Download Electric current and circuits

Electric current and direct-current
circuits
A flow of electric charge is
called an electric current.
Electric current and direct-current
circuits
Q
I
t
Electric current and direct-current
circuits
1C
SI unit  ampere =
A
s
Electric current and direct-current
circuits
When electric charge flows
through a closed path and
returns to its starting point
the path is called an electric
circuit.
Electric current and direct-current
circuits
When electric charge flows
through a closed path in one
direction the path is called a
direct-current circuit.
Electric current and direct-current
circuits
When electric charge flows
through a closed path and
periodically reverses direction
the path is called a
alternating-current circuit.
Electric current and direct-current
circuits
A battery produces a
difference in electric potential
between its terminals
through chemical reactions.
Electric current and direct-current
circuits
The symbol for a battery is
Electric current and direct-current
circuits
The terminal designated +
corresponds to the higher potential,
while the terminal designated by a –
corresponds to the lower potential.
Electric current and direct-current
circuits
By convention we say that the
direction of the current is the
direction in which a positive charge
would move.
Figure 21-4
Direction of Current and Electron Flow
Electric current and direct-current
circuits
The electromotive force (emf)
(ξ) is the potential across the
terminals (voltage) of a battery
under ideal conditions.
Electric current and direct-current
circuits
The charges that actually move
through a conductor, are electrons.
Electric current and direct-current
circuits
In a real conductor there is always
some resistance to electron flow,
and a potential difference is
necessary to keep them flowing.
Electric current and direct-current
circuits
Ohm’s Law relates the potential(V),
resistance (R)and current (I)in a
circuit
Electric current and direct-current
circuits
Ohm’s Law
V  IR
Electric current and direct-current
circuits
Ohm’s Law
V 1V
R    1 = 1ohm
I A
Electric current and direct-current
circuits
Ohm’s Law
L
R   
 A
Electric current and direct-current
circuits
Ohm’s Law
Unit for resistivity is
m
Electric current and direct-current
circuits
When an electric charge moves
across a potential difference the
potential energy changes by the
amount
U  (Q)V
Electric current and direct-current
circuits
U (Q)V
power  P 

 IV
t
t
SI unit; watt, W
Electric current and direct-current
circuits
Other expressions for electric
power
2
V
PI R
R
2
Electric current and direct-current
circuits
Other expressions for electric
power
2
V
PI R
R
2
Electric current and direct-current
circuits
Resistors in a series are
connected end to end.
Example 21-5
Three Resistors in Series
Electric current and direct-current
circuits
The equivalent resistance for
resistors in series is just the
sum of the individual
resistances
Electric current and direct-current
circuits
For the example given
Req  R1  R2  R3
Electric current and direct-current
circuits
Each of the resistors
connected in series has the
same current going through it.
Electric current and direct-current
circuits
Resistors connected in
parallel are connected across
the same potential difference.
Example 21-6
Three Resistors in Parallel
Electric current and direct-current
circuits
The equivalent resistance for
resistors in parallel is
calculated by adding the
reciprocal values of the
individual resistors.
Electric current and direct-current
circuits
This gives the reciprocal of
the equivalent resistance
Electric current and direct-current
circuits
For the example given
1
1 1
1
  
Req R1 R2 R3
Electric current and direct-current
circuits
The current going through
individual resistors connected
in parallel is not necessarily
the same.
Electric current and direct-current
circuits
The sum of the currents will
be equal to the current
calculated for the individual
resistors.
Electric current and direct-current
circuits
For circuits that contain
resistors connected both in
series and in parallel, we first
calculate the equivalent
resistances.
Electric current and direct-current
circuits
We then treat the result as if it
were just another resistor in
series. Ex.21-7 on page 693.
Example 21-7
Combination Special
Electric current and direct-current
circuits
The sum of the voltage drops
in a circuit must be equal to
the voltage applied to the
circuit.
Figure 21-16
Capacitors in Parallel
Electric current and direct-current
circuits
The equivalent capacitance
for capacitors in parallel is
just the sum of the individual
capacitances
Electric current and direct-current
circuits
For the example given
Ceq  C1  C2  C3
Electric current and direct-current
circuits
The sum of the individual
charges on the capacitors is
equal to the charge on the
equivalent capacitor.
Figure 21-17
Capacitors in Series
Electric current and direct-current
circuits
The equivalent resistance for
capacitors in series is
calculated by adding the
reciprocal values of the
individual capacitors.
Electric current and direct-current
circuits
This gives the reciprocal of
the equivalent capacitance.
Electric current and direct-current
circuits
For the example given
1
1
1
1
 

Ceq C1 C2 C3
Electric current and direct-current
circuits
Active example 21-3 p 700.
Electric current and direct-current
circuits
Kirchoff’s rules
1. The sum of the currents entering a
junction, must equal the sum of the
currents leaving that junction (result
of charge conservation).
Electric current and direct-current
circuits
Kirchoff’s rules
2. The algebraic sum of the potential
differences around a closed loop is zero. The
potential increases in going from the negative
to the positive terminal of a battery, and
decreases when crossing a resistor in the
direction of the current. (energy
conservation).
Electric current and direct-current
circuits
Batteries – all non-ideal batteries
have an internal resistance. The
voltage measured across the
terminals of a battery will be less
with current flowing than without
current flowing.
Electric current and direct-current
circuits
Ammeters are connected in series
with the part of the circuit being
tested. The ideal resistance of an
ammeter is 0 .
Electric current and direct-current
circuits
Voltmeters are connected in series
with the part of the circuit being
tested. The ideal resistance of a
voltmeter is ∞.