Electricity plays a vital role in our daily lives, from natural phenomena like lightning to everyday electrostatic effects such as a charged comb bending a stream of water or attracting pieces of paper. This topic introduces the concept of an electric field, how charged particles behave within it, the definition and calculation of electric field strength, as well as the fundamental concepts of electric current and electric potential difference.
See how electric fields form around charges and parallel plates, how charged particles behave, and the core relationships for field strength, current and potential difference.
An electric field is the region around a charged particle where any electric charge in the region will experience an electric force.
Field lines begin on a positive charge and end on a negative charge. They never cross each other. Around an isolated positive charge they point outward; around a negative charge they point inward.
Electric field strength, \(E\), is the electric force acting on a unit positive charge placed at that point.
For two parallel charged plates, the field is uniform, straight, parallel and equally spaced. Its strength is given by \(E = \frac{V}{d}\).
General electric field strength
\(E\) = electric field strength (N C⁻¹), \(F\) = electric force (N), \(q\) = quantity of electric charge (C).
Uniform electric field between two parallel charged plates
\(V\) = potential difference between two parallel plates (V), \(d\) = distance between two parallel plates (m). S.I. unit: volt per metre (V m⁻¹). Note: 1 V m⁻¹ = 1 N C⁻¹.
Electric current
\(I\) = current (A), \(Q\) = total charge (C), \(t\) = time (s). S.I. unit: coulomb per second (C s⁻¹) or ampere (A).
Potential difference
\(V\) = potential difference (V), \(W\) = work done (J), \(Q\) = amount of charges flowing (C). S.I. unit: joule per coulomb (J C⁻¹) or volt (V).
Answer in your mind, then press “Check Answer”.
What is the definition of electric field strength?
Why does a candle flame spread more towards the negatively charged plate when placed in a uniform electric field?
A current of 0.2 A flows through a light bulb for 1 hour. Calculate the electric charge that flows through the bulb.
A neutral metal-coated ball placed centrally between two parallel plates will start oscillating spontaneously as soon as the power is turned on.
The ball is electrically neutral initially and experiences equal opposing forces; it must be displaced to touch one plate first to acquire an initial charge before oscillation begins.
Electric field lines can cross each other where electric forces are strong.
Electric field lines never cross each other because the resultant electric force at any single point has only one unique direction.
Potential difference and electric current are the same thing.
Current is the rate of flow of charge, \(I = \frac{Q}{t}\), whereas potential difference is the work done per unit charge to move charges between two points, \(V = \frac{W}{Q}\).
Objective: Check your understanding of electric field strength, current and potential difference.
1 What will happen to the electric field strength between two parallel plates if the distance between them is halved while the potential difference remains constant?
2 Why does the spread of a candle flame in an electric field skew more towards the negative plate than towards the positive plate?
3 The potential difference between two points is defined as the work done in moving one of charge from one point to another.
4 Electric field lines always point from a positive charge and inward toward a negative charge.
Drag each quantity to the correct formula or definition. If using a phone, tap the answer first, then tap the matching box.
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