section 6. electromagnetism - edubuzz.org

Physics N4 Unit 1: Pupil Notes of 11

Section 6. Electromagnetism Permanent Magnets

A magnet is any material or object that produces a magnetic field. Permanent magnets are formed from materials which can be magnetised to form a permanent magnetic field. They are made from ferromagnetic materials such as iron, nickel, cobalt and rare earth alloys such as samarium and neodymium. The magnetic fields affect neighbouring objects along magnetic field lines as shown below.

Electromagnetism

When an electric current flows in a conductor it has two effects. The first of these, is that heat is produced. The other effect is that a magnetic field is created. The shape of the magnetic field depends on the shape of the conductor and the direction of the magnetic field depends on the direction of the current flow.

The magnetic field is the area around the current carrying conductor in which it’s magnetic force can be felt. Magnetic field lines can be drawn to show the shape and direction of the magnetic field. Plotting compasses can be used to show these magnetic field lines.


Magnetic field around a current carrying wire.

The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire. Magnetic field of a coil of wire – solenoid A long straight coil of wire can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. Such coils, called solenoids, have an enormous number of practical applications. The field can be greatly strengthened by the addition of an iron core. Such cores are typical in electromagnets. The magnetic field is concentrated in a nearly uniform field in the centre of a long solenoid. The field outside is weaker.

The Earth’s magnetic field

The Earth’s core contains a lot of molten iron that scientists believe is responsible for the Earth’s magnetic field. This is important for navigation and protects the earth from the solar wind which can affect our communication systems and satellites.


Practical uses of magnetism and electromagnetism

Loudspeaker

A light voice coil is mounted so that it can move freely inside the magnetic field of a strong permanent magnet. The speaker cone is attached to the voice coil and attached with a flexible mounting to the outer ring of the speaker support. An electrical signal is applied to the voice coils of the loudspeaker (electromagnet), making them vibrate with a pattern that follows the variations of the original signal. This also causes the cone to vibrate and in turn the air creating sound. A permanent magnet is used as an electromagnet would distort the sound.

Relay

A relay is an electrically operated switch. The majority of relays use an electromagnet to operate a switching mechanism mechanically. Relays are used where it is necessary to control a high-power circuit by a low-power signal or where several circuits must be controlled by one signal. For example relays are used in car ignition circuits, the low-power ignition swicth activates a relay which starts the high-power motor.

Electric Motor

Electric motors involve rotating coils of wire which are driven by the magnetic force exerted by a magnetic field on an electric current. They transform electrical energy into mechanical energy. They are used in a wide range of applications from watches, power tools, pumps and ship propulsion. Simple electric motors can use permanent magnets but the vast majority use electromagnets to provide the magnetic field.


Magnetic Storage

Hard disk drives which are found in a large proportion of consumer electronics from PCs to digiboxes make use of magnets to store data. A hard disk drive records data by magnetizing a thin film of ferromagnetic material on a disk or platter. The alignment of micrometer-sized magnetic regions in the disks called magnetic domains in binary patterns represents the data stored. Techniques using both electromagnet and permanent magnets can be used to read and write data. Magnetic storage is also used on magnetic recording tapes and credit card strips.

MRI Machines (Magnetic Resonance Imaging) An MRI scanner is a tube in which the patient lies inside and is surrounded by a large, powerful magnet where the magnetic field is used to align the magnetisation of some atomic nuclei in the body, and radio frequency fields to systematically alter the alignment of this magnetisation. An electromagnet is used as the strength of the magnetic field has to change.

At the cutting edge

Maglev Train

Electromagnets are used to produce Magnetic Levitation or Electromagnetic suspension which provides both lift and thrust to propel vehicles along a guide track. The vehicles can achieve speeds of above 580 km/h and there is an offset of power consumption due to the reduction of drag and friction. More recently (2008) new technology is being developed to use permanent magnets to increase cost effectiveness.


Section 7. Electrical Power

The diagram below will help you to think about what is going on when we connect

an electrical circuit to a power supply.

The electrical appliances we use in our homes, industry and schools use electrical

energy from the mains supply or from batteries. The appliances change electrical

energy into a form that is useful for what we are doing and in many cases into other

forms of energy that are not so useful. For example, a lamp changes electrical

energy into heat and light. The most useful, and main, energy change for a lamp is

the change of electrical energy into light. The more light we get from our lamp (and

less heat given off) we say the bulb is more efficient at changing electrical energy

into light.


When we are using electrical appliances, it is useful to have an idea of how much energy they will require. This leads to the definition of electrical power. Power is defined as the amount of energy transformed per second, as shown in the equation below

Symbol Name Unit Unit Symbol P Power watts W E Energy joules J t time seconds s

Different appliances will transform more or less electricity. Often the highest powered ones will be those which transform electrical energy into heat energy, for example a hair dryer. We often describe this as the power consumption.

