EMF: Electromotive force (E)
Potential Divider Circuits
Current I = ∆Q Charge (c) E = V2 x t Power (P) - Energy
J/s - watt
Amps (A) transferred per unit of
∆t Time (s) time.
P = ∆E
Current = rate of flow of charge -electrons in a wire
-electrons in beam
R2 V2 ∆t
P = V2
NB:∆ = small change in
P = IV
If R1 = R2 then V1 reads 2.5V
N=I = charge/sec E = ItV V2 reads 2.5V
Making good sensors
q As R2 increases then V2 increases & V1
• High resolution -the smallest change the sensor can
No. of ions/ sec decreases
detect. Precision of a sensor depends upon resolution.
V1 + V2 ≡ 5V
2 Chapter 2 • Appropriate output for a given input (Sensitivity) -ratio
E = I RT of change of output to change of input. (Eg Oscilloscope
Sensing R1 = V1
• Rapid response time –time taken to respond to a
Potential Difference - difference in energy per R2 v2
change in input.
coulomb between two places in a circuit V2 = R2__ • Small unsystematic fluctuations (random error) in results
Vin R1 + R2 (Noise, random error, fluctuations) -present in all
V = ∆E Energy transferred (J)
experiments, the size limits the precision of measurements.
∆Q Averages improve the final results.
Energy flowing (c) • Small systematic error –very hard to detect, include zero error
and error due to disturbing influences. ‘Smart’ systems process
information to compensate for disturbing influences.
No. of charge Drift Source : Some Ohm’s Law –resistance and conductance
P.d. J/c or Volts (V) carriers resistance • The conductance or resistance can be
I= E_______ calculated at any given p.d.
I = nAvq Rexternal + Rinternal • If the conductance or resistance are
Output constant, independent of the current
G=I V = I x Rexternal or p.d., the conductor is said to obey
V = IR
Area (m ) Charge
V = E - I x Rinternal Ohm’s law, or to be ‘ohmic’.
• Ohm's law thus says that the
Source: High Source: Low conductance, and resistance, of a
A1 resistance resistance given component is constant. The
Rinternal ≈ ∞ Rinternal ≈ 0 same value can be used in calculations
R=V V R1 R2 R2 I≈0
Rexternal + Rinternal •
what ever the current or p.d.
Most metals are ohmic at constant
I A1 A2 G=1 A1
V = I x Rexternal ≈ 0 Output temp.; ionised gases are not.
A1 V = I x Rexternal ≈ E See also Chapter 4