Current and Circuits

• Circuit Diagrams


  Circuit symbols are used to represent electrical components in circuit diagrams
• These symbols are universally recognised by all scientists in any language
• Commonly used circuit symbols

  • Cells & Batteries
  • Switch
  • Voltmeters & Ammeters
  • Fixed resistor
  • Variable resistor
  • Light-dependent resistor (LDR)
  • Thermistor
  • Potentiometer
  • Lamp
  • Light-emitting diode (LED)
  • Heating element
  • Motor
  • Earth (ground)
• Drawing Circuit Diagrams


  • Circuit diagrams represent the arrangement of components in a circuit
  • Must include an energy source, a closed path or complete circuit, and electrical components
• Measuring Current


  • Electric current is measured using an ammeter
  • Ammeters should always be connected in series within a circuit
• Measuring Potential Difference


  • Potential difference (or voltage) is measured using a voltmeter
  • A voltmeter is always set up in parallel to the component being measured
• The standard circuit symbols are given in the data booklet, so you do not need to memorise them
• When asked to draw a circuit diagram, make sure to draw the wires as straight lines with a straight edge or a ruler and make it as neat as possible, especially the circuit symbols
• Electric Current


  • Electric current is the rate of flow of charge carriers and is measured in units of amperes (A) or amps
  • Charge can be either positive or negative and is measured in units coulombs (C)
• Conventional current


  • The direction of conventional current in metal is from positive to negative
  • In electrical wires, the current is a flow of electrons
  • Electrons are negatively charged so they flow away from the negative terminal of a cell towards the positive terminal
• Direct current (DC)


  • Direct current flows from the positive to the negative terminal of the battery in a circuit
  • Electrons flow in the opposite direction
  • The potential difference in the circuit causes the current to flow
• The equation for current is: I = Δq/Δt
• When will 8 mA of current pass through an electrical circuit?

  • When a charge of 4 C passes in 500 s
• Conventional current flow is always from the positive to the negative terminal
• Electric Potential Difference


  • Potential difference (p.d.) is a measure of the electrical potential energy transferred by an electron as it moves between two points in a conductor
  • The definition of potential difference, also known as voltage, is: The work done per unit charge on moving a positive charge between two points along the path of the current
• Electronvolt (eV)


  • The amount of energy needed to move an electron through a potential difference of one volt
  • One electronvolt is equal to 1.6 × 10^-19 joules
• Conductors


  • A conductor is a material that allows charge (usually electrons) to flow through it easily
  • Metals are excellent conductors of electricity because current is the rate of flow of electrons and the more easily electrons are able to flow in a material, the better it is at conducting electricity
• Electronvolt (eV)


  Unit that will appear in future topics such as Spectra & Atomic Energy Levels
• The units of electronvolt (eV) will appear in future topics such as a in Spectra & Atomic Energy Levels
• Conductor


  A material that allows charge (usually electrons) to flow through it easily
• Conductors


  • Silver
  • Copper
  • Aluminium
  • Steel
• Conductors tend to be metals
• Different materials have different properties of conductivity
• Conductors on the atomic scale

  • Positively charged metal ions within a sea of delocalised electrons
• Current


  The rate of flow of electrons
• Metals are excellent conductors of electricity because current is the rate of flow of electrons, and the more easily electrons are able to flow in a material, the better it is at conducting electricity
• Insulator


  A material that has no free charges, hence does not allow the flow of charge through them very easily
• Insulators


  • Rubber
  • Plastic
  • Glass
  • Wood
• Some non-metals, such as wood, allow some charge to pass through them
• Despite not being very good at producing an electrical current, insulators are able to conduct static electricity when an insulator builds up charge on its surface and the charge can be transferred when it comes into contact with a conductor
• Electric resistance


  As electrons move through a conductor within a circuit (or any other component), they transfer some of their electrical potential energy to the positive ions of the metal, which resist their flow, resulting in an increase in the kinetic energy of the atoms in the lattice and heating up of the wire
• The greater the heating effect, the higher the resistance
• Copper has a low electrical resistance, making it an ideal material to make wires from
• All electrical components have resistance to different degrees, including the wires and batteries
• Ideal voltmeter


  Has infinite resistance, such that no current passes through it
• Ideal ammeter


  Has zero resistance, such that all the current passes through it
• Unless otherwise stated, voltmeters and ammeters will always be ideal in exam questions, meaning you do not need to consider the resistance of an ammeter or voltmeter as part of the total resistance of the circuit
• When non-ideal meters are used, the resistance will always be constant
• Resistance


  The ratio of the potential difference across the component to the current flowing through it
• Ohms (Ω)


  The unit for resistance