# Physics AS electricity

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## Current, Potential difference and Resistance

Current is the rate of flow of charge

! remember ! - current flows from + to - ( opposite to electron flow )

(change in) Q = I x (change in) t ... or ... I = Q / t

measured in colombs ( 1 c = amount of charge that passes in 1 sec if the current is 1 amp )

Potential difference ( v ) = work done ( energy converted ) per unit charge moved

V = W / Q        (work done in joules - measured with a voltmeter)

! remember ! - pd across parallel components is the same

1 volt = 1 joule of energy moving 1 colomb ...... 1v=1JC-1

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## Current, Potential difference and Resistance

Everything has resistance

pd across an electrical component, allows a current to flow

'resistance' = how difficult it is for current to flow through

R = V / I       (resistance is measured in ohms )

For an Ohmic Conductor, R is a constant

( provided physical conditions ( temp ) remain constant) the current through an ohmic conductor is directly proportional to the potential difference across it ... I <> V

the current - pd graph, is a straight line through (0,0) ... doubling th pd, doubles the current - resistance is a constant 2 of 19

## I / V characteristics

I / V Graphs show how Resistance Varies

refers to a graph that shows I against V, showing how current flowing through a component changes as the pd increases

The I / V graph for an ohmic conductor is a straight line through the origin

At constant temp, the current through an ohmic conductor is directly propotional to the voltage  3 of 19

## I / V characteristics

The I/V characteristics for a filament lamp is a curve (I/V) = A curve that starts steep but gets shallower as the voltage rises. (V/I) = curve that starts shallow, the gets steeper as the current and voltage increase

Filament lamp = coiled up length of metal wire, which gets hot. (curent flowing through the lamp increases temp )

! the resistance of a metal increases as the tempreture increases !

Diodes only let current flow in one direction  4 of 19

## I / V characteristics

Semiconductors are used in sensors

nowhere near as good at conducting electricty as metals ( fewer charge carriers, however more can be released )

...meaning they are excellent sensors for detecting changes in their enviroment

Thermistors and Diodes are semiconductors

The resistance of a Thermistor depends on the Temperature

a thermistor is a resistor with a resistance that deoends on its temperature ( as resistance decreases, temp goes up  5 of 19

## Resistivity and Superconductivity

Three things determine resistance

1) Length. ( the longer the wire, the more difficult it is to make current flow )

2) Area. ( the wider the wire, the easier it will be for the electrons to pass along it )

3) Resistivity. ( this depends on the material, could make it easier or harder...depends on enviromental factors as well, like temp and light intensity)

!! The resistivity of a material = resistance of 1m length with a 1m cross sectional area !! - measured in ohm-meteres.

P = RA / L

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## Resistivity and Superconductivity

To find the Resistivity of a wire you need to find its resistance

( use a micrometer to measure diamter of wire ( average value ) divide by 2 to get the radius ( in m) and find area )

1) Clamp test wire to a ruler ( from 0 ) and attach circuit

3) Record the length of the test wire connected in the circuit, the voltmeter and ammeter

4) calculate the resistance ( R = V / I

5) repeat and calculate an average reistance for length

6 ) Repeat for several different lengths between 0.1 and 1m

7) Plot results on a graph of resistance against length with line of best fit

The gradient of the line of best fot os equal to R / L = P / A. Multiply the gradient of the line of best fit by the area of the wire to find the resistivity.

8) The resistivity of a mateiral depends on its temp. Current flowing causes an increase in temp, which leads to random error.

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## Resistivity and Superconductivity

Supercoductors have zero resistance

• Normally all materials have some resistance, even conductors like silver
• The resistance means whenever current flows through them, they heat up, some of the eletcrical energy is wasted as thermal energy, To lower the resistivity, cool them down
• If you cool some materials down (like mercury) below a 'transition temperature' the resistance disapears entirely and they become a superconductor
• Without any resistance, there would be no enery lost to heat. This means that a magnetic field can be started, take away the magnet and the current would carry on flowing forv
• However must normal conductors have a transition temp of 10 kelvin (-263 degrees ) its very hard and expensive to get down to this temp
• Some creations like metal-oxide, scientist have managed to develop into room temp supercinductors, at about 144 kelvin ( -133 degrees )

Uses of superconducting wires

1. Power cabels to transmit electricty without any loss of power

2. Really strong electromagnets that dont need a constant power source

3. Fast electronic circuits

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## Electrical Energy and Power

Power is the rate of transfer of energy

1 watt = 1 joule per second ... P = E / T

P = V I .....

