# Electromotive Force. The electromotive force (emf), ε, of a battery is the maximum possible voltage that the battery can provide between its terminals

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1 Direct Current When the current in a circuit has a constant magnitude and direction, the current is called direct current 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

2 Electromotive Force The electromotive force (emf), ε, of a battery is the maximum possible voltage that the battery can provide between its terminals The emf supplies energy, it does not apply a force The battery will normally be the source of energy in the circuit

3 Sample Circuit We consider the wires to have no resistance The positive terminal of the battery is at a higher potential than the negative terminal There is also an internal resistance in the battery

4 Internal Battery Resistance If the internal resistance is zero, the terminal voltage equals the emf In a real battery, there is internal resistance, r The terminal voltage, ΔV = ε - Ir

5 Active Figure 28.2 (SLIDESHOW MODE ONLY)

6 EMF, cont The emf is equivalent to the open-circuit voltage This is the terminal voltage when no current is in the circuit This is the voltage labeled on the battery The actual potential difference between the terminals of the battery depends on the current in the circuit

7 Load Resistance The terminal voltage also equals the voltage across the external resistance This external resistor is called the load resistance In the previous circuit, the load resistance is the external resistor In general, the load resistance could be any electrical device

8 Power The total power output of the battery is P = IΔV = Iε This power is delivered to the external resistor (I 2 R) and to the internal resistor (I 2 r) P = Iε = I 2 R + I 2 r

9 Resistors in Series When two or more resistors are connected end-to-end, they are said to be in series For a series combination of resistors, the currents are the same in all the resistors because the amount of charge that passes through one resistor must also pass through the other resistors in the same time interval The potential difference will divide among the resistors such that the sum of the potential differences across the resistors is equal to the total potential difference across the combination

10 Resistors in Series, cont Potentials add ΔV = IR 1 + IR 2 = I (R 1 +R 2 ) Consequence of Conservation of Energy The equivalent resistance has the same effect on the circuit as the original combination of resistors

11 Equivalent Resistance Series R eq = R 1 + R 2 + R 3 + The equivalent resistance of a series combination of resistors is the algebraic sum of the individual resistances and is always greater than any individual resistance If one device in the series circuit creates an open circuit, all devices are inoperative

12 Equivalent Resistance Series An Example Two resistors are replaced with their equivalent resistance

13 Active Figure 28.4 (SLIDESHOW MODE ONLY)

14 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 I = I 1 + I 2 The currents are generally not the same Consequence of Conservation of Charge

15 Equivalent Resistance Parallel, Examples Equivalent resistance replaces the two original resistances Household circuits are wired so that electrical devices are connected in parallel Circuit breakers may be used in series with other circuit elements for safety purposes

16 Active Figure 28.6 (SLIDESHOW MODE ONLY)

17 Equivalent Resistance Parallel Equivalent Resistance = + + +K R R R R eq 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

18 Resistors in Parallel, Final In parallel, each device operates independently of the others so that if one is switched off, the others remain on In parallel, all of the devices operate on the same voltage The current takes all the paths The lower resistance will have higher currents Even very high resistances will have some currents

19 Combinations of Resistors The 8.0-Ω and 4.0-Ω resistors are in series and can be replaced with their equivalent, 12.0 Ω The 6.0-Ω and 3.0-Ω resistors are in parallel and can be replaced with their equivalent, 2.0 Ω These equivalent resistances are in series and can be replaced with their equivalent resistance, 14.0 Ω

20 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

21 More about the Loop Rule Traveling around the loop from a to b In (a), the resistor is traversed in the direction of the current, the potential across the resistor is IR In (b), the resistor is traversed in the direction opposite of the current, the potential across the resistor is is + IR

22 Loop Rule, final In (c), the source of emf is traversed in the direction of the emf (from to +), and the change in the electric potential is +ε In (d), the source of emf is traversed in the direction opposite of the emf (from + to -), and the change in the electric potential is -ε

23 Loop Equations from Kirchhoff s Rules 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

24 Kirchhoff s Rules Equations, final In order to solve a particular circuit problem, the number of independent equations you need to obtain from the two rules equals the number of unknown currents Any capacitor acts as an open branch in a circuit The current in the branch containing the capacitor is zero under steady-state conditions

