Electric fields — AQA GCSE Physics
Test yourself on Electric fields with AQA GCSE practice questions.
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Electric fields explained
An electric field is the region around a charged object in which another charged object experiences a force, and that exact wording is what mark schemes want.
Read the full explanation
The field is not something you can see, but it is how the force is carried across the gap, so it exists whether or not a second charge is there to feel it. It is drawn as field lines with arrows showing the direction of the force on a small positive charge, so the lines point away from a positive object and towards a negative one. For an isolated charged sphere the lines are straight and radial, evenly spaced all the way round, starting at right angles to the surface. The closer together the lines are drawn, the stronger the field at that point.
The electric field is strongest close to the charged object.
Field strength is not the same everywhere: it is greatest right at the surface of the charged object and falls away with distance. On a field line diagram this shows up as spacing, because close to the sphere the radial lines are crowded together, and further out the same number of lines has spread over a much larger area, so they lie further apart. Closely spaced lines mean a strong field and widely spaced lines a weak one. The practical consequence is that a second charge placed near the object feels a large force while the same charge further away feels a much smaller one. It is also why sparks start at the nearest point, where the air sits in the strongest field and breaks down first.
The further away from the charged object, the weaker the field.
Move away from a charged object and the field strength decreases. The reason is geometric: the field lines spread out radially, so the same lines cover a larger and larger area as the distance grows, and the field they represent is more dilute. A charge placed far from the object therefore feels only a small force. This behaviour is shared with other radial fields you meet, because the magnetic field of a bar magnet also weakens with distance from the poles, which is why the same diagram conventions serve both. A one-mark question may simply ask how field strength varies as distance from a charged dome increases, and the required answer is 'decreases', but be ready to justify it from the spreading of the field lines.
A second charged object placed in the field experiences a force.
The field is only detectable through what it does to another charge. Place a second charged object in the field and it feels a force: away from the first object if the two charges are alike, towards it if they are opposite. The direction of the field line at that point gives the direction of the force on a positive charge, and a negative charge feels a force in the opposite direction to the line. The size of the force depends on how strong the field is at that point and on how much charge the second object carries, so the same object feels a larger force nearer the first charge. This is what non-contact force means in practice: the first object sets up the field, and the second responds to the field where it sits.
The force gets stronger as the distance between the objects decreases.
Bring two charged objects closer and the force between them grows, because the second object moves into a region where the first object's field is stronger. The increase is steep rather than steady, since halving the separation does far more than double the force, which is why a charged rod feels like nothing at arm's length but snatches at a scrap of paper a centimetre away. The same rule governs sparking: as an earthed conductor is moved towards a charged dome, the field in the gap grows stronger until the air breaks down. Notice that the two objects always feel forces of the same size however different they are, so the small object does not feel a bigger force just because it is the one that moves.
Students should be able to: draw the electric field pattern for an isolated charged sphere explain the concept of an electric field explain how the concept of an electric field helps to explain the non-contact force between charged objects as well as other electrostatic phenomena such as sparking.
For an isolated charged sphere, draw straight lines radiating from the surface, evenly spaced all the way round, each meeting the surface at a right angle, with arrows pointing outwards for a positive sphere and inwards for a negative one. Lines must never cross, and they should be drawn closer together near the surface to show the stronger field there. The field concept then does two jobs. It explains the non-contact force, because the first object fills the space around it with a field and any charge placed in that space feels a force, so nothing has to reach across the gap. It also explains sparking, because a strong field near a highly charged object pulls electrons free from air molecules, so the air conducts and charge crosses the gap.
Your focus
- State the definition of an electric field in the form the mark scheme accepts.
- Draw the field pattern around an isolated negatively charged sphere with correctly directed arrows.
- Explain why the field exists even when no second charge is present.
Show all 18 objectives
- Explain how the spacing of field lines on a diagram shows where the field is strongest.
- Compare the force on an identical charge placed at two different distances from a charged sphere.
- Predict where breakdown of the air will begin around a charged dome and justify the choice.
- Rank marked points at different distances from a charged sphere by the field strength at each, and use the ranking to say where a small charged object would feel the largest force.
- Justify the fall in field strength with distance using the spreading of radial field lines.
- Sketch a radial field around a charged sphere with spacing that correctly shows the field weakening.
- Predict the direction of the force on a positive and on a negative charge placed at a marked point in a field.
- State the two factors that determine the size of the force on a charge placed in an electric field.
- Explain how the field model accounts for a force acting across a gap between two objects.
- Explain why a spark jumps only once an earthed conductor has been brought close to a charged dome, referring to the potential difference across the gap and the breakdown of the air.
