Date post: | 16-Dec-2015 |
Category: |
Documents |
Upload: | georgia-golden |
View: | 214 times |
Download: | 1 times |
Define the current.
Understand the microscopic description of current.
Discuss the rat at which the power transfer to a device in an electric current.
212-11-2014FCI
2-1 Electric current
2-2 Resistance and Ohm’s Law
2-3 Current density, conductivity and
resistivity
2-4 Electrical Energy and Power
312-11-2014FCI
The resistance of an ohmic conductor is proportional to its length, L, and inversely proportional to its cross-sectional area, A
◦ ρ is the constant of proportionality and is called the resistivity of the material
4
LR
A
12-11-2014FCI
For most metals, resistivity increases with increasing temperature
◦With a higher temperature, the metal’s constituent atoms vibrate with increasing amplitude
◦The electrons find it more difficult to pass through the atoms
512-11-2014FCI
For most metals, resistivity increases approximately linearly with temperature over a limited temperature range
◦ ρ is the resistivity at some temperature T◦ ρo is the resistivity at some reference
temperature To To is usually taken to be 20° C is the temperature coefficient of
resistivity
6
)]TT(1[ oo
12-11-2014FCI
Since the resistance of a conductor with uniform cross sectional area is proportional to the resistivity, you can find the effect of temperature on resistance
7
)]TT(1[RR oo
12-11-2014FCI
A class of materials and compounds whose resistances fall to virtually zero below a certain temperature, TC
◦ TC is called the critical temperature
The graph is the same as a normal metal above TC, but suddenly drops to zero at TC
12-11-2014 9FCI
In a circuit, as a charge moves through the battery, the electrical potential energy of the system is increased by ΔQΔV
As the charge moves through a resistor, it loses this potential energy during collisions with atoms in the resistor◦The temperature of the resistor will
increase
1012-11-2014FCI
Consider the circuit shown
Imagine a quantity of positive charge, Q, moving around the circuit from point A back to point A
1112-11-2014FCI
Point A is the reference point◦ It is grounded and its potential
is taken to be zero
As the charge moves through the battery from A to B, the potential energy of the system increases by QV◦ The chemical energy of the
battery decreases by the same amount
1212-11-2014FCI
As the charge moves through the resistor, from C to D, it loses energy in collisions with the atoms of the resistor
The energy is transferred to internal energy
13
When the charge returns to A, the net result is that some chemical energy of the battery has been delivered to the resistor and caused its temperature to rise
12-11-2014FCI
The rate at which the energy is lost is the power
From Ohm’s Law, alternate forms of power are
14
QV I V
t
22 V
I RR
12-11-2014FCI
The SI unit of power is Watt (W)◦“I “ must be in “Amperes”,” R” in ohms
and “V’ in Volts
The unit of energy used by electric companies is the kilowatt-hour◦This is defined in terms of the unit of
power and the amount of time it is supplied
◦1 kWh = 3.60 x 106 J
1512-11-2014FCI
The same potential difference is applied to the two
lightbulbs shown in Figure .Which one of the following statements is true?
(a) The 30-W bulb carries the greater current and has the higher resistance.
(b) The 30-W bulb carries the greater current, but the 60-W bulb has the higher resistance.
1612-11-2014FCI
(c) The 30-W bulb has the higher resistance, but the 60-W bulb carries the greater current.
(d) The 60-W bulb carries the greater current and has the higher resistance.
1712-11-2014FCI
(c). Because the potential difference ∆V is the
same across the two bulbs and because the
power delivered to a conductor is P= I V, the 60-
W bulb, with its higher power rating, must carry
the greater current.
The 30-W bulb has the higher resistance because
it draws less current at the same potential
difference.
1812-11-2014FCI
1. The electric current “I “ in a conductor is defined as
where dQ is the charge that passes through a cross section of the conductor in a time interval dt. The SI unit of current is the ampere (A), where 1 A = 1 C/s.
The average current in a conductor is related to the motion of the charge carriers through the relationship
21
where n is the density of charge carriers, q is the charge on each carrier, vd is the drift speed, and A is the cross-
sectional area of the conductor.12-11-2014FCI
2. The current density in an ohmic conductor is proportional to the electric field according to the expression
22
The proportionality constant σ is called the conductivity of the
material of which the conductor is made. The inverse of & is
known as resistivity ρ (that is, ρ = 1/ σ).
The last equation is known as Ohm’s law, and a material is said
to obey this law if the ratio of its current density J to its applied
electric field E is a constant that is independent of
the applied field.
12-11-2014FCI
23
3. The resistance R of a conductor is defined as
where ∆V is the potential difference across it, and I is the
current it carries. The SI unit of resistance is volts per
ampere, which is defined to be 1 ohm; that is, 1Ω = 1 V/A.
If the resistance is independent of the applied potential
difference, the conductor obeys Ohm’s law.
12-11-2014FCI
4. If a potential difference ∆V is maintained across a circuit element, the power, or rate at which energy is supplied to the element, is
24
Because the potential difference across a resistor is given by ∆V = IR, we can express the power delivered to a resistor in the form
The energy delivered to a resistor by electrical transmission appears in the form of internal energy in the resistor.
12-11-2014FCI
1. The charge carriers in metals areA. electrons.B. positrons.C. protons.D. a mix of protons and electrons.
12-11-2014FCI 25
2. A battery is connected to a resistor. Increasing the resistance of the resistor
will
A. increase the current in the circuit. B. decrease the current in the circuit. C. not affect the current in the circuit.
12-11-2014FCI 26