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3rd/4th form – Electric circuits
Conductors and insulators
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An electric current is a flow of charge.
These charges are often electrons.
Electrons carry a negative charge.
Electrons are usually bound to atoms, but some
more strongly than others
• In a conductor some charges are able to flow so
we can use them in an electric circuit.
• In an insulator the electrons are held tightly in
position and so are unable to move. These
materials do not conduct electricity.
Conductors and insulators
• Generally, most metals are good
conductors and most non-metals are not.
• An electric cable makes use of both types
of material.
Metal wires to
carry electricity
Plastic insulation to
isolate metal wires
• There is a third category, semi-conductors,
which are also very useful (not GCSE)
Conduction electrons
• Metals have lots of ‘free’ electrons
– weakly bound outer electrons
• so they are good conductors.
• Normally they move around randomly
• When an electric field is applied they
move in a common direction
Counting charges
• The charge on a single electron is tiny
• We define a “useful” quantity of charge
which allows sensible measurements,
called the Coulomb.
• 1C is equal to the charge on 6.24151×1018
electrons
– That’s 6,241,510,000,000,000,000 electrons!
– Ammeters need to go in the circuit so the
current can flow in and out of them.
Electric current
• An electric current is a flow of electric
charge (pushed along by a voltage).
• Where do these charges come from?
• In a circuit, all the wires and components
are full of electrons
– so as soon as a power supply is connected a
current starts flowing
– No time delay with long wires!
– (Electrons move quite slowly)
Electric Current
• Current is a flow of charge
• We can define current as the number of
coulombs flowing past a point in 1 second
Current (A)
Q
I
t
Charge (C)
time (s)
• When 1C of charge flows through a wire in
1s, the current is 1A.
Examples
1. What is the current when 4 C of charge
flows for 2 seconds?
2. What is the charge flowing through a wire in
5 s if the current is 3 A?
Measuring current
AMMETER
An ideal ammeter
does not affect the
circuit, no energy
is transferred to it:
it has zero
resistance
• Ammeters need to go in the circuit so the current
can flow in and out of them.
• To just indicate the flow of current without
measuring it we can use an indicator light.
Conventional Current
• Charges come in two flavours, + and –
• Conventional current is defined as the
direction in which + charges flow
• so in a wire, the conventional current is in
the opposite direction to the electron flow
Circuits: a reminder
• Circuit diagrams are simplified
drawings
• Circuit diagrams are drawn with a
ruler!
– Connections must connect
– There are symbols for each component
Circuit symbols
• You’ve just
got to
learn
them!
• Full list is
on p. 242
Electric current
• When a circuit with a battery is competed,
the battery “pushes” the charges around.
• Electric current is never “used up” as
electrons flow around a circuit.
– The current is the same through all
components in series circuit
– All ammeters read the same:
– (note
A
connect in series)
Electrical Energy
• Electricity is useful because it can be easily
converted into other types of energy.
• A battery or power supply gives electrical
energy to the electrons in a circuit.
• Other circuit components then convert this
to different forms of energy.
Potential Difference (“Voltage”)
• The voltage between two points in
a circuit is a measure of how much
energy is transferred to or from the
charges as they pass between
those points.
• The voltage of a power supply is a
measure of how hard it “pushes”
charge
– So a larger voltage supply means a
larger current will flow in a circuit
Voltage
• Voltage is a measure of the energy
change experienced by charges
• We define voltage as the number of joules
transferred per coulomb of charge
voltage (V)
E
V
Q
energy (J)
charge (C)
• When charge passes through a p.d. of 1V,
1J of energy is transferred per coulomb.
Examples
1. When 2C of charge pass through a battery
they acquire 24J of energy. What is the
voltage of the battery?
2. The p.d. across a lamp is 3V. How much
energy is transferred when 5C of charge
pass through it?
3. (harder) How much energy is transferred
when a current of 2A flows through a lamp
with a p.d of 3V across it for 1 minute?
