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# AS and A Level: Fields & Forces

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## What are gravitational fields?

1. 1 A gravitational field is a region where a mass experiences a force. The field strength, g, at any point in the field is given by g=F/m and the value of g on the Earth’s surface is taken to be 9.81Nkg-1.
2. 2 Field lines point towards the centre of the Earth and are radial. Over small distances, near Earth's surface, g can be considered constant so field lines are parallel and the field is uniform.
3. 3 G was calculated by Henry Cavendish by measuring the force of attraction between two lead spheres of known mass and separation. The force between two masses is given by F = Gm1m2/r2 and this is called Newton’s law of universal gravitation.
4. 4 Inside the Earth, g falls from 9.81 to 0 Nkg-1 so we cannot use the inverse square law for r < RE.
5. 5 Combining Newton’s law with circular motion can be used to calculate distance to geostationary satellites.

## What are electric fields?

1. 1 An electric field is a region where a charge experiences a force. The field strength E at any point in the field is given by E = F/Q. The force between two charges is given by Coulomb’s law.
2. 2 For radial fields, E = 1/ Q/r2 and this is another inverse square law. For uniform fields, E = V/d.
3. 3 Uniform electric fields can be set up to accelerate charges. The work done accelerating a charge through a p.d. V is given by W = QV. The unit of energy can be given in Joules (J) or electronvolts(eV).
4. 4 When a charge enters a uniform electric field, such as between the deflection plates of an oscilloscope, there will constant acceleration and so suvat equations can be used.
5. 5 For all electric fields, equipotential lines are drawn perpendicular to field lines. For radial fields, always show at least 3 equipotential lines as concentric circles with increased spacing.

The equipotential lines can be experimentally determined using conductive paper, metal electodes and a voltmeter to map out points of equal potential. You should be able to draw equipotential patterns for two point charges.

## Similarities and differences between gravitational and electric fields.

1. 1 Gravitational forces are always attractive but electric forces can be both attractive and repulsive. There are no negative masses but there are negative charges.
2. 2 The ratio of the strength of the two forces is huge. For two electrons, FE/FG is approximately 1042. This demonstrates how much stronger the electric force is compared to the gravitational force over the same distance.
3. 3 Both fields obey an inverse square law.
4. 4 Over short ranges, electric forces dominate but over much larger distances, say between planets and their moons, gravitational forces dominate because the attractive and repulsive electric forces tend to cancel out.

• Marked by Teachers essays 1
• Peer Reviewed essays 9
1. ## Investigating the forces acting on a trolley on a ramp

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Finally, the angle of the ramp was altered and the measurements taken again for three different angles. By measuring the vertical height at the point where the light gate was and a fixed distance from this, it was possible to calculate the exact angle at which the ramp rested by using simple trigonometry. Theory There were two ways to approach the experiment. It is possible to calculate gravity (g) by considering the 'conservation of energy', which calculates the gravitational potential energy of the trolley and uses this information to find the acceleration of the trolley. In this experiment, the variable is the vertical component of the ramp.

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2. ## Modeling a basketball shoot in the lab

Putting t into equation (6), the equation of trajectory is, y= x tan ? - g/2u2 cos2 ? This shows that the object can be described as a parabolic path since it is comparable with the equation y= - kx2 + c. Theory- PROJECTILE MOTION AT AN ANGLE A general case of projectile motion occurs when the projectile is fired at an angle. y=0; 1. Upward direction is positive. Acceleration due to gravity (g) is downward thus g = - 9.8 m/s2 2. Resolve the initial velocity vo into its x and y components: vox = vo cos ?

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3. ## Investigation to determine the viscosity of glycerol.

Viscosity is basically the resistance between the particles within a fluid. In a solid molecules are tightly packed i.e. there is a strong intermolecular force. Therefore when you move a solid, it moves as a whole as all the particles move together. In a fluid this is not the case. This is because due to lower intermolecular force the particles move at different velocities in relation to each other. This feature can be understood by discussing what happens when pouring a jug of water into a glass. In liquids the molecules flow in layers.

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4. ## What Affects the Strength of Magnetism Exerted By an Electromagnet?

The Earth has a magnetic field. There are also poles named 'Geographically' North Pole and South Pole. The North Side of the compass needle will naturally point to the geographic North Pole. However, this would prove whole theory of magnetism incorrect. We have therefore decided that Magnetic South is Geographic North. Thus, the North Pole of a compass needle points 'Geographic North' and 'Magnetic South'. There are certain 'Rules of Magnetism' which have been founded. One of the most important is that: Lines of force will never cross. This becomes evident when we sprinkle 'Iron Filings' around a magnet.

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5. ## Rules and Laws in Football

The area bounded by these lines and the goal line is the goal area. The Penalty Area A penalty area is defined at each end of the field as follows: Two lines are drawn at right angles to the goal line, 16.5 m (18 yds) from the inside of each goalpost. These lines extend into the field of play for a distance of 16.5 m (18 yds) and are joined by a line drawn parallel with the goal line. The area bounded by these lines and the goal line is the penalty area. Within each penalty area a penalty mark is made 11 m (12 yds)

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6. ## Investigating a factor affecting the voltage output of a transformer.

of turns on the secondary coil Increasing number of secondary coil turns increases the number of times the magnetic field lines are cut, increasing the secondary voltage * Primary voltage Increasing primary voltage increases the current, increasing the speed at which the magnetic field alternates, increasing the number of times the magnetic field lines are cut by the secondary coil in a given time, increasing the secondary voltage * Cross-sectional area of secondary coils Increasing cross-sectional area of secondary coils increases the number of magnetic field lines cut in a given time, increasing secondary voltage * Distance of separation between

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7. ## Einstein's theory of relativity.

A book may be jarred from a seat and fall without being dropped. A ball will travel differently. One way of stating the principle of this theory is to say that the laws of mechanics are the same for an observer in a smoothly moving train as for the observer at a fixed position on the ground. Physicists would say: if two systems move uniformly relative to each other, then all the laws of mechanics are the same in both systems. This principle may be called the classical relativity principle. This principle is as old as the ideas of mechanics and physics.

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8. ## Magnetic Resonance Imaging.

Basic idea Hydrogen nuclei are subjected to a pulse of radio waves, which causes them to briefly emit low-intensity radio waves. These are detected by the MRI scanner, which measures the signal as the patient is scanned by a changing magnetic field. The signal is processed, producing an image of where the hydrogen atoms in water molecules and lipids are located. Nuclei used in NMR contain an odd number of protons or neutrons and so possess intrinsic spin. (Spin is an inherently quantum property.

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9. ## Aurora- Light of Mystery.

This is not absolutely true because vacuum is a space with nothing, no even air, but there are actually particles existing in the interstellar space. At sea level the density of air molecules is 1 quintillion per cm3; at 100km roughly where aurora occurs, density is 1 trillion per cm3; in the interstellar space the density is merely 1 per cm3. Therefore as a result, most of outer space is filled with electrically conductive plasma. Besides of particles, there are magnetic field, electric current, electromagnetic force, gravitational forces...

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10. ## Maglev Trains And The Technology Behind Them (magnetism)

The repulsive force between magnets with like poles facing explains how permanent magnets can be used to provide a levitating and thrust force to the train. Magnet Strength We know that magnets have a north and south pole and if we bring them together they will repel each other. We can see this effect in the picture below with the area between the two magnets having the iron filings being repelled away from the area in between the two magnets.

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11. ## Investigating Electromagnets

An A.C current would demagnetise the core while a D.C. current would magnetise a core. This is because the D.C. current is direct and therefore all the electrons are arranged in the same order creating a magnetic field. When the current is A.C, the electrons are free to move where they want so there is no magnetic field. When an electromagnet has a current induced in it, it becomes magnetised and the greater the current induced the greater the strength.

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