Physics IGCSE Revision(Part 3)

By: Momina

Key pointsFormulae

Simple Easy

Brief Intro on Magnets

Magnetic material- is a type of material that can be magnetized and is attracted to other magnets.

Strong metals contain1- iron2- nickel3- cobalt

They are called “Ferromagnetics”

Iron and alloys of iron are called ferrous. (Ferrous in Latin means iron)Aluminium, copper, and other non-magnets are called non-ferrous.

Properties of magnets:

• Have a magnetic field around them• Has two poles exerting forces on other

magnets.– Like poles repel– Unlike poles attract

• Attract magnetic materials by inducing magnetism in them.

What is induced magnetism?Some metals like iron and steel are attracted to other magnets because if there is a magnet near by, they themselves get magnetized. Magnetism is INDUCED in them.– When steel is pulled away from a magnet, it keeps its induced

magnetism causing it to become a permanent magnet.(hard magnet)

– When iron is pulled away from a magnet, it looses its induced magnetism meaning that iron was only a temporary magnet.(soft magnet)

Magnetic Effects of Current

• When an electric current is passed through a wire an magnetic field is produced. The features of this magnetic field are:– They are circles– Field is strongest close to the wire– Increasing current increases strength of field.

Right-hand grip rule

Electromagnets• These are types of magnets that can be

switched on and off.

Iron coreCoils

The strength of the magnetic field can be increased by:- Increasing the current.- Increasing the number of turns in the coil

Magnetic Relay

• When electricity is passed through the coil end wires, it induced a magnetic field in the iron ROD. This attracts the iron STRIP causing both metal contacts to touch.

Metal contacts.

Circuit BreakerCircuit breaker- it is an automatic switch cutting off the current within a circuit if it rises above a specified value.

- In the case on the left, the pull of the electromagnet has become so strong that it has attracted the soft iron armature. This causes the contacts to open and stop the current.

- If u press the reset button, the contacts close once again.

Magnetic force on the current

• Copper is a non-magnet feels no force of the magnet

But..• If it has a current passing

through it, there will obviously be a force on the wire.

The wire moves ACROSS the field. It is not attracted to it.

Force is increased if:-Current is increased-Stronger magnet is used-Length of wire in field is increased.

Flemings Left Hand Rule

Electric motors

• A motor is made up from a coil of wire which is positioned between the two poles of the magnet.

• When the current flows through the coil, it creates a magnetic field. This magnetic field that is produced interacts with the magnetic field produced by the 2 permanent magnets.

• The combination of these two magnetic fields exert a force, pushing the wire at right angles to the permanent magnetic field.

An electric motor transfers electrical energy to kinetic energy.

Improve turning effect

Increasing Turning Effect

• Increase the current• Use a stronger magnet• Increase the number of turns on the coil• Increase the area of the coil.

Electromagnetic Induction

• A magnetic field can be used to produce current.

When the wire is moved across the magnetic field a small EMF(voltage) is created. This is called electromagnetic induction.“EMF is induced”

Induced EMF increased by:-Moving wire faster-Using stronger magnet-Increasing length of wire.

Induced Currents

Fleming’s right hand rule:

Difference between the left hand and the right hand rule:-When current causes motion the left hand rule applies-When motion causes current the right hand rule applies

Generators• The coil rotates• Magnetic fields are cut• EMF is generated• Causes current to flow• Coil rotates– upwards,

downwards, upwards causing the current to flow backwards, forwards, backwards.

Increasing EMF:- Increasing the number of turns

on coil- Increasing area of coil- Use stronger magnet- Rotate coil faster

Coils and Transformers

• Moving magnet induces EMF• Magnetic field SAME effect.• Mutual induction: when coils are magnetically

linked so that changing current in one coil causes an induced EMF in the other.

Simple Transformer

- Alternating current flows through primary coil

- This sets up an altering magnetic field in the core.

- Coils of the secondary coil ‘cut’ the altering magnetic field thus inducing an alternating voltage in the output coil.Turns in output coil

=

Input voltage

output voltage

Turns on input coil

Step-up and Step-down transformers

Step-up: this is when the number of output coils is greater than the number of input coils which means that there will be a greater output voltage as opposed to input voltage.

