What is a field?
Where gravitational, electric and magnetic fields come from, and how we measure how strong they are.
The Moon never touches the oceans, yet it lifts them twice a day. A magnet moves a paper clip with no contact. How does one object act on another across empty space?
Which of these creates a magnetic field?
A 4.0 kg mass is placed in a gravitational field and feels a force of 36 N. The gravitational field strength is…
A small positive test charge of 2.0 × 10⁻⁶ C feels a force of 6.0 × 10⁻⁶ N toward the west. The electric field strength there is…
A positively charged metal sphere is fixed in place. At a point nearby, the electric field points…
The units of gravitational field strength g and electric field strength E are, in that order…
A friend says: “There is no gravitational field in an empty room, because nothing in the room is being pulled.” Explain what is wrong with this.
A trolley moves at constant velocity. The net force on it is…
The unit N s is equivalent to…
Key points
- • Mass makes a gravitational field. Charge makes an electric field. A moving charge (a current) makes a magnetic field.
- • A field is a 3D region around a source where another object experiences a force.
- • Field strength is the force per unit mass (g = F / m) or per unit charge (E = F / q).
- • g points toward the mass. E points away from +Q and toward −Q, the way a positive test charge moves.
Formulas
- g = FG / m
- E = FE / q
Key terms
- Field :
- a region where another object experiences a force
- Field strength :
- force per unit mass or per unit charge
- Test charge :
- a small positive charge used to define the direction of E
Common errors
- • Thinking a field only exists once a test object is in it.
- • Forgetting that a negative charge is pushed opposite to E.
- • Mixing up the source (creates the field) and the test object (feels the force).