Chapter 24: Electric Current. Definition of current. A current is any motion of charge from one region to another. Suppose a group of charges move perpendicular to surface of area A. The current is the rate that charge flows through this area:. Current. Units: 1 A = 1 ampere = 1 C/s.
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A current is any motion of charge from one region to another.
Current
Units: 1 A = 1 ampere = 1 C/s
Current
Microscopic view of current (cont’d)
Current
Current per unit area
Units: A/m2
Vector current density
V/A ohm
Ohm’s law
Resistivity
Substance
r (Wm)
Substance
r (Wm)
silver
graphite
silicon
copper
glass
gold
teflon
steel
Conductors, semiconductors and insulators
In a metal the free electrons that carry charge in electrical conduction also
provide the principal mechanism for heat conduction.
Resistivity
reference temp. (often 0 oC)
temperature coefficient of resistivity
Resistivity
Material
Material
a (oC)1
a (oC)1
aluminum
iron
lead
brass
manganin
graphite
silver
copper
Resistivity
r
r
r
T
T
T
Tc
semiconductor
superconductor
metal
Resistance
resistance
1 V/A=1W
I
A
L
Resistance
I
I
V
V
semiconductor
diode
resistor that obeys
Ohm’s law
Example: Calculating resistance
b
a
Resistance
r
A
Complete circuit and steady current
+
+
+
+
+
+
+
+
I

I
I







Electromotive Force (emf) and Circuit
Maintaining a steady current and electromotive force
Units: 1 V = 1 J/C
Electromotive Force (emf) and Circuit
source of emf
b
a

+
current flow
Vab=e– Ir (terminal voltage, source with internal resistance r)
e–Ir = IR or I = e/ (R + r)
Electromotive Force (emf) and Circuit
R
c
d
c
d
I
Battery
r
a
b
b
a
+
−
Electromotive Force (emf) and Circuit
Potential in a resistor circuit in realistic situation
c
d
R
I
Battery
r
b
a
b
a

+
Electromotive Force (emf) and Circuit
r
V
R

+
e
I r
IR
0
d
c
b
a
a
b
Energy and Power in Electric Circuit
dUe is electrical potential energy lost as dQ traverses the resistor and falls in V by ΔV.
R
Electric power = rate of supply from Ue.

+
Energy and Power in Electric Circuit
battery
(source)
net electrical
power output
of the source
I
I
rate of electrical
energy dissipation
in the internal
resistance of the
source
a +
b 
rate of conversion of
nonelectrical energy
to electrical energy in
the source
headlight
(external circuit)
total electrical power input
to the battery

+
battery
small emf
I
rate of conversion of
electrical energy into
noneletrical energy in
the battery
rate of dissipation
of energy in the
internal resistance
in the battery
I
Energy and Power in Electric Circuit
Fn
a +
b 
+
v
alternator
large emf
A
ammeter
a
b
A
V
: measures current
through it
: measures pot
ential difference
Electromotive Force (emf) and Circuit
voltmeter
V
Electric Conduction
Electric Conduction
Electric Conduction
To find the resistance at 1000 C:
But
so we have :
Exercise 1
Calculate the resistance of a coil of platinum wire with diameter 0.5 mm and length 20 m at 20 C given =11×10−8 m. Also determine the resistance at 1000C, given that for platinum =3.93×10−3 /°C.
Where we have assumed l and A are independent of temperature 
could cause an error of about 1% in the resistance change.
A 1000 W hair dryer manufactured in the USA operates on a 120 V source. Determine the resistance of the hair dryer, and the current it draws.
The hair dryer is taken to the UK where it is turned on with a 240 V source.
What happens?
This is four times the hair dryer’s power rating – BANG and SMOKE!