Step-down: this is when the number of output coils is less than the number of input coils which means that there will be less output voltage as opposed to input voltage.

Power Through a Transformer

= x Output current

Output voltage

Input voltage

x Input current

SummaryTransformers:Current + Motion=Magnetic Field

Electromagnetic Induction:Magnetic Field + Motion = Current

Generator (Electric Motor):Current + Magnetic Field = Motion

Important electronic components

1. Resistors – keep currents + voltages at levels desired by the electronic component

2. Capacitor – store small amounts of electric charge

3. Diodes – allow the current to flow in only one direction

4. Light-emittingg diodes (LED) – glow when a small current is passed through them.

Continued…

5. Transistors – used to amplify signals

6. Integrated circuits(micro-chips)– contain complete circuits with :a) Resistorsb) Transistorsc) Other components

7. Relays– electromagnetic switches.

Diodes

• Can be used to change AC to DC Rectification

• Lets forward parts of the AC through but blocks the backwards part.

• Forming a DC

Input Output1 resister

Potential Divider

• A potential divider only delivers a portion of the voltage.

Reed Switch

• A reed switch is operated by a magnetic field.

Transistors as switches

• It is a semiconductor device made of silicon.

Emitter

Base

Collector

The NPN resistor

• In the diagram above there are actually two circuits put together as one. The first circuit is the one with the base and the emitter ( input circuit) and the second is the circuit with the collector and the emitter (output circuit).

1. Two input connections joined together no current flow2. Input the base greater than 0.6Vlamp switches on3. 1000 ohm resister is present to protect the input to the resistor, allowing input to

be higher than 0.6V to 5V without harming transistor.4. Little current is needed in the input circuit.

b

e

c1000 ohms 6 V

Logic Gates

Thermionic Emission

• Basically what happens in thermionic emission is that the tungsten filament is heated to 2000 degrees Celsius. Some electrons that are hot enough escape the surface of the white hot surface. These then pass through the vacuum and on the screen.

The Oscilloscope

• The Cathode Ray Oscilloscope uses (as mentioned on previous slide) an electron gun and the X and Y plates to adjust where the stream of electrons go.

• The X-Plates move the beam horizontally (Left or Right)• The Y-Plates move the beam vertically (Up or Down)

• The Y-plates are connected to a Y input terminal. These are connected to an AC supply.

Examples of things that use electron beams:- Television- X-ray tube

Atoms

Nuclear Radiation

AlphaAlpha particles are made of 2 protons and 2 neutrons.This means that they have a charge of +2, and a mass of 4.Alpha particles are relatively slow and heavy. They have a low penetrating power - you can stop them with just a sheet of paper.Because they have a large charge, alpha particles ionize other atoms strongly

Beta•Beta particles have a charge of minus 1, and a mass of about 1/2000th of a proton. .•They are fast, and light. •Beta particles have a medium penetrating power - they are stopped by a sheet of aluminum•Beta particles ionize atoms that they pass, but not as strongly as alpha particles do.

Gamma•Gamma rays are waves, not particles. This means that they have no mass and no charge. •Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly.•Gamma rays do not directly ionize other atoms•We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles. Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy.

In a Magnetic Field

What is Background Radiation?

• Background radiation comes from naturally decaying substances such as soil, rocks, air, food and drink.

• It is detected by a Geiger Muller Tube

Radioactive decay- Alpha Decay

Radioactive decay- Beta Decay

Half-Life• This is the amount of time taken for the nuclei

of a radioactive substance to decay.

Nuclear Fusion

This does not take place on Earth so far. It is the process that powers the stars.

Nuclear Fission

What can Radioactivity be used for?

1. Tracers2. Radiotherapy3. Testing for cracks4. Thickness monitoring5. Carbon Dating- after an organism dies the amount of C-

14 inside it begins to decay. It can be used to find out how old a substance is.

6. Dating Rocks

This is the physics syllabus Complete

Best of luck for your IGCSE exams

Especially my fellow candidates of May/June 2011!!!