# Electrical Circuit

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## Electrical Charges and Fields

- some insulating materials become charged when rubbed together

- electrons are transferred when objects become charged

insulating materials that become positively charged when rubbed lose electrons

insulating materials that become negatively charges when rubbed gain electrons

- like charges repel, unlike charges attract

- the force between between two charged objects is a non-contact force

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## Current and Charge

- every component has its own agreed symbol, a circuit diagram shows how components are connected together

- a battery consists of two or more cells connected together

- the size of an electric current is the rate of flow of charge

- the equation for the electric current of a circuit is:

current = charge flow / time taken

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## Potential Difference and Resistance

- potential difference across a component:

potential difference = energy transferred / charge

- resistance is:

resistance = potential / current

- OHMs law states the current through s resistor at constant temperature is directly proportional to the potential difference across the resistor

- reversing the potential difference across a resistor reverses the current through it

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## Component Characteristics

- the resistance of an appliance:

resistance = potential difference / current

- a filament lamps resistance increases if the filaments temperature

- diode : forward resistance low ; reverse resistance high

- a thermistors resistance decreases if its temperature increases

- an LDRs resistance decreases if the light intensity on it increases

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## Series Circuits

- for components in series:

the current is the same in each component

the total potential difference is shared between the components

adding their resistance gives the total resistance

- for cells in series acting in the same direction, the total potential difference is the sum of their individual potential differences

total resistance = resistance (1) + resistance (2)

- adding more resistors in the series increases the total resistance because the current through the resistors is reduced and the total potential difference across them is unchanged

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## Parallel Circuits

- for components in parallel:

the total current is the sum of the currents through the separate components

the potential difference across each component is the same

- the bigger the resistance of a component, the smaller the current that will pass through that component

- the current through a resistor in a parallel circuit is:

current = potential difference / resistance

- adding more resistors in parallel decreases the total resistance because the total current through the resistors is increased and the total potential difference across them is unchanged

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