Electromagnetically launched rockets? How shocking!

NASA is investigating the use of rail guns to launch spacecraft and hypersonic airplanes. A rail gun is a system that converts electromagnetic energy into mechanical energy. Consider a simple rail gun in the figure below. A conducting bar of length 1 m and mass 1 kg which can slide without friction is connected to two rails, which are in turn hooked up to a voltage source. The voltage source keeps a steady current of 100 Amperes flowing clockwise through the loop and the whole system sits in a constant magnetic field of 1 Tesla perpendicular to the plane of the loop. If I release the bar from rest, how fast is it going in m/s after 10 seconds?

Details and assumptions

  • You may neglect air resistance.


The answer is 1000.

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10 solutions

Mulubrhan Desta
May 20, 2014

In general, we have F = I ( L × B ) F=I(L\times B) . Since the direction of current and magnetic field direction are perpendicular, we have F = I L B = ( 100 A ) ( 1 m ) ( 1 T ) = 100 N F=ILB=(100A)(1m)(1T)=100N . From Newton's second law: F = m a F=ma we have a = F / m = 100 N / 1 k g = 100 m / s 2 a=F/m=100N/1kg=100m/s^2 . From laws of motion, v = u + a t = 0 + ( 100 m / s 2 ) 10 s = 1000 m / s v=u+at=0+(100m/s^2)10s=1000~m/s .

While there were a number of correct solutions to this problem, this solution was featured because it explains not only the numerical calculations, but also the physical concepts and algebraic expressions. Solutions are meant to explain to those who don't know how to do a problem, so this syntax of "Principle -> algebraic expression-> numerical answer" is great when writing solutions.

David Mattingly Staff - 7 years ago
Anupam Ah
May 20, 2014

the main principle involved in this problem is -" when a current carrying conductor is placed in a magentic field ,a force is experienced in the conductor"this force is called lorentz force.This principle is also used in motors and other electromagentic machines.....

lorentz force is calculated by the equation F=BILsin(theta)

B is magnetic field I is the current through the conductor L is the length of the conductor theta is the angle between length vector of the conductor and magnetic field

in this question we have I=100A L=1m B=1T theta=90 degrees

therefore force =1 100 1*sin(90) = 100N

given the mass of the conductor as 1kg accelaration experienced = F/m= 100 m/s^2

final velocity after 10 sec can then be calculated by

v=u+at u=0 therefore v=at =100*10 =1000 m/s

Mark Theng
May 20, 2014

The total force on the bar is ( I × B ) d l = ( 100 A ) ( 1 T ) ( 1 m ) = 100 N \int (I \times B) \, \mathrm{d}l=(100\mathrm{A})(1\mathrm{T})(1\mathrm{m})=100\mathrm{N} . Thus it has a constant acceleration of 100 N / 1 k g = 100 m s 2 100\mathrm{N}/1\mathrm{kg}=100\mathrm{m}\,\mathrm{s}^{-2} . After 10 seconds, its speed would be 100 m s 2 × 10 s = 1000 m s 1 100\mathrm{m}\,\mathrm{s}^{-2} \times 10\mathrm{s}=1000\mathrm{m}\,\mathrm{s}^{-1} .

Muralidhar Kamidi
May 20, 2014

The magnitude of force on the conductors is: dF = i dl x B.

Since the flow of current in each conductor is perpendicular to the magnetic field, the \sin \theta of the cross product is replaced by unity.

Thus, dF = i dl B

However, since the conductors are all linear, the integral of the above equation yields F = i l B, for each conductor. Applying the right hand screw rule, we see that the forces on the "rails" cancel each other. The conducting element opposite the projectile must be fixed (there can be no translational motion otherwise). Thus the acceleration on the projectile is given by,

a = F/m = i l B/m = 100 A * 1 m * 1 T/ 1 kg = 100 m s^{-2}

Accelerating from rest (u=0), at this rate for 10 seconds would give a final velocity of 100 m s^{-2} * 10 s = 1000 m s^{-1}.

Kunal Singh
Sep 29, 2013

Force on the bar is F = B I l F=BIl . Also , F = m a F=ma and a = d v d t a=\frac{\mathrm dv}{\mathrm dt} . Hence , m d v d t = B I l d v = B I l m d t m\frac{\mathrm dv}{\mathrm dt}=BIl \Rightarrow \mathrm dv = \frac{BIl}{m}\mathrm dt . Integrating with proper limits , 0 v d v = 0 t B I l m d t \int_0^v \mathrm dv = \int_0^t \frac{BIl}{m}\mathrm dt

v = B I l t m \Rightarrow v=\frac{BIlt}{m} . Substituting the respective values , we have

v = ( 1 T ) ( 100 A ) ( 1 m ) ( 10 s ) 1 k g = 1000 v=\frac{(1T)(100A)(1m)(10s)}{1kg}=\boxed{1000} m/s .

