Electric Charge

Electric charge Fundamental property of matter Positive or negative Determines electric force

The Electric Force

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

Discussion Question The electric force is like the gravitational force because a. Opposites attract. b. Both decrease as 1/d2. c. Both involve charge. d. Both were identified by Isaac Newton. e. All of these.

Discussion Question The electric force is different from the gravitational force because a. Gravity is only attractive. b. The electric force is much stronger. c. Electric charges can cancel. d. All of the above.

Example Problem The electron has a charge of –1.6  10-19 C and the proton has a charge of 1.6  10-19 C. Their average separation in a hydrogen atom is 5.3  10–11 m. What is the force between them at this distance?

Quick Question What is the force between the electron and proton of a hydrogen atom when the electron is twice as far away? a. Twice as much. b. The same. c. Half as much. d. One fourth 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. + – +

Poll 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. + – +

Poll 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 Poll 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. + – –

Coulomb’s Torsion Balance

Principle of Superposition Force from several charges = sum of forces from individual charges c b Fb Fc a F = Fb + Fc + Fd Fd Fb Fc Fd F d

Vector Fields

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

Electric Field Unit = N/C

Visualizing a Field 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

Field Lines

Question A B C D Particles A–D have the same charge. Which experiences the greatest force from the field? (Green arrows are field lines.)

Example Draw field lines for the electric field about these two objects with opposite charges. + -

Group Work Draw field lines for the electric field about these two objects with equal charges. + +