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All Physics 21 Tips
Physics 21
| What is the basic rule of magnetic poles? | Like poles repel, unlike poles attract. |
| What are ferromagnets? Give examples. | Materials that can be magnetized — iron, nickel, gadolinium, and some alloys. |
| Why do magnetic field lines never cross? | Because that would violate the vector nature of the magnetic field — there can only be one direction at any given point. |
| What two conditions are required for a magnetic force to exist? | An external magnetic field must be present, and the charge must be moving. |
| What is the formula for the magnetic force on a moving charge? | F = qvB sinθ, where θ is the angle between the velocity and the magnetic field. |
| What is the formula for the force on a current-carrying wire? | F = IlB sinθ, where θ is the angle between the current direction and the magnetic field. |
| How does the magnetic force direction differ from the electric force direction? | In a uniform electric field, the force stays the same direction (parabolic path). In a magnetic field, the force is always perpendicular to velocity (circular path). |
| What does the first right-hand rule determine? | The direction of force on a moving positive charge — thumb points along current, fingers point towards magnetic field, palm faces the direction of the magnetic force. |
| How do you apply the right-hand rule for a negative charge? | Reverse the arrangement by 180°; the force is perpendicular to the back of the hand. |
| What is the magnetic field formula for a long straight current-carrying wire? | B = μ₀I / 2πr — the field is inversely proportional to the distance r from the wire. The closer you are to the wire, the stronger the magnetic field will be. |
| What is the second right-hand rule used for? | Finding the direction of the magnetic field around a current-carrying wire — thumb points in the direction of current, fingers curl in the direction of B. |
| What is the magnetic field at the center of a circular coil? | B = N(μ₀I / 2r), where N is the number of loops and r is the radius. |
| What is a solenoid and what is its magnetic field formula? | A solenoid is a long helical coil of wire. Its field is B = μ₀nI, where n is the number of turns per unit length. |
| What is the torque on a current-carrying coil in a magnetic field? | τ = NIAB sinφ, where φ is the angle between the magnetic field B and the normal to the loop. There is a big difference between the angle that the magnetic field makes with the plane of the loop and the one that is made with the normal of the loop |
| What is the magnetic dipole moment? | M = NIA (number of loops × current × area). |
| When is torque on a current loop zero? When is it maximum? | Zero when φ = 0° or 180° (B parallel to normal). Maximum when φ = 90° (B perpendicular to normal). |
| How does a mass spectrometer work? (NOT ON SHEET) | Ions are vaporized, ionized, and accelerated through a potential difference V. They enter a magnetic field and follow semicircular paths — the radius reveals their mass using m = qB²r² / 2V (modified version of qvBsin0 = mv²/r) |
| How does an electric motor keep spinning continuously? | A disconnected cylinder (commutator) and carbon brushes reverse the current direction every half turn, maintaining torque in the same rotational direction. |
| Tip about the magnetic field and the radius on a current carrying wire | The closer you are to the wire, the stronger the magnetic field. |
| Tip about magnetic fields due to electric currents | The stronger the current, the stronger the magnetic field. And the smaller the radius, the stronger the magnetic field at the center of the coil. Remember that when using the 2nd RH rule, you must place your fingers INSIDE the coil. |
| Tip about uniform magnetic field | The direction of a force in a uniform magnetic field is always perpendicular to the direction of the velocity of the particle which keeps it in a circular motion. Centripetal force always makes a 90 degree angle with velocity of moving object |
| The forces at the top and bottom of a loop are typically zero, then... | You can use the first right hand rule to see if the force will be going out or into the page (perpendicular from the palm). |
| If current arrow points down on the right, what direction is it? If it points down on the left? | Right would be clockwise, left would be counterclockwise. |
| Equation for magnetic moment of a coil | M = NIA |
| Dots = | Out of page, point your fingers out |
| Crosses = | Into the page, point your fingers in |
| Magnetic Field Demo: | Magnetic field between the magnet poles points from the N pole towards the S pole. If magnetic field direction is reversed at 180 while the direction of the current stays the same, the direction of the force reverses and acts on the wire straight down. |
| How to determine the magnetic field strength and its direction at a point (P) away from certain wires? | Fingers (and knuckles) towards point (P), curled around the wire, thumb in direction of the current. Figure out if each B is pointing in or out. Use B = μ₀I / 2πr for strength. Add for total strength (make one pos and other neg depending. on in or out) |
| For simplicity... | Make in positive and out negative |
| Let's say you have two wires and they're asking for the magnitude and direction of the force for one of them? (Part One) | Set up the F = IlBsinθ equation and put in all values. To find B, you can use B = μ₀I / 2πr and for the angle, use the second hand rule with the OTHER wire, pointing towards the first. If it points in, that's 90. Put that into thh B = μ₀I / 2πr. |
| Let's say you have two wires and they're asking for the magnitude for one of them? (Part Two) | Use the first right hand rule on the original wire for the direction of the force, knowing your fingers have to point inwards due to the magnetic field. |
| Easiest way to differentiate between first right hand rule and second | First right hand rule is the stretched hand and the second is the curved |
| How to determine the direction of motion in which a particle travels - clockwise or counterclockwise - and the radius of the circle? | Use the first right hand rule to determine the direction of the force and use that to figure out if it'll be clockwise or counterclockwise. Use F = mac, equate F to Fm = qvBsin0 and mac = mv^2/r. |
| Tip about sinθ | Almost always 90 in the case of forces for moving charges |
| If the magnetic field is mounted in such a way that is horizontal... | So if B is horizontal, the loop spins on a vertical axis — and the horizontal sides (parallel to that vertical axis) produce no torque. Only the vertical sides do the work. |
| If the magnetic field is mounted vertically... | If B is vertical, the loop spins on a horizontal axis — and the vertical sides produce no torque. Only the horizontal sides do the work. |
| If the left hand side is under a force that is pointing into the page and the right hand side is under a force pointing out of the page, then... | Then the loop spins about an axis that is parallel to the long side, direction is clockwise. |
| If the left hand side is under a force that is pointing out of the page and the right hand side is under a force pointing into the page, then... | Then the loop spins about an axis that is parallel to the long side, direction is counterclockwise. |
| If the top section has a force inward and the bottom section has a force outward, then... | The loop is going to spin along an axis that is parallel to the short side, counterclockwise. |
| How to find the maximum torque that the magnetic field can exert on the coil? | Use NIAB sinφ, the maximum value would be 90 degrees, equaling 1. |
| When you have horizontal wires and you have to find the magnitudes of the net magnetic fields at two points, what should you do? | First use the second right hand rule for both of the vectors at point A (knuckles point towards A). Do B net = B1 + B2 (set one or the other neg if needed), then use B = μ₀I / 2πr. Do the same for point B. |
| Three particles are moving perpendicular to a uniform magnetic field and travel on circular paths. How would you find the algebraic sign of each charge and which charge has the largest magnitude? | Use the first right hand rule, whatever molecules are coming out from the back of the hand are negative and those underneath or adjacent are positive. Because the smaller the r, the smaller the charge, the one that's least spread out has the largest mag |
| You have an equilateral triangle, if the top dot is out then... | The force would be towards the right and if in, it would be towards the left. |
| You have an equilateral triangle, if the bottom right dot is out then... | The force would be straight down. |
| You have an equilateral triangle, if the bottom left dot is in then... | The force would be straight down. |
| Equilateral Triangle Tip | When solving for B net, solve for x and y components first, then add them together via pythag for magnitude and angle of the field vectors. |
| How to find the required magnitude of the E-field for particles to remain undeflected? | Undeflected means particles go straight through the B-field without being deflected one way or another. You would use -qdelta V = Kef - Kei, with Kei cancelling out due to rest. Solve for v. |
| How to find the required magnitude of the E-field for particles to remain undeflected? (Continued) | Then use 1st right hand rule to find force. You need an equal force to keep it from being deflected. Do qE=qvBsin(90). Solve. |
Created by:
smurtab