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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%