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P2.3.1- Static electricity

Electrical circuits

Calculate current, potential difference or resistance

I=Q/t

When certain insulating materials are rubbed together, thwy become electrically charged.

When materials gain electrons, they become negatively charged. The material that lost electrons becomes positively charged.

When two charged objects are brought together, they exert forces upon another.

Objects can either attract or repel one another.

Objects with positive charges attract, and objects eith negative charged repel.

Electrical charges can move easily through some materials, such as metal.

V=IxR

  • An electrical current is a flow of charge.
  • The size of an electrical current is the rate of flow of electrical charge.
  • The size of the current is given by the equation:

I= the current in amps.

Q= the charge in coulombs.

t= time in seconds.

The potential difference (voltage) between two points in an electrical

circuit is the amount of 'work done'.

V= potential difference in volts (V)

W= 'work done' in joules.

Q= is the charge in coulombs

The current through a component relies upon the amount of resistance.

The greater the resistance, the smaller the current for a given potential difference across the component.

V=W/ Q

V=potential difference in volts.

R= resistance in Ohms

I= charge in coulombs

The total potential difference of cells in a circuit is the sum (adding) of the numbers. (depending on the direction in which they're added.)

Thermistors/ LDR

For components connected in a series:

  • The 'total resistance' is the sum of the resistance of each component.
  • The same current goes through each component.
  • the total potential difference of the supply is shared between the components.

Thermistors are temperature sensors.

  • At low temps. the resistance is high, and a little current can flow through the thermistor.
  • At high temps. the resistance is low, and a lot of current can flow through the thermistor.
  • Thermistors can be used in incubators, to ensure that the temperature is correct, or fire alarms, to alert people of excess heat that could indicate a fires presence.

Resistance graphs

Light Dependant Resistors detect light level.

The resistance of an LDR decreases as the light level increases.

  • In low light levels, the resistance of the LDR is high, and a little current can flow through.
  • In brighter light levels, the resistance of the LDR decreases, and more current can flow through.

On a resistance graph, the current is the vertical axis. The potential difference is the horizontal axis.

Resistor at a constant temp.

The graph for a resistor is is a straight line, because the resistor follows Ohm's law. Ohm's law states that the potential difference is directly proportional to the current.

Filament bulb

Series Circuit

These bulbs have a coil of wire called a filament wire inside. The wire heats up when an electrical current passes through.

It doesn't follow Ohms law. It's resistance increases as the temperature increases, which means that they're not directly proportional.

The Diode

There is only one path for the charge to flow along, if one bulb breaks, neither will get any charge.

Diodes are used to regulate the potential difference in circuits, and to make logic gates. Light Emitting Diodes (LEDs) give off light and are often used in computers and television sets.

LEDs use a smaller current. This makes them more popular.

Diodes have very high resistance, going in only one direction, meaning that current can only go in the other direction.

There is a higher resistance as light intensity increases.

  • An electric current flows when a electrons move through a conductor of electricity, such as a metal wire.
  • The electrons can collide with the ions in the metal.
  • This makes it more difficult for the current to flow. This is resistance.

Parallel Circuit

The charge can take two pathways. That way, if one bulb breaks, the flow of charge can still get to the other bulb.

  • The potential difference is the same in each component.
  • The current of the whole circuit is the sum of the currents
  • The resistance of a long wire is greater than the resistance of a short wire because electrons collide with ions more often.
  • The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire has fewer electrons to carry the current.

P2.3 Currents in Electrical Circuits

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