Electricity

Cambridge IGCSE

Static Electricity

• Def: a stationary electric charge, typically produced by friction, that causes sparks or crackling or the attraction of dust or hair.

• Triboelectric effect The triboelectric effect is a simple process in which an object becomes electrically charged by rubbing against another object.

Insulators & Conductors• Conductors Materials that permit electrons to flow freely from particle to particle. Eg. metals, aqueous solutions of salts, graphite, and the human body

Insulators & Conductors• Insulators Materials that impede the free flow of electrons from atom to atom and molecule to molecule.Eg. plastics, Styrofoam, paper, rubber, glass and dry air.

Electrostatic induction

• Modification in the distribution of electric charge on one material under the influence of nearby objects that have electric charge.

• Advantages to charging something by induction.– The originally charged object never loses any

charge so it need not be recharged. – The induced charge can be quite strong and

subsequent charges will be equally strong

Comparison of charging by Conduction & charging by Induction.

Charged object touches the electroscope. Charged object does not touch the electroscope.

Electroscope ends up similarly charged to the object used to charge it.

Electroscope ends up oppositely charged to the object used to charge it.

The first charge is strong but gets weaker each time the electroscope is recharged. (This is due to the original object giving up some charge every time it is touched.)

The first charge is strong and stays strong each time the electroscope is recharged. (This is due to the original object not losing any charge in the process.)

Electric current • Def: An electric current is a flow of electric

charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma.

Ampere

• Ampere is the unit of current, which is defined by using the magnetic effect.

• Measured by a Ammeter• SI unit : A • Symbol: I • It is named after André-Marie Ampère (1775–

1836), French mathematician and physicist, considered the father of electrodynamics.

Coulomb • Coulomb is equal to the

quantity of charge transferred in one second across a conductor in which there is a constant current of one ampere.

• SI unit : C• Symbol : Q• Named for French chemist Charles-Augustin de

Coulomb (1736-1806), who devised a method of measuring electrical quantity

• Q = I x t

Series Circuits

• In a series circuit, the current through each of the components is the same, and the voltage across the circuit is the sum of the voltages across each component.

Parallel Circuits

• In a parallel circuit, each device is placed in its own separate branch.

Types of Current

• Direct current (DC) which is a constant stream of charges in one direction.

• Alternating current (AC) that is a stream of charges that reverses direction.

Direct Current

• The current in the circuits is moving in a constant direction.

• Refined by Thomas Edison in the 1800s.

Alternating current

• Alternating current is an electric current in which the flow of electric charge periodically reverses direction

Cells, batteries & e.m.f

• Electrical cell is a device that is used to generate electricity, or one that is used to make chemical reactions possible by applying electricity.

• Electric battery is a device consisting of one or more electrochemical cells with external connections provided to power electrical devices.

• A battery has positive terminal, or cathode, and a negative terminal, or anode

Cells, batteries & e.m.f

• Electromotive force, (denoted and measured in volt), is the voltage developed by any source of electrical energy such as a battery or dynamo. It is generally defined as the electrical potential for a source in a circuit.

Frequency

• The number of periods or regularly occurring events of any given kind in unit of time, usually in one second.

• SI unit : Hz• Symbol : F

Potential difference

• A potential difference, also called voltage, across an electrical component is needed to make a current flow through it.

• Cells or batteries often provide the potential difference needed.

Calculate the potential difference

• The potential difference between two points in an electric circuit is the work done when a coulomb of charge passes between the points.

• V = W ÷ Q– V is the potential difference in volts, V– W is the work done (energy transferred) in joules,

J– Q is the charge in coulombs, C

Resistance • An electron traveling through the wires and loads of

the external circuit encounters resistance. • Resistance is the hindrance to the flow of charge.

For an electron, the journey from terminal to terminal is not a direct route.

• While the electric potential difference established between the two terminals encourages the movement of charge, it is resistance that discourages it.

Ohm’s law

• The electric potential difference between two points on a circuit (ΔV) is equivalent to the product of the current between those two points (I) and the total resistance of all electrical devices present between those two points (R).

Resistors

• A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. Resistors may be used to reduce current flow, and, at the same time, may act to lower voltage levels within circuits.

