igcse triple physics 2011 vers. 1.0 magnetims ... electromagnetism_notes… · © nelkin &...

© Nelkin & Cooke’s Physics Notes IGCSE Triple Physics 2011 Vers. 1.0

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Magnetims & Electromagnetism

Magnetism .................................................................................................................................................... 2

Magnetic Fields ..................................................................................................................................... 2 Electromagnetism ......................................................................................................................................... 4

The magnetic field around a straight wire ............................................................................................ 4 The magnetic field around a flat coil .................................................................................................... 4 The magnetic field around a solenoid................................................................................................... 5

Strength of the electromagnetic field ............................................................................................... 6 Using ac electricity ............................................................................................................................ 6

Force on a current-carrying wire in a magnetic field ................................................................................ 6 The motor effect ................................................................................................................................... 6 Flemmings Left Hand Rule .................................................................................................................... 7 Force on a moving charged particle in a magnetic field ....................................................................... 8 Ntk: A Way of Seeing Why there is a Force .......................................................................................... 8 A simple electric motor ......................................................................................................................... 9

Uses of Electro-magnets ......................................................................................................................... 10 The relay .............................................................................................................................................. 10

Electromagnetic Induction .......................................................................................................................... 13 Generation of Electricity ......................................................................................................................... 14 Transformers ........................................................................................................................................... 15

Structure of a Transformer ................................................................................................................. 15 Step-up and Step-down Transformers ................................................................................................ 15 Power and Current on the Transformer ............................................................................................. 16


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Magnetism A magnetic material that is a material that is attracted by a magnet. There are three magnetic materials: iron, nickel, cobalt. Steel consists mainly of iron, and should therefore not be mentioned additionally. Plastic, copper, aluminium, rubber, etc. are non-magnetic materials. Note: any of the materials above are always magnetic. The word does not mean that they have been turned into permanent magnets. Magnetised is the word to use if you want to say that something has become a magnet. Magnetically hard materials are those that retain their magnetism once they have been magnetised. That means all permanent magnets consist of magnetically hard materials. For some applications it is more useful to have materials that do not retain their magnetism when repeatedly exposed to a magnetic field. This is the case for magnetic door locks: the armature, which is the iron attracted to the electromagnet, should not turn into a magnet itself. If it did, it would not be possible to open the door anymore. Therefore a so called magnetically soft material is used. Magnetically soft materials do not retain the magnetism.

Magnetic Fields

The area in which magnetic forces act is called a magnetic field. To show strength, direction and shape of the field, it is represented by magnetic field lines. Where the density of field lines is high (i.e. the field lines are close together), the field is strong. This can be seen at both poles. The further the field lines are apart, the weaker the field. The field lines always point from the North to the South pole outside the magnet. Note: inside an electromagnet the field lines would point from South to North.

Magnetic field lines can be made visible using iron filings. Note: field lines never cross. Unlike poles attract (north and south), like poles repel (north and north or south and south).

A uniform magnetic field is one in which the field lines are parallel and equidistant.

This can be achieved by placing a North and a South pole close to each other, and it

only applies to the area right between the two poles. As you move out, the field lines


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become curvy.

Induced Magnetism

When a magnet is brought close to paper clips, these are attracted to the magnet. A chain of paper clips can be formed. The top paper clip acts like a magnet and attracts the second paper clip despite the fact that it was not a magnet beforehand. The second paper clip then also acts as a magnet for the third one, etc. This acting like a magnet when in a magnetic field is called induced magnetism. There are north and south poles on the paper clips as well due to the induced magnetism. When the paper clips are pulled away from the magnet they will no longer act as magnets which shows that induced magnetism is just temporary and only when the body is placed in the magnetic field.


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Electromagnetism Magnetism and electricity are related. There is a magnetic field around every current. As soon as the current ceases to flow the magnetic field disappears. Electromagnets are therefore magnets that can be switched on and off. To understand electromagnetism we will first look at the magnetic field around a straight wire before looking at coils.

The magnetic field around a straight wire

To determine the direction of magnetic field around a straight wire choose the rule that you remember

the best - the screwdriver or cork screw rule. Imagine you are screwing in a screw in the direction of

current flow. The clockwise motion of the screwdriver is

the same direction as the magnetic field.

Another rule is the right hand rule, which will also predict the field direction. The thumb shows the direction of the current. The fingers show the direction of the magnetic field.

