topic 10 electricity

• circuit diagrams

  materials to conduct electricity :

  • must contain charged particles
  • those charged particles must be able to move when a potential difference is provided
• circuits

  • these electrons are negatively charged so they move towards positive terminal of battery
  • when working with circuits , however the conventional direction of current is used .
  • current goes from positive to negative terminal
• series circuit

  • in a series circuit , there is just one route from the current to flow round the circuit
• parallel circuit

  • in a parallel circuit , there are multiple different routes the current can flow round the circuit
  • these points where the current split is called a junction
• circuit symbols

  A) cell

  B) battery

  C) open switch

  D) closed switch

  E) filament bulb

  F) fixed resistor

  G) variable resistor

  H) voltmeter

  I) ammeter

  J) fuse

  K) diode

  L) LED

  M) LDR

  N) Thermistor

  
• current

  • electric current is measured in units called ampetes ( often shortened to amps , A )
  • use ammeter to measure
  • must be connected in series to measure current flowing through circuit or its branch
• electrical current = Rate of flow of charge in a circuit

  (charge per unit time , flowing past a particular point )
• current in series

  • current in series circuit , through single loop is the same
  • current before junction must be equal to current after a junction
  • current at junction will split depending on resistance of branch
  • charge must be conserved ( based on the conservation of charge law)
  • hence , if charge is always conserved , so must current
• potential difference

  • also called voltage
  • energy per unit charge - measured in volts using voltmeter connected in parallel
• rules of potential difference

  1. P.D across all components in a series circuities' equal to the P.D provided by power source
  2. P.D across each component in a series circuit is shared and adds up tp total P.D provided by power source $V^t =$$V^1 +$$V^2 +$$V^3 +$$....$
  3. The P.D across each branch of a parallel circuit has the same P.D as the power source - branches in parallel have the same P.D
• Potential difference

  • energy must be conserved ( based on conservation of energy law )
  • energy cannot be created or destroyed
  • hence energy provided by a power source must equal to the total energy given to components in a circuit
• Current , charge and energy

  • in metals , current is a flow of electrons , each electron has a very small charge , only -1.6 x 10 ^19 C
  • millions and millions of electrons flow through metal wire to produce 1 C
  • electric charge is measured in coulombs ( C )
  • one coulomb is the charge that passes a point in a circuit when their current is of 1 amp for 1 second
  • charge = current x time
  • Q = I x T
  • Q= coulombs
  • I = amperes
  • T = seconds
• electrical resistance

  • a resistor is a component or device that adds resistance to a circuit
  • resistance is defined as the restriction of a flow of current in an electrical circuit
  • it is measured in units called ohms , Ω
  • potential difference = current x resistance
  • V = I x R
  • V - potential difference
  • I - current
  • R - resistance
• resistance rules in series

  • voltage is shared
  • depending on the resistance of each component
  • current is the same
  • depending on the total resistance of each component
  • resistance is addative
• resistance rule in parallel

  • voltage is the same
  • each branch gets the same amount of voltage
  • current is shared
  • split across each branch and depending on the resistance
  • in parallel, total resistance is less than the individual components
• ohms law

  • the P.D through a compound is directly proportional to the current flowing through it
  • the constant of proportionality is called resistance
  • V = IR -> for an ohmic conductor , I-V graph is a straight line
• I-V characteristics - resistors

  • is a straight line graph ,
  • resistors is an ohmic conductor ( P.D is directly proportional to current)

  
• I-V characteristics - filament lamps

  • current and voltage not proportional
  • resistance changes as the potential difference and current varies
  • bulb does not follow ohms law
  • as voltage increases , the resistance of the filament lamp increases
  • I = V / R , so the gradient of the graph is 1/R , and the gradient is decreasing
  • hence resistance will increase

  
• i-v characteristics filament light

  • as current flows through the circuit , the electrons collide with the metal ions , transferring some of their kinetic energy to the metal ions
  • metal ions vibrate more as they gain K.E
  • as the average K.E increases , so does their temperature
  • as temp increase :
  • -> metal ions vibrate more
  • -> more collisions
  • => more restriction in the flow of current
• I-V characteristics - Diode

  • the current in a diode is not proportional to the voltage
  • when the negative voltage is applied current is zero
  • a small voltage is required in the positive direction to cause a current ( threshold voltage )
  • for a negative V : the gradient is 1/R
  • gradient is equal to 0 - therefore resistance is very large

  
• I-V characteristics - Light dependent resistors - LDR

  • made of a semiconductor material
  • an LDR's resistance depends on the light intensity incident on it . As light intensity increases , the resistance of the LDR will decrease
  • they commonly used in light sensing circuits such as those in street lamps or cameras
  • as the light intensity increase the resistance decreases
  • at a decreasing rate

  
• I-V characteristics - Thermistor

  • thermistors are also made of semiconductor materials
  • their resistance decreases as temperature decreases
• electrical power

  • power = energy transferred per second
  • E = VIT
  • P = IV , v = IR
  • power (W) = current (A) x potential difference (V)
  • substitute
  • P = I^2R (sub V = IR into P = IV)
• Mains Safety

  • mains means principle - hence primary source of electricity for household
  • the electricity that comes from the main is 230 v with a frequency of 50Hz , its an alternating current (A.C)
  • electricity comes in two forms, alternating current ( A.C ) and direct current ( D.C)
  • in A.C , the voltage and current change with time
  • in D.C , the voltage and current remain constant with time
  • A.C -> magnitude varies in time between maximum and minimum value
  • D.C -> magnitude is constant
  • frequency = number of waves / time = 1/T

  
• fuses - main safety

  • fuses or circuit breakers are connected to the live wire entering the house and breaking the current if it becomes too large
  • a large current will transfer a lot of power which can become very dangerous
  • they are connected to the live wire so that the whole circuit stays at 0V if there is a blown fuse
  • if not all sockets in the house remain dangerous
• fuses main safety

  • each plug has a smaller fuse
  • a live wires is considered dangerous because there P.D is 230v between it and other wires
  • if you make the connection between these wires the electrons will flow through your body as it is grounded and having a lower resistance , transferring electrical energy into you which causes burns and potentially stop the heart
  • fuses are used to prevent a large current passing through as they have a range of rating 3A, 5A , 13A which prevents a larger current passing as it melts the fuse breaking the circuit
• uk mains plugs

  • three main wires
  • live wire ( brown ) - voltage changes between 230v and -230 v at 50Hz - A.C current
  • neutral wire ( blue ) -connects back to the power station and should remain at 0v
  • earth wire ( green ) - connected to a large metal spike pushed into the ground . attached to any metal casing so that charge cannot build up and become dangerous