25 Problem-Solving Hints Kirchhoff s Rules Draw the circuit diagram and assign labels and symbols to all known and unknown quantities. Assign directions to the currents. The direction is arbitrary, but you must adhere to the assigned directions when applying Kirchhoff s rules Apply the junction rule to any junction in the circuit that provides new relationships among the various currents

26 EX. V Let V = 0 i V V R = 0 R V V VR = V rise drop 0 V = VR = ir V i = R V Let V i = 0 R V V = V R drop V drop 0 ir V = 0 V i = R Kirchhoffs current rule ( node rule ) The sum of the currents entering any node ( junction ) must be equal to that of leaving currents. ( conservation of energy) i 2 i 1 i 3 i i i = ( i = i + i )

27 EX. 6i+ 18 4i 10i 6 4i= 0 24i = 12 1 i= ( A ) 2

28 Resistance in series V R 1 R n V R eq By loop rule, V ir ir... = 0 V = ir = i R R eq k k V = = i k k R k k 1 2 Resistance in parallel By node rule, V R1 Rn V R eq V V 1 i = i1+ i2 + i = = V R R R R eq 1 1 = R R eq V V = = i V R k k k k 1 2 k k

29 i i R motion R motion Δ V= -ir Δ V=+ ir Resistance Rule: For a move through a resistance in the direction of the current, the change in the potentialδ V = ir. For a move through a resistance in the direction opposite to that of the current, the change in the potential Δ V =+ ir (27 6)

30 Ex: V e R 1 a i i 1 R 4 b R2 R 3 1 i2 i3 =? d i i 3 2 In general, for n unknowns take number of sum of nodes and loops = n. Solve these n independent equations. i V Sol. 2 By Kirchhoff s rules Node rule: node b i1 = i2 + i3l (1) Loop rule:loop aebda V i1r1 i2r3 i1r4= 0 L (2) Loop abed: ir 2 3 ir 3 2 = 0 L (3) i 1 0 i1 R R R 0 i = V i = R i 3 R2 3 0 i 3 c Sol. 1 By Ohm s law i i 1 1 = = i R i 3 R, 2 V // R i i = + R R R

31 RC Circuits A direct current circuit may contain capacitors and resistors, the current will vary with time When the circuit is completed, the capacitor starts to charge The capacitor continues to charge until it reaches its maximum charge (Q = Cε) Once the capacitor is fully charged, the current in the circuit is zero

32 Quick Quiz 28.1 In order to maximize the percentage of the power that is delivered from a battery to a device, the internal resistance of the battery should be (a) as low as possible (b) as high as possible (c) The percentage does not depend on the internal resistance.

33 Quick Quiz 28.1 Answer: (a). Power is delivered to the internal resistance of a battery, so decreasing the internal resistance will decrease this lost power and increase the percentage of the power delivered to the device.

34 Quick Quiz 28.2 In the figure below, imagine positive charges pass first through R 1 and then through R 2. Compared to the current in R 1, the current in R 2 is (a) smaller (b) larger (c) the same

35 Quick Quiz 28.2 Answer: (c). In a series circuit, the current is the same in all resistors in series. Current is not used up as charges pass through a resistor.

36 Quick Quiz 28.4 With the switch in this circuit closed (left), there is no current in R 2, because the current has an alternate zero-resistance path through the switch. There is current in R 1 and this current is measured with the ammeter (a device for measuring current) at the right side of the circuit. If the switch is opened (Fig. 28.5, right), there is current in R 2. What happens to the reading on the ammeter when the switch is opened? (a) the reading goes up (b) the reading goes down (c) the reading does not change

37 Quick Quiz 28.4 Answer: (b). When the switch is opened, resistors R 1 and R 2 are in series, so that the total circuit resistance is larger than when the switch was closed. As a result, the current decreases.

38 Quick Quiz 28.8 In using Kirchhoff s rules, you generally assign a separate unknown current to (a) each resistor in the circuit (b) each loop in the circuit (c) each branch in the circuit (d) each battery in the circuit

39 Quick Quiz 28.8 Answer: (c). A current is assigned to a given branch of a circuit. There may be multiple resistors and batteries in a given branch.

