P2.3.2 Electrical circuits
Current Electricity ­ Electric Circuits
Static electricity ­ the charges stay put.
+
+
Current electricity ­ the charge is flowing.
Conventional current is a flow of positive charge from positive to negative.
positive end
negative end
• Which one is true?
• Who still uses
the other one?
Electron flow is a flow of negative electrons from negative to positive
Electron flow is correct.
conventional current
electron flow
Electrical Current
This is a flow of charge.
The unit of current is the ampere, A, which is usually shortened to amp
We use an ammeter to measure current.
André­Marie Ampère
(1775–1836)
A
Always used in series
Negligible resistance
The unit of electric charge is the coulomb, C
The symbol for charge is Q!
Current is the rate of flow of charge past a point in the circuit, so
Current (in A) = Charge (in C)
time taken (in s)
A charge of 8.0C passes through a bulb in 4.0 seconds.
Calculate the current through the bulb.
I = Q = 8C = 2A
t
4s
+
or
Battery of cells
Cell
specific
number
Ammeter
Fixed resistor
A
Voltmeter
Variable resistor
Diode
Lamp/bulb
Fuse
Heater
V
two or
more cells
Potential difference (a.k.a. voltage)
....provides the energy to make the charge flow.
gy
er
en
p e u
iv
Increasing the
potential difference (pd) will increase the current.
g
ns
(1) potential difference/
voltage gives electrons energy
tro
lec
) e
(2
Potential difference/
voltage is a bit like the difference in height that powers a roller­coaster:
(3) Low energy electrons return
Alternatively:
Voltage/potential difference is the "electrical push" that gets sets the charge flowing to create a current
Big voltage
big push
big current
When charge falls through a voltage/potential difference, it gains energy ­ it can do work:
The energy transferred to a bulb is 24J when 8.0C of charge passes through it.
Calculate the potential difference across the bulb
V = W = 24J
Q 8C
= 3V
Resistance
Georg Simon Ohm
(1789 – 1854)
Resistance is how much a circuit component resists the flow of current.
Increasing the resistance decreases the current.
Resistance (R) is measured in ohms (Ω)
Current/Voltage graph for a wire/resistor
• Ensure all meters read positive
A
• Reverse the battery pack. Both meters should now read negative
• Record V and I for one cell, then two, then three, then four cells
• Plot your results
You'll
need
the
negative
sectors
current (in A)
• Record V and I with one cell, then two, then three, then four cells
V
known value resistor
voltage (in V)
For a wire, current is directly For a wire or fixed proportional to the resistor, current is directly voltage
proportional to the (straight line voltage
through the origin)
(straight line through the origin)
A
V
Directly proportional:
• goes through the origin
• straight line, constant gradient
The wire obeys Ohm's Law, so it is an Ohmic conductor
The graph shows that:
Ohm's Law
α
V I
The constant of proportionality is the resistance, R, so:
Scary
A­level
= R I
V Required for GCSE
Georg Simon Ohm
(1789 – 1854)
V
I
Please be so good as to complete the table
x
R
Resistor
Current (A)
Potential difference (V)
Resistance (Ω)
W
2.0
12.0
6.0
X
4.0
80
20
Y
2.0
6.0
3.0
How resistance works ­ Higher Tier
Metals consist of wandering delocalised electrons....
.... and stationary metal ions
+
+
A potential difference makes the electrons drift...
positive
negative
...creating a current
slow down.
bo
the metal ions, the electrons
ing
When electrons collide with
This reduces the flow of
charge which reduces the
current.
The collisions ARE the
resistance.
More collisions
More resistance
The electrons' kinetic energy is converted to heat
Factors affecting resistance - temperature
Think:
Blocking in
basketball
Easy flow - low resistance
Restricted flow - higher resistance
Factors affecting resistance - length
Longer wire
More collisions
More resistance
Factors affecting resistance - cross-sectional area
Easy flow
- low resistance
Restricted flow
- high resistance
Factors affecting resistance - material
Loads of delocalised electrons
None to speak of
Material
Resistivity(ohm/meter)
Silver
0.0000000159
Copper
0.000000017
Gold
0.000000024
Carbon
0.000035
Polystyrene
0.000035
Glass
1010 ­ 1014
Hard Rubber
1013
inert gas
The filament bulb
glass
tungsten filament
thick wire supports
metal
case
contact
insulator
metal contact
The bulb only obeys Ohm's Law at low currents. At higher currents, the line becomes a curve.
A
The bulb is a non­ohmic conductor.
V
When the current is large, the wire gets hot which increases its resistance
Diodes
current blocked
tive
ve
ti
posi
current
flows
nega
current flows
forward
biased
diode
reverse The diode is an electrical valve.
biased
diode It only allows current to flow in one direction.
Actually, they are not zero, just really small
negligible current
due to huge resistance
A
V
Using Diodes as rectifiers
Full wave AC
Current goes both ways
AC power supply
Rectifiers converts AC to DC
w
flo
blocked
w
Probably not on the exam
flo
AC power supply
Half wave rectification
Current can only flow one way
Full wave rectification
+
­Power
Power
supply
supply
­
+
Current goes the other way
Definitely not on the exam
Current goes one way
I
I
I
Wire or
Diode
Bulb
Resistor
V
V
non­ohmic
V is directly proportional to I.
