Electric Circuits

• Electric current is the rate of flow of electrical charge. In metal wires discharge is carried by electrons
• Charge flow (C)


  Current (A) x time (s)
• How to reverse the current through a resistor

  Reverse the potential difference across the resistor
• Resistance

  How hard it is to flow through something
• A lamp's resistance

  Decreases if the lamp's temperature increases
• Diode

  Forward resistance low, reverse resistance high
• Thermistor resistance

  Decreases if the temperature increases
• LDR resistance

  Decreases if the light intensity increases
• Resistor

  • Constant Resistor = Ohmic
  • Resistance changing as current changes = Non-ohmic
• Direct current

  Flows in one direction only
• Alternating current


  Repeatedly reverses its direction of flow
• Electricity Supply in U.K: Frequency: 50Hz, Pd:230V
• DC Supply in home

  • Mobile Phone
  • Electric toothbrush
• Series Circuit


  • Potential difference of lamps shared between the lamps Vcell = V1 + V2
  • Current is same through each lamp i1=i2
• Parallel Circuit

  • Potential difference across each lamp is the same vcell=v1+v2
  • Total current through circuit is the sum of the current through each lamp itotal=i1+i2
• Series Circuit


  • Lamps both on or off
• Parallel Circuit


  • Lamps can be switched on and off separately
• Series Circuit: 1.5V, 2 ohms, Rtotal = 4 ohms, Current = 1.5A
• Earth wire

  Green and yellow, Potential difference with the earth = 0V, Safety wire, Only carries current if there's a fault
• Live wire

  Brown, Potential difference with the earth = about 230V, Carries the alternating potential difference
• Neutral wire

  Blue, Potential difference with the earth = 0V, Completes the circuit
• Plastic is used in plugs as plastic is a good electrical insulators
• Wires are copper insulated copper surrounded by flexible plastic
• 10W

  10000 kW
• Less energy is lost from power station to home using step-up transformer.
• National Grid

  1. Power station

  2. Step-Up transformer

  3. High-voltage cables

  4. Step down transformer

  5. Homes
• In National Grid energy is lost in the power cables due to the resistance of the wires
• Step-up transformer


  Increase potential difference (25kv to 400kv)
• Step-down transformer


  Reduces potential difference to 230V for home use.
• 2 components needed to measure resistance

  Ammeter, Voltmeter
• Used to detect light in circuit

  LDR (Light Dependent Resistor)
• What Circuit component changes length of wire for higher or lower resistance.

  Variable Resistor
• Light dependent resistor is are used control circuits as the light intensity

  increases the resistance decreases so the current can be changed and circuits can be switched on and off
• Circuit diagram used to measure the resistance across a lamp

  
• Step-up Transformers Efficiency 


  1.Energy lost due to resistance in power cables

  2. step up transformer increases potential difference so current is reduced.

  3.Which reduces energy loss
• Trampoline Springs

  Elastic potential energy increases
• Work done to child

  Total energy transferred by child
• Linear Relationship

  as X increases, Y increases
• In circuit Equations

  Convert 290mA to 0.290A