- Explain the rise in force as the separation falls, using the strength of the field in which the second object sits.
- Explain why both objects feel forces of equal size even when only one of them moves.
- Draw a labelled field pattern for an isolated negatively charged sphere with correct spacing and arrows.
- Explain how the field model accounts for the force between two charged objects that are not touching.
- Explain sparking in terms of a strong field making the air between two objects conduct.
Electric fields exam tips
Marking Points
- one mark for the electric field as the region around a charged object where another charged object experiences a force
- one mark for the field existing whether or not a second charge is placed in it
- one mark for field lines drawn radially and at right angles to the surface of a charged sphere
- one mark for arrows pointing away from a positive object and towards a negative object
- one mark for the field being strongest at or nearest to the surface of the charged object
- one mark for closely spaced field lines representing a strong field
- one mark for a larger force on a charge placed close to the object
- one mark for linking the strongest field to where the air breaks down and sparking begins
- one mark for the electric field strength decreasing as distance from the charged object increases
- one mark for justifying this by the field lines spreading over a larger area
- one mark for a smaller force on a charge placed further away
- one mark for a diagram showing lines drawn further apart at greater distance
- one mark for the second charged object experiencing a force when placed in the field
- one mark for the direction of that force depending on whether the charges are alike or opposite
- one mark for the force being larger where the field is stronger
- one mark for the field line direction giving the direction of the force on a positive charge
- one mark for the force increasing as the separation decreases
- one mark for explaining this as the second object moving into a stronger region of the field
- one mark for the two objects always experiencing forces equal in size and opposite in direction
- one mark for linking the stronger field at small separation to the air breaking down and a spark forming
- one mark for radial straight lines drawn at right angles to the surface of the sphere
- one mark for arrows drawn in the correct direction for the sign of the charge on the sphere
- one mark for the field defined as the region where another charged object experiences a force
- one mark for a strong field near the surface causing the air to break down so that a spark forms
Examiner Tips
- 💡Learn the definition word for word: the region around a charged object where another charged object experiences a force.
- 💡Put an arrow on every field line you draw, because a line without one may score nothing.
- 💡Start every line on the surface of the sphere and draw it at right angles to that surface.
- 💡Use line spacing to justify any statement about field strength taken from a diagram.
- 💡If asked where a spark will start, answer at the point where the field is strongest.
- 💡Do not call the field bigger; use stronger, and refer to field strength.
- 💡Read whether the question asks about field strength, force or charge, because they are different quantities.
- 💡One word may be enough for a one-mark version, but add the field-line justification when more marks are offered.
- 💡Compare with the magnetic field of a bar magnet if you need to check your reasoning.
- 💡Note the sign of the test charge before deciding the direction of the force.
- 💡Quote both factors when asked what the force depends on: field strength and the charge placed in it.
- 💡Use the field-line arrow for a positive charge, then reverse it for a negative one.
- 💡Write 'increases' and give the field reason, because a bare 'gets bigger' may not earn the explanation mark.
- 💡Remember the forces on the two objects stay equal in size even when only one of them moves.
- 💡Link a spark question back to the field in the gap rather than to the distance alone.
- 💡Use a ruler for radial field lines and draw at least six of them, evenly spaced around the sphere.
- 💡Check the sign in the stem before adding arrowheads, because it is the easiest mark to throw away.
- 💡Bring the field into every explanation of a non-contact electrostatic effect.
Common Mistakes
- defining the field as the force itself rather than the region in which a force acts
- drawing field lines that cross one another
- drawing arrows pointing towards a positive sphere
- leaving the arrows off the field lines entirely
- drawing radial field lines evenly spaced at all distances, so the diagram shows a uniform field
- saying the field is strongest halfway between two objects with no justification
- confusing field strength with the size of the charge on the object
- stating that the field is strongest where the lines lie furthest apart
- saying the field stops completely a short distance from the object
- claiming the field strength decreases because the charge on the object decreases
- drawing field lines that stay parallel and equally spaced away from a sphere
- answering about the charge getting smaller when the question asked about field strength
- saying the second object must touch a field line before a force can act
- giving the force on a negative charge in the same direction as the field line
- assuming the force is the same size everywhere in the field
- describing the second object as attracted in all cases, whatever its charge
- describing the increase as directly proportional to the decrease in distance
- saying the lighter object feels the larger force because it accelerates more
- claiming the force only appears once the objects touch
- confusing the strengthening field with an increase in the charge on the objects
- drawing the pattern of a bar magnet, with lines looping from one side of the sphere to the other
- drawing lines that start away from the surface, or that cross each other
- omitting arrows, or drawing them outwards on a negatively charged sphere
- explaining sparking without referring to the field or to the air becoming conducting