The magnetic field around a flat coil

When the wire is bent to a single loop, the field changes to the following shape (see left diagram). The field around a flat coil would be exactly the same as the diagram on the right shows.


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Remember: outside the coil the field lines point from North to South, so on the right of the flat coil would be the North pole, on the left the South pole.

The magnetic field around a solenoid

When the coil is not flat, but cylindrical, it is called a solenoid. The field of the solenoid is similar to the field of

a bar magnet. The coil has a north pole at one end and a south pole at the other. Remember, that we show the lines of force coming out of the north pole and going into the south pole. Wrapping the wire in a coil concentrates and increases the magnetic field, because the additive effect of each turn of the wire. Coiled wire increases magnetic field. A coil of wire used to create a magnetic field is called a solenoid.

The direction of the magnetic field outside the solenoid is from north to south.

The direction of the magnetic field inside the solenoid is from south to north.

Electromagnets are formed when a current passes through a solenoid with an iron core.

There are 2 ways to determine the location of the North and South poles for a solenoid. How to remember the direction of the magnetic field When a current passes through the solenoid, it becomes an electromagnet. So one end is a north pole and the other end is a south pole. There is a little trick for remembering which end becomes which pole.

There are 2 ways to tell where the North or South poles are if you know the current direction.


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1) It's a question of seeing whether the letter S or N will point in the same direction as the current.

When we look at one end of the coil, the current is going anticlockwise. In this case, we can put in the letter N and it will point in the same direction as the current. An S will not. So this is the north pole.

If we look at the other end of the coil, the current is going clockwise. In this case we can fit in an S. So this is the south pole.

2) The right-hand grip rule determines the north pole:

The fingers point in the direction of the current.

The thumb indicates the north pole.

(Ampere's Rule for a solenoid (right-hand rule for a solenoid) states

that if the solenoid is grasped in the right hand in such a way that the

fingers curl in the direction of the current, the right thumb points in the

direction of the north pole of the core. (Magnetic field lines point from

south to north inside the core, in the same direction that the right

thumb is pointing).

Strength of the electromagnetic field Wrapping the wire around an iron core greatly increases the strength of the magnetic field. Increasing the current and incrasing the number of turns of the coil also increases the strength of the electromagnet. Using ac electricity If ac electricity is used, the electromagnet has the same properties of a magnet, except that the polarity reverses with the AC cycle.

Force on a current-carrying wire in a magnetic field

The motor effect

The motor effect is the effect that a current carrying wire, in the presence of a magnetic field, will experience a force. A simple experimental demonstration will show you that this is true. Place a wire that is connected to a power pack in between the poles of a horseshoe magnet. Turn on the power and the wire moves. Often the movement is only very slight because a typical horseshoe magnet is not very strong.

The force depends on a number of things:

How strong the magnetic field is.


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How much current is flowing through the wire. The angle formed between the wire and the direction of a the magnetic field.

The first two points are pretty much obvious, so lets look at the third point in a little more detail. The magnetic field of a horseshoe magnet points pretty much in a straight line from the north pole to the south pole. If the wire cut's this field at right angles the resulting force will be a maximum. If the wire runs parallel to the field from the north to the south pole (or vice versa) there will be no force.

Having said that at GCSE you only really need to think about situations where the field and the wire cut each other at right angles. The force is always at right angles to both the field and the current flowing in the wire. This means that if you draw the direction of the magnetic field and the wire on a piece of paper the force will be out of the plane of the paper pinting straight up or down {More about how you work out which way later).

Look at the diagram. For simplicity, all of the horseshoe magnet except the two ends hasn't been drawn. Also the power pack and connecting wires are not shown either. The magnetic field is ging into the screen. The current is travelling from right to left. The black line represents the force, and therefore the direction that the wire moves.

Q1) Name one way that the force on the wire could be increased.

Q2) Name as many ways as you can to reduce the force on the wire.

Q3) Name two ways of reversing the force on the wire, so that it pushes the wire up rather than down.

Flemmings Left Hand Rule

This rule allows you to work out which direction the force will point in. Arrange your left hand with your thumb, first finger, and second fingers all pointing at right angles to one another.

Point your First finger in the direction of the magnetic Field.

Point your SeCond finger in the direction of the Current

The thumb will then give you the direction of the Thrust or so if you can’t spell you can use …. THorce. Note: Remember you have to use your left hand for this!


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Q4) A student uses her right hand instead of her left. What effect will that have on the force she works out?

Force on a moving charged particle in a magnetic field

When we talk about a current in a wire, we mean that electrons are moving in the wire. When charged particles like electrons, ions or alpha or beta particles move through a magnetic field, they experience a force as well. This of course will only be the case if the movement of the particles is not parallel to the magnetic field (along the field lines).

Ntk: A Way of Seeing Why there is a Force

You will recall that a current carrying wire is surrounded by its own magnetic field. The diagram below shows the wire end on in a magnetic field of two magnets that are NS facing. The field due to the magnets is shown in blue, and the field due to the current in the wire is shown in black.

Notice the direction of the two fields as shown by the arrows. On top of the wire the fields are both going in the same direction. They add up making an overall strong field. Underneath the wire, they go in opposite directions. They cancel each other out to some extent making an overall weaker field. The new field is shown in the diagram below.

See how the field above the wire is stronger. The lines are closer together. Below the wire the field is weaker (due to partial cencelling out) the field lines are further apart. The force pushing the wire downwards away from the strong field into the weak field. It's as if the field lines try to repel each other. They don't like being squashed together and try to straighten out. They also act as if they are made of elastic bands, they don't like being stretched out of shape. (This is just a model of what's going on. The lines aren't real, they don't actually try to push each other away, but I find it a way of helping me understand what's going on. If it doesn't help you, don't use it) Q5)Use the idea of field lines not likeing being squashed together to explain why two magnets with like poles facing each other repel. (Q6)Use the idea of field lines being made of elastic to explain why two magnets with opposite poles facing attract one another.


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A simple electric motor

Ok so far we have been looking at the force that results when we put a current carrying wire in a magnetic field. in this section we will look at a practical use for this force.As you have probably aready guessed from the name of this page, the practical use is going to be an electric motor.

Look at the diagram above. A rectangular loop of wire is sitting inside a magnetic field. We can consider the current in the four sections of the loop and work out which way the force acts.

On the left hand side of the loop the current is flowing into the page or screen. The magnetic field will be going from left to right so from Flemming's left hand rule the force will be downwards.

On the back and front of the loops the current is parallel to the magnetic field so the is zero force.

On the right hand sisde of the loop the current is coming out of the page or screen. The field is still going from left to right so the force will upwards.

The net result of these different forces is that there will be a turning moment that makes the coil rotate by 90°. At that point the upwards and downqwards forces will be acting along the same line and the coil will stop turning. Another way to think about it is to consider the loop as a tiny little one turn soleniod. The solenoid will have a little north pole and a little south pole and will therfore move until it's north pole lines up with the south pole of the magnet on the right, and it's south pole lines up with the north pole of magnet on the left.

This is all very interesting but not much use as a motor. We want something that keeps turning all the time the current flows. They way this is achieved is by the use of the commutator - a circular metal ring that is split into two halves. The ends of the wire loop turn around inside the commutator. They are in electrical contact with it. One side of the commutator is connected to the positive output of a power pack or battery .


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the other half of the commutator is connected to the negative.

Let's look at what happens as the coil turns inside the commutator:

1. The coil turns clockwise because of the forces on the two halves as explained above. 2. As the coil turns the wires slip around on the inside of the commutator. 3. When the coil is in a vertical position there is no force on it, but its momentum carries it forward a

little. 4. The wires inside the commutator make contact with the other half of the ring. I.e. the wire that was

in contact with the positive half of the commutator now touches the negative half and vice versa. 5. This causes the current to flow the other way around the coil, which reverses the little solenoid

magnet. 6. The coil therefore tries to line itself up with the magnetic field by continuing to turn so that it

ultimately points in the other direction. 7. Once the coil is vertical, and facing the other way, the wires touch their original half of the

commutator. and the whole cycle repeats over and over.

Q7)A student set's up am electric motor and turns it on. The coil turns clockwise. List two ways she could reverse the direction.

Uses of Electro-magnets

The relay


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The Electric Bell

The Motor

This occurs when a current-carrying conductor is placed in a magnetic field. The result is a force on the

conductor.

The direction of the force on the wire is determined by Fleming's left-hand rule.

A coil mounted on an axle rotates between the poles of a

magnet.

The ends of the coil are connected to a commutator.

Apply the left-hand rule.

The turning force exerted on the coil is increased by:

o increasing the current

o increasing the number of windings


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o using stronger magnets

The loudspeaker

The coil is inside a magnetic field.

The direction and size of the current changes.

