Electromotive Forces & Potential Difference

• The electromotive Force (e.m.f.) - the electrical work done by a source in moving a unit charge around a complete circuit

  • is the name given to the potential difference of the power source in a circuit aka the voltage supplied by a power supply
  • measured in volts (V)

  
• EMF equation: E = W/Q

  • E = electromotive force (e.m.f.) (V)
  • W = energy supplied to the charges from the power source (J)
  • Q = charge on each charge carrier and these are electrons in circuits (C)
• As charge flows around a circuit, energy is transferred from the power source to the charge carriers, and then to the components

  • This is what makes components such as bulbs light up
  • The potential difference between two points in a circuit is related to the amount of energy transferred between those points in the circuit
• Potential difference - the work done by a unit charge passing through a component

  • measured in volts (V)
• Equation for PD: V = W/Q

  • V = potential difference (p.d.) (V)
  • W = energy transferred to the components from the charge carriers (J)
  • Q = charge on each charge carrier and in circuits these are electrons(C)
• Potential difference is measured using a voltmeter, which can be either

  • Digital (with an electronic read out)
  • Analogue (with a needle and scale)
• • Voltmeters are connected in parallel with the component being tested
  • The potential difference is the difference in electrical potential between two points, therefore the voltmeter has to be connected to two points in the circuit

  
• Analogue voltmeters are subject to parallax error

  • Always read the meter from a position directly perpendicular to the scale
• Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary

  • Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements
  • They should be checked for zero errors before using
• Digital voltmeters can measure very small potential differences, in mV or µV
• Digital displays show the measured values as digits and are more accurate than analogue displays

  • easy to use because they give a specific value and are capable of displaying more precise values
  • digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down
• Digital voltmeters should be checked for zero error

  • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results
• When you are actually building a circuit in class, always save the voltmeter until last
• Electrical potential is compared to the ground

  • the ground has 0 electrical potential
• Every time an electron reaches a component, it loses energy

  • difference in the amount of energy an electron had before
• EMF is not a force - called a force because it "drives" the electrons allowing them to move
• If you measure a battery with a current running through it, the energy is smaller than the original value as resistance generates heat which wastes energy
• Which factor affects the magnitude of the induced e.m.f?

  The speed at which the coil cuts across the magnetic field lines
• Define EMF


  It is the chemical energy transferred to electrical energy. This energy is supplied by the source/batter/cell in driving a unit charge around a complete circuit.
• Define potential difference


  Work done by a unit charge passing through a component.