Electrostatic force & Electric Fields Jan 26, 2017

What do we already know about the electrostatic force? Quick Review: What do we already know about the electrostatic force? The electrostatic force is the force between stationary charges. Opposite charges attract. Same charges repel. We also know that this force is fairly strong and that it can act over a distance (but that the strength of this force decreases with the square of distance) Today, we will learn how to quantify the electrostatic force.

Calculating Electrostatic force: Coulomb’s Law Coulomb’s Law allows us to calculate the force between any two charged objects. Notice the similarities between the equations for electrostatic force and gravity: What do the variables represent? k is “Coulomb’s law constant, and it has a value of q1 and q2 are the amount of charge (measured in C) of each object r is the distance between the objects (measured in m) In both cases, the force increases with the size (mass or charge) of each object and force decreases as the distance between the object increases. Don’t forget: Force is a vector, so it has direction. You can reason out the direction of the force by remembering the opposites attract and like repel.

We do: Calculating Electrostatic Force A charge of +2 mC and a charge of -3 mC are separated by 0.1m. What is the force between them? If the charges are each 0.001kg, what will be their acceleration? 2) Two charges are separated by a certain distance. If the distance between them is halved, how will the force change? F = 8.99 X 109 X 2X10-6 X -3 X 10-6 / (0.1)2 F = -5N (attraction!) a = F / m = -5 N / 0.001 kg = 5000 m/s2 The force will be quadrupled (four times bigger)

(“-“ = attractive force) You Do– Calculating Electrostatic Force 1. A woman accumulates a charge of 2.0 x 10-5 C when sliding out of the seat of a car. A man has accumulated a charge – 8.0 x 10-5 C while waiting in the wind. What is the force between them if they are 6.0 m apart? (“-“ = attractive force) 2. A positive charge is 0.5 m away from a negative charge. If the size of the positive charge is doubled and the distance is between them is also doubled, how will the force between the two charges change? The force will be halved.

Electrostatic force when there are multiple charges Calculating the force between two charges is easy. What do you do if there are multiple charges? You can find the total force on any charge adding the forces from each other charge. Use Coulomb’s law to find the force from each individual charge, and make a free body diagram! Example: Charge Q is in between two other charges, arranged in a line. What is the total force on Q? d = 0.05 m d = 0.08 m 60 min after example q1 = -5.0 mC Q = -2.0 mC q2 = 4.5 mC Use Coulomb’s Law to find F1 and F2 Since both forces are pointing in the same direction, they can just be added Total F = 49 N Draw FBD for Q F1 F1 = 36 N F2 F2 = 13 N Q

Electric Fields What is a “force” field anyway? In physics, a ‘force’ field describes an area where objects experience a force-at-a-distance such as gravity, electrostatic force, or magnetism. In each case, the field is generated by a certain type of particle: Gravity by a mass Electrostatic by a charge Magnetism by a moving charge The particle changes the space around it so that other particles of the same type will experience a force if they come into that area. First 5 min … stop after earth moon system Why is this a good visualization of a gravitational field? Would it work for an electric field? How is an electric field / electric force different than gravitational field/force? Watch me!

Electric Field Lines Electric field lines show the direction of the force on a positive ‘test charge’ Electric field lines point towards negative charges Electric field lines point away from positive charges

Electric Field Lines The density of lines shows the relative strength of the electric field This has two implications: larger charges have more field lines radiating 2) As you move farther away from the charge, the strength of the field (density of lines) decreases

Net Electric Field Lines If we have two or more charges creating an electric field, we add the vectors from each charge. NOTE: Electric field lines can never cross! That would mean that a test charge would go in two directions at once. How are electric and gravitational fields different? How are they the same? Play with me!

Question 1 - show with fingers B a) What is the charge of q1? Thumbs up for +, down for - What is the charge of q2? Thumbs up for +, down for - Which charge is larger? d) Where will a positive charge at position A move? e) Where will an electron at position B move? A How can you tell?

Question 1 - show with fingers B a) What is the charge of q1? Thumbs up for +, down for - negative b/c field lines going towards the charge What is the charge of q2? Thumbs up for +, down for - Which charge is larger? d) Where will a positive charge at position A move? e) Where will an electron at position B move? A 2 – field lines are denser How can you tell? Up Up

Question 4 What is the direction of the electric field at point C? Left Right Zero Away from positive charge (right) Towards negative charge (right) y Net E field is to right. C x

Question 5 What is the direction of the electric field at point A? Up Down Left Right Zero A x

Question 6 What is the direction of the electric field at point B? Up Down Left Right Zero y B x

Question7 What is the direction of the electric field at point A, if the two positive charges have equal magnitude? Up Down Left Right Zero A x