Chapter 8: Electric fields
Exercise 8.2 Calculating force and field strength We define electric field strength at a point in a field in terms of the force on a positive charge placed at that point. This exercise tests your understanding of the equations that define electric field strength and how to apply them. Note: elementary charge, e = 1.6 × 10−19 C. 1 Electric field strength is defined by the equation: E=F Q a State the quantities represented by E, F and Q, and give the units of each. b Rearrange the equation to make F its subject. c Deduce an equation for the acceleration a of a charged particle of mass m in an electric field. Note: use the equation that relates F, m and a. 2 a Calculate the electric field strength when a force of 2.0 × 10−9 N acts on a charge of 4.5 × 10−6 C. b Calculate the force on an electron placed in a field of strength 2.0 × 104 N C−1. 3 The field strength is the same at all points in a uniform electric field. A uniform electric field can be produced by applying a potential difference between two parallel plates. The field strength is given by E = V/d. a State the quantity represented by each of the symbols E, V and d, and give the units of each. b Calculate the field strength between two parallel metal plates separated by a distance of 20.0 cm when there is a p.d. of 5.0 kV between them. Your answer can be in V m −1 or N C−1 as they are the same. c What p.d. is needed to produce a field strength of 500 V m−1 between two parallel metal plates separated by 1.0 cm? d What force will be exerted on a particle of charge +2e placed between two parallel plates separated by a distance of 140 mm when there is a p.d. of 400 V between them? You can do this in two steps: first calculate the field strength. e Calculate the force on the charge shown in this diagram: + 10 kV
5.0 mm
+ +2.0 × 103 C
State the direction of the force.
Exercise 8.3 Moving in an electric field This exercise considers charges moving in an electric field. A charged particle moving in a uniform electric field is like a mass moving in a uniform gravitational field (in other words, like a projectile). Remember that the usual laws of motion apply to a charged particle moving in an electric field.
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