Physics exam 1

Protons in the atomic nucleus are very close together, around 1 femtometer apart. How can they stay so close together if we know that they repel electromagnetically

At nuclear scales, the strong nuclear force is a much stronger attractive force than the repulsive electromagnetic force.

Ampere-Maxwell law

Changing electric fields generate magnetic fields

Gauss's Law

Electric charges generate electric fields

T/F: Given the same intensity of light, radiation pressure from totally reflected light is half as much as from totally absorbed light

False

T/F: When the length of a wire doubles and its cross sectional area doubles, its resistance doubles.

False Cross sectional area and resistance are inversely related, if one increases the other decreases.

T/F: When the angle between a wire's current and magnetic field doubles, the force doubles

False Force is proportional to current

T/F: For a uniform electric field and uniform current density both parallel to the length of a wire, if the electric field magnitude doubles, the current density doubles

False current density is directly proportional to the electric field.

T/F: A transformer with more primary turns than secondary turns will produce a secondary voltage higher than the primary voltage.

False current increases in this scenario, meaning the voltage will decrease regardless if it is primary or secondary.

T/F: Gauss's law is valid only for symmetric charge distributions, such as spheres and cylinders

False. Gauss's law applies to any surface.

Gauss's Law for Magnetism

Magnetic charges do not exist

Faraday's law

Reduces to Kirchhoff's loop law in the case of magnetostatics

Why is it safe to ride in a car during a lightning storm

The car's metal exterior surrounds you like a Faraday cage, preventing an electric from being produced inside the cabin and thus keeping any charges outside.

Which describes the electric field near a large sheet with uniform charge

The field vectors are perpendicular to the sheet and directed TOWARDS the sheet

Two neutral metal spheres on wood stands are touching. A negatively charged rod is held directly above the top of the left sphere, not quite touching it. While the rod is there, the right sphere is moved so that the spheres no longer touch. The rod is withdrawn. Afterward, what is the charge state of each sphere?

The left sphere is charged positively, the right sphere is negatively charged. When a negatively charged rod is held above the top of left sphere, the rod will attract positive charges and repel negative charges.

T/F: equipotential surfaces are always parallel to the electric field.

This is false. Equipotential surfaces have equal potentials everywhere on them. For stronger fields, equipotential surfaces are closer to each other. These equipotential surfaces are always perpendicular to the electric field direction, at every point.

T/F: Any net charge on a conductor resides on it's surface, even if it is NOT in electrostatic equilibrium

This is false. the electric field has to be zero inside a conductor at equilibrium (if it were nonzero, the free charges inside would actually be accelerated, and you would not be in equilibrium)

If a gaussian surface is completely inside the volume of a solid, electrostatic conductor, the electric field must always be zero at all points on that surface.

This is true. The electric field has to be zero inside the conductor at equilibrium.

T/F: Electric current through a wire is steady and smooth where as the actual motions of the electrons that form the current are haphazard and chaotic

True As each electron moves uniformly through a conductor, it pushes on the one ahead of it, such that all the electrons move together as a group.

T/F: For a refracted light ray, the angle of incidence is smaller than the angle of refraction if the incident material has a larger index of refraction than the refracting material.

True first medium is denser and the second is rarer. When light goes from denser to rarer, i<r. incidence angle<refraction angle

T/F: The angular frequency at which an LC circuit oscillates is halved if the inductance doubles and the capacitance doubles

True: these variables are proportional

An electron is moving to the right when suddenly a magnetic field is switched on that points out of the page. Describe the resulting path of the electron

a counter clockwise circle in the plane of the page.

The speed at which an electric field is set up in a wire upon turning on a light switch is ____________ and the drift speed of an electron in a wire is ______________.

approximately the speed of light, approximately one millimeter per minute.

electric force

charge times electric field

Lorentz force law

charged particles move in a circular path through a magnetic field

_________ for an inductor can not discretely change and ___________ for capacitor can not discretely change. Both of these facts are a direct consequence of _____________.

current, voltage, energy conservation.

magnetic charge

does not exist

T/F: the current flowing into one end of a resistor is greater than the current flowing out of the other end of the resistor

false since the resistor can't store charge, the rate at which an amount of charge is entering through one terminal must be the same as the rate at which a same amount of charge is exiting by the other terminal.

gaussian surfaces

gaussian's law in evaluating electric fields is that it applies to any surface. the net electric flux passing through a closed Gaussian surface depends only on the amount of charge inside that surface, not on its size or shape.

electric current

generates a magnetic field

a stationary, point electric charge

generates a spherically symmetric electric field.

changing magnetic field

generates an electric field around a closed loop

A positive charge and a negative charge are moved through an electric field. Both are moved in the direction opposite to the electric field direction. The work the electric field does on the positive charge is ____________ and the work the electric field does on the negative charge is _____________.

negative, positive. The force depends on the type of charge. If the charge is positive the force is also positive which means the force is directed towards the electric field. When the charge is negative, the force has a negative sign meaning the force is directed opposite the electric field.

When a negatively charged plastic rod is brought near one end of a neutral copper rod, the near end of the copper rod becomes ___________ charged and the far end becomes ______________ charged.

positively, negatively The object that offers the most space in which to "hang out" will be the object that houses the greatest number of excess electrons.

According to the conservation of energy, these things CANNOT change discretely

resistors: N/A inductors: current capacitors: voltage

According to the conservation of energy, these things CAN change discretely

resistors: current, voltage inductors: voltage capacitors: current

Fill in the blanks regarded charges embedded in an electric field: a positive charge experiences an electric force in the ____________ direction as the electric field and a negative charges experience an electric force in the ___________ direction as the electric field.

same, opposite positive charges experience an electric force in the same direction as the electric field. And negative charges experience an electric force in the opposite direction.

A magnet dropped north-end down into a copper tube induces eddy currents in the copper tube that induce magnetic fields that point...

up below the magnet, down above the magnet. The induced magnetic field opposes the magnetic field of the cause. At a cylindrical surface around the magnet, the axis of which is the connecting line between the north and south pole, the magnetic field points down.

resistors in parallel have the same _________, resistors in series have the same ___________.

voltage, current when a resistor is connected serially, the same amount of current flows through each resistor element but the voltage across each resistor is determined by the magnitude of the resistance of each resistor. In a parallel connection, the same voltage is applied across each resistor but different current.

Given the same __________, a parallel plate capacitor stores ___________ energy with a dielectric between its plates than without

voltage, more capacitance is directly proportional to the dielectric constant of the medium.