1. SHREE KANKESHWARIDEVI INST. OF TECH.JAMNAGAR
RATHOD SANDEEP
Electrical Engineer

2. CURRENT
ELECTRICITY

3. Current flow versus Electron flow
Electrons flow this way.
Conventional current flows this way.

4. What formula relates Charge, Current and Time?
A current of 1 Ampere is flowing when 1 Coulomb of charge flows past a point in a circuit in 1 second.
Charge = current x time
(C) (A) (s)
If a current of 5 A is flowing then 5 C of charge pass a point in 1 second.
In general, if a steady current I (amperes) flows for time t (seconds) the charge Q (coulombs) passing any point is given by
Q = I x t

5. Worked example
A current of 150 mA flows around a circuit for 1minute. How much electrical charge flows past a point in the circuit in this time?
Solution

6. For you to do!! Convert the following currents into amperes:
a) 400 mA b) 1500 mA.
Ans. = a) 400 mA = 0.4 A b) 1500 mA = 1.5 A
2. What charge is delivered if a current of 6A flows for 10 seconds?
Ans. = 60 C
What charge is delivered if a current of 300 mA flows for 1 minute(60 seconds)?
Ans. = 18 C

7. What is Ohm’s Law? What is the formula for Ohm’s law? V = I x R
The voltage dropped across a resistor is directly proportional to the current flowing through it, provided the temperature remains constant.
What is the formula for Ohm’s law?
Voltage (V) = Current (A) x resistance (Ω)
V = I x R

8. Worked example on Ohm’s Law
8 Ω
2 A
V = ?

9. Ammeters and Voltmeters
Ammeters measure current and are placed in series in a circuit. A V
Voltmeters measure voltage and are placed in parallel in a circuit.

10. Rules for Resistors in SERIES

11. Examples on Resistors in Series
No. 1
9 Ω
6 Ω
Ans. = 15 Ω
No. 2 4Ω
6 Ω
3 Ω
Ans. = 13 Ω

12. Rules for Resistors in PARALLEL

13. Examples on Resistors in Parallel
6 Ω
No. 1
Ans. = 3 Ω
6 Ω
No. 2
12 Ω
Ans. = 6 Ω
12 Ω

14. For you to do!!!!
No. 3
16 Ω
6 Ω
16 Ω
Ans. = 14 Ω

15. 6 Ω
6 Ω
No. 4
12 Ω
Ans. = 6 Ω
No. 5
10 Ω
2 Ω
3 Ω
10 Ω
2 Ω
Ans. = 9 Ω

16. Rules for SERIES CIRCUITS
Same current but ……
split voltage between them.

17. Equal resistors share the voltage between them!!?
6 V
6 V
6 V

18. Rules for PARALLEL CIRCUITS
Same voltage but ……
split current between them.

19. What will be the currents flowing through each ammeter?
Equal resistors
? A

20. Electrical Power Electrical Power = Potential difference * current
Watts Volts Amps
E.g. A study lamp is rated at 60 W, 240 V.
How much current is the bulb carrying?
Solution
60 W = V * Current
60 W
Current = = A
240 V

21. Transformers A transformer is a device for increasing or decreasing
an a.c. voltage.

22. Structure of Transformer

23. Circuit Symbol for Transformer

24. How Transformer works Laminated soft iron core Output voltage (a.c.)
Input voltage
(a.c.)
Primary coil
Secondary coil

25. Laminated iron core – this links the two coils magnetically.
All transformers have three parts:
Primary coil – the incoming voltage Vp (voltage across primary coil) is connected across this coil.
Secondary coil – this provides the output voltage Vs (voltage across the secondary coil) to the external circuit.
Laminated iron core – this links the two coils magnetically.
Notice that there is no electrical connection between the two coils, which are constructed using insulated wire.

26. Two Types of Transformer
A step-up transformer increases the voltage - there are more turns on the secondary than on the primary.
A step-down transformer decreases the voltage - there are fewer turns on the secondary than on the primary.
To step up the voltage by a factor of 10, there must be 10 times as many turns on the secondary coil as on the primary. The turns ratio tells us the factor by which the voltage will be changed.

27. Formula for Transformer
Where Vp = primary voltage
Vs = secondary voltage
Np= Number of turns in primary coil
Ns = Number of turns in a secondary coil.

28. Worked example No. 1 The diagram shows a transformer
Worked example No. 1 The diagram shows a transformer. Calculate the voltage across the secondary coil of this transformer.
Step-up transformer!

29. Solution

30. Worked example No. 2 A transformer which has 1380 turns in its primary coil is to be used to convert the mains voltage of 230 V to operate a 6 V bulb. How many turns should the secondary coil of this transformer have?
VP = 230 V
NP = 1380
VS = 6 V
NS = ?
Obviously, a Step-down transformer!!

31. Solution