Transcript Electric charges in motion

Electric charges in motion
CHAPTER 20
Continuos flow of charges:
electric current
 Flow of charges between two regions with opposite net charges
 electrons (negative charges) flow from the lower electrical
potential points to ones with higher electric potential
 positive charges flow from points with higher electric potential
to points with lower electric potential
 result of flow --> annihilation of charges
 to maintain a constant flow, one must maintain a constant
potential difference: constant tension
 use of a Van der Graff generator (we invest mechanical energy
to get electric energy)
 use of batteries (we invest chemical energy)
Batteries
 Dissimilar metals in contact (or in an
electrolyte) produce an electric potential
difference
 Volta’s battery: Silver, Zinc and paper
soaked in a salt solution
 batteries are composed of many cells
(many layers)
 one cell’s potential difference depends on
the choice of the metals
two ways to connect cells:
 I. In series
-resulting voltage (tension) is the sum of
the cells tension
 II. In parallel
- does not increase the voltage, increase
the effective size of the battery--> will last
longer
 batteries that can be recharged: storage
batteries (car battery)
 non-rechargeable batteries: dry cells
AC and DC currents
 Currents produced by batteries: direct currents (DC)
Characteristics:
- direction of the flow of positive and negative charges does not
change in time
- direction of current (direction of flow for positive charges) is
constant in time
- potential difference (voltage) between two points of the circuit
does not change sign in time
 Household electricity-->produced by generators: alternating
current (AC)
Characteristics:
- direction of the current reverses periodically in time
- voltage (tension) between two points of the circuit changes sign
periodically in time
- 60 cycle alternating current: current reverses direction 120 times a
second (two times during one cycle)
Electric circuits
 Composed by
- voltage source
- conducting wires
- useful elements like:
bulbs, motors, etc…
 In order to have current we have
to make a complete (closed)
cycle
 the ends of the useful elements
should be connected by wires to
the poles of the batteries
 current must be able to flow
from one pole to the other
(through the useful elements)
Water model for electric circuits
 Battery --> pump; wires--> pipes; useful elements --> a paddle
wheel that turns when the water is flowing
 important electric quantities: current, charge and voltage
 important quantities for water flow: flow rate, water, height
 current --> flow rate; charge--> water; voltage--> height
 flow rate: water quantity/time; electric current: charge/time I
 units for electric current: C/s--> Ampere (A)
Q

t
 DIFFERENCES: break in the electric circuit stops the
current (not in the water model); in electric circuits both type
of charges can move in opposite direction (water flows in one
direction only)
Electrical Resistance
 Characterizes the resistance of the materials against the flow of
electric current
 if the resistance of an electric circuit is small the current intensity will
be high (water model: large diameter and short length pipes)
 if the resistance of an electric circuit is high, the current intensity will
be low (water model: small diameter and long pipes)
V
 Electrical resistance (R) of a circuit defined as:
R
I
Ohm’s law
V: voltage at the sides of the circuit; I: electrical current intensity
 units for resistance: V/A --> Ohm ()
 electrical resistance of wires: proportional with length and inversely
proportional with diameter, depends strongly on material properties
and temperature (resistivity: resistance of a wire with unit length and
diameter)
 materials with high resitivity: resistors
 materials with low resistivity: conductors
 Ohm’s law can be applied for each tinny portion of an electrical
Superconductivity
 In normal state all conductors exhibit resistance to the passage of
electric current
 The electrical resistance of some materials however goes to zero
at very low temperatures --> they become superconductors
 discovered in 1911 by Kamerlingh Onnes
 normal superconductivity in simple metals --> 3-10 0K (-270 0C)
(liquid Helium temperature --> not suitable for practical
applications)
 superconductivity in some alloys and compounds up to 32-39 0K
 superconductivity at low temperatures explained by the BCS
model
 Hit in superconductivity 1986 in : High Tc superconductors
(ceramics) become superconductors at temperatures as high as
125 0K (liquid nitrogen temperature --> suitable for practical
applications) --> no accepted theory for them
 problems today: low critical current, making wires, raising more Tc
 important property of superconductors: expel magnetic field
(possible magnetic levitation)
Basic electric circuit types
 Short circuit --> creating a loop
between two points with very small
resistance (current will flow in this
loop)
 Series circuit--> connecting more
useful elements so that there is only a
single path from one pole of the
battery to the other (current flowing
through each element is the same)
Rtotal  R1  R2  ...
 Parallel circuit--> each useful element
has its own path from one end of the
battery to the other (voltage on each
useful element the same, closing the
current on one element will not stop
the current on others)
1
Rtotal

1
1

 ...
R1 R2
 Combined circuits (serial and
parallel, many possible
configurations)
 Household circuits (most
elements are connected in
parallel)
 Fuses: elements that will interrupt
the current if this exceeds a given
value
 Drawing electrical circuits:
symbols for different circuit
elements
Electric power
 We buy electric energy (E)
 Power (P): is the amount of electric energy used per unit time
 unit for Power: J/s--> Watts (W)
E
 electric devices in the house are rated by their power usage P 
t
light bulbs:60, 75 or 100 W; washer: >2000W etc….
 Electric power through a portion of a circuit can be calculated as:
2
V
P  V  I  I 2R 
R
 V: the voltage between the sides, I the current which is flowing, R
the resistance:
 The amount of used electric energy: E=P.t (can be measured in J,
or in kWh (kilowatt-hour)-- more convenient
 1 kWh=3.6 x 106 J
 minimizing electric energy loses in the wires: (making R or I small)
 sending the same amount of energy with small I (increasing V-->
transformers)
Summary
 Electric current: continuos flow of electric charges (need an electric current
source: battery or generator)
 two types of currents direct (DC) and alternative (AC)
 electric charges can flow continuously only when there is a complete circuit
 electric current: amount of charge flowing through the wire per unit time
 the voltage between two points is the electric potential difference
 the resistance of the circuit determines the electric current (units: )
 Ohm’s law: V=IR; R characterizes the wires and useful elements.
 Most charges flow through the path with the least resistance
 Charge conservation implies current conservation in electric circuits
 the resistance of wires increases with increasing length, decreases with
increasing cross-section and depends on the material type and temperature
 basic circuits: serial and parallel or combined ones
 power is the rate of using energy, Energy=Power x time
 power can be measured in Watts, household energy is measured in kWh
Home-Work Assignments:
Part I:514/2-5,7-10,14-16; 515/17-32; 517/1,2,5-12
Part II: 515/33-40; 516/41-50; 517/53-60; 517/13-17; 518/18-26