The coil moves to changes in the current.

The moving coil moves the cone in and out.

This causes vibrations of air molecules,

which generate sound.


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Electromagnetic Induction While electromagnetism deals with the magnetic field created by a current, electromagnetic induction is about the current that flows due to a change in the magnetic field.

If you move a wire connected to an ammeter through a magnetic field, a current is induced in the wire, i.e. it flows because of the change of the magnetic field. If you take several loops of the wire (coil), this effect becomes even stronger (the current of the single loop is multiplied by the number of turns on the coil). Instead of moving the current through a magnetic field you can also move the magnet through fixed loops. If you move a magnet into a coil it induces an emf (electromotive force), a scientific word for voltage used in the context of electromagnetic induction. This is how you can induce a stronger emf:

1. use a stronger magnet 2. moving the permanent magnet faster 3. more coils 4. using a coil with a larger cross-sectional area

This is how you change the direction of the induced emf: 1. Turn the permanent magnet around 2. Move the permanent magnet in the opposite direction

As the permanent magnet is moved into the coil the magnetic field in the coil changes. We say that the coil cuts through the magnetic field lines of the permanent magnet.


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Generation of Electricity If you turn a coil in a magnetic field, the right and left part will move vertically through the field lines (cut through the field). An emf is induced in the coil. If the two ends of the wire are connected to two slip rings, an alternating current will flow in a connected circuit (ac generator). This is the case as the motion of each side of the coil will change direction relatively to the magnetic field. To change this generator into a dc generator the slip rings need to be exchanged for a split ring. The split ring allows the direction of the current in the attached circuit to remain the same as the brushes will always touch the other half of the split ring when the current would change direction using slip rings.

a.c. generator d.c generator Alternatively the magnet itself can be spun next to a coil or two. This is what happens in a dynamo, the mini generator that used to supply the electrical energy for the light bulbs on a bicycle. The following diagram shows the setup of a dynamo.

The induced voltage (emf) increases with

1. The speed at which the magnet/coil turn (basically how many fieldlines are cut per unit time). 2. The number of turns in the coil 3. The strength of the magnet


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Transformers Transformers are devices that increase or reduce voltage. Power stations are connected to the National Grid that allows the distribution of the electrical energy throughout the country. Stepping-up the voltage on the pylons reduces the energy loss by heat in the cables. Transformers are also used in your hi-fi at home or in your desktop computer. The mains electricity of 230V is reduced to 5V, 12V and other values needed for the electronic equipment inside.

Structure of a Transformer

Two coils are placed on a soft-iron core, which could be square-shaped. The coil with the input voltage is called the primary coil, the coil with the output voltage is called the secondary coil. An alternating current in the primary coil produces an alternating magnetic field in the core. This alternating field induces an emf (voltage) in the secondary coil, just like the moving magnet generated an emf in the coil before. The voltage in the secondary

coil depends on its number of turns. The formula that links voltage V and the number of turns N is:

turnsondary

turnsprimary

voltageondaryoutput

voltageprimaryinput

sec)(sec

)(

As a formula this can be written as:

S

p

S

p

N

N

V

V

Vp: primary voltage, Vs: secondary voltage Np: primary turns, Ns: secondary turns That means: if I want to double the voltage, the number of turns on the secondary coil needs to be double the ones on the primary coil. The images show step-down transformers.

Step-up and Step-down Transformers

As there is less loss of electrical energy when it is transferred at a high voltage, step-up transformers are used to connect the power output of the power station to the National Grid. As 400,000V is far too dangerous for homes (despite insulation), the voltage is then reduced to 230V close to the homes with a step-down transformer. Which coil would have the higher number of turns: the primary or the secondary coil of a step-up transformer?


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Power and Current on the Transformer

For an ideal transformer of 100% efficiency the input power (primary) is the same as the output power (secondary).

Pp=Ps Power = voltage x current as you learnt in the electricity chapter. If Pp = Vp x Ip, then this formula can be re-written as:

VpxIp = Vs x Is Rearranging this formula leads to:

p

s

s

p

I

I

V

V

In other words this means that the ratio of currents is inversely proportional to the ratio of voltages. If the voltage on the pylons is increased from 25,000V to 400,000V, i.e. by factor 16, then the current decreases by factor 16. Ntk: The energy loss on the power lines due to heat increases with the square of the current.

igcse triple physics 2011 vers. 1.0 magnetims ... electromagnetism_notes… · © nelkin &...

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