Electricity
Specific Learning outcomes
By the end of this unit you should be able to
Static Electricity
Differentiate between conductors and insulators
Recognize that electric charges are either positive, negative
Understand that objects which have equal numbers of positive and negative charges are neutral
Recall that electrons and negatively charged and that only electrons move when insulators become charged
State that insulators can become charged when friction between them transfers electrons from one to another
Recognize that a negative overall charge will be acquired when an object gains excess electrons, and that a positive overall charge is caused by a deficit of those electrons which would normally neutralise the positive charges in the atomic nuclei
Understand that similar (like) charges repel each other and that different (unlike) charges exert an attractive force on each other
Explain why a charged object can sometimes attract an uncharged object explanations could include drawing a suitable diagram
Illustrate the distribution of charge on an imperfectly insulating object in the presence of a large positive or negative nearby charge
Understand how an object can become charged by contact
Recognise that contact with ‘earth’ (or ‘grounding’) allows electrons to be supplied or lost to neutralise an existing charge
Explain that electrical sparks or lightning are caused by electrical charges travelling through the air
Explain the role of lightning conductors in protecting structures from the effects of lightning discharge
Use the concepts above to discuss common phenomena such as getting a shock on a trampoline, or hair standing on end when charged with static electricity
Have some familiarity with experimental applications of Van de Graaf generators and electroscopes; apply the concepts above to explain observations made during such experiments
Current electricity
Explain that electric current can flow freely in an electrical conductor
Identify some common conductors such as copper, aluminium or steel
State that it is electrons that are the moving charges in a (metallic) conductor
State that the unit of electrical current is the ampere, and that an ampere represents a certain number of electrons flowing per second (coulomb concept or electronic charge size concept not required)
Recognise that charge will only flow if there is available energy to cause them to flow (such as supplied by a battery, power supply or static electrical charge) and a complete pathway (circuit) in which to flow
Recognise that volts are the amount of electrical potential energy per unit charge
Understand that devices which turn electrical energy into another form (such as heat or light) will cause the electrical current to have a difference in electrical potential energy across it; apply the term potential difference or voltage to this difference
Use the term 'resistor' to describe a device which causes a difference in electrical potential energy in a current passing through it; use the term resistance for this property
Understand that the higher the resistance the more energy it takes to cause one amp of current to flow; define the unit of electrical resistance as the ohm Ω and one ohm as being the resistance that will allow 1 amp of current to flow with 1 volt of potential difference
Apply the equation V = I R to this relationship and use the equation to solve problems involving voltage, current and resistance
distinguish between series and parallel connections in a circuit
compare and contrast the properties of series and parallel circuits Range: current and voltage in components, effect of a broken circuit, total resistance
be aware that in series the total resistance is the sum of individual resistances; use this to calculate total resistance in a series circuit applying the relationship RT = R1 + R2 + …
be aware that all devices in series will have the same amount of current passing through them, and the voltage across individual components will add up to the total voltage of devices in series; apply this to circuit problems
be aware that the total of currents in branches of a parallel circuit will be the amount of current at the point where the branches connect; apply this to circuit problems
be aware that the total resistance in a parallel circuit is less than the lowest resistor Note: students are NOT required to calculate resistance in parallel
understand that a resistor is consuming electrical power (i.e. converting electrical energy to other forms a rate of a certain amount of energy per time)
use the relationship P = V I to solve problems involving electrical power Note: these problems may involve first finding V or I from other information
use electrical power calculations to find the amount of energy produced in a given time period by applying the equation P = E/t; solve any problems that require rearranging this formula
solve problems to find current, voltage, resistance, power or energy in a circuit by applying the principles above. Problems may involve multple steps, for example using resistance to find current before then using the current to find power.
