Download Electricity Lecture 2

Electrical Current
Electrical current (measured in
amps) is the rate of flow of charge.
Electrical current is a flow of electric
charge through a medium.
This charge is typically carried by
moving electrons in a conductor
such as wire, or…
It can also be carried by ions in an
electrolyte, or by both ions and
electrons in plasma.
Electrical Current
What is 1 ampere? (a.k.a. 1 amp?)
It is 1 coulomb of charge passing
through a wire cross-section over 1
second. How many electrons?
It is about 6.2x1018 electrons passing
a cross-section during one second
Current I=Q/t
1 A = 1 C/s
The unit is the
ampere (A).
Electrical Current
Electrical current in amps is the
rate of flow of charge.
Conventional d.c. current flows
out of the positive terminal of a
battery, around a circuit, and
into the negative terminal.
In metals, the charge carriers
are electrons, which move in
the opposite direction.
Modelling current (using people!)
This shows:
Purpose of current is
to move energy from
a supply to a user
Current keeps going.
Current is NOT used
up in components.
Electrical Current
Current is measured using an ammeter
- a device used in series that does not
impede current flow but which has a
deflection proportional to current.
Current I=Q/t
The unit is the
ampere (A).
1 A = 1 C/s
Quick experiment
Prove to yourself that current in a loop circuit is the
same everywhere.
Potential Difference
Potential difference (p.d.) in volts
is the “driving force” that makes
current flow around a circuit.
Quite similar to a gravitational
potential difference of a waterfall
Potential
difference is V.
The units is the
volt (V).
1 V=1 J/C
Potential Difference
Potential
difference
has
symbol V.
The units
are volts
(V).
1 V=1 J/C
V = W/Q
It is the energy delivered to
the circuit per unit charge.
Often called “voltage”.
The voltage generated by
the chemical reactions in a
battery is called the
electromotive force
(e.m.f.).
Potential Difference
Potential difference is
measured using a
voltmeter, which goes in
parallel with the circuit.
Potential
difference has
symbol V.
The units are
volts (V).
1 V=1 J/C
V = W/Q
V
Visual models of pd and emf:
(1) A simple circuit
Visual models of pd and emf:
(2) A series circuit
Visual models of pd and emf:
(3) A parallel circuit
Visual models of pd and emf:
(4) A more complex circuit
Sketch these! Then try Q1
Electrical Resistance
Symbol - In electric circuits resistors is usually shown as:
All conductors offer
resistance to the
flow of electrical
current.
Electrical Resistance
An electrical component
specifically made to
limit current is called a
resistor.
Resistance is the pd
required for unit current
to flow.
If V is proportional to I,
R is constant: Ohms
law.
Resistance
symbol R.
The units are
ohms (W).
V=IR
Electrical Power
How to estimate how
much electrical power
was used or delivered?
We know the voltage
supplied and the current
used. What about power?
Electrical Power
Electrical power is the
energy delivered per unit
time in a circuit.
The power is the voltage
times the current. The unit is
the watt, W.
1 W = 1 J/s.
Power P=VI
Example: The iron uses 8 A.
What is its power?
Since the standard voltage
supply is 240V, then the
iron’s power is 8A x 240V =
1920W (Almost 2kW).
Electrical Power
Using equation P=VI
and Ohm’s law I=V/R
lets express power in
several different forms.
I=V/R
P=IV=(V/R)V=V2/R
Lets rearrange Ohm’s
law as V=IR
V=IR
P=IV=I(IR)=I2R
Power, P=VI
=I2R =V2/R
The unit is the
watt (W).
Electrical Power
In a resistor, the power delivered
manifests itself as heating caused
by electrons colliding with atoms
and giving them energy.
Power, P=VI
=I2R =V2/R
Why do we have huge
energy release in short
circuit? If voltage is fixed
then the power is inversely
proportional to the
resistance: P=V2/R.
The unit is the
watt (W).
Electrical Power
Electrical power is the
energy delivered per unit
time in a circuit.
The power is the voltage
times the current. The unit is
the watt, W.
1 W = 1 J/s.
Power P=VI
Since V=IR then the power
is
P
=I2R
=V2/R.
In a resistor, the power
delivered manifests itself as
heating caused by electrons
colliding with atoms and
giving them energy.
Power, P=VI
=I2R =V2/R
The unit is the
watt (W).
Try Qs 2, 3 and 4
Resistance and resistivity
Resistance R is a property
of a particular resistor.
Resistivity r is a property
of material of which the
resistor is made.
We can write:
R=rL/A
r is the resistivity in Wm
A cross-sectional area, L
length
Resistance: symbol
R.
R= rL/A
The units for
resistance are ohms
(W).
V=IR
Resistance and Resistivity
The unit of resistance is
the ohm (W).
What is the unit of
resistivity?
Let’s rearrange R=rL/A as
r=R A/L.
A is the cross sectional
area, thus, it is measured
in m2. L is the resistor
length, thus, it is measured
in m.
 Thus, the unit A/L is m,
while the resistance unit is
ohm.
 Thus, the resistivity units
are ohm m. (Ωm)
Resistance is R.
R= rL/A
The units for
resistance are
ohms (W).
The units for
resistivity are
ohm m
Ohm’s law: V=IR
Resistance and Resistivity
All conductors offer
resistance to the flow of
electrical current.
R=rL/A
r is the resistivity in Wm
A cross-sectional area, L
length
The unit of resistance is the
ohm (W).
An electrical component
specifically made to limit
current is called a resistor.
The current that a potential
difference drives through a
resistor is given by
I=V/R –ohms law.
Resistance is R.
R= rL/A
The units are
ohms (W).
V=IR