1. Direct Current Circuits
Chapter 18
Direct Current Circuits

2. Chapter 18
emf
Resistors in Circuits
Kirchhoff’s Rules
RC circuits

3. Sources of emf What is emf? What is the SI units?
What is terminal voltage?

4. Internal Resistance What is internal resistance?
How do I calculate terminal voltage?
What about the circuit?
What is an open circuit?
What is a load resistance?
The case for R>>r

5. Example
A battery having an emf of 9 V delivers 117 mA when connected to a 72 ohms load. Determine the internal resistance of the battery.

6. Resistors in Series What is a series connection?
What happens to current?
What about voltage?
What about equivalent resistance?

7. Equivalent Resistance – Series: An Example
Four resistors are replaced with their equivalent resistance

8. Resistors in Parallel What is parallel connection?
What happens to current?
What about voltage?
How do I calculate equivalent resistance?

9. Equivalent Resistance – Parallel Example

10. Equivalent Resistance – Complex Circuit

11. Parallel Circuits
What is the advantage?

12. Gustav Kirchhoff 1824 – 1887 Invented spectroscopy with Robert Bunsen
Formulated rules about radiation
Formulated the Kirchhoff’s rules

13. Kirchhoff’s Rules
Why do you need kirchhoff’s rules?

14. Statement of Kirchhoff’s Rules
Junction Rule
The sum of the currents entering any junction must equal the sum of the currents leaving that junction
A statement of Conservation of Charge
Loop Rule
The sum of the potential differences across all the elements around any closed circuit loop must be zero
A statement of Conservation of Energy

15. More About the Junction Rule

16. Setting Up Kirchhoff’s Rules
Assign symbols and directions to the currents in all branches of the circuit
If a direction is chosen incorrectly, the resulting answer will be negative, but the magnitude will be correct
When applying the loop rule, choose a direction for transversing the loop
Record voltage drops and rises as they occur

17. Convention for loop rule

18. Examples

19. RC Circuits
What is an RC circuit?
What is a time constant?

20. Charging Capacitor in an RC Circuit

21. Discharging Capacitor in an RC Circuit
Charge

22. Example

23. Electrical Safety Electric shock can result in fatal burns
Electric shock can cause the muscles of vital organs (such as the heart) to malfunction
The degree of damage depends on
the magnitude of the current
the length of time it acts
the part of the body through which it passes

24. Effects of Various Currents
5 mA or less
Can cause a sensation of shock
Generally little or no damage
10 mA
Hand muscles contract
May be unable to let go a of live wire
100 mA
If passes through the body for just a few seconds, can be fatal

25. Ground Wire
Electrical equipment manufacturers use electrical cords that have a third wire, called a case ground
Prevents shocks

26. Ground Fault Interrupts (GFI)
Special power outlets
Used in hazardous areas
Designed to protect people from electrical shock
Senses currents (of about 5 mA or greater) leaking to ground
Shuts off the current when above this level