6.7 Uniform Magnetic field

Know how to use two permanent magnets to produce a uniform magnetic field pattern

If the lines of a magnetic field are close together the magnetic field is strong, likewise further apart magnetic field lines show a weak field. If the magnetic field lines are parallel to each other we have a field of constant strength – a uniform field

Making a Uniform magnetic field

An easy way to produce a roughly uniform magnetic field is to hold two opposite poles close to each other so that they attract [see image below].

6.6 Magnetic field patterns

Sketch and recognise the magnetic field pattern for a permanent bar magnet and that between two bar magnets.

Things to note:

• The arrows always point from North to South
• The lines are closest at the poles.

6.5 Magnetic materials

Understand that magnetism is induced in some materials when they are placed in a magnetic field.

Practical

1. Stroke a magnet along a steel bar and an iron bar
2. Try picking up some bar clips
3. Bang both bars on the desk
4. Now try picking up the paperclips again

Through this experiment we have magnetised the steel and iron bars so that we can pick up lightweight magnetic objects.

Tap the magnetised bars onto a table gently and try to pick up the paperclip again. What you should notice is that the steel retains some of its magnetism and can pick up the paperclip but the iron bar loses its magnetism completely. This is because steel is magnetically hard.

6.4 Magnetic Field Line

Understand the term ‘Magnetic field line’

Using iron filings we can see the magnetic field of the magnet. These “line” we see traveling from the North pole to the South pole of the magnet are called the Magnetic field lines.

The video below shows how the lines look in 3 dimensions. 

6.3 Magnetic materials

Recall the properties magnetically hard and soft materials

Magnetically hard materials can be permanently magnetised by a strong magnetic field. Steel and other special alloys of iron are magnetically hard.

Magnetically soft materials can be magnetised very easily, but the magnetism induced in them is only temporary and easy to remove. Iron is magnetically soft material.

6.2 Magnets

Recall that magnets repel and attract other magnets, and attract magnetic substances

Magnets and Magnets:

Magnets can either attract or repel other magnets: you should already know that opposite poles attract i.e. South and North attract, but South and South repel

Magnetic substances:

There 5 magnetic materials: 3 elements and 2 compounds

• Iron (Fe : Element)
• Cobalt (Co : Element)
• Nickel (Ni : Element)
• Steel (an alloy or iron)
• Magnetite/Lodestone (Fe₃O₄ : an oxide of iron)

Topic 6

Magnetism and Electromagnetism

Objectives:

6b) Magnetism

6.2 recall that magnets repel and attract other magnets, and attract magnetic substances

6.3 recall the properties of magnetically hard and soft material

6.4 understand the term ‘magnetic field line’

6.5 understand that magnetism is induced in some materials when they are placed in a magnetic field

6.6 sketch and recognise the magnetic field pattern for a permanent bar magnet and that between two bar magnets

6.7 know how to use two permanent magnets to produce a uniform magnetic field pattern

6c) Electromagnetism

6.8 recall that an electric current in a conductor produces a magnetic field round it

6.9 describe the construction of electromagnets

6.10 sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current

6.11 appreciate that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field

6.12 recall that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple d.c. electric motors and loudspeakers

6.13 use the left hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field

6.14 recall that the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current.

6d) Electromagnetic induction

6.15 recall that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it; also recall the factors which affect the size of the induced voltage

6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field; also describe the factors which affect the size of the induced voltage

6.17 recall the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides

6.18 explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy

6.19 recall and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer:

 6.20 recall and use the relationship (for 100% efficiency):

5.19 Boyle's Law

5.19 use the relationship between the pressure and volume of a fixed mass of gas at constant temperature:

p₁ = Pressure at the beginning
V₁ = Volume at the beginning
p₂ = Pressure at the end
V₂ = Volume at the end

NB: Can use any units for V and p as long as they are constant at the beginning and end.

Watch the video below: 

We can clearly see that as we reduce the pressure in the vacuum the volume of the gas increases significantly, also note that the Volume is constant and the mass of the gas is constant.

While we’re on the topic of a vacuum, you might like to take a look at the video below “Nothing” by Vsauce a great YouTube channel for quirky science videos:

Conclusion: Pressure is inversely proportional to volume 

5.18 Gay-lussac's law

5.18 use the relationship between the pressure and Kelvin temperature of a fixed mass of gas at constant volume:

p₁ = Pressure at the beginning

T₁ = Absolute temp at the beginning

p₂ = Pressure at the end

T₂ = Absolute temp at the end

NB: The units of temperature must be Kelvin, but any unit of pressure can be used

Pressure is directly proportional to absolute temperate (when gas is at a constant volume)  

5.17 Kelvin and Pressure

5.17 describe the qualitative relationship between pressure and Kelvin temperature for a gas in a sealed container.


