Millikan measured the elementary charge (e = 1
Electric Fields - OCR A-Level Physics
- Millikan measured the elementary chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).The magnitude of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). carried by a single electron or proton. Equal to 1.6 × 10⁻¹⁹ C. (e = 1.60 × 10⁻¹⁹ C) by balancing the weight of charged oil drops against the electric force from parallel plates.
- When a drop is held stationary: electric force = weight, so QE = mg, giving $Q = mg/E$.
- The mass was found from Stokes' law by observing the terminal velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.The constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. reached when the driving force on an object is exactly balanced by resistive forces, so the resultant force is zero. of the drop with the field off.
- Millikan found that chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). was always a whole-number multiple of e, proving charge is quantisedExisting only in discrete values. Charge is quantisedExisting only in discrete values. Charge is quantised because it can only exist as whole-number multiples of the elementary charge e. because it can only exist as whole-number multiples of the elementary chargeThe magnitude of charge carried by a single electron or proton. Equal to 1.6 × 10⁻¹⁹ C. e..
$$\begin{aligned}
QE &= mg \quad \Rightarrow \quad Q \\
&= \frac{mg}{E} \\
&= \frac{mgd}{V}
\end{aligned}$$
Common Mistake
MEDIUM
Wrong: Millikan measured the charge on a single electron directly.
Right: He measured the total charge on many different oil drops and found that all charges were integer multiples of 1.60 × 10⁻¹⁹ C. The elementary chargeThe magnitude of charge carried by a single electron or proton. Equal to 1.6 × 10⁻¹⁹ C. was deduced from the highest common factor.
Right: He measured the total charge on many different oil drops and found that all charges were integer multiples of 1.60 × 10⁻¹⁹ C. The elementary chargeThe magnitude of charge carried by a single electron or proton. Equal to 1.6 × 10⁻¹⁹ C. was deduced from the highest common factor.