The wire obeys Ohm's Law ­ it is an ohmic conductor
At high currents the bulb no longer obeys Ohm's Law. This is because the resistance of the bulb filament increases when it gets very hot.
When the resistance is high you get a smaller increase in current as the voltage goes up
V
non­ohmic
The diode has a low resistance when it is forward biased and a very high resistance when it is reverse biased
Series Circuits ­ Cells in series
1.5V
1.5V 1.5V
1.5V
1.5V
1.5V
V
V
V
1.5V
3V
4.5V
Potential difference sources in series add together
Parallel Circuits ­ Cells in parallel
1.5V
1.5V
1.5V
1.5V
V
1.5V
The p.d.s are the same
1.5V
but more cells will last
V
longer
1.5V
1.5V
V
1.5V
Normal brightness
Double brightness
Torch design
Normal brightness
(lasts twice as long)
Double brightness
(lasts twice as long)
Current in series
e
­
e
3A
e
­
e
e
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
­
e
e
e
moves, they all have
­
e
to move.
e
­
same at every point
e
­
in a series circuit
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
e
­
­
­
­
3A
A
• The current is the
­
e
­
e
• If one electron
­
A
e
e
­
e
e
e
­
e
­
­
e e
­
­
­
­
­
Current in parallel
e
3A
­
­
­
e
­
­
­
­
e e
­
1A
1A
­
­
e e e
­
e
­
e
­
­
­
e e e
­
e
­
e
­
­
­
­
e e e e
­
e
­
e e e e e e
­
­
­
­
­
e e e e e e­
­
­
­
­
­
­
e e e e e
e
­
­ ­
­
­
­
­
­
­
­
3A
­
­
­
­
e e e e­ e e­ e­
­
­
­
e
e e
e
e
e ­ ­ ­ ­ ­ ­
e e e e e e
2A
e
e e e
e ­ e­ ­ ­ ­ e­
e e e e e e
­
­
­
­
­
­
­
­
­
­
e
e
e e e e e
e
e ­e e ­e e
­
­
­
­
­
­
­
­
­
­
­
e e e e e e e e e e e e e
­
­
­
­
­
The total current into a junction = the total current out of the junction
1A
3A
This is Kirchhoff's Law
2A
These bulbs
are in series
so they have
the same
current
through them
e
3A
­
e e
­
­
­
e e e
­
e
­
­
­
e e e e e e
­
­
­
­
­
­
e e e e e e
­
­
­
­
­
­
e e e e e
e
­
­
­
e
­
e
­
­
­
e e e
1A
­
­
e
­
e
­
­
­
­
­
­
­
­
­
e e e e
1A
e
­
­
­ ­
­
­
­
­
­
­
­
­
­
­
­
­
­
e
e ­e ­e e e e­ e­ e­ e­ e­ e­ e­ ­
­
­
­
e e e e e e e e e e e e e
1A in series
1A
Two components are in series if you
can get from one to the other
without going through a junction
­
3A
­
­
­ ­
e e e e ­ e e ­e­
­
­
­
e e ee
e ­­ e­ ­ ­ ­
ee e e e e
2A
­
e
e ­ e­ e­ e­ e­ e­
e e e e e e
­
­
junction
2A
in parallel
Work out the currents flowing through each bulb
A
4.5A
2
battery
of cells
I =___A
2
battery
of cells
A
3A
1.2
I =___A
2
I =___A
2.5
I =___A
2.5A A
I =___A
3
A
1.2A
1.2A
1.8
I =___A
I =___A
3.5
1
1.5
I =___A
battery
of cells
3
1.5A A
2
I =___A
2A A
Potential difference in series
• The potential difference
provided by the supply is
36V
split up amongst the
components
18V
6V
• The split does not have to
be equal
( - it depends on the resistance of the components)
12V
Potential Difference in parallel
12V
This power supply creates a 12V
potential difference across the circuit
12V
12V
6V
6V
5V
This component gets all 12V
4V
These components share 12V
3V
They don't have to share equally though
( - depends on their resistance)
Work out the potential differences across each bulb
12V
2
battery
of cells
3V
V
9
V =___V
1
V =___V
2.5
V
2.5V
4
V =___V
12V
battery
of cells
V =___V
12
V =___V
9.5
Easy
V =___V
3
8
V =___V
V
4V Quite
easy
6V Vicious!
battery
of cells
3
3.5
V =___V
2.5
V =___V
V
2.5V
3.5
V =___V
Resistors in series
Resistances add together in series
72V
The current in this
circuit would be:
6Ω
18Ω
12Ω
Total resistance = 18 + 12 + 6 = 36Ω
I=V
R
= 72V = 2A
36Ω
Resistors in parallel
Don't ask!
Light dependent resistors (LDRs)
LDR
symbol
Power
supply
igh
rk
Da
-h
An LDR's resistance alters with light intensity
a
ist
res
Light
nce
sensitive
Well lit
- low re
sis
circuit
tance
The darker
it gets, the
brighter
the bulb
Thermistors a.k.a. "Thermal Resistors
Power
supply
ld
igh
-h
Co
Thermistors respond to temperature changes
res
nce
a
ist
Hot
- low resist
ance
Temperature
The colder
sensitive
it gets, the
circuit
brighter
the bulb