How to Understand Electricity: Volts, Amps and Watts Explained on Appliances

© Eugene Brennan

This comprehensive guide for consumers and students explains everything about volts, amps and watts and how they apply to home appliances and circuits. The equations are really quite simple, and you'll find some examples on how to apply them to home appliances.

Electricity 101: Understanding the Basics

In this tutorial, you'll learn about:

• Volts, watts, amps
• Power consumption of appliances and kilowatt hours (kWh)
• Ohm's law and resistance
• Resistivity and how it affects the resistance of a material
• Fuses and how they protect wiring and appliances
• How electricity is produced
• Devices used to measure voltage, current and resistance
• The effects of electric and magnetic fields
• Conductors, insulators and superconductors
• The basics of AC and DC
• Arcs and sparks
• Power supplies and voltage regulation
• Tracking electricity usage in the home

What Is Electricity and an Electric Current?

All matter is made from basic building blocks called atoms. A simplistic model of an atom, known as the Rutherford–Bohr model, Bohr model or Bohr diagram has a central nucleus made up of particles called protons and neutrons. The nucleus is surrounded by orbitals containing electrons. In some materials such as metals, electrons are bound loosely to the nucleus so they can detach and move when a voltage is applied. These materials are known as conductors and can conduct electricity. The flow of electrons is called an electric current.

Conceptual image of atom with protons and neutrons in the central nucleus and electrons in outer orbitals. Geralt, public domain image via Pixabay.com

Electrons with a negative charge flowing through a conductor. © Eugene Brennan

In reality, electrons don't actually move from one end of a conductor to another. Instead they transfer energy between each other just like the steel balls in this Newton's cradle, while moving negligibly themselves. DemondeLuxe, Creative Commons Image 3.0 unported

 

How Does Current Flow in a Circuit?

In the photo below, an AA cell, which is an example of a voltage source, powers a torch bulb. Electric current first flows out the top of the battery, through the wire and bulb and then returns via the bottom wire. So it always flows in a loop and two wires are needed to connect the voltage source to the load. 

In this example of a simple circuit, an AA cell forces current through the wires and lights up a bulb. © Eugene Brennan

Schematic or Circuit Diagram 

We can represent this circuit in a simple manner using a schematic or circuit diagram. Looking at the schematic below, a voltage source V will force a current I around the circuit through the load (the bulb in this case) whose resistance is R.

The load R could be an electrical appliance such as a heater, bulb, LED, motor or component in an electronic circuit. The lines joining the source to the resistance would be the connecting wires inside an appliance or power flex, or tracks on a printed circuit board.

A schematic of of a simple circuit. The voltage source V causes current I to flow around in a loop through the resistance or load R. © Eugene Brennan

 

Current Measured in Amps

The current I measured in amps is given by the equation:

I = V/R

So current is simply equal to voltage divided by resistance. This is called Ohm's Law and we'll examine it in more detail later and see some examples.

 

Voltage (measured in volts) forces current (measured in amps) through the resistance of a circuit (measured in ohms).

Current Direction in a Circuit

Conventionally we think of current flowing out the positive terminal of a source such as a battery. However current is a flow of sub-atomic particles called electrons which are negatively charged, so current actually flows the other way, from the negative terminal of the battery.

What Are Examples of Voltage Sources?

• Battery
• Mains voltage at a socket outlet
• Alternator or DC generator (dynamo)
• Solar cell
• Thermopile
• Laboratory power supply

The Water Pipe Analogy for Explaining Electricity

Voltage and current are like water pressure and water flow rate respectively, and reference is often made to pumps and water pipes as an analogy to explain electrical circuitry. A pump forces water through pipes and when it pumps harder, more water is forced through. The walls of pipes cause resistance to the flow of water. Similarly a voltage source "pumps" electric current through conductors and the current depends on the "pressure" of the source.

What's the Difference Between Watts, Amps and Volts?

Like any discipline, electrical engineering has jargon or specialised terminology.
 

What Are Volts?

Voltage is the pressure in a circuit and measured in volts. Think of a pump in a water pipe. The greater the pressure and the force which the pump exerts, the greater will be the flow of water through the pipe. Similarly a voltage source is like a pump and pushes electrons around the circuit. The higher the voltage applied to a circuit, the greater the current which will be forced through it.

