# Electricity

Used for formulae and general circuit rules.

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## Formulae

I    = Change in Q             V = W             (Ohm's Law) R = V

Change in t                     Q                                         I

Emf(e)   = E                Resistors in Series= R1+R2+R3...

Q

Resistors in Parallel 1/R= 1/R1+ 1/R2 + 1/R3... REMEMBER Inverse answer!

Resistivity rho= RA                  Power: (P=IV)  (P= I squared R)  (P= V squared )

L                                                                                    R

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## Energy and Potential Difference

When a torch bulb is connected to a battery (plural, 1 is a cell)  electrons deliver energy from the battery to the bulb. Each electron takes a fixed amount of energy from the battery as it passes through it.  After delivering the energy to the bulb they re-enter the battery via the positive terminal to be re-supplied with energy.

Each electron passing through a circuit does work to pass through the component and therefore uses some or all of its electrical potential energy.  The work done is equal to the loss of electrical potential energy.  Work done/unit charge is the potential difference OR voltage.

Therefore: Voltage = Work Done/ Charge.

Rearranging gives: W=QV for the work done or energy transfer when Q passes through a component which has a p.d across its terminals.

The Emf of a source of electricity is defined as electrical energy produced per unit charge passing through the source.  The unit is the volt.

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## Electrical Power and Current

Consider a component or device that has a p.d across its terminals and a current through it.  In time (delta t)

The charge flow through it, Q = I x delta t

the work done by the charge carriers, W=QV (Work Done: W= IV x delta t)

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