1. Voltage and Current Division

2. Power and Energy
Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.
P=VI
P=I2R=V2R
Which is the best fuse to use (3A, 5A or 13A) with a 1.15 kW electric fire at a potential difference of 230 V?

3. To calculate the consumption of an electrical appliance in kWh, you have to take into account three factors:
the capacity of your electrical appliance, expressed in watt.
the number of hours that the appliance is in use in one day.
the number of days per year when the appliance is in use.
The calculation is as follows:
[number of  hours’ use] x [number of days’ use] x ([capacity of appliance expressed in watt] / 1,000) = number of kWh

4. Example
A radio alarm is on all the time and therefore uses energy continuously.
An electric fire needs 2 kW. It is switched on for 3 hours. If each kWh costs 10c, how much does it cost to run the fire?
cost = power × time × cost of 1 kWh
       = 2 kW × 3 h × 10c
       = 60c

5. Resistors in series share the same current
Voltage Dividers
Resistors in series share the same current
Vin

6. Resistors in series share the same current
Voltage Dividers
Resistors in series share the same current
From Kirchoff’s Voltage Law and Ohm’s Law : + V1 - V2 _
Vin

7. Resistors in series share the same current
Voltage Dividers
Resistors in series share the same current
From Kirchoff’s Voltage Law and Ohm’s Law : + V1 - V2 _
Vin

8. Voltage Division
The voltage associated with one resistor Rn in a chain of multiple resistors in series is: or where Vtotal is the total of the voltages applied across the resistors.

9. Voltage Division
The percentage of the total voltage associated with a particular resistor is equal to the percentage that that resistor contributed to the equivalent resistance, Req.
The largest value resistor has the largest voltage.

10. Example 1
Find the V1, the voltage across R1, and V2, the voltage across R2. + V1 - V2 _

11. Example 1 + V1 - V2 _ Voltage across R1 is: Voltage across R2 is:
Check: V1 + V2 should equal Vtotal + V1 - V2 _ 20
8.57 V V = 20 V

12. Example 2 Find the voltages listed in the circuit to the right. + V1 -

13. Example 2 (con’t) + V1 - V2 V3
Check: V1 + V2 + V3 = 1V

14. Symbol for Parallel Resistors
To make writing equations simpler, we use a symbol to indicate that a certain set of resistors are in parallel. Here, we would write R1║R2║R3 to show that R1 is in parallel with R2 and R3. This also means that we should use the equation for equivalent resistance if this symbol is included in a mathematical equation.

15. Current Division All resistors in parallel share the same voltage +
Vin _

16. From Kirchoff’s Current Law and Ohm’s Law :
Current Division
All resistors in parallel share the same voltage
From Kirchoff’s Current Law and Ohm’s Law : +
Vin _

17. Current Division All resistors in parallel share the same voltage +
Vin _

18. Current Division
Alternatively, you can reduce the number of resistors in parallel from 3 to 2 using an equivalent resistor. If you want to solve for current I1, then find an equivalent resistor for R2 in parallel with R3. +
Vin _

19. Current Division +
Vin _

20. Current Division
The current associated with one resistor R1 in parallel with one other resistor is:
The current associated with one resistor Rm in parallel with two or more resistors is:
where Itotal is the total of the currents entering the node shared by the resistors in parallel.

21. Current Division
The largest value resistor has the smallest amount of current flowing through it.

22. Example 3
Find currents I1, I2, and I3 in the circuit to the right.

23. Example 3 (con’t)
Check: I1 + I2 + I3 = Iin

24. Example 4
The circuit to the right has a series and parallel combination of resistors plus two voltage sources.
Find V1 and Vp
Find I1, I2, and I3 I1 + V1 _ I2 I3 + Vp _

25. Example 4 (con’t)
First, calculate the total voltage applied to the network of resistors.
This is the addition of two voltage sources in series. I1 + V1 _ +
Vtotal _ I2 I3 + Vp _

26. Example 4 (con’t)
Second, calculate the equivalent resistor that can be used to replace the parallel combination of R2 and R3. I1 + V1 _ +
Vtotal _ + Vp _

27. Example 4 (con’t)
To calculate the value for I1, replace the series combination of R1 and Req1 with another equivalent resistor. I1 +
Vtotal _

28. Example 4 (con’t) I1 +
Vtotal _

29. Example 4 (con’t)
To calculate V1, use one of the previous simplified circuits where R1 is in series with Req1. I1 + V1 _ +
Vtotal _ + Vp _

30. Example 4 (con’t) To calculate Vp:
Note: rounding errors can occur. It is best to carry the calculations out to 5 or 6 significant figures and then reduce this to 3 significant figures when writing the final answer. I1 + V1 _ +
Vtotal _ + Vp _

31. Example 4 (con’t) Finally, use the original circuit to find I2 and I3.+ V1 _ I2 I3 + Vp _

32. Example 4 (con’t) Lastly, the calculation for I3. I1 + V1 _ I2 I3 + Vp

33. Summary
The equations used to calculate the voltage across a specific resistor Rn in a set of resistors in series are:
The equations used to calculate the current flowing through a specific resistor Rm in a set of resistors in parallel are: