Gravitation Field Theory
What is the standard gravitational field strength on Earth's surface?
9.8 N/kg
What is the formula for electric field strength in a uniform field?
E = V / d
Which field can repel: gravitational or electric?
Electric
Which way do electric field lines go — from positive or negative?
positive to negative.
Force on a 2 C charge in a 4 N/C (newton/coulomb) field?
F = qE = 2 * 4 = 8 N
State Newton’s Law of Universal Gravitation and name all variables.
F = (G * m1 * m2) / r^2, where F is force, G is gravitational constant, m1 and m2 are masses, r is distance.
In which direction do electric field lines point around a negative charge?
Toward the charge
Give one similarity and one difference between gravitational and electric field equations.
Similarity: inverse-square law. Difference: gravity only attracts; electric can attract or repel.
What does it mean if electric field lines are close together?
The field is stronger.
Gravitational Potential Energy of a 5 kg object 10 m high?
PE = mgh = 5 * 9.8 * 10 = 490 J
If the mass of one object is doubled, how does gravitational force change?
It doubles.
Compare electric field strength at distances r and 2r from a point charge.
At 2r, the field is 1/4 as strong.
How do field lines differ in shape between the two fields?
Gravitational: radial inward. Electric: radial inward or outward depending on charge.
If a charge moves in a direction where the voltage doesn’t change, how much work is done?
Zero — no voltage change means no work is done.
If voltage stays the same, W = q × ΔV = 0.
Gravitational force between two 3 kg masses 2 m apart?
F = (6.67*10^-11 * 3 * 3) / (2^2) = 1.5*10^-10 N
Explain why astronauts feel 'weightless' while orbiting Earth, even though gravity still acts.
They are in free fall, accelerating with their spacecraft, so no normal force acts on them.
Define electric potential energy and explain how it changes when two like charges move apart.
It decreases as they move apart because repulsive potential energy is reduced.
Why can electric potential be both positive and negative, but gravitational potential is always negative?
Electric potential depends on charge sign; gravity is always attractive.
A particle has a mass of 2.0 kg and a charge of -3 C. It is placed in a region where the gravitational field is 10 N/kg downward and the electric field is 5 N/C downward.
What is the net force acting on the object (magnitude and direction)?
Step 1: Gravitational force = 2.0 kg x 10 N/kg = 20 N downward
Step 2: Electric force = -3 C x 5 N/C = -15 N → 15 N upward (negative charge goes opposite to the field)
Step 3: Net force = 20 N down minus 15 N up = 5 N downward
A 1.5*10^-6 C charge moves across 20 V. Find work done.
: W = qV = 1.5*10^-6 * 20 = 3.0*10^-5 J
Derive g = GM/r^2 and find g at 8000 km from Earth’s center.
From F = GMm/r^2 and F = mg, g = GM/r^2. Plug in G = 6.67*10^-11, M = 5.97*10^24, r = 8.0*10^6 → g ≈ 6.23 N/kg
A point charge Q = 4.0*10^-6 C is 0.2 m away. Find E.
E = kQ/r^2 = (9.0*10^9)(4.0*10^-6)/(0.2^2) = 9.0*10^5 N/C
What are both the law for both fields with examples.
F = GMm/r^2. Electric: F = kQq/r^2
There are two parallel metal plates: the top one is positive and the bottom one is negative.
A small positive charge is placed exactly in the middle between them.
The electric field between the plates is uniform and points downward.
Gravity also pulls the object downward.
Question:
Will the positive charge accelerate faster, slower, or at the same rate as it would under gravity alone? Explain your reasoning using field direction and forces.
The charge will accelerate faster than under gravity alone.
Why?
Gravity pulls it downward.
The electric field also pulls it downward (because the field is down and the charge is positive).
So both forces act in the same direction, making the total (net) force stronger than gravity alone → more acceleration.
An object has a mass of 1.5 kg and a charge of +2 C. It is placed in a region where the gravitational field is 9.8 N/kg downward and the electric field is 4 N/C upward.
What is the net force acting on the object (magnitude and direction)?
Step 1: Gravitational force (downward)
Force of gravity = mass x gravitational field strength
= 1.5 kg x 9.8 N/kg = 14.7 N downward
Step 2: Electric force (upward)
Force from electric field = charge x electric field strength
= 2 C x 4 N/C = 8 N upward
Step 3: Net force (they are in opposite directions)
Net force = 14.7 N (down) minus 8 N (up)
= 6.7 N downward