Current of electricity

• electric current is the rate of flow of electric charge
• I= Q/T

  I= current (A)

  Q= charge (C)

  T= time taken (s)
• An electric charge is a property that many subatomic particles and ions possess
• conventional current is when positive charge flow out of positive terminal of a battery to negative terminal
• Electric current can be measured by an ammeter or a digital multi-meter
• the ammeter is connected in series
• electron flow is the flow of electrons from the negative terminal of the battery to positive terminal
• direction of current in a circuit is a conventional current
• electric current is a scalar quantity
• an ideal ammeter has zero resistance
• volt is the SI unit for potential difference and electromotive force
• the electromotive force (e.m.f) of an electrical source is the work done by source in driving a unit charge around a complete circuit
• E= W/Q

  E= e.m.f of electrical source (V)

  W= work done (J)

  Q= amount of charge (C)
• dry cells (battery) can be arranged in parallel or series
• when batteries are arranged in parallel, the total e.m.f is equal to that of a single cell
• when negative terminal of one cell is connected to the positive terminal of another cell, the total emf is the sum of the emf of each cell
• the potential difference (pd) across a component in a circuit is the work done per unit charge in driving charges through the component
• Potential difference

  V= W/Q

  V= voltage/ p.d across a component

  W= work done (J)

  Q= amount of charge (C)
• emf is present even when no current is drawn from the source

  p.d across any electrical component is zero in the absence of current
• the electrons that flow through a circuit is comes from the wire
• the battery supplies the energy that moves the electrons
• energy from chemical potential store of the battery is transferred electrically to the internal store of the filament
• resistance of a component is the ratio of potential difference across the current flowing through it
• R= V/I

  R= Resistance (ohm)

  V= p.d across the component (V)

  I= current flowing through the component (A)
• resistance is a scalar quantity
• R= PL/A

  R= resistance

  P= resistivity (depends on material ONLY)

  L= length

  A= cross-sectional area
• Variable resistor

  current enters from bottom and exits from top

  
• Filament lamp (non-ohmic)

  
• Semi-conductor diode (non-ohmic)

  
• How to find resistance from I/V graph

  • inverse gradient
• ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it. This is provided that all physical conditions and temperature remain constant
• Aim

  Determine the resistance of an unknown ohmic resistor

  1. Set up the apparatus according to the circuit diagram below
  2. Adjust the rheostat to the maximum resistance so that the initial current is small. This also minimises heating of the rheostat
  3. Record the ammeter and voltmeter readings
  4. Adjust the rheostat to reduce the current by 1A. Record the ammeter and voltmeter readings
  5. Repeat step 4 to obtain four more readings

  Plot V/V against I/A. Determine the gradient of the graph

  Observation:

  • Gradient is linear which gives value of resistance
• For experiment