What Are Amps?

An electric current is due to the movement of electrons through a conductor and load and is measured in amps. High current means lots of electrons flowing through the circuit. The water analogy is water flow rate in gallons per minute or litres per second

What Is a Load?

This is the device connected to a voltage source. It could be a motor, bulb, heater, LED, or an electronic resistor.

What Are Watts?

Power is the rate at which energy is consumed by a load and is measured in watts. A kilowatt is 1000 watts, also abbreviated to kW. Low powers are measured in milliwatts (mW) or thousandths of a watt.

What Are KWh or Kilowatt Hours?

Kwh are a measure of energy consumption. KWh are sometimes called units and are what you pay for on your electricity bill. A 1 kilowatt (1000 watt) appliance uses 1 kilowatt hour of electricity (1 kWh) in one hour. Similarly a 500 watt device uses 1 kWh of electricity in 2 hours.

What Are Some Commonly Used Voltages?

Commonly used voltages of devices and systems

How to Convert Between Volts, Amps and Watts

Now let's examine the quantities which are usually of interest when dealing with appliances, such as volts, amps and watts and how to convert between them. If you look at the casing of an appliance (see photo below) you can usually find a specification label or panel which indicates the voltage supply, frequency, wattage and possibly current. On some appliances e.g. TVs and washing machines, this panel may be mounted at the back of the device.

This is the equation we need to remember. Once we know it, we can rearrange to find the other two.

Examples of Working Out Watts, Amps and Volts for Appliances

How to Work Out Watts

Watts = Volts x Amps

e.g. A 120 volt appliance takes 2 amps, what is the power?

Power in watts = 120 x 2 = 240 watts

How to Work Out Amps

Amps = Watts / Volts

e.g. A 240 volt appliance consumes 480 watts of power, How much current does it draw?

Current in amps = 480 / 240 = 2 amps

How to Work Out Volts

Volts = Watts / Amps

e.g. A 720 watt appliance draws 3 amps, What voltage is it running on?

Voltage in volts = 720 / 3 = 240 volts

So it's really that simple. Notice I have chosen values in the examples so that everything works out nicely. You only really need to remember the first equation and if you know basic algebra you can rearrange to give the other two equations. However as you can see, you always need to know two of the quantities before you can work out the third quantity. From looking at the Google Analytics statistics and the questions which land people on this webpage, I often see questions asked such as "how many watts are in 480 volts?", which obviously makes no sense!

For high powered appliances, power is often specified in kilowatts ( abbreviated to kW)

1 kilowatt = 1000 watts

What Is a kWh? How to Calculate Energy Consumption of Appliances

Power is the rate at which a device uses energy. So for instance an air conditioning unit, shower or kitchen range/cooker uses electrical energy much faster than a light bulb. Power is normally written on a label or embossed into the plastic casing of an appliance.

Energy Used = Power x Time

So to figure out the energy usage of an appliance, you multiply its power rating by the time period for which it is running. The standard unit of energy is the joule or calorie, but generally energy used in the home by appliances is measured in kWh, also known as "units". To work out the number of kwh, you divide the power in watts by 1000 to convert to kilowatt (kW) and then multiply by time in hours to give kWh. So:

Energy in kWh = Watts / 1000 x Time in Hours

Kilowatt hours, kWh or units are what you pay for on your bill. Your electricity meter counts and displays the number of units used by all the appliances and lighting in your home.

e.g. A 2500 watt drier runs for 3 hours a day, how many kWh does it consume and if electricity costs 24c per unit, what is the cost of running it?

kWh = watts/1000 x time = 2500 / 1000 x 3 = 7.5 kWh or units

Cost = 7.5 x 24c = 180 cent

Some appliances don't run continuously. Examples are devices controlled by a thermostat such as refrigerators, freezers, ovens in cookers and air conditioning systems. The time for which the appliance is powered on and consuming power is called the duty cycle and it is often quoted as a percentage. So for instance a fridge which stays on half of the time has a duty cycle of 50%.

Labels on Appliances Showing Volts, Current and Power Rating

Typical electrical appliance labels/panels indicating voltage and current rating, power rating and frequency in hertz. © Eugene Brennan

How to Convert Horsepower to Watts

Horsepower is a measure of ... you guessed it! ... power!

Just as an engine's mechanical output can be measured in horsepower, so can the power of an electric motor.

1 Horsepower = 746 Watts

E.g. A fractional horsepower motor in a washing machine is rated at 1/2 horsepower.

So the power output of the motor = 746 watts x 0.5 = 373 watts

A motor is not 100% efficient, in other words not all the electrical power input is converted into mechanical power at the output shaft, some being wasted as heat in the windings.

Kilowatt hour electricity meter for measuring energy used in a home. © Eugene Brennan

Recommended Books

Introductory Circuit Analysis by Robert L Boylestad covers the basics of electricity and also more advanced topics such as AC theory, magnetic circuits, electrostatics and electronic simulation using SPICE based software. It's well illustrated and suitable for high school students and also first and second year electric or electronic engineering students.

Introductory Circuit Analysis. Source image, Amazon

 

Ohms's Law and Electrical Resistance

A simple circuit with a voltage source and load. The load has a resistance measured in ohms. © Eugene Brennan 

In the circuit above, a voltage V pushes a current I around the circuit and through the load. As you may remember, this could be a device such as a bulb, electrical heater, motor, LED or other electrical appliance. The load resists the flow of current and the magnitude of its resistance is R ohms.

So

I = V / R

or

R = V / I

This is known as Ohm's law and basically says that the current is proportional to the voltage and inversely proportional to the resistance (As resistance increases, current decreases and vice versa) Remember the resistance measured in ohms is just a measure of how the load or appliance in the circuit "resists" the flow of current. In electronic circuits and some electrical appliances, components called resistors have precise values of resistance so that they can be used to control the value of current flowing in a circuit.

Every electrical device or load has resistance. Resistance is like a restriction to the flow of electrons and electricity is dissipated as heat energy in a resistance. For a fixed voltage applied to a load, the higher the resistance, the lower the current. Going back to the water analogy, when you stand on a hose, you increase the resistance and restrict the flow. The only way to restore the flow is by getting the pump to pump harder, and force water through the restriction, i.e. the pump needs to have a higher pressure. Alternatively if you take your foot off the hose, you increase the diameter and lower the resistance and more water can be forced through. In an electrical circuit, if the voltage of the source is increased, more current is forced through the resistance. If the resistance is lowered, more current will flow even if the voltage of the source doesn't change. Even connecting wires in a circuit have resistance, so when high currents need to be carried by a cable, the copper or aluminium cores need to have a sufficiently large cross sectional area (CSA) to avoid overheating.

Calculating the Current Flowing Through a Resistance

The resistance in a circuit is 100 ohms, a voltage of 120 volts is applied, what is the current?

Current = 120 / 100 = 1.2 amps

A load could be an electronic resistor like this one, or an electrical appliance. © Eugene Brennan

How to Work Out Power in a Circuit Knowing the Resistance and Voltage or Current

Remember watts = volts x amps? An alternative way to work out power is from the resistance in ohms:

So if I is the current in amps, V is the voltage, R is the resistance in ohms and P is the power in watts,

Then:

I = V / R from Ohm's law

But also power in watts = volts x amps, i.e. P = VI

So substituting the expression I =V/R into P = VI gives:

P = VI = V(V/R) = V²/ R

similarly

P = VI = (IR)I = I²R

It's unlikely when dealing with appliances in the home to need to use the last two equations. However here is an example.

A 240 volt supply is connected to a load of 100 ohms. What is the power consumption of the load?

Power = V²/ R = (240)² / 100 = 576 watts

Summary of Equations for an Electric Circuit

A summary of equations for an electric circuit

 

What Are Electrical Conductors?

A conductor is a physical medium which carries an electric current. This could be a power cable, prongs on a plug, a liquid such as water, battery acid or ionised gas in a discharge lamp (e.g. fluorescent or sodium lamp). Pure deionised water doesn't conduct electricity, but most water contains ions, hence why it's a hazard in contact with electrical appliances. Many solutions of water and other substances conduct electricity, e.g. water and common table salt.

In the case of a solid conductor such as copper wire, the electrical resistance is proportional to the length of the conductor and inversely proportional to its cross-sectional area. In effect this means that the longer a piece of wire, the higher its resistance. Similarly the greater the diameter of the wire, the lower its resistance. This has implications for conductors used in appliances and power transmission. For example, the gauge of wire used in an extension lead is important, if the wire is too thin, the resistance will be high and the cable can overheat. If a power cable is very long, its resistance may be too high if not properly rated, resulting in an unacceptable voltage drop at the end of the cable (because of the resistance).

What Is Resistivity and How Does It Affect Resistance?

For a conductor with cross sectional area A and length l, the resistance R can be calculated using the equation:

R = ρl / A

ρ (Greek letter "rho") is a constant known as the resistivity and is a measure of how good the material is at conducting electricity. The lower the resistivity of a material, the lower will be the resistance of the conductor.

Copper has the lowest resistivity of most common materials and this is why it is widely used in the manufacture of cables. Silver has a lower resistivity than copper, but it is much more expensive. Aluminium is generally used for overhead cables and although it has a higher resistivity than copper, it is lighter. Gold has a resistivity about 1.5 times that of copper, however it is unreactive and doesn't oxidise (tarnish). A tarnish coating on a conductor increases contact resistance, so this is why gold is often used as a coating on audio / video connectors. Gold is also used for the miniature connecting wires in integrated circuits.
Insulators are conductors with very high and for all practical purposes infinite resistance.

Resistivities of Various Materials

Materials with increasing resistivity.

What Is an Insulator?

An electrical insulator is a material which has a very high resistance because there are no free electrons to carry current. For all practical purposes an insulator can be considered to have infinite resistance. Because resistance is infinite (infinity is represented by the symbol ∞), then current through an insulator is:

Current = Voltage / resistance = voltage / ∞ = 0

Insulators are used to prevent current flow between two electrical points with differing voltage e.g. insulation on the individual cores of a power cable, the plastic of a power plug or glass/ceramic insulators on power lines. They also prevent high voltage from causing electric shock.

What Are Insulators Made Of?

Typical insulating material used for electrical purposes are:

• Plastic
• Ceramic
• Glass
• Glass epoxy (used for PCBs)
• Bakelite (an older style thermosetting plastic)
• Mica

Detail of the insulator string (the vertical string of discs) on a 275,000 volt electricity pylon near Thornbury, South Gloucestershire, England. Adrian Pingstone, Public domain image via Wikimedia Commons

PVC insulation on the cores of a power flex. © Eugene Brennan

 

The insulating black plastic shrouds on the pins of this plug prevent contact with the pins during insertion/removal. © Eugene Brennan

How Is Electricity Made?

Since electricity is a flow of electrons, it isn't really made. Instead it is produced or generated when these electrons are moved.

Electricity is produced from:

• Batteries
• DC generators or AC alternators
• Solar cells
• Thermopiles

Early 20th century (1909) alternators in a hydroelectric power station. Sergei Prokudin-Gorskii - Public domain image via Wikipedia

What Does a Power Plant Do?

A power station, also known as a power plant generates electricity using alternators or solar cells. There are several types of power plants, thermal, hydroelectric, wind, wave, tide and solar.

Power Stations That Use Alternators to Generate Electricity

• Hydroelectric Power Station: In a hydroelectric power station, water flowing through pipes from a dammed lake turns the blades of a turbine attached to the shaft of an alternator. The alternator then generates electricity.

• Thermal Power Station: Fossil fuels such as coal, oil, gas and peat or renewable energy crops like willow are burned and the heat is used to boil water and generate steam at high pressure. The steam passes through pipes to a steam turbine and turns it at high speed. Again the steam turbine is connected to the shaft of an alternator, turning it and generating electricity. Nuclear power stations are also thermal using the heat of nuclear fission to boil water and turn it into steam.

• Wind Farm: A wind farm uses windmills to generate electricity. Wind turns the blades of the windmill which are connected to a metal shaft. This shaft turns an alternator and this generates electricity. Wind farms can have several hundred windmills spread over hundreds of acres. 

