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Edexcel A Level Physics:復習筆記7.2 Electric Field Strength

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年10月11日上午10:50

Electric Field Strength

The?electric field strength?at a point is defined as:

The force per unit charge acting on a positive test charge at that point

The electric field strength can be calculated using the equation:

7.4.3-Electric-Field-Strength-Equation_2

Where:
- E?= electric field strength (N C–1)
- F?= electrostatic force on the charge (N)
- Q?= charge (C)
It is important to use a positive?test charge?in this definition, as this determines the direction of the electric field
Recall, the electric field strength is a?vector?quantity, it is always directed:
- Away?from a positive charge
- Towards?a negative charge
This direction is also denoted by the direction of the electric field

18.1-Radial-E-field-lines

Electric field lines are directed away from a positive point charge and towards a negative point charge

Worked Example

A charged particle is in an electric field with electric field strength 3.5 × 104?N C-1?where it experiences a force of 0.3 N.

Calculate the charge of the particle.

7.4.3-Electric-Field-Strength-Worked-Example

Exam Tip

While the defining equation for electric field strength,?E?=?F / Q?is defined for a positive test charge, it is still useable for negative charges in an electric field. You will find that, if you substitute a negative charge in for?Q, the electric field strength?E?is also negative. This simply means that the vector representing the field points in the?opposite direction?than it would for a positive charge, as you should expect. Make sure you can interpret the direction of electric field lines for your exam!

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