Magnetism
define a magnetic field as a force field that can exert an attractive force on certain substances (e.g. soft iron) or cause a small bar magnet which is free to rotate (e.g. a compass) to orient itself in a particular direction
draw diagrams to represent the direction and strength of the magnetic field around a bar magnet or other simple permanent magnets (e.g. horseshoe magnet) or between two magnets; recall the pattern that iron filings make in the presence of a magnetic field
state that like magnetic poles will repel and unlike ones attract each other; apply the terms ‘north’ and ‘south’ to the poles of a magnet
recognise that a magnet always has a north and south end and that chopping a magnet in half will produce two smaller magnets each with a north and south oriented as the larger magnet
draw magnetic field lines to represent the strength and direction of the magnetic field around the planet Earth
explain why the Earth’s magnetic field will cause a compass to point ‘north’
predict the direction that the needle of a small compass would point when placed near one or more magnets
draw field lines to indicate the direction and strength of the magnetic field around a current carrying wire if given the direction of the current; predict the current direction given the orientation of the field lines
apply the equation B = kI/d to calculate the strength of the magnetic field given a current and distance from a wire; rearrange the equation to find one of the other values given sufficient information (note: the value and units of k will be supplied)I
use tesla as the unit of magnetic field students are not required to recall the derivation of the tesla; use the symbol T to designate tesla supplied field strengths may be given in standard form e.g. 3 x 10-4 T
state that a coil of current carrying wire will act to produce a magnetic field with similar characteristics to a bar magnet
predict the direction of the magnetic field lines in a wire coil given the direction of current around the loops
state that the strength of such a magnetic field produced by a wire coil can be increased by increasing the current, increasing the number of turns in the coil, or replacing the hollow core with a suitable magnetic material such as soft iron
Electrostatics
Static electricity is caused electrical charge building up in substances which are insulators. In an uncharged insulator, the number of positive and negative charges are equal. Such objects are neutral.
Substances can be conductors, which allow charges to move easily, or insulators which make it very difficult for charges to move. Many substances are somewhere in-between.
Conductors and insulators
Matter is made of atoms, which in turn consist of the nucleus (positive) and electrons (negative). In solids, the atoms must stay in place. If the electrons cannot move around by jumping from atom to atom, the substance will be a good insulator.
Substances which form a giant covalent network such as diamond or plastics tend to be the best insulators. Glass and ceramics are also very good insulators. Liquids formed from strongly covalent molecules, such as paraffin oil, are also good insulators and are used in electrical transformers and similar applications.
Ceramic insulator on a high voltage power line
Fluorescent light tubes contain ionised gas, which is a conductor. During the flow of charge some of the energy is converted to radiation which is used for light.
Gases should theoretically be good insulators, but some gases can easily be turned partially into ions; if this happens, the ions (which are charged particles) can easily move and the gas will act as a conductor. In applications where a gaseous insulator is needed, engineers use a gas such as sulfur hexafluoride which is extremely difficult to ionise.
Conductors allow charges to move freely. Liquids and gases containing ions are therefore good conductors because their particles are free to move around. Pure distilled water is not a good conductor, but still contains some ions so is not a good insulator either. Even a tiny amount of dissolved ionic substance, such as acid or salt, will greatly increase the conductivity of water. The gas inside a fluorescent light tube is a good conductor; it contains argon gas mixed with a little mercury vapour (the mercury makes it very easy to ionise). Molten salts are also conductors.
All metals are conductors because their atoms form a solid in which one electron from the valence shell can easily jump from atom to atom. A current in a metal conductor therefore consists of a flow of electrons moving in one direction by ‘hopping atoms’. In the best metal conductors (gold, silver, copper) this takes very little energy. Aluminium and steel (iron) are also fairly good conductors. Some metals, such as lead, are not particularly good conductors but are good enough to be used to join better conductors together in soldered electrical connections.
Solder is an alloy of lead and tin. Although not as good a conductor as copper, it is good enough for the very short distance the current needs to travel, and the solder forms a solid and reliable electrical connection which conducts better than one formed by metal surfaces just in contact.
Electrical wires tend to be made of copper because it is a good conductor, not too expensive and doesn’t corrode too easily. Gold plating is sometimes used where a very good electrical connection is needed but the connection is not soldered (e.g. in high-end AV systems). This is because gold does not corrode and form a coating of oxide which can act as an electrical resistor.
