Transcript Current, Resistance, and Electromotive Force
Current, Resistance, and Electromotive Force
Physics 231 Lecture 5-1 Fall 2008
Current
Current is the motion of any charge, positive or negative, from one point to another Current is defined to be the amount of charge that passes a given point in a given amount of time Current has units of
I
dQ dt Ampere
1
Coulomb
1 sec
Physics 231 Lecture 5-2 Fall 2008
Drift Velocity
Assume that an external electric field E has been established within a conductor Then any free charged particle in the conductor will experience a force given by
F
q E
The charged particle will experience frequent collisions, into random directions, with the particles compromising the bulk of the material There will however be a net overall motion Physics 231 Lecture 5-3 Fall 2008
Drift Velocity
There is net displacement given by v d
D
t where v d is known as the
drift velocity
Physics 231 Lecture 5-4 Fall 2008
Drift Velocity
Consider a conducting wire of cross sectional area A having n free charge-carrying particles per unit volume with each particle having a charge q with particle moving at v d The total charge moving past a given point is then given by
dQ
n q v d A dt
the current is then given by
I
dQ dt
n q v d A
Physics 231 Lecture 5-5 Fall 2008
Current Density
This equation
I
dQ dt
n q v d A
is still arbitrary because of the area still being in the equation We define the
current density J
to be
J
I A
n q v d
Physics 231 Lecture 5-6 Fall 2008
Current Density
Current density can also be defined to be a vector
J
n q v
d
Note that this vector definition gives the same direction for the current density whether we are using the positive or negative charges as the current carrier Physics 231 Lecture 5-7 Fall 2008
Resistivity
The current density in a wire is not only dependent upon the external electric field that is imposed but It is also dependent upon the material that is being used Ohm found that J is proportional to E and in an idealized situation it is directly proportional to E The resistivity is this proportionality constant and is given by
E J
The greater the resistivity for a given electric field, the smaller the current density Physics 231 Lecture 5-8 Fall 2008
Resistivity
The inverse of resistivity is defined to be the
conductivity
The resistivity of a material is temperature dependent with the resistivity increasing as the temperature increases This is due to the increased vibrational motion of the atoms the make up the lattice further inhibiting the motion of the charge carriers The relationship between the resistivity and temperature is given approximately by
0 1
T
T
0
Physics 231 Lecture 5-9 Fall 2008
Resistivity
Let us take a length of conductor having a certain resistivity We have that
E
J
But E and the length of the wire, L, are related to potential difference across the wire by
V
E L
We also have that
J
I A
Putting this all together, we then have
V L
I A
or
V
A L I
Physics 231 Lecture 5-10 Fall 2008
Resistance
We take the last equation
V
A L I
and rewrite it as with
R
A L V
I R
being the resistance The resistance is proportional to the length of the material and inversely proportional to cross sectional area
V
I R
is often referred to as Ohm’s Law The unit for R is the
ohm
or
Volt / Ampere
Physics 231 Lecture 5-11 Fall 2008
Example
Two cylindrical resistors, R 1 of identical material. R 2 and R 2 , are made has twice the length of
R
1 but half the radius of R 1 . These resistors are then connected to a battery V as shown:
V I
1
I
2 What is the relation between I 1 , the current flowing in R 1 , and I 2 , the current flowing in R 2 ?
(a)
I
1 < I 2 (b)
I
1 = I 2 (c)
I
1 > I 2 The resistivity of both resistors is the same (r). Therefore the resistances are related as:
R
2
L
2
A
2 ( 2
A
1
L
/ 1 4 ) 8
L
1
A
1
The resistors have the same voltage across them; therefore
8
R
1
I
2
V R
2
V
8
R
1 1 8
I
1
Physics 231 Lecture 5-12 Fall 2008
Resistance
Because the resistivity is temperature dependent, so is the resistance
R
R
0 1
T
T
0
This relationship really only holds if the the length and the cross sectional area of the material being used does not appreciably change with temperature Physics 231 Lecture 5-13 Fall 2008
Electromotive Force
A steady current will exist in a conductor only if it is part of a complete circuit For an isolated conductor that has an external field impressed on it Physics 231 Lecture 5-14 Fall 2008
Electromotive Force
To maintain a steady current in an external circuit we require the use of a source that supplies electrical energy Whereas in the external circuit the current flows from higher potential to lower potential, in this source the current must flow from lower potential to higher potential, even though the electrostatic force within the source is in fact trying to do the opposite In order to do this we must have an
electromotive force, emf,
within such a source The unit for emf is also Volt Physics 231 Lecture 5-15 Fall 2008
Electromotive Force
Ideally, such a source would have a constant potential difference,
e
, between its terminals regardless of current Real sources of emf have an internal resistance which has to be taken into account The potential difference across the terminals of the source is then given by
V ab
e
I r
internal
Physics 231 Lecture 5-16 Fall 2008
Energy
As a charge “moves” through a circuit, work is done that is equal to
qV ab
This work does not result in an increase in the kinetic energy of the charge, because of the collisions that occur Instead, this energy is transferred to the circuit or circuit element within the complete circuit Physics 231 Lecture 5-17 Fall 2008
Power
We usually are not interested in the amount of work done but in the rate at which work is done This given by
P
V ab I
If we have a pure resistance, we also have from before that
V ab
I R
giving us the additional relations
P
I
2
R
2
V ab R
Physics 231 Lecture 5-18 Fall 2008