What mistakes do students make with Point and spherical charges?

Applying the point-charge model inside the sphere. Put it right by restricting the model to points outside the sphere. Forgetting that the sphere's total charge, not surface charge density, is used in the model. Put it right by using the total charge Q. Assuming the model works for any shape of charged object. Put it right by noting it applies to uniformly charged spheres.

What are the exam tips for Point and spherical charges in Physics OCR A-Level?

Check whether the point of interest is inside or outside the sphere before selecting an option. Remember the model uses the total charge of the sphere, not a fraction of it. If the question mentions a uniformly charged sphere, look for the condition r greater than R.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

(b) modelling a uniformly charged sphere as a point charge at its centre

Full specification wording

Point and spherical charges: (b) modelling a uniformly charged sphere as a point charge at its centre

A uniformly charged sphere has its charge spread evenly over its surface or volume.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 2 of 4

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

A uniformly charged sphere can be modelled as a point charge at its centre for points outside the sphere.
The total charge of the sphere is treated as concentrated at the centre in this model.
The model is valid only for distances greater than the sphere's radius.
Inside a uniformly charged insulating sphere the field is not given by the point-charge model.
The model allows the field equation for a point charge to be applied to a spherical charge distribution.

Common mistakes and how to put them right

Applying the point-charge model inside the sphere.
Put it right by restricting the model to points outside the sphere.
Forgetting that the sphere's total charge, not surface charge density, is used in the model.
Put it right by using the total charge Q.
Assuming the model works for any shape of charged object.
Put it right by noting it applies to uniformly charged spheres.
Confusing the sphere's radius with the distance from the centre.
Put it right by using r for the distance from the centre and R for the radius.

Exam tips

Check whether the point of interest is inside or outside the sphere before selecting an option.
Remember the model uses the total charge of the sphere, not a fraction of it.
If the question mentions a uniformly charged sphere, look for the condition r greater than R.

Try the lesson

The start of the Point and spherical charges: (b) modelling a uniformly charged sphere as a point charge at its centre lesson, made from checked teaching notes for this specification point.

Listen

A uniformly charged sphere has its charge spread evenly over its surface or volume. Outside the sphere, the field it produces is identical to that of a point charge of the same total charge placed at the sphere's centre. This lets you use the point-charge field equation for any distance r greater than the sphere's radius R. Inside a uniformly charged insulating sphere the field behaves differently and is not simply that of a central point charge, so the model applies only outside the sphere. A reliable method is to check the distance from the centre: if r is greater than R, use the point-charge model; if r is less than R, do not. A strong answer earns marks for these points. A uniformly charged sphere can be modelled as a point charge at its centre for points outside the sphere. The total charge of the sphere is treated as concentrated at the centre in this model. The model is valid only for distances greater than the sphere's radius. Inside a uniformly charged insulating sphere the field is not given by the point-charge model. A common slip: Applying the point-charge model inside the sphere. Instead, restricting the model to points outside the sphere. Exam tip: Check whether the point of interest is inside or outside the sphere before selecting an option.

See it

A flowchart starting from a uniformly charged sphere, then comparing the distance r from the centre with the radius R, then asking whether r is greater than R. If yes, the point is outside the sphere, so the sphere is modelled as a point charge at the centre, using the total charge Q and applying the point-charge field equation. If no, the point is inside the sphere, where the point-charge model is not valid. — It shows how checking whether the distance from the centre is greater than the radius decides whether a uniformly charged sphere may be modelled as a point charge at its centre.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A flowchart starting from a uniformly charged sphere, then comparing the distance r from the centre with the radius R, then asking whether r is greater than R. If yes, the point is outside the sphere, so the sphere is modelled as a point charge at the centre, using the total charge Q and applying the point-charge field equation. If no, the point is inside the sphere, where the point-charge model is not valid.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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