Electricity Notes

Units of charge

A coulomb is a unit of charge in the metric system.  It is 6.25 × 1018 electrons.  The symbol for charge is q.

Electric Potential

A charge in an electric field has a potential energy based on the strength of the electric field


Current: the amount of electrical charge flowing through a wire per second.

Current is measured in Amperes: One ampere is one coulomb per second.  The symbol for current is an i.  The symbol for Amperes is just an A.  

If a light bulb is rated to use 0.8 A, that means 0.8 Amperes or 0.8 coulombs per second.

If 0.5 amperes pass through a light during an eight hour night, how many coulombs of charge passed through the light?

i =q/t   0.5 A = q/(8 hrs*3600sec/hr)

0.5 = q/28800

q= 14400 Coulombs of charge

Electromotive Force (EMF)

EMF is the force that impels electrons to move.  EMF is the energy per coulomb of charge.  Remember that energy is also called work in physical science.

EMF = work/q   or EMF = energy (Joules)/charge (Coulomb)

EMF is also called "voltage" because the units of EMF are Volts.

Using i =q/t  we can write q = it   (t is time)

Therefore EMF = energy/ it  (substituted  it for charge q).  Now the energy per unit time is power. 

EMF = 1/i ×energy/time
EMF = 1/i ×power
EMF × i = power

We also use V for EMF (for voltage) hence we often write V × i = power

The voltage (Volts) times the current (Amperes) is the power (Watts)

A typewriter uses 0.8 Amps at 100 Volts.  How many Watts of power does the typewriter consume? iV = Power: (0.8 Amps)(100 Volts) = 80 Watts.  Answer: 80 Watts. 

How many amps (short for amperes) runs through a 100 Watt light bulb on Pohnpei?   The Voltage on Pohnpei is 118 Volts during the day.

P = iV
100 Watts = i (118 Volts)

i (Amperes) = 100 Watts/118 Volts