeters

Ammeters have a very low value, but quite definite resistance.

 Multimeters

Electrical and electronic engineers do not carry separate voltmeters and ammeters with them to where they are working. Instead they will carry about a multimeter which is a combined instrument that can:

      Measure voltage       Measure current       Measure resistance       Measure frequency in some instruments.       Test diodes and transistors in some instruments.

 A typical digital multimeter looks like this.

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An analogue meter is shown below:  

For AC use, a diode is placed in the circuit to rectify the AC to DC to which the meter responds.

  Impedance

In AC, we do not use the term resistance. Instead we use the term impedance.     A simpler way of looking at it is the “effective resistance” of an AC circuit, but be careful when using this. It is given the symbol Z, and its units are ohms ().

Ohm’s Law Resistance is the ratio of the voltage to the current, described in the simple equation R = V/I. In a metallic conductor, we find that if we alter the voltage or the current, the other variable changes in such a way that the ratio remains constant.

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R = V/I

This is Ohm’s Law, which states:

The current in a metallic conductor is directly proportional to the potential difference between its ends provided that the temperature and other physical conditions are the same.

Since only metals have constant resistance as stipulated by Ohm’s law, they are called ohmic conductors. All other materials which conduct current such as electrolytes and semiconductors etc. do have resistance. But they are known as non-ohmic conductors since their resistance is not constant. Hence for non-ohmic conductors the V-I graph is a curve.

Voltage Current Characteristics

We can easily measure voltage and current, using the data to plot voltage current graphs. We use the following circuit.

From this circuit we take readings of voltage and current plotting them as a graph called a VI characteristic.

We normally put the voltage on the y-axis and current on the x-axis. This allows us to determine the resistance from the gradient. This is a voltage current graph for an ohmic conductor:

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The straight line shows a constant ratio between voltage and current, for both positive and negative values.  So when the voltage is negative, the current is negative, i.e. flowing in the opposite direction.  Ohm’s Law is obeyed.

For a filament lamp we see:

The resistance rises as the filament gets hotter, which is shown by the gradient getting steeper.

Can you explain why the shape of this graph suggests that a light bulb does not obey Ohm’s Law? 

A thermistor

(a heat sensitive resistor) behaves in the opposite way.  Its resistance goes down as it gets hotter.  This is

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because the material releases more electrons to be able

to conduct. 

Although it looks similar to the graph above, notice how the gradient is decreasing, indicating a lower resistance.  There is, however, a health warning:

As the current goes up, the thermistor gets hotter. As it gets hotter, it allows more current to flow;

Therefore it gets hotter and so on.

This is called thermal runaway, and is a feature of many semi-conductor components.  At the extreme the component will glow red-hot, then split apart.   The thermistor is used wherever any electronic circuit detects temperature:

Here we see a thermistor protecting a power supply from too high a temperature.

Why does a thermistor not obey Ohm's Law? 

 Diodes are semi-conductor devices that allow electric current to flow one way only.  

 The diode characteristic graph looks like this:

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Note the way it is presented.  However many text books show the graph with the voltage on the horizontal axis and current on the vertical.  Watch out for this bear-trap in the exam.

The diode starts to conduct at a voltage of about +0.6 V.  We call this forward bias.  Then the current rises rapidly for a small rise in voltage. If the current is reversed (reverse bias) almost no current flows until the breakdown voltage is reached.  This usually results in destruction of the diode.

Can you use the graph to explain why a diodes allows a current to flow one way only?  

Resistivity

The resistance of a wire depends on three factors:

the length; double the length, the resistance doubles.

the area; double the area, the resistance halves.

the material that the wire is made of.

Resistivity is a property of the material.  It is defined as the resistance of a wire of the material of unit area and unit length.

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First, the total length of the wires will effect the amount of resistance. The longer the wire, the more resistance that there will be. There is a direct relationship between the amount of resistance encountered by charge and the length of wire it must traverse. After all, if resistance occurs as the result of collisions between charge carriers and the atoms of the wire, then there is likely to be more collisions in a longer wire. More collisions means more resistance.

Second, the cross-sectional area of the wires will effect the amount of resistance. Wider wires have a greater cross-sectional area. This can be attributed to the lower amount of resistance which is present in the wider pipe. In the same manner, the wider the wire, the less resistance that there will be to the flow of electric charge. When all other variables are the same, charge will flow at higher rates through wider wires with greater cross-sectional areas than through thinner wires.

A third variable which is known to effect the resistance to charge flow is the material that a wire is made of. Not all materials are created equal in terms of their conductive ability. Some materials are better conductors than others and offer less resistance to the flow of charge. Silver is one of the best conductors, but is never used in wires of household circuits due to its cost. Copper and aluminum are among the least expensive materials with suitable conducting ability to permit their use in wires of household circuits. The conducting ability of a material is often indicated by its resistivity. The resistivity of a material is dependent upon the material's electronic structure and its temperature. For most (but not all) materials, resistivity increases with increasing temperature.

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