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Current of Electricity
Electric Current
Potential Difference
Resistance and Resistivity
Electromotive Force
What is electric current?
Rate of flow of electrical charges
I = Q/t
Unit: Cs-1
̴Ampere, A
Definition of Coulomb
Q = It
A coulomb is the amount of charge that in
one second crosses a section of a circuit in
which there is a current of one ampere.
Example 1
A fully charge 12-V battery can supply a
total charge of 100kC. The starter motor
of a car requires 200A for an average
period of 2.0 s. The battery does not
recharge because of a fault.
What is the maximum number of times
the starter motor of the car can be used?
Potential Difference
Energy that is used in the
circuit per unit charge
passing through from one
point to another
Energy used by resistors
Potential Difference
Energy converted per
unit charge
Power converted per unit
Units: JC-1, WA-1
̴Volt, V
Potential difference divided by current
Units: VA-1
̴Ohm, Ω
Ohm’s Law
The current through a conductor
is proportional to the potential difference
across it
Provided its temperature remains
Resistivity, ρ
Electrical property of a material
Conducting ability of different materials
R α Length of wire
R α 1/Cross-sectional Area of wire
(See p. 248, Table 14.2)
Example 1
A cylindrical wire 4.0 m long has a
resistance of 31 Ω and is made of metal
of resistivity 1.0 x 10-6 Ωm.
What is the radius of the cross-section of
the wire?
Example 2
Two wires P and Q made of the same
material and of the same length are
connected in parallel to the same voltage
supply. Wire P has diameter 2 mm and
wire Q has diameter 1 mm.
What is the ratio
Electromotive Force (emf)
Energy that is supplied in the circuit per
unit charge passing through from one
point to another
Energy from battery or power supply
Internal Resistance, r
Energy lost per unit charge, Ir
Energy supplied per unit charge, ε - Ir
Example 1
The emf of the battery is 9.0 V. The
reading on the voltmeter is 7.5 V.
What is the current I?
Electrical Power
Energy per unit time
P = VI and derived equations
Unit: Js-1
̴ Watt, W
Example 1
Two heating coils X and Y, of resistance
Rx and Ry respectively, deliver the same
power when 12 V is applied across X and
6 V is applied across Y.
What is the ratio Rx/Ry?
Example 2
Which circuit has greater pd across the
Which circuit has greater power
dissipation in the resistor?
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