Magnetism

Permanent Magnets

Some materials can be defined as magnetic whereas others are non-magnetic. A magnetic material is one which is affected by magnetism.

Some materials which are strongly attracted to magnets (magnetic) are:

• Iron and steel
• Other alloys of iron, cobalt or nickel
• Alloys containing a mixture of iron, colbalt and nickel

Some materials which are not attracted to magnets (non-magnetic) are:

• wood
• plastic
• leather

Properties of Magnets

Poles

Poles are the regions on a magnet to which materials are attracted. All magnets have two poles, a north pole and a south pole. Hence, they are called magnetic dipoles.

The two poles of a magnet are either south-seeking or north seeking. A suspended magnet always settles with its poles pointing in the same direction. The north pole of the magnet will always point toward the geographical north pole of the earth hence it is called the north seeking pole or simply the north pole. The south pole of the magnet will always point toward the geographical south pole of the earth hence, it is called the south seeking pole or simply the south pole. This video explains this some more. Because of the property explained above, magnets are used to make the magnetic compass.

Forces between magnets

If two opposite poles of a magnet are brought into close proximity, a force of attraction between the two magnets is observed. Also, if two similarly poles of a magnet are brought into close proximity a force of repulsion is observed. Therefore, "like poles repel and unlike poles attract".

Magnetic Induction

When an unmagnetised iron alloy is brought near to a magnet it is attracted to the magnet. This is a result of temporary magnetism being induced in the material. Magnetic induction always results in attraction, never repulsion. Also, there is always a pair of induced poles.

Permanent and Temporary Induced Magnetism

Refer to page 276 of your text book.

Iron alloys like steel and magnadur are hard to magnetise, hence they are called hard magnetic materials. Materials which are easier to magnetise such as iron and mumetal are called soft magnetic materials. Soft magnetic materials are used to make temporary magnets whereas hard magnetic materials are used to make permanent magnets.

Magnetic Forces

The magnetic field around a magnet is the region in which forces act on other magnets and on magnetic materials by inducing magnetism in them. The direction of a magnetic field at a particular place is the direction of the force it produces on a free magnetic north pole. Remember, field lines always go from north to south.

Magnetic Field Diagrams

You should be able to draw the field lines:

• Around a strong single magnet
• Around and between two strong magnets which are oriented parallel, anti-parallel, and pole to pole with each other just as you did in your SBA. (Page 280 of your text has a few diagrams)

Read up on this entire topic in your text books people. It is important.

Measurements and Mathematics

Measurement and Significant Figures

When we calculate the value from our results the answer should we written to the same number of significant figures as the original results. For example, is the two sides of a rectangle are measured as 24.2 cm and 18.3 cm, then the are of the triangle is 24.2 cm x 18.3 cm = 442.86 cm²

Since the sides of the triangle were given at three significant figures, then the answer should also be to three significant figures. Therefore, the answer is 443 cm²

Reading Scales

Many readings in physics are taken from a scale on an instrument e.g. thermometers, ammeters, voltmeters etc. When reading a scale you make an estimate when the pointer is not actually on a mark on the scale. In the example below of an ammeter, the result would be taken as 1.34 A.

Accuracy of Results

For an experiment to be useful we must obtain accurate results. There are certain steps that can be taken to increase the certainty of our results. These are:

1. Take the same reading more than once can calculate an average value.
2. Measure a large number of a quantity and calculate the value for one. For example, if we have to find the thickness of a sheet of paper, we can measure the thickness of 300 sheets. We then divide our result by 300 to find the thickness of one sheet.
3. We can select and instrument which is appropriate to the reading. If a current of about 0.4 A is being measured we use an ammeter with a range of 0 to 1 A, not 0 to 5 A.
4. We take care to avoid parallax error. Always try to read scales from directly over the mark.

 Large and Small Numbers

When we have very large and small numbers there are useful alternative ways to write them.

 Standard Form

In standard form we write numbers in two parts as follows:

You should be able to multiply and divide numbers in standard form:

 Prefixes

Prefixes are also used to represent very large and small numbers. The following examples show there meaning.

 Graphs

A common way to present results is to draw graphs. Graphs often provide us with extra information and helps our understanding.

When plotting a graph the axes are labelled with the quantities involved, their symbols and the units of the quantities. A convenient scale must be chosen in order for the results to use up most of the graph.

A small cross or a circled dot can be used to plot the points. Results must be plotted as accurately as possible and should not be rounded off.

A line of best fit must be drawn to join the plotted points together. This line goes as close as possible to as many points as possible. The points should also be 'balanced' about the line with equal numbers of points below and above the line.

A the results are arranged in a curve, a smooth curve must be drawn.

 Common Graphs

If the graph is a straight line passing through the origin, it means that the two quantities are proportional to each other.

If the graph is a straight line but not passing through the origin, the quantities are linearly related to each other but not proportional to each other.

 Gradient and intercepts of a graph

Two important quantities of a straight-line graph are its gradient and its intercepts on the axes.

The intercept is the point where the line cuts the axis. The y-intercept is the point where the line cuts the y-axis and the x-intercept is the point where the x-axis. In the example below, the y-intercept is 4 and the x-intercept is -8.