High voltage wires (like the ones in the header picture) are not always made of copper for reasons of strength and weight.
All metals require some energy to make the electrons move; the better conductors require less energy. There is a special class of conductors in which the electrons require no energy at all to move. These are called superconductors. So far, the only superconductors to be discovered only work at extremely cold temperatures so they are only used in special application such as MRI scanners.
Magnetic levitation is one of the characteristics of superconductors.
Charging an insulator by friction
Some insulators can become electrically charged when they rub together. This requires two different substances, one of which attracts the electrons slightly more strongly than the other. Pairs of substances from a list of glass, wool, silk and plastic are among those most commonly used and asked about in exam questions.
An object becomes charged electrons are lost or gained by friction. For example, if electrons are rubbed off perspex and onto a cloth, the ‘holes’ where the electrons were on the perspex create a positive charge on the perspex rod. The extra electrons stick to the cloth because it is an insulator and they can’t go anywhere, and the cloth becomes negatively charged.
There is no simple way to predict which substance will become positive and which negative. This depends on the relative properties of the two substances. In exam questions, you will either be given that information somewhere in the question or you will just have to assume that the two substances have opposite charges and answer questions using general terms. You are expected to be aware that the two objects rubbing together will have opposite charges.
Exam tip: One of the most common mistakes that students make when answering questions about electrostatics is to state or imply that positive charges move. An object becomes positively charged by losing electrons, and a positively charged object becomes neutral by gaining electrons. Electrons are negative (far too many students talk about 'positive electrons' in exam answers; they get an automatic N grade for that question).
Charges, energy and voltage
When you transfer charge by friction and then pull the charges apart, you are exerting a force over a distance, therefore doing work. The work done is stored as electrical potential energy. The amount of force is quite large considering the very small amount of charge involved, so the amount of stored energy per unit charge is also quite large. The units of potential energy per unit charge are volts; since the amount of energy per charge is very high in electrostatics, the voltage is very high. Since the amount of charge is small, any electrical current is small so doesn't deliver a large shock.
Attraction and repulsion
"Like" charges repel each other i.e. positively charged objects will repel other positively charged objects; negatively charged objects will repel other negatively charged objects.
One fairly common example of this is when a child's hair stands on end because they have become electrically charged by rubbing on the plastic of a trampoline mat. The hairs all have the same charge and repel each other. They stand on end because that is how they can get as far away from each other as possible.
Another example of this is when small objects, such as polystyrene balls or pieces of paper, are placed in a cup on top oaf an electrostatic generator (Van de Graaf generator). The small objects come flying out of the cup because they acquire a charge by conduction from the cup, then they violently repel each other and are repelled by the cup also (since they have the same charge as the cup).
Attraction between charged and uncharged objects
A charged object can attract one that is uncharged. This works best with substances that allow a little movement of the charges, but which are electrically isolated (not able to be supplied with electrons from elsewhere). You will be familiar with the ability of a rubbed pen or ruler to pick up bits of paper.
This happens because the electrons will move in response to the force they feel from the charged object. Consider a positively charged plastic rod brought near a piece of paper:
As the rod approaches, the electrons experience an attractive force and a few will move to the end of the piece of paper closest to the rod. This end of the paper will then be attracted to the rod, while the far end will be repelled.
Because the attracted electrons are closer to the rod than the positively charged area, they experience a larger attractive force than the repulsive force from the other end of the paper. There is now a NET attractive force towards the rod. If that net force becomes large enough, the paper will fly up towards the rod.
Note that if the paper is able to transfer some of those electrons to the rod, the paper will now have an overall positive charge and may now be repelled from the rod and jump away. Whether this happens depends on how free the electrons are to move (which depends on both the rod and the paper).
Since a charged object can attract BOTH an oppositely charged object and an uncharged one, attraction cannot be used by itself to find out the charge on an object.
e.g. you have a positively charged plastic rod and you bring it near a small, suspended pith ball. The ball swings towrds the rod. This means the ball is negative? NO - it could be negative or it could be neutral. You would have to check by bringing a negative rod close and seeing whether it swings away.
