一个P Physics C Electricity : Calculating Magnetic Fields and Forces

研究概念,例子的问题& explanations for AP Physics C Electricity

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Example Questions

Example Question #1 :Calculating Magnetic Fields And Forces

一个n infinitely long wire has a current ofrunning through it. Calculate the magnetic field at a distanceaway from the wire.

Possible Answers:

Correct answer:

Explanation:

For infinitely long wires, the formula for the magnetic field is, whereis the current and线的距离。

The magnetic field is calculated using our given values.

Example Question #1 :Calculating Magnetic Fields And Forces

一个solenoid islong and it is made up ofturns of wire. How much current must run through the solenoid to generate a magnetic field ofinside of the solenoid?

Possible Answers:

Correct answer:

Explanation:

The formula for the magnetic field inside the solenoid is, whereis the number of turns of wire,is the length of the solenoid, andis the current.

We want to find the current so we solve for.

Plug in the values.

Example Question #1 :Calculating Magnetic Fields And Forces

Two parallel wires a distanceapart each carry a current, and repel each other with a forceper unit length. If the current in each wire is doubled to, and the distance between them is halved to, by what factor does the force per unit length change?

Possible Answers:

Correct answer:

Explanation:

Relevant equations:

Step 1: Find the original and new magnetic fields created by wire 1 at wire 2, using Ampere's law with an Amperian loop of radiusor, respectively.

Original

New

Since the wires are parallel to each other and wire 1's field is directed circularly around it, in each case wire 1's field is perpendicular to wire 2.

Step 2: Find the original and new magnetic forces per unit length on wire 2, due to the field created by wire 1.

Original

New

So, the new force per unit length is 8 times greater than the original.

Example Question #11 :Magnetism

一个region of uniform magnetic field,, is represented by the grey area of the box in the diagram. The magnetic field is oriented into the page.

Ps0_movingqinbfield

一个stream of protons moving at velocityis directed into the region of the magnetic field, as shown. Identify the correct path of the stream of protons after they enter the region of magnetic field.

Possible Answers:

一个semi-circular path oriented horizontally into the page

一个semi-circular path oriented vertically upward

一个semi-circular path oriented vertically downward

一个semi-circular path oriented horizontally out of the page

Correct answer:

一个semi-circular path oriented vertically upward

Explanation:

The magnetic force on a moving charged particle is given by the equation:

Isolating the directional component of this equation yields the understanding that the resulting force on a moving charged particle is perpendicular to the plane of the velocity vector and magnetic field vector. Using the right-hand-rule on this cross-product shows that the velocity vector right-crossed into the magnetic field vector into the page yields a magnetic force vector upward on a positive charge. This will result in a semi-circular path oriented vertically upward.

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