Why is gravitational potential always negative?

It is zero at infinity and gravity is attractive, so the potential falls as a mass moves in from infinity.

Is gravitational potential a scalar or a vector?

A scalar, so potentials from several masses add by ordinary addition.

How do you find the change in gravitational potential energy between two points?

ΔE = mΔV, where ΔV is the change in potential.

When can ΔE = mgh be used instead of ΔE = mΔV?

Only for small changes in height where g is almost constant.

Give two similarities between gravitational and electric fields.

Both are radial around a point source and both forces follow an inverse square law (also, potential is zero at infinity).

Give two differences between gravitational and electric fields.

Gravity is always attractive but electric forces can repel; gravity is far weaker and cannot be shielded.

Which quantities correspond to mass and G in the electric force law?

Charge corresponds to mass, and 1/4πε₀ corresponds to G.

Why does gravity dominate for planets but not for atoms?

Large bodies are almost neutral so electric forces cancel and masses only attract; in atoms the electric force is about 10³⁹ times larger.

Gravitational potential and comparison with electric fieldsEdexcel A-Level Physics: Flashcards

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Define gravitational potential at a point.

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Define gravitational potential at a point.: The work done per unit mass in moving a small mass from infinity to that point.
What is the equation for gravitational potential near a point mass M?: V = −GM/r.
Why is gravitational potential always negative?: It is zero at infinity and gravity is attractive, so the potential falls as a mass moves in from infinity.
What is the unit of gravitational potential?: J kg⁻¹.
Is gravitational potential a scalar or a vector?: A scalar, so potentials from several masses add by ordinary addition.
How do you find the change in gravitational potential energy between two points?: ΔE = mΔV, where ΔV is the change in potential.
When can ΔE = mgh be used instead of ΔE = mΔV?: Only for small changes in height where g is almost constant.
Give two similarities between gravitational and electric fields.: Both are radial around a point source and both forces follow an inverse square law (also, potential is zero at infinity).
Give two differences between gravitational and electric fields.: Gravity is always attractive but electric forces can repel; gravity is far weaker and cannot be shielded.
Which quantities correspond to mass and G in the electric force law?: Charge corresponds to mass, and 1/4πε₀ corresponds to G.
Write the equation for the electric force between two point charges.: F = Q₁Q₂/4πε₀r².
Why does gravity dominate for planets but not for atoms?: Large bodies are almost neutral so electric forces cancel and masses only attract; in atoms the electric force is about 10³⁹ times larger.

Exam questions on Gravitational potential and comparison with electric fields

An engineer is planning to launch a probe of mass 450 kg from the surface of the Earth. The Earth may be treated as a uniform sphere of mass 5.97×10²⁴ kg and radius 6.37×10⁶ m, and the gravitational constant is G = 6.67×10⁻¹¹ N m² kg⁻².
The probe is raised from the surface of the Earth to a height equal to the radius of the Earth. Calculate the increase in the gravitational potential energy of the probe.2 marks
In a simple model of a hydrogen atom, an electron of mass 9.11×10⁻³¹ kg and charge −1.60×10⁻¹⁹ C is a distance of 5.3×10⁻¹¹ m from a proton of mass 1.67×10⁻²⁷ kg and charge +1.60×10⁻¹⁹ C. Both particles are treated as point masses and point charges. Take G = 6.67×10⁻¹¹ N m² kg⁻² and ε₀ = 8.85×10⁻¹² F m⁻¹.
Calculate the gravitational force between the proton and the electron.2 marks
A lunar lander of mass 1200 kg is on the surface of the Moon. The Moon may be treated as a uniform sphere of mass 7.35×10²² kg and radius 1.74×10⁶ m. Take G = 6.67×10⁻¹¹ N m² kg⁻². The surface field strength of the Moon is 1.62 N kg⁻¹.
Calculate the gravitational potential at the surface of the Moon and explain why the value is negative.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).