PHYSICS – Electricity: Formulas

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  • Created by: Amy Hills
  • Created on: 21-04-13 13:40

Current (measured in amps [A])

I=Q/t

I=V/R

I=P/V

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Voltage (measured in volts [V])

V=W/Q

V=IR

V=P/I

(W=energy)

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Resistance (measured in ohms [Ω])

R=V/I

R= ρXL/A

ρ=resistivity    L=length in m    A=cross-sectional area in m²

Resistors in series: R= R1+R2+R3...

Resistors in parallel: 1/R= 1/R1+1/R2+1/R3...

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Resistivity (measured in ohm-metres [Ωm])

ρ=RXA/L

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Power (measured in watts [W])

P=VI

P=V²/R

P=I²R

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Total Energy (measured in joules [J])

E=VIt  

E=(V²/R)t

E=I²Rt

Basically: powerXtime=total energy

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Conservation of energy

energy per coulomb supplied by source=

energy per coulomb used in load resistance+energy per coulomb wasted in internal resistance

the energy wasted per coulomb overcoming the internal resistance is called the lost volts (v)

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Root mean square (rms) of an alternating current a

Irms=1/√2 XV0

Vrms=1/√2 X I0

ie.  rms= peak/√2

V0=voltage peak value       I0=current peak value

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Electromotive force (E.M.F) [V] and internal resis

ε =E/Q ε =V+v ε =I(R+r)=IR+Ir V=ε-v V=ε-Ir v=lost volts V=terminal p.d E=electrical energy r=i.r R=load resistance

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Power in an alternating current

P= Irms2R

P=Vrms2/R

P=VrmsX Irms

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Resistance in alternating current

R=Vrms / Irms = V0 / I0

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