Combining cells
• For batteries in series, the
supply voltage is the sum of the
individual batteries
– Adding more batteries in series
increases voltage and therefore
increases current
7.5 V
V
• For batteries in parallel, the
supply voltage is the same as a
single branch, but the battery
capacity is increased (battery
lasts longer)
– Adding more batteries in parallel
does not affect voltage or current
V
3V
Measuring voltage
• To measure the voltage between two
points we connect a voltmeter in parallel
across those two points
Here we are measuring
the voltage across the
resistor R
An ideal voltmeter does not
affect the circuit, no energy
passes through it: it has infinite
resistance
Resistance
• Resistance is the opposition to
current flow displayed by
components
– for a fixed voltage, the larger the
resistance, the smaller the current
• Resistance of connecting wires is
usually so small it is ignored
• Resistors dissipating energy get hot!
– e.g. lamp filament
Resistance
• Resistance is caused by collisions between the
free charges and the lattice of atoms which
makes up the conductor
• Each collision transfers energy to the atoms of
material – material heats up
• A high current means more collisions – resistor
gets hotter
Resistance
• Resistance is a measure of the opposition
to current flow
• We define the resistance of a device as the
voltage needed to push a given current
V
through it
voltage (V)
R
resistance (W)
(Ohms)
I
current (A)
• When a p.d. of 1V causes a current of 1A to
flow through a device, its resistance is 1 W
Examples
• If a lamp has a current of 3A when there is
a p.d. of 12V across it, what is the
resistance of the lamp?
• What is the current through a 100W
resistor with a p.d. of 5V across it?
• A real ammeter has a resistance of 0.5W.
What will the p.d drop across it be when a
current of 5A is flowing?
Ohm’s Law
• “The current through a conductor is
proportional to the current across it,
provided the temperature remains
Voltage (V) 6
constant”
– Generally true for metals
– Not true for all components
5
4
3
2
1
0
0
5
10
15 20 25 30 35 40
Current (mA)
V
Voltage, Resistance & Current
• We have
resistance (W)
V
R
I
voltage (V)
current (A)
• So for a given circuit:
– What happens to the current if we increase the
voltage of the power supply?
– What happens to the current if we increase the
resistance of the components?
Resistors limit current
• They can be used to protect vulnerable
circuit components in case of a fault
• Variable resistors can be used to control
things
Series and Parallel Circuits
• A series circuit has only one
path for the current to flow, all
the components are joined
together in a continuous line.
• A parallel circuit contains
branches, the current splits and
recombines.
– Each branch is unaffected by the
other branches
A
A
Current in series circuits
• Remember, current is never “used up” as
electrons flow around a circuit.
• The current is the same through all
components in series circuit
– All ammeters read the same
A
A
A
A
A
A
Voltage in a series circuit
• The energy transferred to the charge by the
battery = the energy dissipated by all the
components in the circuit
Vbattery  V1  V2  V3
• The largest resistance has
the largest voltage across it
(most energy transferred)
Vbattery
V1
V2
– If all resistances are equal, the battery voltage is
divided equally
V3
Think about this circuit…
• What happens to
V1 as the
resistance is
increased?
• What happens to
V2?
• What happens to
the current?
• What effect will this
have on the lamp?
Vbattery
V1
A
V2
Voltage in a parallel circuit
• All components connected to a battery in
parallel have the same voltage across
them.
Vbattery  V1  V2  V3
• The current through each
component is the same as if the
other components weren’t there.
– We can use V=IR on each branch
in turn
Vbattery
A
Current in a parallel circuit
• The total current through the battery is
equal to the sum of the currents through
each parallel branch
I battery  I1  I 2  I 3
• The smallest current flows
through the branch with the
highest resistance.
Ibattery
I1
I2
I3
Characteristics of circuits
• Series:
– simple
– one switch affects all components
– one broken component affects all
components
– voltage is shared between components
• Parallel:
– components can be switched individually
– one broken component only affects its
own branch
– all branches receive the full supply
voltage