Earthing/grounding
The ground can act as a large supply of electrons, or a large place for them to drain away to. Objects that are 'electrically isolated' are not in electrical contact with the ground. Humans are not good insulators, so if you touch an electrically charged object you will often allow electrons to move through you to neutralise the charge. Sometimes, the charge flowing through you will be enough to give you a shock. The process of neutralising a charge through the ground is called earthing or grounding.
Lightning
Air is not a perfect insulator. An electric charge with sufficient energy will strip electrons away from nearby air molecules, turning them into ions (charged particles). These charged air molecules can now act as the carriers of a current because they can move. We experience this process happening when we see an electrical spark or an electric arc.
Electrical arcing from a Tesla coil; this is a special type of device for producing very high voltages at extremely low currents.
The process of electrons jumping on and off air molecules releases energy in the form of heat and light. This causes us to see light flashes or glowing pathways in the air (this is the actual place the current is flowing). This is often accompanied by a crackling sound caused by the rapid movement of air when it is heated by the energy in the charges.
Certain types of cloud can build up very large electric charges as a result of friction forces acting on spinning water droplets and ice crystals. When this charge has enough energy to ionise the air, it will form a charged pathway through the air along which an electric current flows - a lightning bolt. These pathways can be hundreds of metres long, so they heat up a lot of air (making thunder) and produce a lot of light (the flash).
When the lightning path is between two areas of cloud we don't see the lightning bolt, only the light from it which is reflected and diffused within the clouds. This is called sheet lightning and makes up about 90% of lightning discharges.
A smaller proportion of charged clouds discharge by first inducing an opposite charge in the ground below them as shown in the diagram below. This is cloud to ground lightning and is responsible for the lightning bolts you see.
The top of the building experiences the largest electric forces; electrons here are repelled away down into the ground leaving the high point positive. The strongest electric force field is between the top of the tallest point and the cloud, and it is the likeliest point for the lightning to strike.
For complicated reasons, pointy objects tend to accumulate more charge on the pointy bits, so these are often points where lightning strikes.
Lightning rods and conductors
Lightning bolts usually contain a lot of energy. If the electrical current of the lightning passes through a structure which is a poor conductor it causes that structure to heat up.
This is why trees sometimes explode when struck by lightning. The current mainly flows through the the sap of the wood and changes the water in it into steam. This causes the wood to explode from the pressure build up.
Tree struck by lightning
To prevent this happening. tall buildings are usually equipped with a lightning rod. This is a long metal rod that will concentrate the most charge and therefore be the place that the lightning is likeliest to hit. The rod will be connected to a lightning conductor (wire). The conductor provides a safe, low resistance pathway for the current down to the ground.
The rods are stainless steel or copper, and the conductors usually thick steel wire rope. The conductor is connected to an underground ground rod or grid to dissipate the charge over a large volume. This causes most of the heating to happen in the ground, which can easily absorb the energy, and the building is much less likely to be damaged.
Other electrostatic phenomena
Bendy water
If you hold a charged object, such as a rubbed pen or plastic comb, next to a thin stream of water the water stream will bend towards it. This is true whether the object is positively or negatively charged. The reason has to do with the fact that water molecules have a positive and negative end. In the presence of the electric force-field they will line up so that the oppositely charged end is pointing towards the charged object
This causes an overall net force in much the same way as the net force exerted on a piece of paper when you hold a charged plastic rod near it, even though the overall net charge of the paper is neutral.
The Van de Graaf Generator
This is a device for building up large static electrical charges; the diagram to the left shows how it works but you are not expected to know the details.
There are some YouTube videos that demonstrate the properties of the VdG generator e.g. here; however I am not embedding them on this page in case the URL goes out of date.
The Electroscope
This is a device for detecting electric charge. There are a number of different designs; the gold leaf design is the most sensitive but is not very robust and does not survive well in a school laboratory.
The basic principle is that two pieces of metal are hanging in such a way that they will develop a charge if the disc at the top experiences an electric force field. This causes them to repel each other and move apart. You are not expected to know details but you can find more information here.
