1. Chapter 25
Electric Circuits

2. Direct Current
When the current in a circuit has a constant direction, the current is called direct current
Most of the circuits analyzed will be assumed to be in steady state, with constant magnitude and direction
Because the potential difference between the terminals of a battery is constant, the battery produces direct current
The battery is known as a source of emf

3. Sources of emf
The source that maintains the current in a closed circuit is called a source of emf
Any devices that increase the potential energy of charges circulating in circuits are sources of emf (e.g. batteries, generators, etc.)
The emf (ε) is the work done per unit charge
Emf of a source is the maximum possible voltage that the source can provide between its terminals
SI units: Volts

4. Electric Circuits
A circuit is a collection of objects usually containing a source of electrical energy connected to elements that convert electrical energy to other forms
A circuit diagram – a simplified representation of an actual circuit – is used to show the path of the real circuit
Circuit symbols are used to represent various elements

5. Resistors in Series
When two or more resistors are connected end-to-end, they are said to be in series
The current is the same in all resistors because any charge that flows through one resistor flows through the other
The sum of the potential differences across the resistors is equal to the total potential difference across the combination

6. Resistors in Series
The equivalent resistance has the effect on the circuit as the original combination of resistors (consequence of conservation of energy)
For more resistors in series:
The equivalent resistance of a series combination of resistors is greater than any of the individual resistors

7. emf and Internal Resistance
A real battery has some internal resistance r; therefore, the terminal voltage is not equal to the emf
The terminal voltage: ΔV = Vb – Va
ΔV = ε – Ir
For the entire circuit (R – load resistance):
ε = ΔV + Ir
= IR + Ir

8. emf and Internal Resistance
ε = ΔV + Ir = IR + Ir
ε is equal to the terminal voltage when the current is zero – open-circuit voltage
I = ε / (R + r)
The current depends on both the resistance external to the battery and the internal resistance
When R >> r, r can be ignored
Power relationship: I ε = I2 R + I2 r
When R >> r, most of the power delivered by the battery is transferred to the load resistor

9. Resistors in Parallel
The potential difference across each resistor is the same because each is connected directly across the battery terminals
The current, I, that enters a point must be equal to the total current leaving that point (conservation of charge)
The currents are generally not the same

10. Resistors in Parallel

11. Resistors in Parallel For more resistors in parallel:
The inverse of the equivalent resistance of two or more resistors connected in parallel is the algebraic sum of the inverses of the individual resistance
The equivalent is always less than the smallest resistor in the group

12. Problem-Solving Strategy
Combine all resistors in series
They carry the same current
The potential differences across them are not necessarily the same
The resistors add directly to give the equivalent resistance of the combination:
Req = R1 + R2 + …

13. Problem-Solving Strategy
Combine all resistors in parallel
The potential differences across them are the same
The currents through them are not necessarily the same
The equivalent resistance of a parallel combination is found through reciprocal addition:

14. Problem-Solving Strategy
A complicated circuit consisting of several resistors and batteries can often be reduced to a simple circuit with only one resistor
Replace resistors in series or in parallel with a single resistor
Sketch the new circuit after these changes have been made
Continue to replace any series or parallel combinations
Continue until one equivalent resistance is found

15. Problem-Solving Strategy
If the current in or the potential difference across a resistor in the complicated circuit is to be identified, start with the final circuit and gradually work back through the circuits (use formula ΔV = I R and the procedures describe above)

16. Chapter 25 Problem 20
What resistance should you place in parallel with a 56-kΩ resistor to make an equivalent resistance of 45 kΩ?

17. Kirchhoff’s Rules
There are ways in which resistors can be connected so that the circuits formed cannot be reduced to a single equivalent resistor
Two rules, called Kirchhoff’s Rules (Laws) can be used instead:
1) Junction (Node) Rule
2) Loop Rule
Gustav Kirchhoff
1824 – 1887

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

19. Junction Rule
I1 = I2 + I3
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

20. Loop Rule
When applying the loop rule, choose a direction for transversing the loop
Record voltage drops and rises as they occur
If a resistor is transversed in the direction of the current, the potential across the resistor is – IR
If a resistor is transversed in the direction opposite of the current, the potential across the resistor is +IR

21. Loop Rule
If a source of emf is transversed in the direction of the emf (from – to +), the change in the electric potential is +ε
If a source of emf is transversed in the direction opposite of the emf (from + to -), the change in the electric potential is – ε

22. Equations from Kirchhoff’s Rules
Use the junction rule as often as needed, so long as, each time you write an equation, you include in it a current that has not been used in a previous junction rule equation
The number of times the junction rule can be used is one fewer than the number of junction points in the circuit
The loop rule can be used as often as needed so long as a new circuit element (resistor or battery) or a new current appears in each new equation
You need as many independent equations as you have unknowns

23. Equations from Kirchhoff’s Rules

24. Problem-Solving Strategy
Draw the circuit diagram and assign labels and symbols to all known and unknown quantities
Assign directions to the currents
Apply the junction rule to any junction in the circuit
Apply the loop rule to as many loops as are needed to solve for the unknowns
Solve the equations simultaneously for the unknown quantities
Check your answers

25. Chapter 25 Problem 72
The figure shows a portion of a circuit used to model muscle cells and neurons. All resistors have the same value R = 1.5 MΩ, and the emfs are ε1 = 75 mV, ε2 = 45 mV and ε3 = 20 mV. Find the current through ε3, including its direction.

26. RC Circuits
If a direct current circuit contains capacitors and resistors, the current will vary with time
At the instant the switch is closed, the charge on the capacitor is zero
When the circuit is completed, the capacitor starts to charge, and the potential difference across the capacitor increases, until the charge reaches its maximum (Q = Cε)

27. Charging Capacitor in an RC Circuit
Once the capacitor is fully charged (maximum charge is reached), the current in the circuit is zero, and the potential difference across the capacitor matches that supplied by the battery

28. Charging Capacitor in an RC Circuit

29. Charging Capacitor in an RC Circuit
The charge on the capacitor varies with time
q = Q (1 – e -t/RC )
The time constant,  = RC, represents the time required for the charge to increase from zero to 63.2% of its maximum
In a circuit with a large (small) time constant, the capacitor charges very slowly (quickly)
After t = 10 , the capacitor is over 99.99% charged

30. Discharging Capacitor in an RC Circuit
When a charged capacitor is placed in the circuit, it can be discharged

31. Discharging Capacitor in an RC Circuit
The charge on the capacitor varies with time
q = Qe -t/RC
The charge decreases exponentially
At t =  = RC, the charge decreases to Qmax; i.e., in one time constant, the capacitor loses 63.2% of its initial charge

32. Chapter 25 Problem 34
An uncharged 10-µF capacitor and a 470-kΩ resistor are in series, and 250 V is applied across the combination. How long does it take the capacitor voltage to reach 200 V?

33. Meters in a Circuit – Ammeter, Voltmeter
An ammeter is used to measure current in line with the bulb – all the charge passing through the bulb also must pass through the meter
A voltmeter is used to measure voltage (potential difference) – connects to the two ends of the bulb

34. 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

35. 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

36. Answers to Even Numbered Problems
Chapter 25:
Problem 18
200 kJ

37. Answers to Even Numbered Problems
Chapter 25:
Problem 26
2 A

38. Answers to Even Numbered Problems
Chapter 25:
Problem 30
1 kW

39. Answers to Even Numbered Problems
Chapter 25:
Problem 58
7.5 mJ