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1 of 7 © Boardworks Ltd 2010 Links to the current specifications AQA 3.4.3b Electric field strength: E is defined by E = F / Q; magnitude of E in a radial field given by E = 1 / 4πε0 Q / r2; magnitude of E in a uniform field given by E = V / d. Edexcel Topic 4.4 – Electric and magnetic fields 83. Explain what is meant by an electric field and recognise and use the expression electric field strength E = F / Q. 84. Draw and interpret diagrams using lines of force to describe radial and uniform electric fields qualitatively. 85. Derive and use the force between two charges expression for the electric field due to a point charge. 86. Recall that applying a potential difference to two parallel plates produces a uniform electric field in the central region between them, and recognise and use the expression E = V / d. OCR 2 of 7 5.1.1b Define electric field strength as force per unit positive charge. 5.1.1c Describe how electric field lines represent an electric field. 5.1.1e Apply E = Q/4πε0r2 for electric field strength of a point charge. 5.1.1f Select and use E = V / d for the magnitude of the uniform electric field strength between charged parallel plates. © Boardworks Ltd 2010 Placing a charge in an electric field 3 of 7 © Boardworks Ltd 2010 Electric field strength Electric field strength is defined as the force experienced per unit charge. The charge in the equation refers to the charge of the particle in the field. electric field strength = force / charge E = F/Q Example: What is the electric field strength around a point charge if a 3.20 × 10-19 C charge experiences a force of 7.30 × 10-15 N? E = F/Q E = (7.30 × 10-15 ) / (3.20 × 10-19 ) E = 2.28 × 104 N C-1 4 of 7 © Boardworks Ltd 2010 Electric field strength: uniform fields The electric field between two parallel plates (e.g. capacitor plates) is uniform: a charge will experience the same force wherever it is placed between the plates. The electric field strength depends on two factors: the voltage between the plates and the distance between them. electric field strength = voltage / distance E = V/d The units of E in this instance are volts per metre (V m-1). 5 of 7 © Boardworks Ltd 2010 Electric field strength: radial fields 6 of 7 © Boardworks Ltd 2010 Electric field strength calculations 7 of 7 © Boardworks Ltd 2010