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Chosen Hill School - Mathematics, Science and Technology
Learning together USING MATHEMATICS
Using Formula in Electronics 1 RESISTORS
You will need to refer to the ‘Summary of formulas used in basic electronics helpsheet.
1
a) Copy the circuit diagrams shown below.
b) Identify which diagram shows resistors combined in parallel, and which in series, and label
your two diagrams with the appropriate description.
R1
R1
R2
R2
2. Resistors are used to control the flow of electricity. Calculate the total resistance across the
circuits shown below:
100
2K
10K
200
10K
250
500
5K
400
10M
5M
600
Information
Electric current, like water or air, flows from a higher pressure to where the
pressure is lower. In electric circuits the ‘pressure’ is measured by the
Voltage.
The Voltage can be produced by batteries, where each cell produces 15
volts or by the generators of the Electricity board.
When the National Grid was established in the late 1920's early 1930's, the
cables worked at 132 kilovolts. In the new millennium, 275 & 400 kilovolt are
now standard.
Transformers then reduce this pressure so that the standard mains voltage in
a house is 240volts.
Ohm’s Law, is a relationship discovered between the current, the voltage
and the resistance, discovered in 1827 by Georg Simon Ohm. He discovered
that the voltage drop (change in pressure) across a resistance was given by
the formula V = IR (where I is the current measured in amps and R is the
value of the resistance, measured in Ohms).
The Current (flow of electricity) is constant throughout a circuit in series.
When a circuit is in parallel, the sum of the currents in the ‘parallel channels’
must equal the current in the single channel eg x
x+y
y
+12V
+12V
600
+15V
+ 9V
5K
5K
600
10K
750
500
5K
0V
0V
0V
0V
450
550
+3V
3
4
350
0V
+ 6V
1K
0V
a)
b)
c)
d)
e)
Rearrange the formula to make I the subject of the formula
Calculate the total resistance across each of the circuits shown above
Use Ohm’s Law to calculate the current flowing through each of the circuits shown above
Rearrange the formula to make R the subject of the formula
A Light Emitting Diode (LED) is a device that gives off light when an electrical current
passes through. It has limitations in that it can get damaged if the current exceeds 20 mA.
Write down an inequality that must be satisfied by these conditions
f) What size resistor must be used to run the diode off a 9V battery in order to guarantee the
current never exceeds 20mA
Use Ohms Law to show that the total resistance of two resistors in series is given by
R = R 1 + R2
R1R2
but that the total resistance, when the resistors are in parallel, is given by
R=
R1  R2
R1R2
5 Show that R =
is equivalent to the formula 1 = 1 + 1
R
R1
R2
R1  R2