Appliance Power

transformation/W Oven 3000

Dishwasher 1400 Iron 1100

Hair Dyer 1500 Microwave 1000

TV 250 Stereo 60

Filament Lamp 100 Energy Saving Lamp 11

Drill 750 Fridge 1400

Worked example

1. A 1500 W hairdryer is used for 5 minutes, how much energy is transformed?

P t E

= 1500W = 5 x 60 = 300 s = ?

P = 1500 =

E = E = E =

E/t E/300

1500 × 300 450000 450 kJ


Efficiency

The more ‘useful’ energy we get when we change from one form of energy to

another, the more efficient the process is. We can write equations for how efficient

energy and power transformations are below:

% Energy efficiency = (useful Eout /Ein) x 100%

% Power efficiency = (useful Pout / Pin) x 100%

The highest possible efficiency of a system is 100 %. This means that all of the

energy or power we put into a system is transferred into useful energy or power

out.

The higher the percentage efficiency of a system, the better!

When we turn on a filament lamp, the desired transformation is from electrical energy to light energy. This is the main transformation, but the lamp gets very warm when it is switched on. This transformation from electrical to heat energy is undesired.

This sankey diagram shows the energy transformation in a filament lamp.

This is a similar diagram but for an energy saving lamp. Very little energy is ‘lost’ as heat.

Any energy which is transformed into a form other than the one we want could be said to be ‘lost’ as it was not intended.

We can calculate the efficiency of the lamps using the formula above e.g.

100 J Electrical

Energy

100 J Electrical

Energy 90 J ‘lost’ Heat

Energy

10 J Useful Light

Energy

75 J Useful Light

Energy

25 J ‘lost’ Heat

Energy


For the filament lamp


= (10 /100) x 100%

= 10%

For the energy saving lamp


= (75 /100) x 100%

= 75%


Section 8. Gas laws and the kinetic model

The particles (atoms) in solids are held firmly in place, however, in a liquid the atoms

can tumble over each other allowing the liquid to flow and to match the shape of its

container. In a gas, the atoms have more energy and are able to travel at high

speeds in all directions filling the container they are in. The atoms in a gas can move

independently since there are only weak forces of attraction between the atoms.

Gas Pressure

A gas consists of very small particles, which are all very far apart and which all move randomly at high speeds. The study of gases by treating them as particles free to move in any direction is known as Kinetic Theory. These particles collide with one another and with anything else they come in contact with. Each time a particle collides with a wall of the container, it exerts a force on the wall. Thus with particle continuously bombarding the container walls, the gas exerts a pressure on the container.

Diagram showing gas particle collisions in a container.

If we think about a flat tyre: there is little air inside tyre, there are few collisions and

therefore low pressure. If we pump up the tyre, we add more air particles; there

are more collisions and therefore higher pressure.

http://www.docstoc.com/docs/113002286/Kinetic-Molecular-Theory---PowerPoint


Gas Laws Pressure and Volume (Boyle’s Law)

Using the apparatus below, the volume of a gas can be changed and any corresponding change in Pressure measured. The mass and temperature of the gas are fixed.

As the syringe is pushed, the volume decreases, so the pressure increases. We can

explain this in terms of the kinetic model. We have seen how the pressure exerted

by the gas on its container arises from the collisions of molecules with the container

walls. If we have the same number of molecules in a smaller container, they will hit

the walls more frequently.

Temperature and Pressure

We will now look at the relationship between the temperature and pressure of a fixed mass of gas, with the volume of the container kept constant. According to the kinetic theory, the average speed of the gas particles increases with increasing temperature. The hotter the gas, the faster the gas particles are moving. We stated earlier that when a particle collides with a wall of a container a force is exerted. The force exerted depends in the speed of the particle, so the faster the gas particles are moving, the greater the pressure exerted by the gas on the walls of the container.

Pressure sensor connected to computer Syringe


Volume and Temperature (Charles' Law)

Consider a fixed mass of gas trapped in a cylinder with a frictionless piston. If the gas is heated, the average speed of the gas particles increases. This means the force exerted on the piston when gas particles collide with it increases. It also means the number of collisions per second will increase. The increased force and rate of collisions act to push out the piston, increasing the volume inside the cylinder and hence the volume of the gas. Once the piston has been pushed out, the rate of collisions will decrease, as

particles now have further to travel between collisions with the piston or the walls.

Thus the pressure remains the same - the effects of greater impulse and lower rate

of collision balance each other out. The overall effect is that heating a gas causes it

to expand.

Applications: inflating tyre / ball, breathing in and out, firing bullets – gunpowder

ignites, rapid expansion of gas, bullet fires out of barrel at high speed.

Scuba divers (issues with surfacing too quickly)

Free divers: what is the record? What are the problems?

Pressurised aircraft cabins: why do flight attendants often hand out sweets on

landing?

Why mountaineers use oxygen supplies whilst climbing

Recycling and using air on-board submarines

Manufacture, operation and use of weather balloons

Research heating and cooling of gases; for example why do anti-perspirant

canisters feel cold when you hold down the nozzle to release gas?

section 6. electromagnetism - edubuzz.org

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