The gradient is found using the formula below:

Newton's Cradle

The video below demonstrates the Principle of Conservation of Linear Momentum. Use your observations to do lab #7.

You can also check the link for the blog post called lab no. 7. There you will be able to carry out the experiment for yourself.

Micrometer and Vernier Callipers

The video below shows how a micrometer and vernier calliper are read. Enjoy!

Hope this helps.

Logic Gates

Logic gates are the building blocks of digital electronics and are used to build telecommunication devices, computers, etc.

AND Gate

The truth table for the AND gate is below:

OR Gate

The truth table for the OR gate is below:

NOT Gate

The truth table for the NOT gate is below:

Combining NOT gate with AND and OR gates

The NOT gate can be combined with the AND and OR gates to produce a different output.

NAND Gate

NAND = NOT AND. The NAND gate corresponds to an AND gate followed by a NOT gate.

The truth table for the NAND gate is below:

NOR Gate

NOR = NOT OR. The NOR gate corresponds to an OR gate followed by a NOT gate.

The truth table for the NOR gate is below:

Measurement and Units Revision

Fundamental Quantities and Base Units

In science there are five fundamental quantities, or base quantities. All other quantities, called derived quantities, are related to these five base quantities.

To measure a physical quantity we compare it with a standard known as the unit, called base unit, of the quantity. 

A quantity is written as its value followed by its unit, e.g. the height of the girl is 1.40 m. This system of units we use is called the Systeme International d'Unites (International System of Units) or in short, the SI system.

Derived Quantities

We can multiply or divide base quantities with their units to produce derived quantities with their units. For example, the area of a rectangle with sides 0.6 m and 0.5 m  is given by the product of its sides, i.e. (0.6 m) x (0.5 m) = 0.3 m²

Remember in algebra that y  X  y = y²

Similarly, metre x metre equals metre squared, i.e. m  X  m  = m²

Length

Length can be measured using various instruments. You can use a ruler to measure to the nearest millimetre. However, a ruler is not suitable for measure very short distances, such as the diameter of a wire or the thickness of a sheet of paper. In these cases, a micrometre screw gauge or calipers are used.

Micrometer Screw Gauge

The main scale is marked in millimetres (mm). As the screw rotates once the micrometre opens 0.5 mm. Each of the 50 divisions on the rotating scale is 0.01 mm. A micrometre screw gauge can be used to measure the diameter of a wire and even the diameter of the human hair. If you have forgotten how to read a micrometer screw gauge check your textbook. Also, you can come to me for any help.

Vernier Callipers

The vernier calliper can also be used to measure the diameters of objects. It has a main scale and a vernier scale. The vernier scale can slide along the main scale. Read up on how to use the vernier scale. Again, if you need assistance, you can contact me.

Area

The area of a rectangle is the product of the lengths of the sides.
 

Remember the formula: Area = length x width

With irregular shapes we can divide the area into small squares, of known size, and estimate the total number of squares.

Volume

To find the volume of rectangular solids (cuboids) we measure the lengths of the sides. Remember the formula: volume = length x width x height

For irregular solids another method is employed. We simply immerse the object into water. The volume of water displaced is equal to the volume of the object.

In the above example, the volume of the stone is 100cm³

For larger irregular objects, the eureka can method (displacement can method) is used. The volume of the object is equal to the volume of water that overflows. 

The volume of liquids is found by using a measuring cylinder and looking carefully at the bottom of the meniscus. Pipettes and burettes can also be used.

Mass

To measure mass balances are used. There are different types of balances: lever balance, triple beam balance, and chemical balance. The scale on a lever arm balance is non-linear, i.e. the marks are not evenly spaced. Note that mass and weight are different.

Density

Density can be defined as the mass per unit volume of a substance. Blocks of materials can have the same volume but different masses. A wooden block may have a mass of 48g while a block of iron of the same volume has a mass of about 420g. This is because the materials have different densities.

Remember the formula for density is mass divided by volume and its unit is kilogram per metre cubed, i.e.  Kg m^-3

Relative Density

Relative density is the comparison of the density of a material with the density of water. It is found by dividing the density of the substance by the density of water. Relative density is a ration and it has no units.

Time

A stopwatch or stop clock is used to measure intervals of time. Your reaction time causes inaccuracy but you can increase the credibility of your results by repeating timings and averaging your results.

Circuit and Components

Hello Students

Remember that long list of circuit components you were given? Well, here is a list of ALL you are required to know by CXC.

I liked this explanation of an electric circuit:

This next video explains the difference between series and parallel circuits.

I've added this one cause I think you guys can try it at home. Pretty cool huh?

 Why do you think it works?

Okay, that's it for now.

Happy Studying Folks!

Important Notice

So, I need to mark your labs so that it can be a part of your mid-term grade. Note that all labs from 1 to 7 are due on Thursday 13th February 2014. No excuse will be accepted.

Lab no. 7

So, here is a link that you can try to help you with lab no. 4. If you scroll down the page there is a simulation of the newton's cradle that you can fiddle around with.

Good luck with this.