Transcript Lab 3 Resistance and Ohm`s Law

Lab 3 Resistance and Ohm’s Law
Ying Yi PhD
1
PHYS II @ HCC
Current of water
Mass
Current 
t
2
PHYS II @ HCC
Average current
Average current is equal to the amount of charge
divided by the time interval:
Q
I av 
t
Q is the amount of charge that flows through an area A
in a time interval t and that the direction of flow is
perpendicular to the area.
SI unit: coulomb/second (C/s), or the ampere (A)
3
PHYS II @ HCC
Current and Voltage Measurement
Note that: Be sure to always start your measurements
using the highest multimeter scales(say 10A and 1000V),
and increase the sensitivity one scale at a time to obtain
the highest accuracy without overloading the meters.
4
PHYS II @ HCC
Resistance
V
R
I
Resistance is the ratio of the voltage across the
conductor to the current it carries. It is a
constant over a wide range of applied voltage or
currents.
SI unit: ohms (Ω)
Q: Graph shape with ΔV as the ordinate and I as the abscissa?
5
PHYS II @ HCC
Resistor
A conductor that provides a specified resistance in an
electric circuit. The symbol for a resistor in circuit
diagrams is a zigzag line:
l
R
A
 is called the resistivity of the material. Good electric
conductor have very low resistivity.
6
PHYS II @ HCC
Resistors in series
The current is the same in the two
resistors because any charge that
flows through R1 must also flow
through R2.
 ∆Vab+ ∆Vbc= ∆Vac
Ohm’s law
IR1  IR2  IReq
R1  R2  Req
Req  R1  R2  ......
7
PHYS II @ HCC
Resistors in parallel
8
PHYS II @ HCC
Resistors in parallel
The potential difference across the
resistors are the same because each is
connected directly across the battery
terminals. ∆V= ∆V1= ∆V2
Charge is conserved! I=I1+I2
Ohm’s law
V V V


Req
R1
R2
1
1 1
 
Req R1 R2
9
PHYS II @ HCC
1
1 1
   ......
Req R1 R2
Objectives
 Definition of the concept of electrical resistance
 Demonstration of the dependence of the resistance
on the length, cross sectional area, and resistivity of
the wire
 Demonstration of the equivalent resistance of
resistors in series and in parallel arrangements
10
PHYS II @ HCC
Equipment List
 Resistance coils
 Multimeter
 DC supply
 Wire leads with alligator clips
11
PHYS II @ HCC
R1
𝑙1
𝑙2
𝑙3
𝑙4
12
PHYS II @ HCC
R2
R3
R4
= 10𝑚, 𝑑1 = 0.0006439, 𝜌1 = 1.72 × 10−8 Ω. 𝑚
= 10𝑚, 𝑑2 = 0.0003211, 𝜌2 = 1.72 × 10−8 Ω. 𝑚
= 20𝑚, 𝑑3 = 0.0006439, 𝜌3 = 1.72 × 10−8 Ω. 𝑚
= 20𝑚, 𝑑4 = 0.0003211, 𝜌4 = 1.72 × 10−8 Ω. 𝑚
Data
I (A)
VR1
VR2
VR3
VR4
0.50
1.00
1.50
2.00
Combination
R1R2R3R4 Series
R2R3R4 Series
R1R2 Parallel
R1R3 Parallel
R2R3 Parallel
13
PHYS II @ HCC
I (A)
V (volts)
Figures
R1 and R2 in parallel
14
PHYS II @ HCC
R1 and R3 in parallel
Calculations
R1
R2
R3
R4
Rtheo=ρl/A
Rexp
0.5503
% error
Combination
R1R2R3R4 Series
R2R3R4 Series
R1R2 Parallel
R1R3 Parallel
R2R3 Parallel
15
PHYS II @ HCC
(Re)exp1
(Re)exp2
(Re)exp
(Re)theo
% error