• Wave and Tide Generation: Wave energy generators use the motion of waves to operate an electric generator. Tidal generators are like undersea windmills and use the flow of water during rising and ebbing tides to turn giant under water "propellers". Like a windmill, the propeller is connected to an alternator that generates electricity.

A wind farm. BKnight97, CC by SA 4.0 via Wikimedia Commons

 

Power Stations That Don't Use Alternators to Produce Electricity

• Solar Farm: Solar panels are large flat panels made of special semiconductor material. When sunshine lands on the panels, they produce an electric current. The larger the area of the panel, the greater is the electricity produced. Just like wind farms, solar-generating farms can be spread over a large area and consist of hundreds of panels. However people can also have solar panels fixed on their roof to generate some of their electricity requirements. Solar panels are becoming more efficient, which means that they can produce useful amounts of electricity even on cloudy days.

Solar panels don't have any moving parts unlike an alternator.  RosiePosie, public domain image via Pixabay.com

What Is a Thermopile?

A thermopile is an array of thermocouples connected together, usually in series. A thermocouple works on the principle of the Seebeck Effect to produce electricity. Thermopiles aren't really used to produce energy commercially, but are the only method of generating the electricity required by probes in deep space, far from the Sun. Solar power isn't an option and batteries wouldn't have the capacity to last the mission. So radioisotope thermoelectric generators (RTG) are used consisting of a nuclear source to produce heat and a thermopile to produce electricity.

What Is the Voltage Supplied to Our Homes?

In general, the voltage supply to your home is nominally 230 or 120 volts. Voltage in the USA is 120 volts, but two "hots" are supplied to homes so that a 240 volt supply is also available between the hots. The higher voltage is used for high powered appliances such as washers, driers, kitchen ranges (cookers) and air conditioning. 120 volts is used for lower power and portable devices. It is also safer because in the event of an electrical shock, less current flows through the body so there is a lesser risk of electrocution.

In countries where 230 volts is standard, generators or step down isolating transformers are used to provide a 110 volt supply for power tools. This is normally mandatory on construction sites. Again the idea of the lower voltage is to lessen the danger of electrocution, if for example a power flex is inadvertently cut, or a tool gets wet.

Utility voltage by country. SomnusDe, Public domain, via Wikimedia Commons

What Are Fuses For?

As we will see later, electrical cables, appliances, wires inside appliances, components etc all have resistance. This resistance produces heat when current flows through it. Any electrical conductor can get excessively hot if too much current flows and in the case of wires, this can cause the plastic insulation covering the cable to melt or even catch fire. So fuses are used in series with a cable or appliance to limit current flow and make everything safe. Fuses are like a "weak link" in a chain and blow before damage can occur. They have a specified rating and this is not the current they blow at, but the current they will carry without blowing. Once current exceeds the rating of the fuse, the fuse will blow. The length of time it takes for the fuse to blow is proportional to the current. So minor overloads can result in a fuse blowing in minutes, but if there is a large current or short circuit scenario, the fuse will blow in seconds or milliseconds.

Breaking Capacity of Fuses

Fuses have a max current they can carry without the encapsulation of the fuse rupturing. So fuses on the secondary of domestic power supplies in TVs, battery chargers and other electronic appliances are often glass types because the supply will only source a relatively small amount of energy if there is a fault. Ceramic types are used to resist the heat and shock that occurs when the inrush current can be perhaps hundred or thousands of amps, potentially feeding a huge amount of instantaneous power. If a short circuit occurs in an appliance, it's quite possible that the utility transformer in your street can feed current of this magnitude into the short. So for example the BS1362 fuse in a UK style plug has a ceramic body. Blown fuses should always be replaced by the same type, ceramic if necessary, to avoid a fire occurring.

Fuse Types

In general, fuses are fast blow (F) and time lag (T). Time lag types are often used for power supplies in electronic equipment because the capacitors take a surge of current as they charge up, which would blow a fast acting fuse.

How Many Amps Are Supplied to Our Homes?