Increasing the temperature of particles in a sealed container causes them to move with larger amounts of kinetic energy so collide with the walls more often and at higher speeds, this means that when the particles come into contact with the container they apply a larger amount of force onto the area and therefore the pressure increases

Increase in temperature results in an increase in pressure (if volume is constant)

Example: Cloud formation

• Place a little water in the bottom of a 1½ litre plastic bottle
• Squeeze a few times
• Introduce a small amount of smoke
• Squeeze and release several times
• When you squeeze, the cloud disappears; when you release the cloud reforms.

Explanation

• When the pressure increases the temperature increases and vice versa.
• The smoke particles are nucleating sites on which the water can condense

Use the Java app below to test the idea. 

Click to Run

5.16 Kelvin and Kinetic energy

5.16 understand that the Kelvin temperature of the gas is proportional to the average kinetic energy of its molecules.

What the graph above shows is that the average speed of the particles squared (v²) is directly proportional to the temperature of the particles in Kelvin. What this means is that as we increase the absolute temperature of the particles we can predict the average speed of the particles.

5.15 Temperature and Speed

5.15 understand that an increase in temperature results in an increase in the speed of gas molecules

What is Temperature?

Temperature is a measure of the average kinetic energy of the particles in a substance. What this means is that as we increase the temperature of a gas the particles have more kinetic energy and therefore move around at a greater speed and collide with each other more often, it also means that they collide with the walls of the container more often and with a greater force, so looking back at the previous video [see 5.12] an increase in force and a constant surface area results in an increase in pressure \ increase temperature (of a gas) => increased pressure (of the gas).

5.14 The Kelvin Scale

5.14 describe the Kelvin scale of temperature and be able to convert between the Kelvin and Celsius scales

Describe: The Kelvin scale begins at -273°C and the intervals between Centigrade and Kelvin are equal (i.e. ΔK = Δ°C)

Converting:
To Kelvin: Kelvin = Centigrade + 273
To Centigrade: Centigrade = Kelvin – 273

Examples
Convert these temperatures

1. 20°C = 20 + 273 = 293K
2. 150°C = 150 + 273 = 423k
3. 300K = 300 – 273 = 27°C
4. 650K = 650 – 273 = 377°C

5.13 Absolute Zero

5.13 understand that there is an absolute zero of temperature which is -273°C

Watch the video below: http://www.youtube.com/watch?v=ZvrJgGhnmJo

Using particle theory we can explain why the gas in the balloon contracts. Obviously the temperature inside the balloon decreases; meaning that the average kinetic energy of the particles also decreases. If the particles have less KE they will collide with the walls of the balloon with less force (and less collisions per second). Because the walls of the container are flexible, the volume decreases because the particles aren’t applying enough force on the balloon to keep it inflated.

This is known as Charles’ law. A law stating that the volume of an ideal gas at constant pressure is directly proportional to the absolute temperature. 

5.12 Gas Molecules

5.12 Recall that molecules in a gas have a random motion and they exert a force hence a pressure on the walls of the container


Watch the video below:

When a gas is introduced to the container the gas particles colldie with all the walls of the container; when particles collide with the wall attached to the needle they have enough force to push the wall and in turn move the needle. The needle on the meter is measuring pressure; as the gas particles collide with the walls they apply a force on the walls, These walls have a surface are, so we can measure the pressure because p=F/A

5.11 Brownian motion

5.11 understand the significance of Brownian motion.

What is Brownian motion?

The English Botanist Robert Brown presented the first evidence that matter consists of tiny particles in motion. Brown was studying pollen grains suspended in a liquid with a microscope and noticed the haphazard movement of the grains, this similar motion can also be seen when smoke particles in air are observed under a powerful microscope. The zigzag motion is due to unequal bombardment between the suspended particles and the molecules of the surrounding medium. This irregular motion of suspended particles is known as Brownian motion.

Why is it significant?

Brownian motion was the first step into proving atomic theory, and although it was Einstein that finally described the physics behind the phenomenon, the motion was named after Brown because he was the first to test this theory. So without Robert Brown, we may perhaps have not had such a conclusive theory until a much later date, or we may have had a different theory all together.

