Topic 6 - Fields and forces

Question 1

What is Newton’s universal law of gravitation?

Answer 1

The forces acting between two point masses (m₁ and m₂) are proportional to the product of the masses and inversely proportional to their separation (r) squared

Question 2

What is graviational field strength?

Answer 2

The force per unit mass that would be experienced by a small test mass placed at that point

Question 3

What is the gravitational field strength due to point mass?

Answer 3

Question 4

What is the graviational field strength at the surface of a planet?

Answer 4

r_P = radius of the planet

Question 5

How is the gravitational field strength due to two point masses determined?

Answer 5

Question 6

Are there two type of electric charge?

Answer 6

Yes. (positive and negative)

Question 7

What is the law of conservation of charge?

Answer 7

The total charge in any isolated system remains constant

Question 8

What is an electrical conductor?

Answer 8

A material through which charges can flow, making an electric current

Question 9

What is an electrical insulator?

Answer 9

A material through which charges cannot flow

Question 10

What is Coulomb’s law?

Answer 10

The greater the distance between two charged objects, the smaller the electrical force between them

• Only applies to point charges

k = Coulomb’s constant

Question 11

What is the expanded version of Coulomb’s law?

Answer 11

ε₀ = permittivity of free space

k = Coulomb’s constant

Question 12

What are the differences and similarities between graviational and electric forces?

Answer 12

1. The electrical force between two isolated point charges is much larger than the gravitational force beteen two isolated point masses
2. There is only one kind of mass but two kinds of charge
3. As masses get larger, the gravitational forces get larger as well. Electrical forces do not increase with size
4. On the microscopic scale, gravitational forces are negligible and electrical forces dominate. On the large scale, electrical forces are negligible and gravitational forces dominate
5. The value of the gravitational force between two masses is always the same.The value of electrical force depends on the permittivity of the medium

Question 13

What is electric field strength?

Answer 13

The force per unit charge that would be experienced by a small positive test charge placed at that point

Question 14

What is the electric field strength due to one or more point charges?

Answer 14

k = Coulomb’s constant

Question 15

What produces magnetic fields?

Answer 15

Moving charges

Question 16

Describe the magnetic field due to a current in a straight wire

Answer 16

Question 17

What is Fleming’s left hand rule?

Answer 17

Question 18

What is magnetic field strength?

Answer 18

Also known as magnetic flux density

Unit: NA^–1m^–1 or T (tesla)

L = length of the conductor in the field

I = current in the conductor

sinθ = direction of the force relative to the field

Question 19

Describe the forces between currents in parallel wires

Answer 19

• Definition of ampere
• Forces will be attractive if the currents are in the same direction
• Forces will be repellent if the currents are in opposite directions

Question 20

What is the force on an invidual charge moving in a magnetic field?

Answer 20

F = qvBsinθ

Question 21

What happens to the force on a charge in a magnetic field if the charge stops moving?

Answer 21

The force is zero

Question 22

What is the direction of current for moving electrons?

Answer 22

Opposite to their velocity

(current is the direction of movement of positive charges)

Question 23

What kind of conditions are necessary for a charged particle to move in a circular path in a magnetic field?

Answer 23

The force needs to be perpendicular to motion

Question 24

What is the relationship between the speed of a charge and the radius of its circular path in a magnetic field?

Answer 24

The faster a charge moves, the greater the radius of the circle

Question 25

How can the radius of the circle of a charge moving in a magnetic field be calculated?

Answer 25