40 RC Circuits A direct current circuit may contain capacitors and resistors, the current will vary with time When the circuit is completed, the capacitor starts to charge The capacitor continues to charge until it reaches its maximum charge (Q = Cε) Once the capacitor is fully charged, the current in the circuit is zero

41 Charging an RC Circuit As the plates are being charged, the potential difference across the capacitor increases At the instant the switch is closed, the charge on the capacitor is zero Once the maximum charge is reached, the current in the circuit is zero The potential difference across the capacitor matches that supplied by the battery

42 Active Figure (SLIDESHOW MODE ONLY)

43 q E ir = 0 The current i = dq E dq R q = 0 C dt dt C dq q R + = E dt C If we rearrange the terms we have: This is an inhomogeneous, first order, linear differential equation with initial condition: q (0) = 0. This condition expresses the fact that at t = 0 the capacitor is uncharged. (27 15)

44 τ = RC Differential equation: Initial condition: Solution: Here: τ = dq R dt q (0) = 0 ( t / τ 1 ) q = CE e RC q + = E C (27 16)

45 Charging a Capacitor in an RC Circuit The charge on the capacitor varies with time q = Cε(1 e -t/rc ) = Q(1 e -t/rc ) τ is the time constant τ = RC The current can be found I( t) = ε e R trc

46 Time Constant, Charging The time constant represents the time required for the charge to increase from zero to 63.2% of its maximum τ has units of time The energy stored in the charged capacitor is ½ Qε = ½ Cε 2

47 Active Figure (SLIDESHOW MODE ONLY)

48 Discharging a Capacitor in an RC Circuit When a charged capacitor is placed in the circuit, it can be discharged q = Qe -t/rc The charge decreases exponentially

49 Discharging Capacitor At t = τ = RC, the charge decreases to Q max In other words, in one time constant, the capacitor loses 63.2% of its initial charge The current can be found dq Q trc I t = = e () dt RC Both charge and current decay exponentially at a rate characterized by τ = RC

50 Galvanometer A galvanometer is the main component in analog meters for measuring current and voltage Digital meters are in common use Digital meters operate under different principles

51 Galvanometer, cont A galvanometer consists of a coil of wire mounted so that it is free to rotate on a pivot in a magnetic field The field is provided by permanent magnets A torque acts on a current in the presence of a magnetic field

52 Active Figure (SLIDESHOW MODE ONLY)

53 Galvanometer, final The torque is proportional to the current The larger the current, the greater the torque The greater the torque, the larger the rotation of the coil before the spring resists enough to stop the rotation The deflection of a needle attached to the coil is proportional to the current Once calibrated, it can be used to measure currents or voltages

54 Ammeter An ammeter is a device that measures current The ammeter must be connected in series with the elements being measured The current must pass directly through the ammeter

55 Ammeter in a Circuit The ammeter is connected in series with the elements in which the current is to be measured Ideally, the ammeter should have zero resistance so the current being measured is not altered

56 Ammeter from Galvanometer The galvanometer typically has a resistance of 60 Ω To minimize the resistance, a shunt resistance, R p, is placed in parallel with the galvanometer

57 Active Figure (SLIDESHOW MODE ONLY)

58 Ammeter, final The value of the shunt resistor must be much less than the resistance of the galvanometer Remember, the equivalent resistance of resistors in parallel will be less than the smallest resistance Most of the current will go through the shunt resistance, this is necessary since the full scale deflection of the galvanometer is on the order of 1 ma

59 Voltmeter A voltmeter is a device that measures potential difference The voltmeter must be connected in parallel with the elements being measured The voltage is the same in parallel

60 Voltmeter in a Circuit The voltmeter is connected in parallel with the element in which the potential difference is to be measured Polarity must be observed Ideally, the voltmeter should have infinite resistance so that no current would pass through it

61 Voltmeter from Galvanometer The galvanometer typically has a resistance of 60 Ω To maximize the resistance, another resistor, R s, is placed in series with the galvanometer

62 Voltmeter, final The value of the added resistor must be much greater than the resistance of the galvanometer Remember, the equivalent resistance of resistors in series will be greater than the largest resistance Most of the current will go through the element being measured, and the galvanometer will not alter the voltage being measured

63 Household Wiring The utility company distributes electric power to individual homes by a pair of wires Each house is connected in parallel with these wires One wire is the live wire and the other wire is the neutral wire connected to ground

64 Household Wiring, cont The potential of the neutral wire is taken to be zero Actually, the current and voltage are alternating The potential difference between the live and neutral wires is about 120 V

65 Household Wiring, final A meter is connected in series with the live wire entering the house This records the household s consumption of electricity After the meter, the wire splits so that multiple parallel circuits can be distributed throughout the house Each circuit has its own circuit breaker For those applications requiring 240 V, there is a third wire maintained at 120 V below the neutral wire

66 Short Circuit A short circuit occurs when almost zero resistance exists between two points at different potentials This results in a very large current In a household circuit, a circuit breaker will open the circuit in the case of an accidental short circuit This prevents any damage A person in contact with ground can be electrocuted by touching the live wire

67 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 touching the live wire the part of the body in which the current exists

68 Effects of Various Currents 5 ma or less can cause a sensation of shock generally little or no damage 10 ma muscles contract may be unable to let go of a live wire 100 ma if passing through the body for 1 second or less, can be fatal paralyzes the respiratory muscles

69 More Effects In some cases, currents of 1 A can produce serious burns Sometimes these can be fatal burns No contact with live wires is considered safe whenever the voltage is greater than 24 V

70 Ground Wire Electrical equipment manufacturers use electrical cords that have a third wire, called a ground This safety ground normally carries no current and is both grounded and connected to the appliance

71 Ground Wire, cont If the live wire is accidentally shorted to the casing, most of the current takes the low-resistance path through the appliance to the ground If it was not properly grounded, anyone in contact with the appliance could be shocked because the body produces a low-resistance path to ground

72 Ground-Fault Interrupters (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

73 The constant τ is known as the time constant of the circuit. If we plot q versus t we see that q does not reach its terminal value CE but instead increases from its initial value and it reaches the terminal value at t =. Do we have to wait for an intercity to charge the capacitor? In practice no. qt ( = τ ) = ( 0.632) CE ( ) ( ) qt ( = 3 τ ) = qt ( = 5 ) = CE CE τ If we wait only a few time constants the charge, for all practical purposes has reached its terminal value CE. i dq E = = e t / τ The current dt If we plot i versus t we R get a decaying exponential (see fig. b)

74 q o O q (27 17) τ = t i RC + - RC circuits:discharging of a capacitor Consider the circuit shown in the figure. We assume that the capacitor at t = 0 has charge q o and that at t = 0 we throw the switch S from the middle position to position b. The capacitor is disconnected from the battery and looses its charge through resistor R. We will write KLR starting at point b and going in the counterclockwise direction: q ir = 0 C dq dq q = R + = 0 dt dt C Taking into account that we get: i

75 This is an homogeneous, first order, linear differential equation with initial condition: q(0) = qo. The solution -/ t τ is: q = qoe where τ = RC. If we plot q versus t we get a decaying exponential. The charge becomes zero at In practical terms we only have to wait a few time constants. ( ) ( ) ( ) q( τ ) = q, q(3 τ) = q, q(5 τ) = q o o o t =

76 Ohm s law 1 Which statement (s) is (are) true?when a long straight conducting wire of constant cross-section is connected to the terminals of a battery. The electric field A lines are uniformly distributed over the crosssectional area of the conductor B inside the wire is of constant magnitude and its direction is parallel to the wire C both of the above D neither of the above

77 Quick Quiz 28.9a Consider the circuit seen below and assume that the battery has no internal resistance. Just after the switch is closed, the potential difference across which of the following is equal to the emf of the battery? (a) C (b) R (c) neither C nor R.

78 Quick Quiz 28.10a Consider the circuit in the figure below and assume that the battery has no internal resistance. Just after the switch is closed, the current in the battery is (a) zero (b) ε/2r (c) 2ε /R (d) ε/r (e) impossible to determine.

79 Quick Quiz 28.10a Answer: (c). Just after the switch is closed, there is no charge on the capacitor. Current exists in both branches of the circuit as the capacitor begins to charge, so the right half of the circuit is equivalent to two resistances R in parallel for an equivalent resistance of 1/2R.

80 Quick Quiz 28.10b After a very long time, the current in the battery is (a) zero (b) ε/2r (c) 2ε/R (d) ε /R (e) impossible to determine.

81 Quick Quiz 28.10b Answer: (d). After a long time, the capacitor is fully charged and the current in the right-hand branch drops to zero. Now, current exists only in a resistance R across the battery.

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