Current Electricity
A flow of electric charge is called an electric current. In a metal, an electric current consists of electrons hopping from atom to atom with a general flow is in one direction.
The unit of electric current is the ampere, symbol A. One ampere of current represents about 6 x 1018 electrons per second flowing past a given point.
Thin wires can only carry a small amount of current without getting hot and potentially causing a fire.
For this reason, most electrical circuits have safety devices which turn off the current when a threshold is reached. For example, a home multi-box has a circuit breaker which will trip when a threshold of 10 amps is exceeded.
Wires carrying large currents need to be thicker. For example, jumper cables for vehicles need to be very thick wire, and large diesel engines need thicker cables than small petrol ones.
The electrons in the wire travel quite slowly, but the energy they carry travels at the speed of light. This is similar to the way that adjusting a tap on a garden hose causes the flow at the other end to change straight away, even though it takes several seconds for the water from the tap to reach the end of the hose (as long as the hose is already full of water; a wire is always 'full of' electrons.
Electrons travel from the negative terminal of a power supply to the positive terminal. However, when discussing current we use a theoretical current called the 'conventional current' which is made of imaginary positive charges. The conventional current flows from positive to negative. This will be important later when we discuss magnetism, as the current direction influences magnetic field and it is the conventional current that you will be given or expected to work out.
A current can only flow if two conditions are met:
there must be an unbroken circuit i.e. a continuous path from one terminal of the power supply to the other
there must be an energy source to make the charge flow. This can be a cell, a battery, a power supply or some other energy source such as a solar cell.
A switch works by breaking the circuit and preventing the current from flowing.
Drawing circuit diagrams
Circuits are usually drawn using symbols to represent the components and straight lines to represent the wires. Below are the symbols for the components you will be expected to know for NCEA Level 1:
There isn't a particularly standard symbol for a power supply, so you may be given one that differs slightly from the one above. In a cell or battery, the long line represents the positive terminal and the current goes from positive to negative (this is only important in electromagnetism questions).
How do we translate an actual circuit into a diagram?
Below is a diagram of a circuit showing the actual way a student might set it up.
The four cells are connected end-to-end so make a four cell battery. An example of a circuit diagram for this circuit is shown on the right. Notice a few points:
although there are two wires connected to the rheostat in the picture, it is not shown that way in the diagram. As a general rule, devices should only have one wire on either side of them.
the same applies to bulb 3
corners should be square
no devices on corners
Common circuit drawing errors
As a general rule, they are looking for understanding rather than precision drawing when you are asked to to draw circuits in NCEA. However, the common errors below are a 'bad look' or may get you marked incorrect:
You may connect more than one wire to a bulb or other device when you set up a circuit, but the correct way to show it is as shown on the diagram on the right. This is because if you draw it as on the in the incorrect diagram, it isn't clear which terminal the extra wire is connected to (you would draw this for a three terminal device such as a transistor, but you won't get these devices in NCEA).
The same rule applies when connecting a device in parallel, such as a voltmeter.
Also, don't put devices on corners:
Series and parallel
There are two basic ways that devices can be connected in a complete circuit:
Series circuit
In a series circuit, current must pass through one device to get to the other. There is only one possible pathway for the current
Parallel circuit
There is at least one place where there is more than one pathway for the current
Applications of series and parallel circuits
Series circuits are easier to wire up and are useful where low voltage devices are to be used with a higher voltage power supply. A well known traditional application of this was to wire 12 V lamps in a series of 20 for use with a 240 V mains power supply. This ensured that no transformer was needed and the amount of current was small. Twelve volts is the common voltage for car lamps of various descriptions, so lamps for this voltage are common and cheap.
However, if one bulb 'blows', they all go out. Unless you can easily tell which one it is, you have no choice but to change each bulb in turn until you find the one that is faulty.
Modern LED Christmas tree lights tend to be wired in parallel. Part of the reason for this is that they use far less current than old fashioned incandescent bulbs, so don't require the thick wiring that would be needed if incandescent bulbs were used in parallel with a transformer. They are often used outdoors, where a transformer is required anyway for safety reasons. They also tend to be in chains of 100 or more, where individual testing of possible failed bulbs would be impractical.