Typically for a 230 volt supply to a home, the main fuse rating is 80 to 100 amps at the consumer unit. So this is the maximum current that will flow before the fuse blows. At 80 A and 230 volts, this allows a power draw of 230 x 80 = 18.4 kW.

A High Breaking Capacity (HBC) BS1362 fuse, used as standard in a UK style plug. © Eugene Brennan

Testing and Measuring Voltages

How Do You Check Voltage With a Multimeter?

A multimeter is an instrument that can measure voltage, current, resistance and possibly additional parameters. You can also use it to check continuity of cables and check fuses. If you don't know how to use one, read my guide How to Use a Digital Multimeter (DMM) to Measure Voltage, Current, and Resistance. Multimeters normally have a continuity range also, and this comes in useful for checking breaks in cables, fuses and loose connections.

 

Fluke digital multimeter. © Eugene Brennan

How to Test a Live Wire

For this it's best to stay safe and use a non-contact volt tester or phase tester screwdriver. These will indicate if voltage is e.g > 100 volts. A multimeter can only measure the voltage between live and neutral or live and earth (ground) if these conductors/terminals are accessible, which may not always be the case.

A non-contact detector is a standard tool in any electrician's tool kit, but useful for homeowners also. I use one of these for identifying which conductor is live whenever I'm doing any home maintenance. Unlike a neon screwdriver (phase tester), you can use one of these in situations when live parts/wires are shrouded or covered with insulation and you can't make contact with wires. It also comes in useful for checking whether there's a break in a power flex and where the break occurs.

Note: It's always a good idea to use a neon tester to double check that power is definitely off when doing any electrical maintenance.

How Can I Measure Electricity Usage in My Home?

An electricity usage monitor or tracker tells you everything you want to know about your appliance behaviour. The parameters are displayed on an LCD and include voltage, current, power consumption, kWh used, cost of running and run time of appliance. The latter is useful for troubleshooting fridges, freezers, air conditioners etc which are controlled by a thermostat and switch on and off. A failed thermostat or waterlogged insulation can cause an appliance to run constantly, so this problem can be identified.

Power consumption monitoring adapter. © Eugene Brennan

What Exactly is Meant By "Electrical Energy Consumption"?

What happens when an appliance is powered from electricity? Scientists tell us that energy cannot be destroyed, it just changes from one form to another. This process happens all the time - on Earth and throughout the Universe. For instance a rock on the edge of a cliff has potential energy, because of its altitude above the ground. If it falls over the edge of the cliff, it starts to pick up velocity, i.e. gains kinetic energy (motion energy) while losing potential energy. When it hits the ground, this energy is dissipated as heat (think of the heat produced by an asteroid impact). Similarly when an appliance is plugged in, the electricity doesn't get wasted or "consumed", in the sense of being destroyed, it simply changes form. So in the case of a lamp, it ends up as light energy or as heat energy when a heater is used. Electrical energy can also be converted to sound in a loudspeaker or electromagnetic radiation (microwave oven or radio transmitter), all forms of energy. Electrical energy can also be converted to kinetic energy in an electric motor or to potential energy when an elevator is raised in a building.

Power is a measure of the rate at which energy is used. So for instance a 1000 watt heater or high powered hvac air conditioning system uses energy at a higher rate than a 60 watt light bulb.

How is Electricity Converted to Other Forms of Energy?

Converting energy from one form to another

What Is a Superconductor?

When certain materials are subjected to very low temperatures, their resistance falls to zero.

Since V = IR, if R is zero, then V becomes 0 even if I is non zero.

The consequences of this are that a current can flow even if the voltage source is removed. Because resistance is zero, and no heat is dissipated, huge currents can be carried by thin cables. Superconductors are used for example in MRI machines to carry the high currents required by powerful magnets.

 

Superconducting cables. RRama, CC BY-SA 2.0 FR , via Wikimedia Co

What Are AC, DC and Three-Phase?

The current produced by a power source can take one of two forms, AC or DC. The power source could be a battery, electrical generator, power transmitted along service cables to your home or the output of a signal generator, a device used in laboratories or by test personnel when testing or designing electronic systems.

DC Explained

This stands for direct current so the current provided by the source only flows one way. A DC source will have a nominal value voltage level and this voltage will fall as the source is loaded and outputs more current. This drop is due to inherent internal resistance within the source. The resistance is not due to an actual resistor, but can be modelled as such, and is composed of actual resistance of conductors, electronic components, electrolyte in batteries etc.

Examples of DC sources are batteries, DC generators known as dynamos, solar cells and thermocouples.

AC Explained

This stands for "alternating current" and means that the current "alternates" or changes direction. So current flows one way, reaches a peak, falls to zero, changes direction, reaches a peak and then falls back to zero again before the whole cycle is repeated. The number of times this cycle happens per second is called the frequency. In the U.S. the frequency is 60 Hertz (Hz) or cycles per second. In other countries it is 50 Hz. The electricity supply in your home is AC.

The advantage of AC is the ease by which it can be transformed from one voltage level to another by a device known as a transformer.

AC sources include the electrical supply to your home, generators in power stations, transformers, DC to AC inverters (allowing you to power appliances from the cigarette lighter in your car), signal generators and variable frequency drives for controlling the speed of motors. The alternator in a vehicle generates electricity as AC before it is rectified and converted to DC. New generation brushless, cordless tools convert the DC voltage of the battery to AC for driving the motor.

Reducing Costs of Transmitting Electricity Over the Grid

Because AC can so easily be transformed from one voltage to another, it is more advantageous for power transmission over the electricity grid. Generators in power stations output a relatively low voltage, typically 10,000 volts. Transformers can then step this up to a higher voltage, 200,000, 400,000 volts or higher for transmission through the country. A step up transformer, converts the input power to a higher voltage, lower current output. Now this decrease in current is the desired effect for two reasons. Firstly, voltage drop is reduced in the transmission lines because of the lower current flowing through the resistance of cables (since V = IR). Secondly, reducing current reduces power loss as current flows through the resistance of the distribution cables (remember power = I²R in the equations above?). Power is wasted as heat in transmission cables, which obviously isn't wanted. If current is halved, power loss becomes a quarter of what it was previously (because of the squared term in the equation for power), If current is made 10 times smaller, power loss is 1% of what it was, and so on.

The AC waveform of the the domestic supply to our homes is sinusoidal. © Eugene Brennan

AC voltage is sinusoidal. © Eugene Brennan

Transformer in an electrical sub-station. The function of a transformer is to either increase or decrease voltage. Image: Rainer Knäpper, Free Art License via Wikimedia Commons

What Is Three-Phase Voltage?

Very long distance transmission lines may use DC to reduce losses, however power is normally distributed nationwide using a 3 phase system. Phase voltages have a sinusoidal waveform and each of the phases is separated by 120 degrees. So in the graph below, phase 1 is a sine wave, phase 2 lags by 120 degrees and phase 3 lags by 240 degrees (or leads by 120 degrees). Only 3 wires are needed to transmit power because it turns out that no current flows in the neutral (for a balanced load). The transformer supplying your home, has 3 phase lines as input and the output is a star source so it provides 3 phase lines plus neutral. In countries such as the UK, homes are fed by one of the phases plus a neutral. In the US, one of the phases is split to provide the two 'hot' legs of the supply.

Why Is 3 Phase Used?

• More power can be transmitted using just 1.5 times the number of wires
• Motors powered by 3 phase are smaller than a similar single phase motor of the same power
• Evening of output torque smooths operation and results in less vibration of motors powered by 3 phase
• Neutral conductor can be reduced in size because of lower current flow
• Neutral is unnecessary for transmitting power between substations and transformers

Note: A "phase" is a single winding in an alternator, winding in a transformer, coil in a motor or resistive load. The phase voltage is the voltage across a phase.

3 Phase Formulas

If Vₚ is the phase voltage

and VL is the line voltage between each phase

Delta load:

Vₚ = VL
Iₚ = IL / √3

Star load:

Vₚ = VL / √3

Iₚ = IL

 
For both star and delta balanced loads with a power factor of 1:

Power = √3VLIL

Delta Star Transformer

A Delta-star (also known as delta - wye or delta Y) transformer is often used for producing a 3 phase, or single phase and neutral supply to homes and industry. The incoming supply is typically 11kv and output phase voltage is 230 volts (in countries which use this voltage).

3 Phase voltages. Each phase is sinusoidal with a phase difference of 120 degrees. J JMesserly modification of original svg by User:SiriusA, Public Domain image via Wikimedia Commons

Delta-Star(Wye) transformer which can supply single or 3-phase supply. © Eugene Brennan

 

Three-phase power lines. Each overhead line is a single phase. Wing-Chi Poon, Creative Commons Attribution-Share Alike 2.5 Generic via Wikimedia Commons

 

What Are Other Effects When a Current Flows?

As mentioned above, when current flows through the resistance of a load, it gets hot. This is sometimes the desired effect, e.g. an electrical heater. However it is an unwanted effect in lamps, because the desired function of the device is to convert electricity to light, and not produce heat as a byproduct. Excessive current in power cables during an overload can potentially cause a fire if protective devices such as fuses or MCBs (Miniature Circuit Breakers) aren't included in line with the cable.

What Does Current Flow Produce?

Magnetic fields. This phenomenon is used in a device called a solenoid or electromagnet which is basically like a spool or coil of wire through which a current flows. Electromagnets are used in the old style, non-electronic, door and phone bells, water inlet valves on washing machines, relays (a switch operated by an electromagnet), starter motors on vehicles and in salvage for lifting iron and steel. Magnetic fields are also the principle upon which all motors work.

Electric fields. Current flowing through a conductor also produces an electric field. An extreme example of this is the high intensity field produced under a high voltage power line which is sufficient to illuminate a fluorescent tube held in the hand.

Magnetic field lines around a conductor. © Eugene Brennan

The electric field under a high voltage power line is sufficient to produce an electric discharge in a fluorescent tube. Image BaronAlaric GNU_Free_Documentation_License version 1.2 via Wikimedia Commons

How Do Switches Work and What Are Sparks?

As you've discovered, if resistance is increased in a circuit, current decreases. If you just break the conductor in a circuit and create an air gap, the magnitude of the resistance for all practical purposes is infinite because air is a good insulator and no current will flow. I.e.

Current = Voltage / Resistance = Voltage / ∞ = 0

So this is how a switch works. Two contacts, usually made of brass in a domestic switch, are pressed together when the switch is on and closed. When the switch is turned off, the contacts rapidly separate and interrupt current.

What Are Sparks?

Imagine two electrodes or points in a circuit separated by an air gap (e.g. the gap in an automotive spark plug). If voltage is high enough, the air between the two points becomes so stressed by the electric field that it becomes ionized, i.e. atoms have their electrons ripped off. These electrons are then able to traverse the gap, attracted by the positive electrode and in doing so, collide with other gas molecules and release more electrons. Eventually an avalanche of electrons occurs (all of this happening in a split second) and the result is called a spark or spark discharge A spark produces a flash of visible light, heat, UV radiation and sound and it's temperature can be about 5000 deg C, hotter than the surface of the sun. The voltage required to produce a spark is about 3000 volts per mm between rounded electrodes in air. Sparks can be small, e.g. automotive spark plug or gas lighter, or much larger.

An example of a large spark is lightning. When clouds get charged up, voltage becomes so high that a spark jumps from cloud to cloud or cloud to ground. The sound we call thunder is caused by the explosive heating and expansion of air by the electrical discharge.

Sparks occur in an air gap when voltage exceeds the breakdown voltage of the gap. When two electrodes are separated, current tends to continue to flow and heating of the metal electrodes causes material to vaporise and also ionise the air. This results is a continuous spark discharge called an arc which is similar to a spark. If the electrodes are separated sufficiently, the arc won't be sustained and will stop abruptly. Arc welding makes use of an arc between two electrodes to melt metal. Switches must also be designed so that their contacts separate sufficiently apart and quickly enough so that arcs are rapidly quenched and reduce damage to the contacts. In substations, large air gaps or oil filled circuit breakers are necessary to quench the high current arcs which occur when high voltage is switched.

References

Boylestad, Robert L. (1968). Introductory Circuit Analysis. (6th ed. 1990) .Merrill Publishing Company, London, England.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualised advice from a qualified professional.

© 2012 Eugene Brennan