P2.3 Currents in Electrical Circuits

When certain insulating materials (e.g. silk, wool, nylon and polythene) are rubbed against each other they become electrically charged- the charges are fixed or static because the materials are insulators, meaning the electrons cannot travel through the materials.

Negatively charged electrons are rubbed off one material onto the other. The material that gains electrons becomes negatively charged, and the material that loses electrons is left with an equal positive charge. 

Two electrically charged objects exert a force on each other when brought together- two objects carrying the same type of charge repel, and two objects that carry different types of charge attract.

Electrical charges can move easily through conducting materials, e.g. metals.

An electric current is a flow of electrical charge, and will only flow through a conductor. The size of the electric current is the rate of the flow of electric charge. A current of 1A means 1C of charge passes a point every second.

I = Q / t

• I = Current, in A
• Q = Amount of electric charge, in C
• t = Time, in s

Whebn an electric charge flows it does work and energy is transferred. The potential difference (voltage) is the amount of energy transferred by each C of electric charge between two points. A potential difference of 1V means that 1J of work is done by each C of charge.

V = W / Q

• V = Potential difference (voltage), in V
• W = Work done, in J
• Q = Electric charge, in C

A resistor is an electrical component that reduces the current in a circuit.

Current-potential difference graphs are used to show how the current through a component varies as the potential difference across it is changed. 

The current-potential difference graph for a resistor at a constant temperature is a straight line through the origin - The current through a resistor at a constant temperature is directly proportional to the potential difference across the resistor- this is Ohm's Law.

V = IR

• V = Potential difference, in V
• I = Current, in A
• R = Resistance, in (ohms)

The arrangement of components in an electric circuit affects the current and potential difference

Series Circuits

• The same current flows through all the components
• Potential difference of cells is the sum of the potential difference of each cell
  • Only true if the cells are set up the same way round so that the + terminal of one cell is connected to the - terminal of the next
• Total resistance in a series circuit is the sum of the resistances of each of the components

Parallel Circuits

• Potential difference across each component is the same
• The total current through the whole circuit is the sum of the currents through the separate components

Filament Bulbs

• Outer electrons become delocalised , leaving metal ions - electrons can flow and carry a charge
• The resistance increases with the temperature 
  • When potential difference across the filament increases, electrons move more rapidly
  • Electrons collide with the ions that make up the structure of the metal filament
  • Electrons transfer energy to the ions so the temperature of the filament rises 
  • Ions vibrate more and obstruct the electrons, so resistance increases
• At high potential difference, the current through the bulb increases less for each equal increase

The current through a diode flows in one direction onlu- the diode has a very high resistance in the reverse direction

An LED (light emitting diode) will only give out light when the current flows in the forward direction. There is an increasing use of LEDs as they require a smaller current than other forms

The resistance of LDRs (Light dependent resistors) falls as the intensity of light falling on it increases. This means that an LDR allows more current to flow in the light compared with in the dark. Used in street lights

The resistance of a thermistor decreases as the temperature rises. This means that the current through a thermistor rises as the temperature rises. Used in thermostats that control the temperature of appliances