All of your lights and power plugs at home are wired in parallel. So are nearly all of the devices, such as headlights, in a car. There are two good reasons for this. The most obvious is that failure of one part of the circuit doesn't affect other branches, and devices can be individually switched. The second, less obvious reason, is to ensure stability of voltage: if you were to change one device in a series circuit for another of different resistance, it would change the voltage on any other devices. Many devices will only work properly within a narrow range of voltages.
Resistance
Electrical resistance is the property that causes an electric current to lose energy. The amount of energy per charge that an electric current has is termed voltage. An electric current experiences a change in voltage across a resistance. This change in voltage is called the potential difference because it is a difference in potential energy, but most people just call it the voltage. Its units are volts.
We connect a voltmeter in parallel with the device it measures. No current goes through the voltmeter; it measures the difference in energy between its two sides. This is similar to the way a tyre pressure gauge measures the difference in pressure between the inside and outside of the tyre without letting any air through it.
A tyre pressure gauge doesn’t let any air through it – it is connected in ‘parallel’ like a voltmeter.
A river flow gauge lets all the water through it. It works like an ammeter.
Resistance is defined as the amount of potential difference per amount of current i.e. volts per amp. Mathematically, this is R = V/I where R is the resistance, V the potential difference (in volts) and I the current ( in amps)
The units of resistance are volts per amp, given a special name – the ohm, symbol Ω
Note: if you are answering questions digitally and don’t have the omega symbol for ohms, write the word. Remember to use a lowercase letter at the start of the word ohms.
The equation rearranges into: V = I R and I = V/R
It is the V = I R arrangement of the formula that will be given on the exam paper.
Example: a light bulb is drawing a current of 2 A from a 6 V battery. What is its resistance?
Answer: V = IR so R = V/I = 6V/2A = 3 Ω
Problems may require rearranging the formula:
Example: a LED for a 4 V supply has a resistance of 20 Ω. How much current does it draw?
Solution: V = I R so I = V/R = 4 V/20 Ω = 0.2 A Note: NEVER leave out the zero before the decimal point.
Note on units for this: since an ohm is a volt per amp, you could write - I = V/R = 4 V/ 20 V A-1
You will see in this that the volts on the top and bottom line cancel, leaving the answer in units of 1/A-1, which is A.
Power
Electrical resistance changes electrical energy into heat.
Since voltage is the amount of energy per charge and current is the amount of charge per time, we can work out that voltage x current = amount of energy per time (as the charge cancels out). This energy is the electrical energy which is changed into heat (or light or some other energy form, since power ratings can apply to motors and other devices).
Energy per time is power, so we can take from this that electrical power = voltage x current i.e.
P = V I
Where P is the power in watts, symbol W (joules per second)
V is the potential difference in volts, symbol V
I is the current in amperes, symbol A
The formula rearrange as I = P/V and V = P/I
Example
A car starter motor runs off a 12 V battery and draws 150 A of current. What is the power of the starter motor?
Solution: P = V I = 12 V x 150 A = 1800 W
Some problems will require you to rearrange the formula e.g.
Electric bikes in NZ are not permitted to exceed 300 W unless they are classified as motorbikes (which require registration and a license). Most electric bike batteries work at 35 V. How much current does the battery supply at the maximum power output?
Solution: P = V I so I = P/V = 300 W/35 V = 8.6 A
Note of interest: a pre-metric unit of power is the ‘horsepower’ which is 735.5 W, so the rationale for the power limit on electric bikes seems to be based on ‘half a horsepower’. By contrast, a 50cc motorbike is about one horsepower. Electric motors can produce more torque than a petrol motor, though.
Energy
Power, by definition, is energy per time. This can be expressed in a formula as P = E/t.
In the Mechanics standard, the formula is given as P = W/t where the W stands for work, in the mechanical work sense i.e. the energy change of a force acting through a distance. You may well be asked for the energy used by an electrical device, or some related concept, by applying the power/time relationship. This may well involve finding the power first by applying electric concepts, or by using other information. Mostly, these will be multi-step problems (i.e. Excellence – recall what I said about getting an instant E7 or E8).
