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Chapter 25
Electric CurrentsAnd Resistance
Physics for Scientists & Engineers, 3rd EditionDouglas C. Giancoli
© Prentice Hall
Figure 25-3
Figure 25-4 (a)
Figure 25-5
Figure 25-6
Figure 25-22
Figure 25-23
Figure 25-24
Figure 25-25
3P12-
Resistors & Ohm’s Law
RAρ
=
IRV =∆
parallel 1 2
1 1 1R R R
= +series 1 2R R R= +
Figure 25-7
Figure 26-2
P12-
Measuring Potential DifferenceA voltmeter must be hooked in parallel across the element you want to measure the potential difference across
Voltmeters have a very large resistance, so that they don’t affect the circuit too much
P12-
Measuring CurrentAn ammeter must be hooked in series with the element you want to measure the current through
Ammeters have a very low resistance, so that they don’t affect the circuit too much
P12-
Measuring ResistanceAn ohmmeter must be hooked in parallel across the element you want to measure the resistance of
Here we are measuring R1
Ohmmeters apply a voltage and measure the current that flows. They typically won’t work if the resistor is powered (connected to a battery)
Figure 26-3 (a)
Figure 26-3 (b)
Figure 26-4 (a)
Figure 26-4 (b)
Figure 26-4 (a)
Figure 26-6
Figure 26-8
Figure 26-11
Figure 26-12
Figure 26-40
Figure 26-39
10P12-
(Dis)Charging a Capacitor1. When the direction of current flow is toward
the positive plate of a capacitor, then
dQIdt
= +
Charging
2. When the direction of current flow is away from the positive plate of a capacitor, then
DischargingdQIdt
= −
11P12-
Charging A Capacitor
What happens when we close switch S?
12P12-
Charging A Capacitor
NO CURRENTFLOWS!
0=−−=∆∑ IRCQV
ii ε
1. Arbitrarily assign direction of current2. Kirchhoff (walk in direction of current):
13P12-
Charging A CapacitorQ dQ RC dt
ε − =dQ dt
Q C RCε⇒ = −
−
0 0
Q tdQ dtQ C RCε = −−∫ ∫
A solution to this differential equation is:
( )/( ) 1 t RCQ t C eε −= −RC is the time constant, and has units of seconds
14P12-
Charging A Capacitor
/t RCdQI edt R
ε −= =( )/1 t RCQ C eε −= −
16P12-
Discharging A Capacitor
What happens when we close switch S?
17P12-
Discharging A Capacitor
NO CURRENTFLOWS!
dtdqI −=
0=−=∆∑ IRCqV
ii
18P12-
Discharging A Capacitor
0dq qdt RC
+ =0 0
Q t
Q
dq dtq RC
⇒ = −∫ ∫
/( ) t RCoQ t Q e−=
19P12-
General Comment: RCAll Quantities Either:
( )/FinalValue( ) Value 1 tt e τ−= − /
0Value( ) Value tt e τ−=
τ can be obtained from differential equation (prefactor on d/dt) e.g. τ = RC
20P12-
Exponential Decay
/0Value( ) Value tt e τ−=
Very common curve in physics/nature
How do you measure τ?
1) Fit curve (make sure you exclude data at both ends)
(t0,v0)
(t0+τ,v0/e)
2) a) Pick a pointb) Find point with y
value down by ec) Time difference is τ
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