# Electric Circuits

## Electrical charges and fields

The nucleus of an atom contains positively charged protons and neautral neutrons. Negatively charged electrons move around in the space outside the nucleus

An uncharged atom = equal number of protons and electron

An atom that gains an electron = negatively charged ion

An atom that loses an electron = positively charged ion

Static electricity = when you rub two electrically insulating materials together, electrons are rubbed off one material and desposited on the other

Charging by friction = electrons have a negative charge, so the material that has gained electrons becomes negatively charged. the one that has lost electrons is left with a positive charge.

Acharged object creates an electric field around itself

The forces berween two charged object is a non-contact force

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## Current and Charge

current = charge / time

current is measured in amps

In a circuit that is a single closed loop, the current is the same at every point

A diode allows current to flow in only one direction through the circuit

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## Potential difference and Resistance

Ammeters measure the current passing through a component in a circuit.

an ammeter are always connected in series with a component

The unit of current is A

Voltmeters measures the potential difference across a component in a circuit

Voltmeter are always connected in parallel with the component

The unit for potential difference is V

Potential difference = energy (j) / Charge (c)

Resistance is the opporsitye to current flow

Resistance =  potential difference (v) / current (a)

Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.

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## Current-potential difference graphs

Current potential graphs are used to show how current through a component varies with the potential difference across it

If a resistor is kept at a constant temperature = the current-potential difference graph shows a straight line passing through the origin - the current is directly proportional to the potential difference across the resistor - ohm's law - components that behave like this are called ohmic conductors

At a constant temp a component resistance remains constant, regardless of the direction of the potential difference and current

The greater the resistance of the resistor the less steep the current-potential graph

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## Component characteristics

The line on a current-potential difference graph for a filament lamp is a curve, this is because it is not directly proportional.

The resistance of the filament lamp increases as the current increases, this is because the resistance of the filament increases as its temperature increases

THe current through a diode is in one direction only, in the other direction the diode has a very high resistance so the current is nearly zero

A light-emitting diode emits light when a current passes through it in a forwards direction

The resistance of a thermistor decreases as the temperature increases

the resistance of a light-dependant resistor decreases as the light falling on it gets brighter

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## Series Circuits

Charge stops flowing if there is a break

the current through every component is the same

the current in a series circuit is determined by the potential difference of the power supply and the total resistance of the circuit worked out by: l = v / r

the p.d.s across individual components add up to give the total resistance of the circuit

Rtotal = R1 + R2+...

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## Parallel Circuits

Each component is connected across the supply so if there is a break, charge can still flow in the other parts

The p.d across each component is the same

The current through a component in a parallel circuit = i = v / r

the total resistance through the whole circuit is the sum of the current through the seperate branches.

Many circuits are a mixture of both parallel parts and series parts.

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