Example:
It takes 336 kJ (336,000 J) to heat one litre of water from room temperature (20°C) to boiling point (100°C). If a 230 V electric kettle draws a current of 8 A, how long will it take to boil a litre of water starting at room temperature?
Solution: first work out the power of the kettle
P = V I = 230 V x 8 A = 1,840 W
Step two: since we know that P = E/t it follows that t = E/P = 336,000 J/1,840 J s-1 = 183 s
Note that I used joules per second in place of watts to show that the answer is in seconds. Remember the rule about checking if your answer is reasonable – this is 3 minutes and 3 seconds; not unreasonable for a fairly low-powered kettle.
Magnetism
The first magnets were a naturally occurring rock (lodestone) that had the ability to attract pieces of iron or certain other materials.
Pieces of lodestone could also make iron nails magnetic by ‘stroking’ the nail constantly in one direction. These ‘magnetic’ nails had an unusual property: if they are able to float freely, one end will always point north. This ‘north-seeking’ end is termed the north end of the magnet.
The magnetised nails could also act like lodestones themselves - they would stick together.
A lodestone is an example of a naturally occurring magnet. A magnet produces a magnetic force field, capable of attracting certain substances (iron, nickel, certain alloys, certain iron compounds including iron sand).
You can reveal the pattern of magnetic force field lines by sprinkling iron filings or iron sand onto a piece of paper which covers a magnet.
One way to think of this is to imagine what would happen if you put a tiny compass anywhere around a bar magnet. The direction it points is the direction of an imaginary line we call the magnetic field line.
Attraction and repulsion
Like magnetic poles repel and unlike ones attract.
One way to think of this is to imagine what would happen if you put a tiny compass anywhere around a bar magnet. The direction it points is the direction of an imaginary line we call the magnetic field line.
Magnetic field lines can be used to explain attraction and repulsion: if the lines point the same way, the force is ATTRACTIVE. If the field lines oppose each other, the force is repulsive.
Note that in the picture of the two north poles pointing at each other there is a region in the middle where the magnetic fields cancel out. If you are asked in an exam to draw the orientation of a compass placed in that position, point it to the top of the page. This is because the NZQA apparently believe such a compass would point 'north' in the geographic sense, which in their minds is the top of the page (the physics behind that is dodgy to say the least). In theory, there is no net magnetic field from the magnets in that place (so ideally the Earth's magnetic field predominates). In practice, the field from the magnets is so large compared to that of the Earth that this region is too small to find in reality.
Note that a magnet must ALWAYS have both a north and south end. If you chop a magnet in half you will produce two smaller magnets, each with its own north and south ends pointing the same way as the original magnetic poles. Similarly, you can join magnets together north-to-south to create a single, longer magnet.
A straight magnet is called a bar magnet. You can bend a bar magnet into a U shape to create a ‘horseshoe magnet’. These have the opposite poles next to each other so the field lines go from one to the other.
Magnets should be stored with opposite poles next to each other and a ‘soft’ magnetic material (soft iron) between the poles. This is because the iron ‘concentrates’ the magnetic field lines within it and reinforces the magnetism in the permanent magnets.
Earth's magnetic field
The fact that the north end of a magnetic compass points in a direction fairly close to geographic north means that the Earth itself must have a magnetic field. Applying our reasoning from above about the fact that the north end of a compass will point to the south end of a magnet, this means that the geographic north pole of the Earth is the location of an internal 'south' magnetic pole. The magnetic field lines of the Earth point approximately north, although there is quite a bit of variation. In Europe and North America the difference between magnetic and geographic north is only a few degrees. However, in Auckland, magnetic north is about 20° east of geographic north.
The location of actually moves by a few kilometres each year. The magnetic poles are the places where a compass would point straight down into the ground, or straight up into the air (really high quality compasses sens orientation in 3D).
