Edexcel A-Level Physics Electric Field Strength

Practise Edexcel A-Level Physics electric field strength by connecting electric force, charge, plate voltage and particle acceleration.

Exam points

use E equals F over Q to calculate electric force, charge or field strength
combine E equals V over d with F equals EQ for fields between parallel plates
calculate acceleration or beam deflection by linking electric force to F equals ma

[Maximum number: 5]

Some spiders may be seen suddenly moving up into the air, even when there is no wind. This happens when they let out strands of web.

The spiders are pulled upwards as the electrically charged strands of web interact with the Earth's electric field.

The average strength of the Earth's electric field is $120 \mathrm{~V} \mathrm{~m}^{-1}$ downwards.

A spider moves upwards after letting out a strand of web. Determine the initial upwards acceleration of the spider.

mass of spider $=3.00\times10^{-6}\,\mathrm{kg}$

total magnitude of charge on a strand of web $=3.00\times10^{-7}\,\mathrm{C}$

Use of F=E Q

- Use of W=m g

- F-W to determine resultant force

- Use of F=m a

- $a=2.2 \mathrm{~m} \mathrm{~s}^{-2}$

Example of calculation
$\begin{aligned} & F=120 \mathrm{~V} \mathrm{~m}^{-1} \times 3.00 \times 10^{-7} \mathrm{C}=3.60 \times 10^{-5} \mathrm{~N} & W=3.00 \times 10^{-6} \mathrm{~kg} \times 9.81 \mathrm{Nkg}^{-1}=2.94 \times 10^{-5} \mathrm{~N} & \text { Resultant force }=3.60 \times 10^{-5} \mathrm{~N}-2.94 \times 10^{-5} \mathrm{~N}=6.57 \times 10^{-6} \mathrm{~N} & a=6.57 \times 10^{-6} \mathrm{~N} \div 3.00 \times 10^{-6} \mathrm{~kg} & =2.19 \mathrm{~m} \mathrm{~s}^{-2} \end{aligned}$

(5)

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4.4.93 - Electric field strength question 1