Kevin Fei
Sep 29, 2013

The force of a magnetic field on a current carrying wire is given by F = B I l F = BIl . Using the right hand rule, we calculate that the magnetic force pushes the rod to the right at all times. Now we will use Newton's Second Law to find the acceleration and the final speed of the rod.

F = B I l = m a F = BIl = ma

a = B I l m a = \frac{BIl}{m}

Now, kinematics:

v = v 0 + a t v = v_{0} + at

v = a t = B I l t m = 1 100 1 10 1 = 1000 m / s v = at = \frac{BIlt}{m} =\frac{1*100*1*10}{1}=1000m/s

Let F F be the force acting on the rail gun, L L be the length of the rail-gun, I I be the steady current, and B B be the magnetic field. Then, we obtain: F = I ( L × B ) \vec{F}=I (\vec{L} \times \vec{B} ) However, since L B \vec{L} \perp \vec{B} , the equation reduces to: F = I L B F= ILB The acceleration of the rail gun will then be given by: a = F m = I L B m a= \frac{F}{m}= \frac{ILB}{m} where m m is the mass of the rail-gun.

After time t t , the velocity of the rail gun will then be: v t = a t = I L B m t v_t= at = \frac{ILB}{m} t Plugging in the values, we obtain v 10 s = 1000 m / s v_{10 \ s}= \boxed{1000 \ m/s}

This is by far the simplest and clearest approach. :)

Ahaan Rungta - 7 years, 7 months ago

Log in to reply

Thanks! :)

Sreejato Bhattacharya - 7 years, 7 months ago

conductor resistances should also be neglected. OP forgot to put this assumption in the question.

Kunal Rmth - 7 years, 8 months ago
Sam Leo
Oct 9, 2013

Magnetic force on current: F = BILsinθ , F = force, B = magnetic flux in Teslas, I is the current in Ampres, L is the length of rod in metres and \theta is the angle the current makes with the magnetic field. In this case, because the rod is already perpendicular to the current, sinθ = 1

From the details given in the question, we simply sub in B = 1T, I = 100A, L = 1m to get force acting on the rod to be F = 1(100)(1) = 100N. Acceleration of the rod = F/m = 100/1 = 100 ms^{-1}.

By kinematic eqn v = u+at, we find that v = 0+(100)(10) = 1000s

Ton de Moree
Oct 6, 2013

We have that 1 T = 1 N A m 1T=1 \frac{N}{A*m} and that N = k g m / s 2 N= kg*m/s^2 .

So 1 = 1 a c c e l l e r a t i o n 100 1 1 = \frac{1*accelleration}{100*1} , so accelleration = 100 m/s^2.

Final speed after 10 10 seconds is therefore 10 100 = 1000 10*100=1000 m/s.

Brian Yao
Sep 29, 2013

This problem deals with the concept of motional EMF. This is a typical example of how current in a loop in conjunction with a magnetic field oriented orthogonal to the plane of the loop can induce a force. The magnitude of the force on the bar is F = I l × B F=Il\times B , where F F is the resultant force vector , I I is the current vector , and l l is the length of the sliding bar. Since we only really care about the magnitude of the force (if you want, you can confirm that the bar moves right according to the right hand rule), we take the magnitude of the cross product: F = B I l sin 9 0 o = B I l F=BIl\sin90^{o}=BIl . Thus, F = 1 T × 100 A × 1 m = 100 N F=1 T\times100 A\times1 m=100 N . Now, we can find the acceleration using Newton's second law: F = m a a = F m = 100 N 1 k g = 100 m s 2 F=ma → a=\frac{F}{m}=\frac{100 N}{1 kg}=100 \frac{m}{s^{2}} . Finally, we use kinematics to find the velocity after 10 seconds: v = v 0 + a t v = 0 + ( 100 m s 2 ) ( 10 s ) = 1000 m/s v=v_0+at → v=0+(100 \frac{m}{s^{2}})(10 s)=\fbox{1000 m/s} .

Ignore the first sentence; I meant to say magnetic force on a current-carrying conductor.

Brian Yao - 7 years, 8 months ago

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