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All Physics 22 Tips
Physics 22
| Question | Answer |
|---|---|
| Magnetic Flux: | the amount of field lines passing through the area of the magnetic field. |
| Normal: | unit vector that is perpendicular to the surface. |
| Magnetic Flux is at maximum when: | the magnetic field and the normal are parallel to each other, cosine (0) = 1, cosine (180) = -1. |
| The Magnetic Flux is at zero when: | the normal force is pointing horizontal and the magnetic field is pointing down the page, so the angle made is 90. Cosine (90) = 0, so the flux here will be nothing. |
| When will the magnetic flux be at a fraction of the maximum: | when the angle between the magnetic field and the normal is in between 0 to 90, like 60. |
| The magnetic field through a given loop can change in three different ways → | a) changing the magnetic field, b) changing the area of the loop, and c) changing the angle between the magnetic field and the loop - which can be done by spinning the loop. If you do one, you have to keep the others constant |
| Faraday's Law of Induction | states that a changing magnetic field through a loop of wire induces an electric voltage (EMF) in the wire. The faster the magnetic flux changes, the larger the induced voltage and current. |
| Induced EMF Demo | Changing flux with a magnet induces current in a coil, detected by galvanometer. The N and S poles produce opposite current directions, and pulling the magnet out reverses the current. Essentially, changing magnetic flux controls the induced current. |
| Lenz's Law | States that an induced EMF produces a current whose magnetic field always opposes the change of magnetic flux that caused it, indicated by the negative sign in the equation. |
| When the external magnetic flux increases... | The induced magnetic flux, which is coming from the other side, counteracts the external by adding a flux in the opposite direction. They cancel each other out because the magnetic field is a vector quantity. |
| If you want to find the direction of the induced current… | use the second right hand rule. Point your fingers in the same direction as the induced magnetic field, then hold the loop from the front side and let the thumb rest on top of the coil. Use this to find the INTERNAL magnetic field, not the external. |
| When the external magnetic flux decreases… | the induced magnetic flux counteracts it by restoring the amount of flux that has been taken away. Meaning the ex and in magnetic fields are pointing in the same direction. If you want to find the direction of the current, use the 2nd rule again. |
| If flux is increasing → | induced magnetic field and external magnetic field point in opposite directions. |
| If flux is decreasing → | induced magnetic field and external magnetic field point in the same direction. |
| Tip for Induced Magnetic Field | If you are only given the external magnetic field, determine whether your induced magnetic field will have the same direction or not (based on inc vs. dec), then curl your fingers in direction of induced field. Thumb shows current direction |
| If you're solving for the EMF for a loop that changes by a certain angle... | First find the change in flux from f - i using the angles using the BAcos(theta) formula, then input that into the induced EMF formula. |
| Important EMF formulas | V = Emf - Ir V = IR -> EMF = IR I = EMF/R |
| Motional EMF: | an EMF is induced in a conducting rod or wire which moves through a magnetic field. This can be predicted by the first right hand rule. Theta is the angle between the magnetic field and the velocity. |
| How to predict the direction of motional EMFs with electrons | Use the first right hand rule. Line your thumb up with the current and point your fingers in the direction of the external magnetic field. Remember that because the electron is negatively charged, your force should be perpendicular from the backside hand |
| When the bar is moving towards the right... | the area of the closed loop is going to increase. The magnetic field is constant, the loop area is increasing, and therefore, the magnetic flux is going to increase. |
| How to find the direction of the current with motional EMFs? | Figure out whether flux increases or decreases, draw induced field. Then use the second hand rule → line the curling fingers with the direction of the induced field and line the thumb up with the rail. |
| The loop (and flux by extension) is increasing when... | The rod is moving towards the right. |
| Tip about electrons | The force will ONLY emerge from the back of the hand when there's an electron involved; otherwise, it will come from the palm. |
| Why is it necessary to maintain a constant external force on a moving bar to keep it moving at constant velocity? | Because you need to solve for the magnetic force that counteracts it, which is IlBsin(theta). Remember that you would use the second right hand rule for the induced magnetic field and the first right hand for the external force. |
| Eddy Current Demo: | A plate is moved through a space where there is a magnetic field. When plate is not cut at ALL, it slows entirely. When it's cut into comb shape, it doesn't stop. When it's almost cut into a comb shape, loops can be formed and it slows again. |
| How to find the magnitude and direction of the induced current when the rod is moving at a certain velocity? | Use the EMF = vlBsintheta equation, then solve for I = EMF/R. To find the direction, figure out whether the flux is inc or dec, then use the second right hand rule on the induced magnetic field. |
| How to solve for the magnitude of external force keeping the rod moving at constant speed? | Fext = IlB, use the right hand rule to find the direction of the force, thumb in direction of velocity. |
| Eddy Current | induced current is called an Eddy current when it is produced by a changing magnetic flux. The direction of the magnetic force acting on the induced current (eddy current) always opposes the direction of motion of the plate → which explains the demo. |
| Conclusion of Free-Falling Plate Eddy Current Example: | the plate will slow down when it enters the magnetic field, pick up full speed while completely within the region of the magnetic field because there is no change of flux, and then slow again while the plate starts to leave. |
| When the plate is falling into the magnetic field... | There's an increase in magnetic flux |
| When the plate is falling out of the magnetic field... | There's a decrease in magnetic flux |
| How to calculate direction of current and net force for a plate falling into a magnetic field? | You can use the second right hand rule for the current, then use 1st rule while lining up your thumb with the current. Calculate the magnetic of magnetic force using IlBsintheta. |
| How to calculate direction of current and net force for a plate falling out of a magnetic field? | You can use the second right hand rule for the current, then use 1st rule while lining up your thumb with the current. Calculate the magnetic of magnetic force using IlBsintheta. |
| Super Important Formula that's not included in the formula sheet but essential for motional EMF? | IlBsintheta |
| The net force on a loop would be... | There should be three vectors that make contact with the magnetic field, the one that's different from the others is your primary acting force. |
| When a magnet falls through a conducting copper pipe... | it induces currents in the pipe that create magnetic fields opposing the magnet’s motion. The induced field below the magnet repels it, while the field above attracts it, producing an upward resistive force that slows the magnet as it falls. |
| Tip about Second Right Hand Rule with induced currents... | Remember that you hand should be placed at the front of the loop, meaning the bottommost part. |
| Electric Generator | uses mechanical work to produce energy. Coil is turned by an external torque and an emf is included as the coil rotates through an external magnetic field (because the flux changes due to the constantly changing angle between field and normal). |
| w in Electric Generation Equation | The w is the constant angular velocity, wt is the angle that the face of the loop makes with the direction of the magnetic field |
| Back EMF Generated by An Electric Motor | When a motor is operating, two sources of EMF are present → 1) The applied EMF (V) that provides current to drive the motor. 2) The EMF induced by the generator-like action of the rotating coil, because the magnetic flux is changing. |
| Minimum Current | In the V - EMF = IR equation, Io represents the initial current when the motor is not spinning. The total current I is smallest when the motor is running at full speed because the back EMF is greatest. |
| Back EMF (Simplified) | the voltage produced by a motor’s spinning coil that opposes the applied voltage. As the motor speeds up, the back EMF increases, reducing the current drawn by the motor. |
| Transformers | a transformer is a device used to increase or decrease an AC voltage. It costs of wire, known as the primary and secondary coils. The primary is connected to a source of EMF and the secondary to a device usually referred to as the load. |
| Primary Coils | AC current in primary coil creates a changing magnetic field in the primary side. The magnetic field in the primary side is constantly changing because the AC current is a changing current. When the current is changing, the B field also has to change. |
| Secondary Coils | the magnetic field lines are guided by the iron core all the way to the secondary coil and cause the magnetic flux there to change. |
| If the transformer is assumed to be ideal... | the change in flux in the primary coil will equal the secondary. The power output in the secondary would be equal to the power input in the primary. |
| An ideal transformer has… | 100% efficiency. |
| Main equation for transformers | P lost = I^2(R) Total power generated = IV |
| Transmission of Electric Power | The higher the current, the more power is going to be lost. To keep the power loss at a minimum, you can increase the voltage (which will decrease the current) and the power will decrease → this can be done with transformers. |
| Power Essential Concept | Increase voltage -> decreased current -> power loss stays as low as possible. |
| Voltage is stepped down via multiple transformers | Before getting to the city, the voltage is stepped down gradually using multiple transformers. In this case, the voltage is stepped all the way down to 8000 V, and near homes, stepped down again to 240 V. |
| Transformers Demo | When AC current flows through the primary coil, it creates a changing magnetic field. This changing field induces a current in the secondary coil, which powers the light bulb. |
| How to determine the emf and current in the secondary transformer in an ideal transformer? | You could use the Vs/Vp = Ns/Np relationship, solve for voltage (emf). Furthermore, since it's ideal, IpVp = IsVs, so you can set up a relationship to solve for the current. |
| If the number of turns in the primary has doubled... | Except the voltage in the secondary to double. |
| How to evaluate how much power is saved by the voltage stepped up and then stepping down? | First evaluate how much power is lost when the transformer is not used. Do P = IV, solve for current. Then P lost = I^2(R). Do the same with the transformers with Pinput = Poutput, then I^2(R). Compare. |
| If you have resistance over two wires... | The resistance value should be doubled. |
| How to find the average induced current in a loop with a changing diameter? | Do Emf = IR, then find the change in flux and insert it into the -N delta flux/delta t equation, also use delta A. Then put that into the Emf = IR equation for average I. |
| How to find the EMF induced in a loop as it's moving out of a magnetic field? | Design the question to include four bars on each side, the EMF would be the sum of all four bars. Use the vlBsin(theta) equation. The bar outside of the loop is 0. Imagine that there is an e- moving along each bar and use 1st right hand rule. |
| Remember that if you use first right hand rule and discover that the component of the length of the bar is perpendicular to velocity, then... | The EMF would be zero there. The sides of the loop tend not to have a charge while the topmost does if it's falling straight down. |
Created by:
smurtab