Magnetism and Electromagnetism
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MCAT Physical › Magnetism and Electromagnetism
In a current-carrying wire, which of the following expressions best relates charge, current, velocity and the length of the wire?
Assume 
Explanation
The force of a charge in a magnetic field is given by the equation 
Additionally, the force on a current-carrying carrying wire in a magnetic field is given by the equation 
Because we assume the angle to be 90o , we can set the two force equations equal to each other, and derive the equation 
By manipulating the variables, we can generate the equation 
In a current-carrying wire, which of the following expressions best relates charge, current, velocity and the length of the wire?
Assume
Explanation
The force of a charge in a magnetic field is given by the equation
Additionally, the force on a current-carrying carrying wire in a magnetic field is given by the equation
Because we assume the angle to be 90o , we can set the two force equations equal to each other, and derive the equation
By manipulating the variables, we can generate the equation
Which of the following influences the emf produced in a wire loop that is rotating in a magnetic field?
The size of the loop
The shape of the loop
The material of which the loop is made
The resistance of the loop
More than one of the other options is correct
Explanation
Recall that 
Which of the following influences the emf produced in a wire loop that is rotating in a magnetic field?
The size of the loop
The shape of the loop
The material of which the loop is made
The resistance of the loop
More than one of the other options is correct
Explanation
Recall that
You look at a circular loop of wire such that the plane of the loop is perpendicular to your line of vision. In what direction must a current be going through the loop in order for a magnetic field to be produced in your direction.
Counterclockwise
Clockwise
Clockwise but slowly decreasing
Clockwise and slowly increasing
The direction of the current does not matter
Explanation
This is an application of the right hand rule for magnetic fields produced by current carrying loops. To use the right hand rule, put your right thumb in the direction of the current, and the direction of the magnetic field is the same as the way your other four fingers wrap as you close your fist. The answers which include increasing or decreasing of a current hint at the concept of induction and are incorrect.
You look at a circular loop of wire such that the plane of the loop is perpendicular to your line of vision. In what direction must a current be going through the loop in order for a magnetic field to be produced in your direction.
Counterclockwise
Clockwise
Clockwise but slowly decreasing
Clockwise and slowly increasing
The direction of the current does not matter
Explanation
This is an application of the right hand rule for magnetic fields produced by current carrying loops. To use the right hand rule, put your right thumb in the direction of the current, and the direction of the magnetic field is the same as the way your other four fingers wrap as you close your fist. The answers which include increasing or decreasing of a current hint at the concept of induction and are incorrect.
You look at a circular loop of wire such that the plane of the loop is perpendicular to your line of vision. The loop has a constant current that is running through it clockwise. What will happen if a magnetic field were to be activated that is pointing in your direction?
The current in the loop will continue clockwise but will increase
The current in the loop will continue clockwise but will decrease
The current in the loop will begin to move counterclockwise at the same magnitude of current as before
The current in the loop will begin to move counterclockwise but slowly decrease
The current in the loop will begin to move counterclockwise and slowly increase
Explanation
This question works with the concept of induction. Simply put, the current in a wire will adjust such as to oppose a change in magnetic field. The loop originally has a magnetic field pointing away from the observer. Therefore, with the external magnetic field suddenly activated in the opposite direction (towards the observer), the current in the loop will act to counteract this change and increase while remaining clockwise.
You look at a circular loop of wire such that the plane of the loop is perpendicular to your line of vision. The loop has a constant current that is running through it clockwise. What will happen if a magnetic field were to be activated that is pointing in your direction?
The current in the loop will continue clockwise but will increase
The current in the loop will continue clockwise but will decrease
The current in the loop will begin to move counterclockwise at the same magnitude of current as before
The current in the loop will begin to move counterclockwise but slowly decrease
The current in the loop will begin to move counterclockwise and slowly increase
Explanation
This question works with the concept of induction. Simply put, the current in a wire will adjust such as to oppose a change in magnetic field. The loop originally has a magnetic field pointing away from the observer. Therefore, with the external magnetic field suddenly activated in the opposite direction (towards the observer), the current in the loop will act to counteract this change and increase while remaining clockwise.
An electron moves at 85km/s to the right along the plane of the page, while a uniform magnetic field points into the page. In what direction does the force act on the moving electron?
Upward along the plane of the page
To the right
Downward along the plane of the page
Out of the page
Into the page at an angle
Explanation
This question requires knowlegde of the right-hand rule. Point the fingers of your right hand in the direction of the electron's velocity (to the right). Point your thumb in the direction of the magnetic field (into the page). Your palm should be facing in the direction of the force on a positive particle. However, electrons are negative, so this direction must be reversed, meaning that the direction of the force is upward along the plane of the page.
An electron moves at 85km/s to the right along the plane of the page, while a uniform magnetic field points into the page. In what direction does the force act on the moving electron?
Upward along the plane of the page
To the right
Downward along the plane of the page
Out of the page
Into the page at an angle
Explanation
This question requires knowlegde of the right-hand rule. Point the fingers of your right hand in the direction of the electron's velocity (to the right). Point your thumb in the direction of the magnetic field (into the page). Your palm should be facing in the direction of the force on a positive particle. However, electrons are negative, so this direction must be reversed, meaning that the direction of the force is upward along the plane of the page.