Electricity is a topic which seems to confuse a lot of students. Perhaps because electric current cannot be seen so a lot has to be taken ‘on trust’, perhaps because the topic lends itself to algebraic problem solving with (simple) equations.

 

In order to be able to answer questions about electricity, we must first understand the terminology. Words like current, voltage, energy, power, resistance are familiar but often poorly understood.

 

Charge you may well know as an intrinsic property of some particles. There are two types of charge, named positive and negative because they negate each other. Static electricity involves the accumulation of charge. Current electricity involves the movement of charge. The vast majority of electrical scenarios involve the accumulation or movement of negatively charged electrons but some situations can involve positively or negatively charged ions.

 

Charge is measured in coulombs (C). Coulombs are actually rather large units of charge. The elemental charge on an electron is -1.6 x 10^-19 C (+1.6 x 10^-19 C on a proton). The symbol for charge is Q (which is short for the French word Quantité).

 

Current is the rate of flow of charge. In other words current is the charge transferred per unit time. Current is measured in amperes (A) or Amps for short. One ampere equals one coulomb per second. The symbol for current is I, from the French word Intensité.

 

Current = Charge ÷ Time (seconds) I = Q/t

Charge transferred = Current x Time Q = I x t

 

Voltage, more correctly called Potential Difference or P.d., is related to Energy. P.d. equals the energy transferred per unit of charge flowing. The unit of P.d. is volts (V). One volt equals one joule transferred per coulomb of charge flowing.

 

Potential Difference = Energy ÷ Charge

V = E/Q

 

Energy = P.d. x Charge

E = V x Q

 

Remember Work Done = Energy Transferred, so sometimes you might see:

 

Potential Difference = Work ÷ Charge

V = W/Q

 

Work = P.d. x Charge

W = V x Q

 

 

Power (in any application, not just electricity) is the rate of transfer of energy (or the rate at which work is done.) In other words, it is the energy transferred per unit time (or the work done per unit time.) Power is measured in watts (W). One watt equals one joule of energy transferred/work done per second.

 

Power = Energy ÷ time

P = E/t

 

Energy = Power x time

E = P x t

 

Power = Work ÷ time

P = W/t

 

Work = Power x time

W = P x t

 

For electrical applications we can show that power equals current times p.d.

 

I x V = (Q/t) x (E/Q) = EQ/Qt = E/t = Power

 

Power = Current x P.d.

P = IV

 

The resistance of a component is the ratio of the p.d. across it to the current flowing through it. Materials with zero resistance are called superconductors. Resistance is measured in ohms (Ω)

 

Resistance = P.d. ÷ Current

R = V/I

 

We can substitute this into the previous equation for power to give us:

P = IV = I²R = V²/R