Physics-Electricity

• Current


  The rate of flow of charge in a circuit
• Potential Difference


  Also called voltage. The difference in potential between two points of a circuit. Causes a current to flow.

  
• Charge


  The amount of electricity travelling through a circuit
• Resistance


  Anything that slows the flow of charge around a circuit. Resistance is usually caused by electrons colliding with ions in a material.
• Series Circuit


  A circuit with a single loop of wire
• Parallel Circuit


  A circuit with two or more loops (branches) of wire
• For electrical charge to flow through a closed circuit, the circuit must include a source of potential difference
• An electric current is the flow of electrical charge, usually electrons, around a circuit
• The size of the electric current is the rate of flow of electrical charge
• In a series circuit (one with a single loop of wire) the current is the same at any point of the loop
• Charge flow (Q)


  Coulombs (C)
• Current (I)


  Amp (A)
• Time (t)


  Seconds (s)
• Calculating charge flow


  1. Convert time into standard units

  2. State equation: Q = I t

  3. Substitution: Q = 1.2 x 300

  4. Answer: Q = 360 C
• The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component
• The greater the resistance of the component, the smaller the current for a given potential difference (V) across the component
• The resistance in a circuit will depend on the components used in the circuit as well as the length of wire used in the circuit
• Potential difference (V)


  Volts (V)
• Resistance (R)


  Ohms (Ω)
• Calculating resistance


  1. State the equation: V = I x R

  2. Rearrange: R = V / I

  3. Substitution: R = 9.4 / 0.2

  4. Answer: R = 47 Ω
• Ohmic Conductors


  Resistors where the current through the material is proportional to the potential difference applied across its ends
• Ohm's Law


  The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor
• Ohmic conductors will produce a straight line I – V graph that goes through the origin
• Non-Ohmic Conductors


  • Components such as lamps, diodes, thermistors and LDRs where the resistance changes with the current through the component
• Filament Lamp


  As the current increases, the temperature of filament increases therefore the resistance of the filament lamp increases
• Diodes


  Electrical components that only allow a current to flow in one direction only
• Thermistors


  Resistors where the resistance varies with temperature. The resistance decreases as temperature increases
• LDRs (Light Dependent Resistors)


  Resistors where the resistance varies with light intensity. The resistance decreases as light intensity increases
• Measuring Resistance


  1. Set up circuit with ammeter and voltmeter

  2. Use equation R = V / I to calculate resistance
• To get a range of potential differences and currents a variable resistor can be added into the circuit or the input potential difference changed
• Circuit Diagram Symbols


  • Switch open
  • Switch closed
  • Cell
  • Battery
  • Diode
  • Resistor
  • Variable resistor
  • LED
  • Lamp
  • Fuse
  • Voltmeter
  • Ammeter
  • Thermistor
  • LDR
• In a series circuit the current is the same at any point of the loop
• In a parallel circuit the potential difference across each component is the same
• In a parallel circuit the total current through the whole circuit is the sum of the currents through the separate components on each loop (branch)
• In a parallel circuit the total resistance of two resistors is less than the resistance of the smallest individual resistor
• Series circuit


  Same current at any point of the loop
• Parallel circuit


  The total current through the whole circuit is the sum of the currents in each loop
• Series circuit


  1. Switch

  2. Battery

  3. Two lamps
• Resistance of resistors in series

  Rtotal = R1 + R2
• The total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor