Forces that act across a distance, like gravity, electric forces, and magnetic forces can be explained using the idea of fields. Fields extend through space and can be mapped around objects using a test charge that interacts with the field. Learn about different fields and the types of objects that interact with them. Created by Khan Academy.
- If you hold a ball up in the air and let it go, you know it's going to fall, but why? Nothing is touching it once you let it go. How can there be a force on it? Well, this is because Earth's gravitational force is pulling the ball and gravity is a non-contact force. Non-contact forces don't have to touch an object to exert a force on it. Instead, these forces act over a region. So if an object is in that region, it will be affected by the force. In this case, the ball is in Earth's gravitational field, and so it feels an attractive force towards the Earth and the ball falls to the ground. Field forces include non-contact forces, such as electric, magnetic, and of course, gravitational forces. So since these forces are non-contact, they can exert a force on objects they aren't touching, but how do these objects know if there's a force between them? To explain these non-contact forces, scientists eventually developed the idea that these objects were surrounded by something called a field. So what is a field? A field extends through space from an object with certain physical properties. What are those? Well, for gravitational forces, these affect objects with mass. So any object with mass has a gravitational field surrounding it that points towards the object's center. The further you move away from the object, the less dense the field and weaker the field becomes. Electric forces effect charged objects. So an electric field surrounds any object with a net charge, and the direction of this field will depend on the charge. Magnetic fields will affect magnets and any other material with magnetic properties. Each spot on a field has two things associated with it: magnitude and direction. And these help us predict what forces objects will experience when they're in the field. So let's look at an example to help understand this. Say we have a planet. Now, the planet has a lot of mass so we know it's going to be surrounded by a gravitational field that points towards the center of the planet. I can draw these little field lines that show the direction of the field and its strength. As we move away from the planet, the field will start to weaken, and I'm going to represent that by a less dense field with these arrows. Now, let's say there's an asteroid moving near the planet in this direction. I know that the asteroid, as it's shown here, is in the outskirts of this planet's gravitational field. So it is going to feel some gravitational attraction towards the planet, which we can draw with this vector, Fg, which is force of gravity. Now, because it's attracted to the planet, the astroid will continue to move towards the planet. And the closer the asteroid gets to the planet, the stronger the field and the stronger the force of attraction it will feel. And so in this way, scientists can use fields to help predict behavior of objects experiencing non-contact forces. And all of this may sound kind of odd, but you probably already think about forces this way. For example, if we go back to the ball that you know is going to fall, you knew this because the force of gravity from Earth was going to pull the ball towards the Earth. But now you also know that that's because Earth's gravity is a field force. And so the ball is in the field of gravity for Earth and experiences an attractive gravitational force. So while fields may sound mysterious, they really just mean that a force is felt over a distance. Gravitational, electric, and magnetic forces are all field forces, which means they act over distance and can affect a lot of objects.