Download A3. Revision notes - Practical Electricity

REVISION NOTES
name
cell
battery
lamp
switch
resistor
variable
resistor
PRACTICAL ELECTRICITY
symbol
NATIONAL 4/5
notes
provides electrical energy to the charges flowing
around a circuit
two or more cells connected in series
converts electrical energy into light (+ heat)
energy. It is an OUTPUT device
breaks a circuit to turn it on or off
reduces the flow of charge and so the current
same as a resistor, but resistance can be changed
voltmeter
measures the voltage (potential difference) across
part of a circuit
ammeter
measures the current in a circuit
diode
converts electrical energy to light energy. An
OUTPUT device. Uses very low currents
converts electrical energy to kinetic energy. An
OUTPUT device
converts electrical energy to sound energy. An
INPUT device
converts light energy to electrical energy. An
INPUT device
contains a thin wire which melts if the current
gets too high. A safety device.
a ‘valve’ which only allows charge to flow one way
capacitor
used to store electric charge
thermistor
a type of resistor. As the temperature goes up, its
resistance goes down
LDR
a type of resistor. As the light level goes up, the
resistance goes down
relay
a type of switch. Low voltage side powers electromagnet. This closes switch on high voltage side
transistor
(NPN)
an electronic switch. Switches “on” at about 0.7 V
MOSFET
an electronic switch. Switches on at about 2.0 V
LED
motor
loudspeaker
photovoltaic
(solar) cell
fuse
REVISION NOTES
PRACTICAL ELECTRICITY
NATIONAL 4/5
Series circuits (N4/5)
A series circuit has only one ‘loop’. There is only one
path for the current.
In a series circuit, the current is the same at all points.
In a series circuit, the voltages (potential differences)
across the components in the circuit add up to the
supply voltage. We can write this as:
Vs = V1 + V2
Example
Current is the same at all points – so current at Y = 0. 5 A.
Vs = V1 + V2 so
6 V = 4.5 V + V2
so V2 = 1.5 V
Parallel circuits (N5)
A parallel circuit has more than one ‘loop’. The current can
takes more than one path.
In a parallel circuit, the currents in each branch add up to the
current from the supply. We can write this as:
Is = I1 + I2
The voltage across each branch is the same, and is equal to the supply voltage.
Example
Is = I1 + I2
so Is = 2.0 A + 3.0 A
so Is = 5.0 A
Voltage across each branch is the same, so Vs = 12.0 V.
Using voltmeters and ammeters (N4/5)
Ammeters are connected in series to measure the current in a
circuit.
Voltmeters are connected across a component to measure the
potential difference (voltage) across that component.
Resistors in series (N4/5)
To find the resistance of several resistors
in series, simply add them up.
RTotal = R1 + R2 + R3
Resistors in parallel (N5)
This is a little more complicated.
1
1
1
1
=
+
+
+⋯
𝑅𝑇
𝑅1 𝑅2 𝑅3
Example
A 10  resistor and a 15  resistor are connected in parallel.
1
𝑅𝑇
So
=
1
𝑅1
+
1
1
𝑅2
𝑅𝑇
=
1
10
+
1
1
15
𝑅𝑇
=
3
30
+
2
1
30
𝑅𝑇
=
5
𝑅𝑇
30
1
=
30
5
RT = 6 
Voltage, current and resistance (N4/5)
These three quantities are linked by Ohm’s Law:
V=IxR
Example
A 20  resistor is connected to a 3 V battery. Calculate the current in the resistor.
R = 20 
V=3V
I=?
V=IxR
3 = I x 20
I = 3/20
= 0.15 A
Special resistors (N4/5)
The resistance of resistors is usually fixed. However, resistance can sometimes
change depending on the conditions.
For a light-dependant resistor (LDR):
as the Light goes Up the Resistance goes Down
(LURD)
For a thermistor:
as the Temperature goes Up the Resistance goes Down
(TURD)
The LED (light-emitting diode) (N5)
Like diodes, LEDs only allow current to pass in one direction. They must be fitted the
right way round in a circuit – ‘pointing’ towards the negative.
LEDs use very little power and so are ideal for different types of lighting.
LEDs can only handle small currents, so must be fitted with a resistor in series to
reduce the current.
Here is an LED, rated at 10 mA and 2 V, connected to a 6 V supply.
2 V across the LED means 4 V across the resistor
(VS = V1 + V2)
10 mA (= 0.01 A) in LED means 0.01 A in the resistor (series circuit)
So using Ohm’s Law for the resistor:
V=IxR
4 = 0.01 x R so R = 400 
Voltage dividers (N5)
12 V
Two resistors in series will share the supply voltage. The resistor
with the largest resistance will take the larger share.
If R1 = R2 = 50 , the voltage across each resistor will be 6 V.
If R1 = 20  and R2 = 40 , then R1 ‘gets’ 4 V and R2 ‘gets’ 8 V.
(R2 ‘gets’ twice as much voltage because it’s twice the size of R1).
If R1 = 5 k and R2 = 1 k, then R1 ‘gets’ 10 V and R2 ‘gets’ 2 V.
(R1 ‘gets’ five times as much voltage because it’s five times the size of R1).
When it’s difficult to spot the relationship, you can use:
𝑉2 = (
𝑅2
) 𝑥 𝑉𝑠
𝑅1 + 𝑅2
to find V2 and then
𝑉𝑠 = 𝑉1 + 𝑉2
to find V1.
Transistors (N5)
Transistors are electronic switches. They are switched ON and OFF by altering the
voltage across them.
There are two main types:
NPN
switches ON at 0.7 V
MOSFET
switches on at 2.0 V
MOSFETs can handle larger currents than NPNs.
Control circuits (N5)
We can combine special resistors, voltage dividers and transistors to make control
circuits. These circuits will respond to changes in light or temperature and switch on
an output device like an LED or a motor.
This circuit switches on the LED if the temperature gets
too high.
As the temperature rises, the resistance of the thermistor
falls. (TURD)
So the voltage across the thermistor falls.
This means the voltage across the variable resistor goes up
(as the two resistors share the supply voltage).
When the voltage across the variable resistor rises above 0.7 V, the NPN transistor
switches ON and the LED comes on.
By adjusting the variable resistor, we can adjust the temperature at which the LED
comes on.
By switching the position of the two resistors, we can make a circuit that turns on
the LED if the temperature gets too low.
Control circuits, capacitors and timing (N5)
Capacitors take time to charge. As a capacitor charges the voltage
across it increases.
So in this example, as time passes, the capacitor gradually charges.
The voltage across it gradually rises. When the voltage reaches
0.7 V, the NPN transistor switches on and the LED lights. We have
built a time delay into the circuit.