Current electricity

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current, charge, pd and resistance

  • current - rate of flow of charge (amps, A; symbol I), due to the passage of charged particles, called charge carriers. 
  • the convention for the direction of current in a circuit is from + to - which is OPPOSITE to electron flow. 
  • Q = It
  • the magnitude of the charge of an electron is 1.6 x 10^-19 SO  for a current of 1A, there are 6.25 x 10^18 electrons passing along the wire each second

Potential difference: 

  • pd - the work done in moving a charge between two points/ per unit charge
  • a voltmeter, in PARALLEL, measures the voltage in (volts, v = j/C)

Resistance:   

  • resistance - a measure of the difficulty of making current pass through a component. (ohm) 
  • resistance is caused by repeated collisions between charge carriers, with each other and with the fixed positive ions in the material. 
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I-V characteristics

  • ohms law - states that the pd across a metal conductor is directly proportional to the current through it, provided the physical conditions do not change. 
  • ohmic conductors are conductors (mostly metals) that obey ohm's law
  • The shallower the gradient on a V/I graph, the greater the resistance. 
  • measuring resistance:

 (http://h2physics.org/wp-content/uploads/2010/04/13.jpg)

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I-V characteristics...

  • filament lamps: The filament is a thin coil of metal wire. When current flows through this, some of the electrical energy is transferred into heat energy and causes the metal to heat up and causes the particles in the metal to vibrate more. Therefore, the resistance increases. The graph for this component levels off at high currents because as R increases, I decreases. 
  • why filament bulbs are more likely to blow: when they are first switched on, it has lower R because it is cold. Therefore the current is larger so the bulb is more likely to burn out.      ALSO the filament heats up very quickly from cold to its operating temp. when its switched on. This rapid temp. change causes the filament wire to blow too. 
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Semiconductors

Semiconductors: a group of metals that aren't as good at conducting electricity as metals, because they have fewer charge carriers available. However, if energy is supplied, more charge carriers can be released and R decreases. There are 3 types:  

  • Thermistors - a component with a resistance that depends on its temperature. 'Negative temperature coefficient' means that the resistance decreases as the temp. increases. Warming the thermistor gives more electrons enough energy to escape from their atoms. So more charge carriers are available and R decreases. 
  • Diodes - including LEDs are designed to let current flow in ONE DIRECTION only - FORWARD or REVERSE. Forward bias is the direction in which the current is allowed to flow. Most diodes require a voltage of about 0.6V before they will conduct, called the threshold voltage. In reverse bias, R is very high and the current that flows is very low. 
  • LDRs - light-dependent resistor. The greater the intensity of light shining on an LDR, the lower its resistance. The light provides energy that releases electrons and lowers the component's resistance.  
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Resistivity

Resistance, R, depends on:

  • Length (L) - the longer the wire, the more difficult it is to make a current flow through it. R is proportional to the length. 
  • Area (A) - the wider the wire, the easier it will be for the electrons to pass along it. 
  • Resistivity (p) - a measure of how much a particular material resists current flow. It depends on the structure of the material as well as environmental factors such as temperature and light intensity. It is a property of the material. 
  • Resistivity - the resistance of a 1m length with a 1m^2 cross sectional area. It is measured in ohm-metres 
  • the lower the resistivity of a material, the better it is at conducting electricity. 
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Superconductors

  • superconductor - a wire or device made of a material that has zero resistivity at and below a critical temperature that depends on the material. Below this temp. the wire has zero R so when a current passes through it, there is no pd across it so the current has no heating effect. 
  • Above the critical temperature, a superconductor material loses this property. Any material with a critical temp. above 77 kelvin (-196 degrees) is a 'high-temperature superconductor'. 
  • However, most 'normal' superconductors have critical temps. below 10k (-263 degrees C). This is very cold and the expensive to create. 
  • uses include:
    • power cables that transmit electricity without power loss (current has no heating effect)
    • Really strong electromagnets for use in medicine and trains etc. 
    • Electronic circuits that work really fast as there's no R to slow current down.
  • power - the rate of energy transfer (watts, W)
  • Energy - subsitute p = E/t into power equations to get:
    • E = VIt
    • E = I^2Rt etc.
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E.m.f.. and Internal resistance

  • In a battery, chemical energy is used to make electrons move. As they move, they collide with atoms in the battery, causing internal resistance. This causes batteries to warm up.
  • Internal resistance is the loss of pd per unit current in the source when current passes through it. 
  • Load resistance - the total resistance of all components in the external circuit. 
  • E.m.f. (electromotive force) is the amount of electrical energy produced per unit charge passing through a source. (volts, v)
  • Terminal pd - the electrical energy per unit charge DELIVERED by the source when it is in a circuit. This is LESS THAN the emf whenever current passes through. The difference is due to the internal resistance of the source. 
  • The 'lost pd' inside the cell (pd across the internal resistance of the cell) is equal to the diff. between the cell emf and the terminal pd. The 'lost' pd is the energy per coulomb dissipated (wasted) inside the cell due to its internal resistance.                                                                                                                    
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Circuit rules

Kirchoff's laws:

1. The total current entering a junction = the total current leaving it.            2. For any complete loop of a circuit, the total e.m.f. around a series circuit = the sum of the pds across each component.

In series:

  • the current is the SAME
  • e.m.f. SPLITS between components
  • the pd SPLITS propoprtionately to the resistance. The total pd is the sum of each pd.
  • the total resistance is the sum of the resistance of each component.

In parallel:

  • The current SPLITS at each junction
  • the pd is the SAME in each component 
  • 1/R total = 1/R1 + 1/R2 + 1/R3 etc...


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Cells in series and parallel

in series: 

  • if cells are connected in same direction, the net emf is the sum of the indiv.
  • if cells are connected in opposite directions, the net emf is the difference between indiv. 
  • the current is found by dividing the net emf by total resistance
  • the total internal resistance is the sum of the individual internal resistances. 

in parallel:

  •  the total current divided by n number of cells = the individual current. 
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The potential divider

  • a potential divider consists of 2 or more resistors in series with each other and a source of fixed pd. 
  • the pd of the source is divided between the components (series)
  • potential dividers can be used to:
    • supply a pd which is fixed at any value between 0 and the source pd
    • supply a variable pd
    • supply a pd that varies with a physical condition such as temperature or pressure. 
  • the ratio of the pds across each resistor is equal to the resistance ratio of the resistors
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Alternating current

  • ac current - a current that repeatedly reverses its direction. In one cycle, the charge carriers move forward, then reverse and then forward again.
  • the frequency os an ac current is the number of cycles per second. Mains electricity has a frequency of 50Hz
  • the time for 1 full cycle = 1/f
  • The peak value of an ac current is the max current or pd, which is same in both directions. 
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Comments

wizdomm

R = \rho \frac{\ell}{A}. \,\!

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