When resistors are connected in series, the current can only flow through one path. The total resistance of a series circuit equals the sum of the individual resistances. The current at any point in the circuit is the same. The voltage across each resistor depends on its resistance and the current flowing through it. The total voltage supplied by the battery equals the sum of the voltages across each resistor. The total power equals the sum of the power dissipated by each resistor.

1. Resistors
Connected in series

2. Battery
R1 R2 R3
Current Flow
3 Resistors connected in
series

3. When a supply is added to a number of
resistors connected together end to end
the current can only take one
route through the circuit.
This is known as a series connection.

4. These rules apply to a series circuit:
The total circuit resistance (Rt), equals the
sum of all the circuit resistors. To work out
the total amount of circuit resistance, add
up all the individual resistor values.
Rt = R1 + R2 + R3 + etc……

5. The total circuit current (I), equals the
battery emf (V) divided by the total
resistance (R). Ie Ohm’s Law.
I = V
R

6. The current will be the same value at any
point in the circuit.

7. The potential difference across each
resistor is proportional to its resistance.
Voltage pushes the electrons through a
resistor. How much is used depends on
the size of the resistor. The bigger the
resistor the more we use.
So V1 = I x R1 , V2 = I x R2 etc……

8. The supply voltage will equal the sum of
all the potential differences across each
resistor.
If you add together all the potential
differences across each resistor (ie the
amount of volts dropped across each
resistor) it should equal the value of the
supply voltage.
Where V = V1 + V2 + V3 + etc….

9. The total power in a series circuit equals
the sum of all the individual powers used
by each resistor.
So P = P1+ P2 + P3 + etc…..

10. Example
A 5 ohm resistor is connected with
a 10 ohm resistor and a 12 V battery in
series.
5 Ω 10 Ω
12 V

11. (1) Find the total resistance.
To find Rt, add together all the values of
each resistance.
Rt = R1 + R2
Rt = 5Ω + 10 Ω
Rt = 15Ω

12. (2) Find the total current.
I = V
R
So I = 12
15
I = 0.8 Amps

13. (3)Find the potential difference across each
resistor.
Across R1
V = I x R
V1 = I x R1
V1 = 0.8 x 5
V1 = 4 Volts
Across R2
V = I x R
V2 = I x R2
V2 = 0.8 x 10
V2 = 8 Volts

14. (4) Find total voltage.
Add together all the voltage drops
Vt = V1 + V2
Vt = 4 + 8
Vt = 12V

15. (5) Find power used by each resistor.
Pt = V x I
Pt = 12 x 0.8
Pt = 9.6 Watts

16. (6) Find power used by R1
P1 = V1 x I
P1 = 4 x 0.8
P1 = 3.2 Watts

17. (7) Find power used by R2
P2 = V2 x I
P2 = 8 x 0.8
P2 = 6.4 Watts

18. Total power
Pt = P1 + P2
Pt = 3.2 + 6.4
Pt = 9.6 Watts