Resistor in series • Resistors can be connected in series; that is,

the current flows through them one after another.

Resistors in Parallel

• Resistors can be connected such that they branch out from a single point (known as a node), and join up again somewhere else in the ciruit.

Colour coded Resistors

Capacitors

• A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store electrical energy temporarily in an electric field.

Charging & Discharging a capacitor

• A Capacitor is a passive device that stores energy in its Electric Field and returns energy to the circuit whenever required. A Capacitor consists of two Conducting Plates separated by an Insulating Material or Dielectric.

Charging • As soon as the switch is closed in position 1 the battery is

connected across the capacitor, current flows and the potential difference across the capacitor begins to rise but, as more and more charge builds up on the capacitor plates, the current and the rate of rise of potential difference both fall. Finally no further current will flow when the p.d. across the capacitor equals that of the supply voltage Vo. The capacitor is then fully charged.

Discharging • As soon as the switch is put in position 2 a 'large' current starts to flow

and the potential difference across the capacitor drops. As charge flows from one plate to the other through the resistor the charge is neutralised and so the current falls and the rate of decrease of potential difference also falls.

Eventually the charge on the plates is zero and the current and potential difference are also zero - the capacitor is fully discharged. Note that the value of the resistor does not affect the final potential difference across the capacitor – only the time that it takes to reach that value. The bigger the resistor the longer the time taken.

Magnetic fieldsProperties of Magnetic Lines • They seek the path of least resistance between

opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loops from pole to pole.

• They never cross one another.• They all have the same strength.• Their density decreases (they spread out) when they

move from an area of higher permeability to an area of lower permeability.

Properties of Magnetic Lines cont.

• Their density decreases with increasing distance from the poles.

• They are considered to have direction as if flowing, though no actual movement occurs.

• They flow from the south pole to the north pole within a material and north pole to south pole in air.

Magnetic fields caused by electric currents

• The magnetic field around a straight wire consists of ‘concentric circles’ (circles around the same centre). These are at right angles to the direction in which the electric current flows.

• A solenoid consists of a long piece of wire made into several coils. Its magnetic field is the same as the magnetic field produced by a simple bar magnet.

Maxwell’s Right Hand rule

• The nature of magnetic field around a straight current carrying conductor is like concentric circles having their center at the axis of the conductor. The direction of these circular magnetic lines is dependent upon the direction of current.

Electromagnets

• An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off.

Electric motors

• 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.

Fleming’s left hand rule

Generators

• Generators induce a current by spinning a coil of wire inside a magnetic field, or by spinning a magnet inside a coil of wire.

Electromagnetic induction

• A bar magnet is kept stationary while the coil is moved back and forth within the magnetic field; an electric current would be induced in the coil.

• Then by either moving the wire or changing the magnetic field we can induce a voltage and current within the coil and this process is known as Electromagnetic Induction

• Is the basic principal of operation of transformers, motors and generators.

Faraday’s law

• Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated.

Lenz’s Law

• A principle stating that an electric current, induced by a source such as a changing magnetic field, always creates a counterforce opposing the force inducing it. This law explains such phenomena as diamagnetism and the electrical properties of inductors.

Fleming’s right hand rule (Dynamo rule)

Transformers

• Alternating current is passed through the primary coil (the input)which creates a changing magnetic field in the iron core.The changing magnetic field then induces alternating currentof the same frequency in the secondary coil (the output).

Transformers • A step up transformer has more turns of wire

on the secondary coil, which makes a larger induced voltage in the secondary coil.

• It is called a step up transformer because the output voltageis larger than the input voltage.

References

• http://study.com/academy/lesson/what-is-static-electricity-definition-causes-uses.html

• https://learn.sparkfun.com/tutorials/what-is-electricity

• http://science.howstuffworks.com/electricity.htm • hyperphysics.phy-astr.gsu.edu/hbase/magnetic/

magfie.html

References

• http://www.physics4kids.com/files/elec_magneticfield.html

• http://global.britannica.com/science/magnetic-field

• https://www.khanacademy.org/science/physics/magnetic-forces-and-magnetic-fields