1. DC Circuits

2. Topics Circuit Symbols 5 principles of DC Circuits
2 tips for solving DC Circuits
Putting it all together – Electricity Problem Solving Toolkit

3. Circuit Symbols

4. Circuit Symbols (pg 324) Sources of E.M.F.: Resistor Meters Switch
Cell & Battery
D.C. Power Supply
A.C. Power Supply
Resistor
Fixed Resistor
Variable Resistor (Rheostat)
Light Dependent Resistor (LDR)
Thermistor
Light Emitting Diode (LED)
Meters
Ammeter
Voltmeter
Switch
Light Bulb (Lamp)
Fuse
Bells
Variable Potential Divider (Potentiometer)

5. Cell (Battery)

6. D.C. Supply

7. A/C Supply

8. Fixed Resistor

9. Variable Resistor (Rheostat)

10. Light Dependent Resistor (LDR)
LDRs decrease their resistance when exposed to light
Often used as components for light-sensitive circuits (e.g. turn on lamps when it is dark)

11. Thermistor
Thermistor’s decrease resistance when exposed to heat

12. Semiconductor Diode Allows current to flow in one direction
Blocks current in opposite direction

13. Light Emitting Diode (LED)
Same as a diode, except now it emits light when current is flowing in the allowed direction

14. Potentiometer Also called “variable potential divider”
Is actually just a wire attached to a metre rule
Comes with an apparatus called a “jockey”
Will use this in the lab to prepare for SPA 2

15. Ammeter

16. Voltmeter

17. Galvanometer

18. Switch

19. Light Bulb

20. Fuse

21. Bell

22. Earth

23. Transformer

24. 5 principles of dc circuits

25. Current in Series
In a series circuit (i.e. no parallel circuits), the current is the same at all points of the circuit

26. Worked Example 1 What is the reading of Ammeter X?
Ammeter Reading = 0.2 A A A
Ammeter X

27. Current in Parallel In a parallel circuit, there must be branches
Current follows the “what goes in must come out” rule

28. Worked Example 2
What is the value of I?
0.3 A I
0.2 A

29. Worked Example 3 What is the value and direction of current in wire X?

30. Potential Difference in Series
Total p.d. is equals to the sum of the individual p.d. components across the series
This is similar to calculating resistance of resistors in series
Note that p.d. across wire (without resistors) is zero

31. Worked Example 4 What are the readings of voltmeters X and Y? V V V
Voltmeter X
1.0 V V
Voltmeter Y

32. Worked Example 5 What is the reading of voltmeter X? V V 1.0 V

33. Potential Difference in Parallel
p.d. is the same across parallel branches

34. Worked Example 6 What is the reading of voltmeter X? V 4.0 V V

35. Worked Example 7 What is the reading of voltmeter X? V 4.0 V V V 1.0 V

36. Potential Divider Principle
The ratio of the resistances is the ratio of the p.d.
Equation form (not recommended to memorize):
V1 = [R1/(R1 + R2)]Vɛ

37. Worked Example 8
What is the reading of the voltmeter?
3 V
1 Ω
1 Ω V

38. Worked Example 9
What is the reading of the voltmeter?
3 V
1 Ω
2 Ω V

39. Worked Example 10
What is the reading of the voltmeter?
5 V
3 Ω
4 Ω V

40. How is a potential divider useful?
Let’s say I only have a 10 V battery, but I only need 5 V of emf for a circuit. I can use a potential divider to “divide up” my 10 V battery into just 5 V.
10 V R R

41. Potentiometer
However, using resistors to divide up emf is inflexible. We cannot change the ratio easily (need to change the resistors manually).
An easier method is to use a potentiometer (or variable potential divider)

42. Potentiometer
50 cm
50 cm R V
R/2
R/2 V

43. Worked Example 11 What is the reading of the voltmeter? 5 V 30 cm

44. Worked Example 12
State and explain what will happen to the lamp as the jockey slides from the 0 cm mark to the 100 cm mark.
5 V
0 cm
100 cm

45. Worked Example 13
Potential divider circuits may also involve the use of LDRs and Thermistors
Design a circuit which switches on a lamp automatically when it turns dark (hint: when bright, p.d. across lamp is low. when dark, p.d. across lamp is high)

46. 2 tips for dc circuits

47. Redraw Circuit Diagrams

48. Replace Resistors in a Cluster

49. Electricity Problem Solving Toolkit
4 equations
Definition of Current
Definition of Resistance
Electrical Power
Electrical Energy
2 arrangements of Resistors
In series
In parallel
5 Principles
Current in Series
Current in Parallel
P.d. in series
P.d. in parallel
Potential Divider
2 tips
redraw diagram
replace parallel resistors