Transcript AS Level Electricity - the basics - revision from GCSE

AS Level Electricity - Circuits
Taking Measurements
• The p.d. across a
component in a
circuit is measured
in volts (V) using a
voltmeter connected
across (in parallel
with) the component.
Taking Measurements
• The current (I)
flowing through a
component in a
circuit is measured
in amperes (A) using
an ammeter
connected in series
with the component.
Current
• A current will flow through an electrical
component (or device) only if there is a
voltage or potential difference (p.d.)
across its ends.
• The bigger the potential difference
across a component, the bigger the
current that flows through it.
Model
• You can think of electrical
potential as being the
topography of the electrical
environment.
• The flow of charged
particles is affected by the
steepness of the ‘slope’.
• The change in volts per
metre is a measure of how
steep the slope between two
points is… the steeper the
‘potential gradient’ the
faster the charge will flow.
Current
• An electric current is a flow of charge
(Q) measured in coulomb (C).
• The charges 'flowing' are usually
electrons (in a wire) but can be ions (in a
solution).
Current
• It is the 'net' flow of charge that
makes the current.
• Charges going in opposite directions
cancel out each other's effect.
• Double-charged ions will make double
the current that single-charged ones
would.
Resistance
• Components resist a current flowing
through them.
• The bigger their resistance, the smaller
the current produced by a particular
voltage, or the bigger the voltage
needed to produce a particular current.
• Resistance (R) is measured in ohms (W)
Resistance
• When electrical charge flows through a
resistor, electrical energy is
transferred as heat according to the
equation P=IV
• This makes components get hotter as
current goes through them.
• A change in temperature can change the
resistance of the component. You need
to appreciate this.
Cells and Batteries
• An electric cell provides the
potential difference for a
battery powered circuit by
changing chemical energy into
electrical energy.
Cells and Batteries
• If more than one electrical cell
is connected together the term
for the power source is ‘battery’
– a single cell is just called an
electric cell.
Cells and Batteries
• A cell’s potential difference
between its terminals has a
chemical source and that this
can ‘run down’ with use or
incorrect storage providing less
of an electrical gradient for the
current (i.e. the voltage
stamped on a battery might not
be correct).
Electrical Energy Transfer
• As an electric current flows through a
circuit, energy is transferred from the
battery or power supply to the
components in the electrical circuit.
• An electric current is a flow of charge.
• Charge (Q), measured in coulomb (C) is a
property of the electrons that move in
the wire. Each electron has a very tiny
charge of 1.6 X 10-19C
Equations you should already
know from GCSE
When electrical charge flows through a
resistor, electrical energy is transferred as
heat.
The rate of energy transfer (power) is given by:
P = IV
Where:
P = power (in watts, W)
V = potential difference (in volts, V)
I = current (in ampere, A)
1 watt is the transfer of 1J of energy in 1s.
Equations you should KNOW
The higher the voltage of a supply, the
greater the amount of energy
transferred for a given amount of
charge which flows.
E = VQ
Where
E = energy transferred (in joule, J)
V = potential difference (in volt, V)
Q = charge (coulomb, C)
Equations you should KNOW:
Q=It
Where:
Q = charge (coulomb, C)
I = current (in ampere, A)
t = time (in seconds, s)
Equations you should KNOW
V=IR
Where:
V = potential difference (in volts, V)
I = current (in ampere, A)
R = resistance (in ohm, W)
Equations you should KNOW
E = Pt
Where:
E = energy transferred (in joule, J)
P = power (in watts, W)
t = time (in seconds, s)
For all equations
you should be able to:
•
•
•
•
•
recall the equation
manipulate it
know the symbols, values and units
use it in calculations
be able to use S.I. Prefixes with the
units
Symbols
connecting wire
connection between two
crossing wires
two crossing wires that are not
connected to each other
switch (open)
switch (closed)
signal lamp
filament lamp
Symbols (cont)
cell
battery
power
supply
fuse
resistor
diode
variable
resistor
thermistor
Symbols (cont.)
ammeter
voltmeter
L.D.R. (light
dependant
resistor)
You have to be able to draw these symbols and incorporate
them into circuits.
They must be drawn carefully.
Never put a symbol in a ‘corner’.
Never leave a gap.
Use a sharp pencil to draw the circuits.
Series Circuits
When components are connected in
series:
• their total resistance is the sum of
their separate resistances
RTOTAL = R1 + R2 + ..........RN;
• the same current flows through each
component;
• the potential difference from the
supply is shared between them.
Parallel Circuits
When components are connected in parallel:
• there is the same potential difference across
each component;
• the current through each component depends
on its resistance; the greater the resistance
of the component, the smaller the current;
• the total current through the whole circuit is
the sum of the currents through the separate
components - this follows from Kirchhoff's
First Law - see below.
Characteristic Curves
• Current-voltage graphs are used to
show how the current through a
component varies with the voltage you
put across it.
• They are called characteristic curves
of the components.
The current through an ohmic conductor (e.g. a wire) is proportional to the
voltage across the resistor at constant temperature.
This is known as Ohm's Law.
The straight line shows proportionality – the fact it goes through the origin
shows it is directly proportional – double the voltage and the current
doubles!
The resistance of a filament lamp increases as the temperature of the
filament increases.
When the filament is very cool the graph is a straight line – it curves most as
the temperature changes rapidly (when it goes through the red glow to white
glow stage). When it is really hot it gets to a steady temperature and the line
straightens out again.
The current through a diode effectively only flows in one direction only. It
acts like a closed switch when connected in forward bias and an open switch
when in reverse bias.
When connected in forward bias its resistance is very low (provided it has a
potential difference of more than 0.6 volts across it).
The diode has a very high resistance in the reverse bias therefore only a
tiny current flows.
Zero p.d. gives zero current.
You also need to KNOW
• The resistance of a light dependent resistor
decreases as the light intensity increases.
• The resistance of a thermistor decreases as
the temperature increases. (There are some
thermistors which behave in the opposite way
to this but all of your questions will be set on
this version).