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4. Resistance, inductors and capacitors
4.1. Resistance and its combination
4.1.1. Resistance
In figure 24, the direction of current and voltage is the same. According to Ohm’s law, the
following formula could be given:




ng formula could be given: 
(a)resister component 
(b) Ideal resister with the Resistance 


∞
ρ

Figure 24. Resistance and its symbol
u =iR
The parameter of resistance is gained: 
R =
u
i
This parameter is called resistance which has the property of holding back the current in circuit.
And it is denoted with the symbol in figure 24b. Here, this relationship could be applied at
very high voltage and current. Some electrictronic materials have a very small range of currents
and voltages where they exhibit linear behavior. In reality, some material is linear only within
a range of values. Outside this range, resistance is not linear. In circuit, we define:(1) having a
0V voltage drop when R=0;(2)having a 0current through resister when R=∞.
Each material has a property called resistivity(ρ) that indicates the resistance of the material.
Conductivity is the inverse of resistivity, and conductance (G) is the inverse of resistance.
Advances in Bioengineering
206

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Conductance is measured in unit called siemens(S) and has the unit of A/V. In terms of
conductance, ohm’s law could be written as:
i =Gu
For formula 
u =iR
, if current is produced by both sides of this equation and they are integrated,
the following equation could be given:
∫
0
t
uidt =
∫
0
t
i
2
Rdt
This formula demonstrates that electrical energy is all consumed by resister component. And
the energy is converted into thermal energy, that’s to say, resister is a consuming-energy
component.
4.1.2. Series and parallel combination of resistance
If the same current flows from one resister to another, the two are said to be in series. If these
two resisters are connected to the third and the same current flows through all of them, then
the three resistors are in series. Consider figure 25 with three resisters in series, an equivalent
circuit can be derived through Kirchhoff’s Voltage Law as follows: