Electricity

• Current

  What is it measured in and with

  Definition

  Equation

  Measure in amps (A) and with an ammeter in series in a circuit

  Rate of flow of Charge at a point in a circuit

  Q(charge, coulombs)=I(current) x t(time, seconds)
• Current In metals

  Convectional Current

  Conservation of Current

  In a metal current is due to a flow of electrons

  In a solution current is due to a flow of ions

  Convectional current is the rate of flow of positive charge and is opposite direction of flow of electrons

  Conservation of current is when current is conserved at a junction because charge is always conserved
• Potential Difference/ Voltage

  What is it measured in and with what

  Definition

  Equation

  Relationship between Voltage and Current

  Is measure in Volts (v)and measured with a voltmeter in parallel to a circuit

  Work done per unit charge in moving between two points of a circuit

  V=E(j)/Q

  V= I x R Greater the voltage, Greater the current
• Resistance

  What is it measure in?

  Equation

  What happens to current when resistance is increased

  It is measured in ohms

  V= I x R

  Greater the resistance the harder it is for current to flow
• In an Ohmic conductor (resistor at a constant temperature)

  In a non- Ohmic conductor (filament lamp)

  In an ohmic conductor as voltage increases so does current they are directly proportional as there is no change in resistance.

  In a non-ohmic conductor resistance changes with voltage and current

  In a filament lamp as current increases so does temperature so electrons vibrate more and collide more causing resistance to increase
• Thermistor

  Light dependent resistor

  Thermistor- resistance decreases as temperature increases

  Light dependent resistor - Resistance decreases as light intensity increases
• Series Circuit

  How are components connected

  How does current flow through components

  How is potential difference shared

  How to calculate total resistance

  Connected end to end in a loop

  Current is the same across all components

  Voltage is shared across all the components

  Total resistance = Resistance of all the components added up
• Parallel circuit

  How are components connected

  How does current flow through components

  How is potential difference shared

  Why is it advantageous

  Connected to power supply in separate branches

  Current is shared between each branch because current can only flow in one direction

  Voltage is the same across every branch

  Means that if one breaks current can still pass through the rest
• Dangers of electricity

  Hazards (3)

  • Damaged insulation can cause a electric shock if come into contact with naked wire or can cause a fire due to a short circuit
  • Overheating can lead to a fire as high current flowing through a thin wire can cause insulation to melt
  • Damp conditions- water can conduct electricity so wet electrical equipment can cause electric shock
• Fuses and Circuit Breakers

  What is a fuse

  Current ratings

  Fuse is a thin piece of wire which overheats and melts if current is too high protecting the circuit

  Fuses have current ratings which should be slightly higher then the one used in the circuit usually 3A , 5A , 13A
• What is a circuit breaker and why is it better then a fuse

  Circuit breakers have an automatic electromagnet which breaks the circuit if current rises over a certain value

  Better then a fuse as it can be reset and used again, it works quicker than a fuse
• Earthing metal cases

  What do they do

  How do they do it

  They create a safe route for current to flow in case of a short circuit which prevents electric shocks

  Earth wires have very low resistance so a strong current will flow through it and then breaks the fuse which disconnects the appliance
• Double insulation

  Appliances with double insulation have a plastic casing

  Or they have been designed so that the earthing wire cant touch the metal casing

  Preventing electric shock
• Energy Transferred

  What is energy measure in ?

  Energy transfer from battery to appliance

  What is power measured in?

  Equation of Power

  Equation of Energy transferred

  Energy is measure in joules

  Chemical to electrical

  Power is measure in watts

  P=I x V

  Power = current x voltage

  Energy transferred = current x voltage x time
• Direct current

  Alternating current

  Direct current -flows only in one direction - provided by a cell

  Alternating current - continuously changes direction - mains electricity
• Electrical Charge

  How do you charge a body

  What happens when two insulators are rubbed together

  Example

  You can charge a body by adding or taking away electrons

  • Friction causes electrons to move from one insulator to the other meaning they both become charged
  • Material which gains electrons becomes negatively charged vice versa
  • Magnitude of charge on both materials is equal as they loose and gain same number of electronsWhen a cloth is rubbed against a rod electrons are transferred from cloth to rod and rod looses electrons making it positively charged
• What is static electricity

  When charges are not free to move and build up
• Examples of Static Electricity

  Lightning - Electrical charge in clouds build up due to friction , when charge becomes large enough, clouds discharge through the air

  Charged balloon on a wall- Positive charged balloon will stick to a wall if moved close enough because positive charges in the wall are repelled into the wall leaving the negative charge close to the balloon causing the balloon to stick

  Comb picking up bits of paper - Rubbing a comb against an insulator causes it to pick up charge , comb repels like charges in the bits of paper leaving it to attract the opposite charges
• Dangers of Electrostatic Charge

  Risk of electric shock

  Fuelling vehicles

  Safety

  Risk of electric shock - If a person touches an object with a large electrostatic charge electrons will flow through the person to the earth

  Fuelling vehicles- If enough charge builds on in the vehicle or pump it can create a spark , This can ignite a fire and causing explosion make sure to earth wire

  Safety is Earthing
• Inkjet printer

  • Droplets of ink are charged and pass between two charged metal plates
  • One has Positive charge other negative the ink is attracted to opposite charges
  • Deflecting them to a specific place on the paper
• Photo copier

  • Image is projected onto the positively charged plate
  • Where light fall onto the plate charge is leaked
  • Negatively charged toner particles are attracted to remaining positive areas
  • Paper is then placed over the plate and toner is transferred onto it making the photo copy