Electricity

• Direct current

  Current flows in one direction

  Produced by direct potential difference

  Eg. Cell or battery
• Alternating current

  Current repeatedly reverses direction

  Produced by an alternating potential difference

  Eg. Mains electricity
• Mains electricity

  Provides an alternating current

  Frequency: 50Hz

  Voltage: 230V
• Wires in plugs

  1. Earth wire, Yellow and green, Safety wire, Stops the appliance becoming live, Carries a current if there is a fault, Potential difference of 0V
  2. Neutral wire, Blue, Completes the circuit, Potential difference of 0V
  3. Live wire, Brown, Carries electricity, Potential difference of 230V
• National grid: Nationwide network of cables and transformers that links power stations to consumers of mains electricity
• Step-up transformers: Increase the potential difference from power stations to transmission cables
• Step-down transformers: Decrease the potential difference from the transmission cables to consumers of mains electricity
• What equation links power, current, and potential difference?

  Power(W)= Current(A) x Potential difference(V)

  P=IV
• What equation links power, current, and resistance?

  Power(W)= Current^2(A) x Resistance(Ω)

  P=I^2R
• What equation links charge flow, current, and time?

  Charge flow(C) = Current(A) x Time(s)

  Q=It
• What equation do we use to find the energy transferred to an electrical appliance when a charge flows through it?

  Energy transferred(J) = Charge flow(C) x Potential difference(V)

  E=QV
• What equation do we use to find the energy transferred by an electrical appliance?

  Power(W) = Energy transferred(J) /Time(s)

  P=E/T
• The unit for charge: Coulombs (C)
• The unit for potential difference: Volts (V)
• The unit for current: Amps (A)
• The unit for resistance: Ohms (Ω)
• The unit for power: Watt (W)
• Charge in an atom

  An atom has no charge because it has an equal number of positive protons and negative electrons

  An atom would become positively charged if an electron is removed

  An atoms would become negatively charged if an electron is added
• Static charge

  Insulating materials can become charged when they are rubbed with another insulating material

  This is because electrons are transferred from one material to another
• Electric fields

  Charged object creates and electric field around itself

  If the object is placed in the electric field of another object, it experiences electrostatic force

  This means they exert a non-contact force on each other

  Like charges repel

  Opposite charges attract
• Spark

  If 2 objects have a very strong electric field between them, electrons are pulled away from air molecules and cause a flow of electrons between the 2 objects
• Drawing electric fields

  Electric fields can be represented using a diagram with field lines, these show the direction of the force

  Always remember:

  Field lines meet at the surface of charged objects at a right angle

  Arrows point away from positive charges and towards negative charges
• Electric current

  Electric current is when charge flows

  The charge in an electric circuit is carried by electrons
• Electric current in circuits

  Current flows from positive terminal of cell or battery to negative terminal, this is known as conventional current

  In a single closed loop, the current has the same value at any point in the circuit
• Why are metals good conductors of electricity?

  Because they contain delocalised electrons
• Potential difference

  A measure of how much energy is transferred between two points in a circuit

  Pd across a component: The work done on it by each coulomb of charge that passes through it

  Pd across a power supply or battery: The energy transferred to each coulomb of charge that passes through it
• Resistance

  Opposition to the flow of current

  Electrons collide with the ions and atoms of the wires and components in the circuit, this causes resistance to the flow of charge

  Long wire has more resistance than a short wire
• What equation do we use to find the resistance of an electrical component

  Potential difference(V)= Current(A) x Resistance(Ω)

  V=IR
• What happens to the resistance of a thermistor as its temperature increases?

  Resistance decreases
• What happens to the resistance of a light dependent resistor when light intensity increases?

  Resistance decreases
• What happens to the resistance of a filament lamp as its temperature increases

  Resistance increases
• Series circuits and parallel circuits

  Series: Components one after another in a single loop, if one component stops working the whole circuit stops working

  Parallel: 2 or more loops through which current can flow, if one branch stops working the other branches are not affected