Current, potential difference and resistance

Standard circuit diagram symbols

Electrical charge and current

For electrical charge to flow through a closed circuit the circuit must include a source of potential difference.

Current is the flow of electrical charge around a circuit.

The charge in an electrical circuit is carried by electrons.

Current is measured in amps using an ammeter.

Current only flows in one direction in a series circuit and is produced by a source of potential difference.


Charge flow=current*time

charge flow, Q, in coulombs,

C current, I, in amperes, A (amp is acceptable for ampere)

time, t, in seconds, s

A current has the same value at any point in a series circuit.

Current, resistance and potential difference

The current through a component depends on both the resistance of the component and the potential difference across the component.


The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component.

Electrons are pushing through particles in wires and components.


Potential difference is the energy transferred for each coulomb of charge between two points in a circuit.

The voltmeter is attached in a parallel circuit.


potential difference = current × resistance

potential difference, V, in volts,

current, I, in amperes, A (amp is acceptable for ampere)

resistance, R, in ohms, Ω

Resistors

The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor.

This means that the resistance remains constant as the current changes.


The current through a diode flows in one direction only. 

 In the opposite direction resistance is very high.


The current through a filament lamp is not directly proportional to potential difference.

  This is because as the temperature increases, the atoms vibrate more.

  This also means that there are more collisions.
  Therefore, more potential difference is needed to push current through the filament.

 As the potential difference increases, the current no longer increases (resistance is increasing.



LDR

  • Resistor that depends on light.

  • Light is needed for current to flow.

  • Therefore, as light intensity increases, the resistance decreases.


Thermistor

  • Resistor that depends on temperature.

  • As the temperature increases, the resistance decreases.