If you are asked in an exam to draw the magnetic field lines around the Earth you must draw them coming OUT of Antarctica and INTO the Arctic. The Antarctic pole must be labelled N and the Arctic one S.
Magnetism and electricity
A compass placed next to a wire will sometimes start to show a deflection if a current passes through the wire. If you investigate the direction of this, you find that the magnetic field lines make a circle around the wire:
The direction the compass needles point is given by a rule called the right hand grip rule: if you held the wire in your right hand, with your thumb pointing in the direction of the current (i.e. towards the negative), your ‘grip’ is the same direction that your fingers curl. The current direction for this is the CONVENTIONAL CURRENT i.e. from positive to negative.
You must draw the field lines as circles with arrows, as in the picture. You should draw at least three circles and show that the inner ones are closer together than the outer ones. You must put on arrows to clearly indicate the field direction.
You may be given a picture of a wire coming out of the page or going into the page. A dot in the middle of a wire indicates current out of page and a cross current into page.
The diagram on the right shows clearly the sort of thing you should draw, circles with arrows and closer together near the wire.
The strength of the magnetic field:
Is stronger closer to the wire i.e. at twice the distance it is half as strong
Is stronger for more current e.g. double the current doubles the field
The strength of the magnetic field at a distance d (in metres) from a wire carrying a current of I amperes is given by the formula B =kI/d
In this formula
"I" is the current, in amperes
"d" is the distance from the wire, in metres (note - you will usually be given the distance in cm or mm and will need to convert to metres first)
“B” is the strength of the magnetic field; its units are called tesla, symbol T.
The letter “k” stands for a constant which has the value 2 x 10-7 T m A-1. Both the formula and the value of k are given in the exam resource sheet.
Note: a tesla could be described as the size of magnetic field that exerts a force of 1 N on each metre of a wire carrying 1 amp of current i.e. 1 T = 1 N A-1 m-1. A field of 1 T is a very strong field; a fridge magnet would have a maximum of about 1 millitesla (0.001 tesla).
When your are given field strengths in exam questions they will usually be given to you in standard form e.g. 4 x 10-4 T. It is generally best to also write your answer in this form. You will need practice at both entering and reading numbers in this form on your calculators, as most students have very little experience with this and don't use their calculators properly.
Example: A starter motor in a car is drawing a current of 150 A. What is the strength of the magnetic field 2 cm from the wire?
Solution: B = kI/d = 2 x 10-7 T m A-1 x 150 A/ 0.02 m = 1.5 x 10-3 T (or 0.0015 T or 1.5 mT)
Example 2: A cable carrying a large current is 15 cm behind a wall. A magnetometer slid along the wall gives a maximum reading of 4 x 10-4 T. What is the size of the current?
Solution: B = kI/d; we need to make I the subject so I = Bd/k = 4 x 10-4 T x 0.15 m/2 x 10-7 T m A-1
= 300 A
Note: many students will need help with calculator use and formula rearrangement for these problems; I will create a help page for this.
Wire loops, coils, solenoids and electromagnets
If you shape the wire into a loop and apply the RH grip rule to the whole of the loop, you can see that the magnetic field lines are concentrated into the centre of the loop:
When you loop the wire around multiple times, each loop produces the same pattern in the field. However, each additional loop makes the field stronger:
A coil of wire for producing a magnetic field like this is called a solenoid, and acts like a bar magnet.
The direction of the field can either be worked out by applying the grip rule to individual loops, as shown in the first diagram above, or by applying a slightly different grip rule: if you hold the solenoid in your right hand, with your fingers curling in the direction of the current flow, your thumb points north (diagram above right).
Placing a core of 'soft' magnetic material such as 'soft' iron or ferrite into the centre of the solenoid will make the magnetic field more effective. The device is then called an electromagnet. It behaves exactly like a bar magnet, except that it can be turned on or off and the strength of the field can be adjusted by adjusting the current through the wire.
The strength of the field depends on
the number of loops
the amount of current
i.e. you could make an electromagnet stronger with the same current by winding the wire around it more times, or you could keep the number of loops around the same but increase the current.
Extra resources: (only available if logged into SHC google network