Download As Unit 1 - School

As Unit 1 - Electricity
What you need to know.
 Current is the rate of flow of charged
particles.
 In metals these are conduction electrons,
most electrons are attached to atoms but
some are not. When a voltage is applied
these electrons are attracted to the +ve
terminal.
 When an electric current is passed
through a salt solution the charge is
carried by ions.
 Unit for Current = Ampere (A) Symbol I
= Coulomb per second
 Unit for Charge = Coulomb (C) Symbol
Q
 (The charge flow in 1 sec when the
current is 1A)
Charge on an electron = 1.6 x 10-19C
Conventional Current
+ve to -ve
This convention is kept even though we now know
that the charge carriers (electrons) are actually
flowing in the opposite direction.
 Insulators - each electron is attached to an
atom and cannot move even when a
voltage is applied.
 Semi Conductor - number of charge
carriers increases with temperature.
Resistance therefore decreases as
temperature increases.
Potential Difference and Power
 Electrons are supplied with Electrical Potential
Energy (EPE) by a battery.
 Electrons do work to pass through a component,
using some or all of its EPE.
 The work done per unit charge is defined as the
POTENTIAL DIFFERENCE or p.d.
EMF
 EMF ( ε) of a source is defined as the electrical
energy produced per unit charge passing
through the source. The unit is also the volt.
Electrical energy produced when charge Q passes
through = W = Qε (Joules)
This energy can be transferred in components with
resistance, as heat energy ( electric heater), due
to charge carriers colliding with atoms, causing
them to vibrate and heat up.
Electric Power
 Current I, time Δt, p.d across component V
 Charge flowing through a device = Q = IΔt
 Work done (Energy) = QV = IΔt V
 Power = Energy = IV so P=IV
time
 Unit = Watts or Joules/Second (W or Js-1)
 Energy Used = Power x Time
Resistance
 Ohms law states that the p.d. across a metallic
conductor is proportional to the current through it
provided the physical conditions do not change.
 R = V/I = p.d across the component
the current through it
 A resistor is a component that is designed to
have a certain resistance regardless of the
current through it.
Measuring Resistance
Plot a graph of p.d. y axis vs
current x axis.
Line of best fit. Gradient = R
Resistivity
Super Conductors: used for power cables
and electromagnets
 A material that has
zero resistivity,
therefore resistance,
at and below a critical
temperature, that
depends on the
material.
 So when a current
passes through there
is no p.d and no
heating effect.
Highest Tc 150K (-123oC)
Tc above 77K (-196oC) are called
high temperature superconductors.
 Resistance and Temperature
 Metals have a positive temperature
coefficient as its resistance increases with
temperature, due to the vibration of the
metal lattice (+ve ions).
 Semiconductors have a negative
temperature coefficient as its resistance
decreases with temperature, due to the
increase of charge carriers.
I is constant (same everywhere).
Component voltages adds up to supply V0 = V1 + V2 + V3
so IRT = IR1 + IR2 + IR3
RT = R1 + R2 + R3
Total resistance
Voltage is constant V0 = V1 = V2
Current Splits so I0 = I1 + I2 (as I=V/R) then
V0/RT = V1/R1 + V2/R2 divide through by V
Gives 1/RT = 1/R1 + 1/R2
Using Resistance to Heat
Power supplied = IV
Gives P = I2R
and pd V= IR
= V2/R
Energy = Power x Time
If the component heats up its temperature
rise depends on the power supplied and
heat transfer to the surroundings.
Emf and Internal Resistance
 The pd across the
terminals, V, is the
electrical energy
produced per unit
charge by the source
when it is in a circuit
with resistance R.
 This will be less than
the Emf of the source
(unless otherwise
stated) due to the
internal resistance (r)
of the source.
RT = R + r and V = IR
Power = IV
Measuring Internal resistance
Measure I and V
Plot V vs I
When I = 0
y = mx + c
V vs I
means that gradient = -r
Y intercept =
More Complicated Circuits
1. Sketch the circuit if not drawn
2. Calculate cell current.
Cell current =
cell emf
total circuit resistance (inc
internal)
3. Work out current and so pd across resistors in
series with cell first (I is constant) using V = IR
4. Work out current through any parallel resistors
Current through each resistor = pd across parallel
resistance
Cells in series
 Cells in same direction: total emf is sum of individual
emfs.
 Cells connected oppositely net emf is the difference
between the individual emfs.
 Total internal resistances are the sum of the individual
internal resistances regardless.
Cells in parallel
 Current through each
= I/n
 pd lost in each = (I/n)
x r = Ir/n
The Potential Divider
Uses:
 To supply a variable
pd from 0 to V0
 To supply a pd that
varies with physical
condition; light,
temperature,
pressure etc.
The ratio of the pds across
each resistor is equal to
the the ratio of the
resistances
 Volume or light control:
 An audio signal is
supplied to the potential
divider, the variable
output from the potential
divider is supplied to the
loudspeaker.
Temperature
Sensor

At constant temperature the pd is shared
between the variable resisitor and thermistor.

By adjusting the variable resistor the pd across
the thermistor can be set at any desired value.

When the temperature changes the reistance
will change so then the pd across it will change.

If the temperature rises the resistance of the
thermistor will drop, and the pd will drop.
Light Sensor
 When light intensity
increases the
resistance of the LDR
falls and so the pd
across it falls.
Alternating Current and Power
An alternating current is one in which the current repeatedly reverses
its direction over time. (Sinusoidal)
 Frequency (f) is the number of cycles per second. (Hz or
S-1)
 Mains = 50Hz (each cycle takes 0.020s)
 Peak value is the maximum pd V0 or
current I0, and depends on the peak pd or
current of the source and the components
within the circuit.
 In mains circuits this peak value is 325V
Alternating Current and Power
P = I2R when I is negative, I2 will still be positive
At Peak current I0 maximum power is I02R
At zero current, zero power is supplied.
Power varies from 0 to I02R.
So the mean Power is 1/2 x I02R
The direct current that would give the same power
as the mean power is called the ROOT MEAN
SQUARE value of the alternating current = I rms
Using an Oscilloscope
 An oscilloscope consists of an electron tube and control
circuits.
 An electron gun fires electrons towards a screen that
fluoresces when electrons hit it.
 A pair of electric plates can be used to deflect the beam
horizontally and vertically.
 The displacement is proportional to the applied p.d.
http://www.virtual-oscilloscope.com/simulation.html#
X-Scale (time base)
 Vertical plates move the beam in the x direction.
 These are connected to the TIME BASE CIRCUIT moving the spot at a
constant speed left to right.
 The x-scale can be calibrated usually in milli or micro seconds per cm.
Y-Scale (y-sensitivity/gain)
 Horizontal plates move the beam in the y direction.
 The p.d. to be displaced is connected to these plates, so the spot moves up
and down, and if the beam is moving left to right, a wave form will be
created.
 The Y-input is calibrated in volts per cm Vcm-1
To measure the peak
p.d. measure height
from the bottom to the
top of the wave.
The amplitude (peak
value is half of this).
If the y-gain is set to
0.5Vcm-1, then what is
the peak value from
this trace?
= 1.25V
To measure the frequency
of the alternating p.d.
measure the time period
(T) for one cycle.
The frequency f = 1/T
If the time base is set to
2mscm-1.
What is the time period? =
10ms or 0.01s
To measure the time period
accurately, measure over a
number of cycles and then
divide by the number of cycles.
What is the frequency? =
100Hz
Measuring D.C. Voltage

A D.C. input voltage will make the
spot move at a constant
displacement from the zero p.d. line.

Knowing the y-gain the D.C. voltage
can be calculated and can be either
positive or negative.
3.3cm
p.d = 3.3 x 0.1 = +0.33V
2.4cm
p.d = 2.4 x 0.1 = - 0.24V