1) Potential difference (v) is the energy trasnferred per columb

2) Current ( I ) defined as the number of coloumbs per second

3) ...So p.d x current is enery transferred per second ( power )

P = V2 / R ...  P = I2 R ...  P = V I

Energy is easy to calculate if you know the power, to calculate the total energy transferred, multipy the power by time

E = V I T ...  E = V2 / R ... E = I2 R T

!! make sure time is in seconds !!

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## E.M.F and Internal Resistance

Batteries have resistance

Resistance comes from electrons coliding with atoms and loosing energy to other forms

In a battery, chemical energy is used to make electrons move. As they move, they collide with atoms inside the battery - so batteries must have resistance

This is called internal resistance - and this is what causes cells to warm up when used

1) The amount of electrical energy produced by coulomb of charge is called its EMF ( measured in volts )EMF = E / Q

2) The potential difference across the load resistance, is the energy transffered when 1 coloumb of charge flows through the resistance. The potential difference is called the terminal pd ( v )

3) If there was no internal resistance, the terminal pd = emf ( in real power supplies, energy would be lost overcoming the resistance)

4) The energy wasted by C overcoming the internal resistance is called the lost volts ( v )

Conservation of energy = energy per C supplied by the source = energy per C trasferred in load resistance + energy per C wasted in internal resistance

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## E.M.F and Internal Resistance

There are lots of calculations with EMF and internal resistcnace

EMF = V + v

EMF = I ( R + r )

V = EMF - v

V = EMF - IR

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## Progressive Waves

• Waves move energy from one place to another.  In a progressive wave the wave front moves through the medium.
• There are two types of waves, transverse and longitudinal.
• Transverse waves: are waves where the displacement of the particles in the medium is perpendicular to the direction the wave is travelling in.
• Longitudinal waves: are waves where the displacement of the particles in the the same direction as the wave is travelling in.
• Amplitude (A) is the maximum displacement of a particle in a wave from its equilibrium position. It is measured in metres (m)
• Frequency (f) is the number of complete waves passing a point in one second. It is measured in hertz (Hz).
• Wavelength  is the distance between two identical points on a wave (i.e. one full wave). It is measured in metres (m).
• Wave speed (c) is measured in metres per second (ms-1).= speed = wave length x frequency
• Points on a wave which are always travelling in the same direction, rising a falling together, are in phase with each other.
• Points on a wave which are always traveling in opposite directions to each other, one is rising while the other is falling, are in antiphase with each other
• If we measure the distance travelled by two waves and then compare those distances, any difference in the distances travelled is called the path difference. Path difference is measured in metres (m).
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## Transverse and Longitudinal Waves

• Mechanical waves occur in a medium (solid, liquid or gas)
• Longitudinal waves are waves where the displacement of the particles in the the same direction as the wave is travelling in. For example sound waves.
• Transverse waves are waves where the displacement of the particles in the medium is perpendicular to the direction the wave is travelling in. For example water waves.
• Electromagnetic waves are oscillating electric an magnetic fields they include radio waves, microwaves, infra-red, visible light, ultra-violet, x-rays and gamma rays. Electromagnetic waves are transverse waves and they all travel at the speed of light ( 3 x 108ms-1  ) in a vacuum.
• Transverse waves can be polarised but longitudinal waves cannot. Unpolarised light is a mixture if waves in different planes. When this light is passed through a polaroid material only light waves in one plane are transmited and the light is now polarised.
• Polaroid sunglasses are popular with fishermen they reduce glare by blocking the reflected polarised light from the waters surface.
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## Refraction

