# What Is the Acceleration of the Metal Bar After Closing the Switch?

• kimberlyann9
In summary: The diagram is wrong, it is showing a VERTICAL magnetic field which means the force on the bar is towards the bottom of the image. To find the acceleration of the bar due to the field, you would need to use V=IR and then use F=ILB-mg to find the force.
kimberlyann9

## Homework Statement

A 2.60-N metal bar, 1.50 m long and having a resistance of 10.0 ohms, rests horizontally on conducting wires connecting it to the circuit shown in the figure . The bar is in a uniform, horizontal, 1.60-T magnetic field and is not attached to the wires in the circuit.

What is the acceleration of the bar just after the switch S is closed?
http://session.masteringphysics.com/problemAsset/1266176/3/27.74.jpg

## Homework Equations

V=IR
Fbar=ILB
Fgravity=mg
Rseries=sum of R
Rparallel=(1/R)^-1

## The Attempt at a Solution

I found the resistance of the parallel resistors(the bar and the 10ohm resistor) to be 5ohms
Then the Req=5ohms+25ohms=30ohms

Use that and the given voltage to find I.
I=4A

Use I to find Voltage of the bar.
V=20V

And then we can find I using the voltage(20V) and resistance of bar.

I=2A

F=Fbar-Fgravity
ma=ILB-mg

I get a=8.29 m*s^-2 but Mastering Physics says it's wrong. Where have I gone wrong?

Last edited:
Hi kimberlyann9, Welcome to Physics Forums.

kimberlyann9 said:

## Homework Statement

A 2.60-N metal bar, 1.50 m long and having a resistance of 10.0 ohms, rests horizontally on conducting wires connecting it to the circuit shown in the figure . The bar is in a uniform, horizontal, 1.60-T magnetic field and is not attached to the wires in the circuit.

What is the acceleration of the bar just after the switch S is closed?
http://session.masteringphysics.com/problemAsset/1266176/3/27.74.jpg

## Homework Equations

V=IR
Fbar=ILB
Fgravity=mg
Rseries=sum of R
Rparallel=(1/R)^-1

## The Attempt at a Solution

I found the resistance of the parallel resistors(the bar and the 10ohm resistor) to be 5ohms
Then the Req=5ohms+25ohms=30ohms

Use that and the given voltage to find I.
I=4A

Use I to find Voltage of the bar.
V=20V

And then we can find I using the voltage(20V) and resistance of bar.

I=2A

F=Fbar-Fgravity
Question: Why do you subtract the force due to gravity here? The bar is resting horizontally on the wire "rails", so it's supported. What is the direction of the force on the bar due to the magnetic field?
ma=ILB-mg

I get a=8.29 m*s^-2 but Mastering Physics says it's wrong. Where have I gone wrong?

The diagram shows a VERTICAL magnetic field, not a horizontal field. This could be why you think you need to subtract mg.
Also did you notice the WEIGHT of the bar (2.60N) is given, you will need mass to use in the F =ma equation

Thanks guys I figured it out.

I would first check my calculations to make sure all of the steps were correct. If I am confident in my calculations, I would then consider other factors that could affect the acceleration of the bar, such as friction or air resistance. I would also double check the given values and equations to ensure they are accurate and appropriate for the problem. If all of these factors check out, I would then consider other physical phenomena that could be at play, such as electromagnetic induction or eddy currents, which could potentially affect the acceleration of the bar. I would also consider reaching out to colleagues or doing further research to see if there are any other factors I may have overlooked.

## 1. What is a magnetic field?

A magnetic field is an invisible force field that is created by moving electric charges. It exerts a force on other moving electric charges and on magnetic materials, such as iron, nickel, and cobalt.

## 2. How are magnetic fields created?

Magnetic fields are created by moving electric charges, such as electric currents. They can also be created by permanent magnets, which have their own magnetic fields.

## 3. What is resistance in relation to magnetic fields?

Resistance is a measure of how difficult it is for electricity to flow through a material. In magnetic fields, resistance can affect the strength and direction of the magnetic field.

## 4. How do magnetic fields impact electrical devices?

Magnetic fields can impact electrical devices in a variety of ways. They can be used to generate electricity, but they can also interfere with the proper functioning of electronic devices. Strong magnetic fields can even damage electronic components.

## 5. How can we protect ourselves from strong magnetic fields?

Strong magnetic fields can be harmful to humans, so it is important to take precautions when working with or near them. This can include using protective equipment, limiting exposure, and following safety guidelines set by regulatory agencies.

Replies
5
Views
6K
Replies
3
Views
1K
Replies
1
Views
2K
Replies
8
Views
2K
Replies
16
Views
1K
Replies
8
Views
6K
Replies
4
Views
2K
Replies
4
Views
2K
Replies
1
Views
15K
Replies
4
Views
8K