Phy Electricity/magnetism

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Conductor of electriciy

Back 1

A substance through which electric charge can flow

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Insulator of electricity

Back 2

A substance through which electric charge cannot flow

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Earthed object

Back 3

Is connected to earth so that if a fault develops in a machine, the current from the live wire can flow to earth, meaning it's potential is zero and the machine remains safe to touch

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Coulomb's law states

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That the force of attraction of repulsion between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them

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Electric field

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-Any region of space where a static electric charge experiences a force other than the force of gravity.

-Always caused by other static charges in the vicinity.

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Electric field line / Line of force

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A line drawn in an electric field showing the direction of the force on a positive charge placed in the field.

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Electric field strength

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Is the force per unit charge at a point in an electric field.

(Force per unit Coulomb)

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The potential difference

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The work done in bringing a charge of +1C from one point to another in an electric field

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Potential at a point

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Is the potential difference between a point and the earth, called the potential of that point.

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Volt

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Potential difference between two points is 1V if 1J of work is done when 1C is brought from one point to the other

1V=1J/C

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Capacitance, unit

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Is the ratio of the charge on a conductor to its potential.

ie C=Q/V

Farad

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Farad

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The unit of capacitance

A conductor has a capacitance of 1 farad if placing a charge of 1C on it raises its potential by 1V

1F=1C/V

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Electric current

Back 13

Is a flow of electric charge

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Electromotive force(emf)

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Is a voltage when applied to a circuit.

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Resistance, unit

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The resistance of a conductor is the ratio of the p.d. across it to the current flowing through it:

R=V/I

Ohm

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Ohm

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A conductor has a resistance of 1ohm if the current through it is 1amp when the p.d. across it is 1volt

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Ohm's Law

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For certain conductors (mainly metals), at a constant temperature, the current flowing through them is directly proportional to the p.d. across them:

V→I ie V/I=a constant

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Resistivity

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If a conductor of length l and cross sectional area A has a resistance R, the constant ρ is given by

ρ=RA/l

Which is the resistivity of the material in the conductor

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Effects of electric current

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Heating effect

Chemical effect

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Ion

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An atom or a molecule that has a charge due to losing or gaining one or more electrons

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Electrolysis

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The breaking up of water into hydrogen and oxygen by passing an electric current through it

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Joule's Law

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States that the rate at which heat is produced in a conductor is directly proportional to the square of the current, provided its resistance is constant:

P&I²

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Semiconductor,

Charge carriers

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A substance whose resistivity is between that of a good conductor and a good insulator. The resistivity of a semiconductor decreases as its temperature increases.

Positive holes and negative electrons.

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Intrinsic conduction

What is the conductor therefore called?

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Is conduction in a pure semiconductor due to electrons moving from negative to positive and an equal number of holes moving in the opposite direction.

An intrinsic semiconductor.

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Thermistor

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Is a semiconductor device whose resistance decreases rapidly with increasing temperature.

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Light dependant resistor (LDR)

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Is a semiconductor whose conductivity is increased when light shines on it.

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Doping

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Is the adding of small controlled amounts of certain impurities to a pure semiconductor to increase its conductivity.

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P-type semiconductor

Eg

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P for positive holes

A semiconductor in which the impurity added produces extra holes available for conduction

Eg boron in silicon.

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N-type semiconductor

Eg

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N for negative electrons

A semiconductor in which the impurity added produces more free electrons available for conduction.

Eg phosphorus in silicon.

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Extrinsic conduction

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Is the increased conduction in a semiconductor due to the addition of impurities.

Semiconductor produced is called an extrinsic conductor.

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Depletion layer

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Is the region at both sides of a p-n junction that contains no free majority charge carriers, due to roaming electrons and holes filling eachother, and so it acts like an insulator.

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P-n junction

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A piece of semiconductor with part of it doped p-type and the rest doped n-type

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Junction voltage

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Is the p.d. that exists across across the p-n junction caused by holes and electrons moving across it when it's formed.

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Diode

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Only allows the flow of current in one direction because of the p-n junction.

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Reverse biased p-n junction

Orientation

Functionality

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N connected to positive side of battery

Will not conduct current

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Forward biased p-n junction

Orientation

Functionality

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N connected to negative side of battery

Allows current to flow as long as the power source has a greater voltage than the junction voltage (and the depletion layer is eliminated)

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Light emitting diode (LED)

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Is a p-n junction which gives off light when forward biased.

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Photodiode

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A reverse biased p-n junction which conducts electricity when light shines upon it.

Current flowing is directly proportional to the intensity of the light.

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Magnetic field

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Any region of space where magnetic forces can be felt. The direction of the magnetic field at a point is the direction of the force on a north pole if it were placed at that point.

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Magnetic field line

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A line drawn in a magnetic field such that a tangent drawn along it at any point shows the direction of the field at that point.

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The magnetic flux density

Unit

Back 41

At a point in a magnetic field, is a vector, whose magnitude is equal to the force that would be experienced by a conductor of length 1m carrying a current of 1A at right angles to the field at that point and whose direction is the direction of the force on a north pole placed at that point.

The Tesla (T)

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Tesla

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The magnetic flux density at a point is 1 Tesla if a conductor of length 1 meter carrying a current 1amp experiences a force of 1N when placed perpendicular to the field.

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The ampere

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Is that constant current which, if maintained in two straight parallel conductors of infinite length, negligible cross section, and placed 1 meter apart in a vacuum, would produce a force on each conductor of 2×10^-7 N/M

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Magnetic flux,

Unit

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B, passing through an area, A,

Φ=BA

Unit: Weber (Wb)

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The Weber

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The flux through 1m² is 1Wb if the magnetic flux density over the area is 1T

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Electromagnetic induction

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Is the name given to the emf that appears in a coil due the the change in the surrounding magnetic field.

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Faraday's law

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Of electromagnetic induction states that the size of the induced emf is directly proportional to the rate of change of flux

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Lenz's law

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States that the direction of an induced current is always such as to oppose the change producing it

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Rms current

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As alternating current varies over time, the rms value gives you the value of current that produces the same heating effect as direct current

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Rectification

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Mutual induction

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When a changing magnetic field in one coil causes an induced emf to appear in a nearby coil.

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Self induction

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When the current passing through a coil changes and causes a change in the magnetic field surrounding that coil, an emf is induced in the coil that opposes the change (aka back emf)

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The Photoelectric Effect

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Is the emission of electrons from the surface of a metal by electromagnetic radiation of a suitable frequency.

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Work Function (Φ)

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Of a metal is the minimum energy required to remove the loosest electron from the surface of that metal.

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Solenoid, sides

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A coil whose length is much longer than its radius

North and south

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Electromagnet

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Solenoid plus soft iron core

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Principle on which an electric motor works

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A current carrying conductor in a magnetic field experiences a force

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Devices based on the principle that a current carrying conductor in a magnetic field experiences a force

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D.c. electric motor

Moving coil loudspeaker

Moving coil galvanometer

Moving coil voltmeter

Moving coil ohmmeter

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The Coulomb

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Unit of electric charge.

The charge that passes when a current of 1A flows for 1s