Chapter 2 Section 3

Work in Electrical Systems

Objectives

• Explain the relationship between work, charge and potential difference (voltage).

• Calculate electrical charge in a circuit.

• Define units of electric current.

• Solve problems in electrical work.

• Identify effects of electrical work.

• Explain efficiency in terms of work in and work out in electrical systems.

Coulombs

• The unit of electric charge is the coulomb (C).

• The fundamental charge on an electron is 1.6 X 10-19 C.

• 1 C = 1/ 1.6 X 10-19 = 6.25 X 1018 electrons or 6,250,000,000,000,000,000 electrons.

Work in electrical systems

• The electric field is given by E = F / q (Sec 1.3). Thus F=Eq.

• Potential difference (voltage) is given by V=Ed (Sec 1.3)

• W = Fd = Eqd• Thus W = Vq

Work - cont

• Thus Work = potential difference X charge moved

• W=Vq

• The work needed to move one coulomb of charge though a potential difference of one volt is one joule.

• 1J = 1 V . C

• 1V = 1 J/C

Electric Charge & Current

• Electric current is the rate at which charge flows through a circuit.

• Current = charge / time; I = q / t

• Current is measured in amperes or amps (A)

• 1 amp = 1 coulomb per second; 1A = 1 C/s

Efficiency

• Efficiency = work out / work in; Wout / Win

• % Efficiency = (Wout / Win) x 100%

• Efficiency can never be equal to or greater than 100%

• Work out will always be less than work in.

Summary

• Electric work is done when a charge is moved through a potential difference.

• W = Vq

• 1 joule = 1 volt X 1 coulomb; 1J = 1V.C

• Current is the rate at which charge flows.

• 1 Amp = 1 coulomb per second; 1A=1C/s.

• % Efficiency = (Wout / Win) x 100%