• The refractive index (n) of a material is the ratio of the speed of light (c) in a vacuum to the velocity of light in the material (cS).
• n = c/cs
• The refractive index of a material is always greater than 1.
• For example; water = 1.33, diamond = 2.42, glass = 1.5 and air » 1
• When a ray of light goes from material (1) into material (2), rather than from a vacuum into a material we talk about the relative refractive index
• Refractive index (relative) when light is travelling from one material to another 1n2
• The relative refractive index can more or less than 1. If we go from material 1 with refractive index (n1) into material 2 with refractive index (n2). Then we can find the relative refractive index 1n2 by dividing the speed of light in material 1 (c1) by the speed of light in material 2 (c2) OR by dividing the refractive index of material 2 (n2) by the refractive index of material 1 (n1) OR by dividing the sine of the incident angle (q1) by the sine of the refracted angle (q2).
• 1n2 = c1/c2 = n1/n2 = sin1/sin2
• This can also be written as n1sin1 = n2sin2
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## Refracation

• When a ray of light goes from a material into an optically less dense material like air. The angle of refraction can become 90o and the ray of light travels along the boundary between the two material. When this happens the angle of incidence is called the critical angle (qc)
• sin1/sin2 = n2/n1
• If the second material is air then; sinc=1/n1
• If the incident angle is greater than the critical angle then light reflects at the boundary between the two material and this is called Total Internal Reflection 15 of 19

## Refraction

• Optical fibres: There is a fine glass core and it is surrounded by a cladding of glass with a lower refractive index than the core. This means that light shone into the core at an angle greater than the critical angle will Total Internally Reflect at the boundary between the core and the cladding. The light then travels down the fibre through a series of reflections before exiting at the other end.
• The optical fibre would work without the cladding as air also has a lower refractive index than the core glass. However the cladding is useful as it protects the core, prevents cross talk and prevents the leakage of light.
• The core should be narrow as this cuts down on multi-mode dispersion which is where light entering the optical fibre at slightly different angles follow slightly different paths and arrive at the other end a slightly different times this causes the pulse of light to broaden out.
• Optical fibres are used in medical instruments called endoscopes and they are used in communications (telephone, Internet, cable TV). The use of optical fibres in communications has improved the transmission of data giving us high speed internet access.
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## Superposition of waves & stationary waves

• When two waves pass through the same point they combine together to either constructively interfere with each other or destructively interfere with each other before passing on past each other and continuing their separate journeys.
• Constructive interference The two waves are in phase with each other and constructively interfere to give a wave of greater amplitude.
• Destructive interference The two waves are  out of phase (anti-phase) with each other and destructively interfere to give a wave of zero amplitude
• Stationary waves are formed by two waves with the same frequency travelling in opposite directions.  17 of 19

## Interference

• Laser light is a source of coherent monochromatic light.
• Coherence – two waves are coherent if the phase difference between them is constant. They must have the same frequency.
• Monochromatic – means having only one wavelength of light present.
• Young’s double slit experiment: When laser light passes through a slit, it is diffracted. If there are two slits present the light will diffract at both slits.
• If a screen is placed on the other side of the slits from the laser an interference pattern is seen. It produces a series of bright and dark fringes.
• Fringe spacing = (wavelength x distance between the slits) / slit seperation
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## Diffraction

• Diffraction happens when a wave hits an obstacle or gap, diffraction is greatest when the gap is about the same size as the wavelength of the wave. The waves bend round the object or spread out when they pass through the gap, this is called diffraction.
• For single slit diffractionWhen monochromatic laser light is shone through a narrow single slit a diffraction pattern is produced consisting of light and dark fringes. It produces a wide central bright fringe. The other bright fringes get dimmer as you move away from the centre.
• A diffraction grating is a piece of glass with lots of closely spaced parallel lines on it each of which allows light to pass through it, this is a transmission diffraction grating.
• Diffraction gratings are used in spectrometers. The diffraction grating splits up the light into a spectra.
• Grating space*sinB=order number*wavelength
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