1. Conceptual Physics Class
Welcome to PEP Mason
Conceptual
Physics
Class
Mrs. Morales

2. Chapter 17
Fields and Forces

3. Chapter 17 – Fields and Forces
17.2 Gravity
17.3 The Electric Field

4. Section 17.1
Fields and Forces

5. What is a field?
In physics, a field is a physical phenomena that has a value everywhere in space.
The degree to which we hear music, loud or soft, has a value everywhere around a stereo.
This means you can describe the loudness with a field.
All interactions between matter and energy occur by way of fields.

6. Fields and Energy
Any field is a form of energy that is distributed through space.
A magnetic field created by either a permanent magnet or an electromagnet has energy since it can exert force (push/pull) over a distance, or do work, on another magnet.
An electrical field similarly exerts force over distance, on charged particles.
A gravitational field exerts a force over distance on objects as well.

7. Fields of the same kind can be added or subtracted.
Adding Fields
Fields of the same kind can be added or subtracted.
The field from an electromagnet can either cancel the field from a permanent magnet or add to it.

8. Inverse Square Law
The degree to which objects in a field experience a given force can be quantified.
Objects closer to the source of the field (whether that source is a magnet, charged particle, massive object) experience a greater force. Objects farther experience less force.

9. Inverse Square Law
The inverse square law states that a field decreases as the square of the distance from the source of the field increases.
While sound, light, gravity and electricity follow this trend, magnetism does not.

10. Inverse Square Law
The magnetic field decreases much more quickly than what is represented by the inverse square law.
The north and south poles of the magnet cancel each other out as you move farther from the magnet.

11. The speed of a field
The magnetic field exerts a force of one magnet on another at the speed of light.
The speed of light is 300 million m/s, so it takes only a tiny fraction of a second for the force to be exerted by one magnet on another when the distance is a few meters.

12. The speed of light
All interactions are carried by fields, and the fastest that any field can spread is the speed of light.
Information like your cell phone number and the number you are calling is coded in pulses of energy.
The information spreads as an electromagnetic field that expands at the speed of light.

13. How do mobile phones work?
Imagine calling a friend on the other side of town. As you chat away, your phone converts your voice into an electrical signal, which is then transmitted as  radio waves and converted back into sound by your friend’s phone. 

14. How do mobile phones work?
A basic mobile phone is therefore little more than a combined radio transmitter and a radio receiver, quite similar to a walkie-talkie or CB radio.

15. How do mobile phones work?
In order to remain portable, mobile phones need to have relatively compact antennas and use a small amount of power. This means that mobile phones can send a signal over only a very short range, just like a walkie-talkie.

16. How do mobile phones work?
The cellular network, however, enables you to chat about the latest Trump “scandal” regardless of how far away your friends are. This is done by dividing up land into a patchwork of ‘cells’ – hexagonal areas of land each equipped with their own phone mast (also called a base station).

17. How do mobile phones work?
These huge phone masts pick up the weak signal from your phone and relay it onwards to another phone mast nearer to your friend. And if you’re on the move while you talk, your phone switches masts as you go without interrupting your call.

18. The Gravitational Field
Section 17.2
The Gravitational Field

19. Gravity The gravitational field is created by mass.
All mass creates a gravitational field.
Gravity is a relatively weak force, so it takes a planet-sized mass to create a field strong enough to exert a significant force.

20. Gravity
The gravitational field is a force field because it creates a force on masses at all points in space.
The force (Fw) on any mass (m) is equal to the mass multiplied by the gravitational field (g).
Remember Fw = mg?

21. Gravity Gravitational force acts in two steps.
Earth, due to its gigantic mass creates a gravitational field.
The Moon feels a force from the gravitational field that causes it to orbit Earth.

22. Gravity
The gravitational field is a vector field because a gravitational force has a direction at all points in space.
Like the magnetic field, you can draw field lines to show the direction of the gravitational field.

23. Gravity
The formula for Newton’s law of gravitation can be rearranged, if we are interested in finding the strength of the gravitational field on the surface of a massive object.
The strength of the gravitational field (g) is given by the quantity Gm2/r2.
If we know the mass and radius of a planet, we can use this quantity to calculate the strength of gravity on that planet.

24. Gravity
The planet Mars has a mass of 6.4 × 1023 kg and a radius of 3.4 million m. Calculate the value of g on the surface of Mars.
Looking for: …the value of g in N/kg for Mars
Given: …the mass (6.4 x1023 kg) and radius 3.4 x106 m) of Mars
Relationships: Use g = Gm2 ÷ r2 and G= 6.67 x10-11 N•m2/kg2
Solution: g = (6.67 ×10−11 N•m2/kg2)(6.4 ×1023 kg)
3.4 ×106 m
= 3.7 N/kg on Mars compared to 9.8 N/kg on Earth

25. Section 17.3
The Electric Field

26. Electric Fields
Like gravity, the force between electric charges is carried by a field, called the electric field.
By convention, we draw the electric field to represent the force on an imaginary positive test charge: how would that test charge behave in the given electric field.

27. Drawing the electric field
Electric field lines follow the direction of the force on a positive test charge.
The strength of the electric field is shown by the spacing of the field lines.
The field is strong where the field lines are close together and weak where the lines are far apart.

28. Drawing the electric field
From the diagram at left, where is the electric field strongest?
Is the field repulsive or attractive to the positive test charge?

29. Coulomb’s law and electric field
The object that creates the field is called the source charge (q1).
The charge you place to test the force is the test charge (q2).
The force (F) on the test charge is equal to the amount of charge (q2) multiplied by the electric field (E) or F = q2E.

30. Coulomb’s law and electric field
The electric field (E) is the result of what is produced by the source charge (q1).
As with gravity, we can rewrite Coulomb’s law so that the electric field is a separate quantity in the formula.

31. Units of the electric field
1 newton per coulomb is the same as 1 volt per meter.
A voltage difference of 1 volt over a space of 1 meter makes an electric field of 1 V/m.

32. The force on a charge in an electric field
The force from the electric field accelerates the charge on which it is acting.
An electron only accelerates for a short distance before it collides with a copper atom.
This is why the constant force from the electric field results in a constant drift velocity for electrons.

33. Calculating the force of an electric field on a raindrop
A raindrop has a static charge of C. In a thunderstorm, the raindrop experiences an electric field of 1,000 V/m. What would be the force on the drop?
Looking for: …force in newtons on the drop
Given: …charge ( C) and electric field (1,000 V/m)
Relationships: Use F = qE
Solution: F = ( C) × (1,000 V/m) = 0.1 N

34. Electric shielding
When a circular conductor is placed in an electric field, no electric field is detected inside the conductor.

35. Electric shielding
If you unwrap a computer network wire, you will find smaller wires wrapped by aluminum foil.
The aluminum foil is a conductor and shields the wires inside from electrical interference.

36. Solar wind and magnetic storms are associated with sunspots.
Weather is magnetic
Solar wind and magnetic storms are associated with sunspots.
Sunspots occur when magnetic fields—caused by the movement of gas within the Sun—break the Sun’s surface.