The video below visually explains the motion: the Red disc represents large smoke molecules and the small ball bearings represent the small particles that are usually to small to see.

5.9 & 5.10 Solids, Liquids (and gases)

5.9 Recall that particles in a liquid have a random motion within a close-packed irregular structure.
5.10 Recall that particles in a solid vibrate about fixed positions within a close-packed regular structure.

State Changes

All of the state changes are shown in the diagram below

5.8 Boiling

Understand that a substance can change state from a liquid to gas by the process of evaporation or boiling.

If the temperature of liquid atoms increases even more the particles eventually have enough energy to break the inter-molecular bonds between the atoms, the particles now fly around at very high speeds (several hundred kilometers per hour) and fill any container they are placed in. this is the gaseous state. The state change between a liquid and a gas is called boiling.

5.7 Melting

Understand that a substance can change state from solid to liquid by the process of melting.


We know that all materials are made of atoms, in a solid state these atoms attract each other and are locked together by the inter-molecular forces between them. But, even in a solid state the atoms are not completely still; they still vibrate on a fixed position.

If these atoms are given more internal (kinetic) energy, the particles vibrate faster and further, if the temperature continues to increase to a point where the inter-molecular forces between particles are not strong enough to hold the structure together, but are strong enough to prevent the atoms from flying apart from each other; a liquid is born. This process of going from a solid state to a liquid is called melting or fusion.

5.6 Pressure Difference

Recall and use the relationship for pressure difference:

Δp = pressure of the fluid (N/m² or Pa)
h = Height of the fluid above loci (depth) (m)
ρ = density of the fluid (kg/m³)
g = gravitational field strength (N/kg)

Proof

• The bottom hole of this column squirts water the furthest
• This is because the water at the bottom has more pressure
• In the formula: Δp = hρg, ρ and g are both constant at all loci, but h is larger lower down \Δp = large.

Questions

The pressure gauge on a submarine in a river was reading 100kPa when it was the surface. If a sailor notices that the gauge is now reading 250kPa, how deep is he? How would the answer change if he were diving in sea water that is slightly denser that fresh water?
Note: ρ_{fresh water} = 1,000kg/m³

If the submarine was in sea water he would be slightly shallower than 15m.

A diver on Saturn’s moon Titan is 50m below the surface of a lake of liquid methane, what is the increase in pressure on him due to his depth in the methane? The density of liquid methane is 0.42g/cm³. The acceleration of gravity on Titan is 1.4m/s².
We are told that the pressure of the atmosphere on Titan is 1600mbar. What is the total pressure on the diver (in kPa)?
Note: 1000mbar = 1bar = 100,000Pa.

5.5 Equal Pressure

Understand that the pressure at a point in a gas or liquid which is at rest acts equally in all directions

Magdeburg Hemispheres

When the pressure inside the sphere is equal to the atmospheric pressure outside, the hemispheres can easily be pulled apart because there is no external force holding the hemispheres together, but if a vacuum is introduced inside the sphere, the pressure atmospheric pressure pushing against the sphere is much greater than the pressure pushing outwards, so the resultant force clamps the sphere closed…

If you’re confused… I tried my best to explain the pressure as a group of men.

1. No vacuum
There are 50 people inside the sphere trying to push the sphere open and 50 outside trying to keep it shut; obviously the sphere will remain in a stable state as neither side has more strength, but when 2 extra people are introduced (the people holding onto the handles) all of a sudden the side trying to open the sphere has more force than the opposing team. So the sphere comes loose and all the little people can escape.

2. Vacuum
Inside the vacuum there are no longer 50 people inside trying to break open the sphere, but the 50 people outside (representing the atmosphere) are still there so the sphere remains firmly closed, and when 2 people are introduced to attempt to open the sphere, the sphere remains closed because those two people alone don’t have enough strength to fight against the 50 people working against them.

5.4 Pressure

Recall and use the relationship between pressure, force and area:

NB: 1N/m² = 1Pa

Using the Formula

Calculate the pressure generated by an ordinary shoe heel (person of mass 40kg, heel 5cm x 5cm), an elephant (of mass 500kg, foot of 20cm diameter) and a high-heeled shoe (person of mass 40kg, heel area 0.5cm­²). Which ones will damage a wooden floor that starts to yield at a pressure of 4000 kPa?

1. Ordinary shoe heel

2. Elephant

3. High-heeled shoe

Clearly one can see that the high-heeled show will do the most damage to the floor.