Electricity and Magnetism SCI 340 L40 Charge 5/5/2019 Electricity and Magnetism Demo equipment Round balloons, long balloons, rabbit fur, bubble soap, 2x4, watch glass, Bakelite rod, glass rod, aluminum soda can Underlying a whole lot of phenomena Ch. 18–24

Outline Stationary charges Moving charges Magnetism Magnetic Induction forces, potential, fields Moving charges current, resistance, circuits Magnetism another effect of moving charges Magnetic Induction Pushing charges with magnetism

Objectives Determine electric force using Coulomb’s Law. Explain forces in terms of electric fields. Determine energies from electric potential.

Electric Charge §18.1–18.2

Conductors and Insulators Conductor: charges can move freely Insulator: charges cannot move §18.3

Charging Charge is conserved Charges can be separated Charges can transfer from one object to another §18.4

Electric Forces Between Objects SCI 340 L40 Charge 5/5/2019 Electric Forces Between Objects Demo: balloon and fur balloon attracts fur balloon repels other balloon always between objects § 18.5

Coulomb’s Law kq1q2 F = d2 Nm2 k = 8.992  109 C2 C = coulomb (unit of electric charge)

Coulomb’s Law kq1q2 F = d2 Force is attractive for opposite charges Force is repulsive for like charges Proportional to the inverse square of the separation

Question A hydrogen atom consists of a positive proton and a negative electron. How does the force between the electron and proton change when the electron moves twice as close? The force becomes twice (2) as much. The force becomes half (1/2) as much. The force remains the same. The force becomes four times (4×) as much. The force becomes one-fourth (1/4) as much.

tells us something about matter Charge Polarization tells us something about matter

Scenario A bag contains equal numbers of positive and negative charges. The charges can move around inside the bag, but they cannot leave the bag. The bag is placed near a very large, immobile + charge. + – +

Quick Question What sort of force exists between the + charges in the bag and the large + charge? The + charges are attracted to the charge. The + charges are repelled by the charge. The + charges are neither attracted nor repelled. + – +

Quick Question What sort of force exists between the – charges in the bag and the large + charge? The – charges are attracted to the charge. The – charges are repelled by the charge. The – charges are neither attracted nor repelled. + – +

Quick Question In which direction do the + charges in the bag accelerate due to the large + charge? Toward the charge. Away from the charge. The + charges will not accelerate. + – +

Quick Question In which direction do the – charges in the bag accelerate due to the large + charge? Toward the charge. Away from the charge. The – charges will not accelerate. + – +

Question After the charges re-distribute, which force to the external + charge will be stronger? The attraction to the – charges. The repulsion to the + charges. The attraction and repulsion will exactly cancel. + – +

SCI 340 L40 Charge 5/5/2019 Question What sort of force exists between the bag overall and the large + charge? The bag is attracted to the charge. The bag is repelled by the charge. The bag is neither attracted nor repelled. Demo: balloon and uncharged objects: wall, aluminum can, soap bubbles, 2”x4” + – +

Group Question What sort of force on the bag will exist if the external charge is negative? The bag is attracted to the charge. The bag is repelled by the charge. The bag is neither attracted nor repelled. + – –

around electric charges Electric Fields around electric charges § 18.6

Electric Field Field E relates the electric force F on an object to its charge q F = qE Field is a vector

Electric field Magnitude is the force in N on a +1 C charge Direction is the direction of the force exerted on a positive charge Vectors point away from positive charges and toward negative charges Unit = N/C

Question A positive charge experiences a force F to the right in an electric field. How does the force change if the field strength doubles? The force becomes 1/4 what it was (F/4). The force becomes 1/2 what it was (F/2). The force remains the same (F). The force becomes 2 what it was (2F). The force becomes 4 what it was (4F). The force reverses direction (–F).

Fields and Newton’s Third Law Field notation is unilateral Remember that forces are always between objects A charge’s field acts on other charges Never on itself

Visualizing Fields Field vectors Field lines

Field Vectors

Board Work Draw field vectors to describe the electric field of a single positive charge. +

Field Lines

Field Lines Magnitude of the force on a charge is greater where field lines are close together Direction of the force is parallel to field lines Force on a positive charge is along field lines Force on a negative charge is opposite field lines

Question Green arrows are field lines. B C D Green arrows are field lines. Particles A–D have the same charge. Which experiences the greatest force from the field? E. All four forces are equal.

Question Green arrows are field lines. Particles A–D have the same charge. Which experiences the greatest force from the field? C A B D E. All four forces are equal.

Board Work Draw field lines to describe the electric field of a single positive charge. +

Board Work Draw field lines to describe the electric field of an electric dipole. + −

Question What is the nature of the electric field near an infinite plane of electric charge?

Conductors exclude E fields Shielding Conductors exclude E fields §18.8

A Conductor in an E Field Charges move in an E field Charges make their own field Charges rearrange until there is zero field inside the conductor Then they stop Conductors Exclude E fields There can never be a static electric field inside a conductor.