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AS and A Level: Fields & Forces
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What are gravitational fields?
- 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 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 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 Inside the Earth, g falls from 9.81 to 0 Nkg-1 so we cannot use the inverse square law for r < RE.
- 5 Combining Newton’s law with circular motion can be used to calculate distance to geostationary satellites.
What are electric fields?
- 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 For radial fields, E = 1/ Q/r2 and this is another inverse square law. For uniform fields, E = V/d.
- 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 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.
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 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 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 Both fields obey an inverse square law.
- 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
Procedure Apparatus * Stand * Motion sensor * Computer with datastudio installed * Slotted masses and mass holder * CD * Pointer * Half metre stick * Balance accurate to 1g * Lab jack * Spring Part One * The experiment is set up as shown. * Datastudio is opened on the computer, and "create experiment" is clicked. * In the "create experiment" window, the motion sensor is selected. * In the measurement window, "Position, Ch1 & 2 (m)" is selected. * In the "display" window, the "graph" selection was double clicked to set up a graph of the results.
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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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air C Displacement of fall C Time taken to fall D Prediction Since the theory suggests that So the square of time should be directly proportional to the inverse of mass. A straight line graph of t2 versus m-1 should give a straight line graph with a gradient of All of these values will be measurable or known, except for the drag coefficient, c. Method Preliminary experiments Determining a size of the paper cone: Three unweighted paper cones were constructed from A4 paper and selotape: w is the width (diameter)
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* 'h' is the distance of the pivot from the centre of gravity in metres - independent variable * 'g' is the acceleration due to gravity - dependant variable By plotting T against h at this point, the graph will be a parabola. This is because T is proportional to h, and the equation is not in the form of y = mx + c, in which case the graph would be a straight line. The above equation can also be written including the mass of the pendulum.
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The object will accelerate to higher speeds before reaching a terminal velocity. Thus, more massive objects fall faster than less massive objects because they are acted upon by a larger force of gravity; for this reason, they accelerate to higher speeds until the air resistance force equals the gravity force . Method The apparatus used in the experiment are a plastic bag, scissors, a set of 5 paperclips, a ruler, stopwatch or wristwatch with ability to read to at least 0.1 s, notebook and pencil.
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Investigate four factors that may affect the strength for electromagnets: the number of turns, the size of the current, the nature of the current (a.c. or d.c.) and the distance between the sensor and the magnet.4 star(s)
3 Connect the circuit as the diagram showed. 4 Twine the wire on the magnet with 20 turns. 5 Turn on the switch and record 5 successive readings on the graphical calculator as 'X1 T' (since the reading changes all the time) 6 Turn off the switch and change only and increase the number of turns on the magnet by 10 turns. 7 Turn on the switch and record the new 5 successive readings on the graphical calculator as 'X2 T' 8 Repeat step 6&7 for another 4 times and correlatively get X3 X4 X5 X6 T. Part 2: Size of the current 1 Measure the room temperature and record as 't'.
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this investigation, I am going to determine the acceleration due to gravity on the earth by using an electronic timer and varying its height of dropping. In this method, a steel ball is hold by a free fall adaptor (ball release mechanism) , when we release the ball, the current to the circuit is switched on and the ball begins to fall. At the same time an electronic timer starts. The ball falls through a receptor pad and this will break down the circuit to stop the electronic timer.
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Elastic potential energy (EPE) is the energy stored in bodies such as springs, elastic and rubber bands. An archer drawing a bow applies EPE to the bow string. science.howstuffworks.com 4. Chemical energy is the stored energy possessed by foods, fuels and batteries. A human eating food is taking in chemical energy. http://en.wikipedia.org/wiki/Battery_ (electricity) 5. Thermal or heat energy is energy that flows by conduction, convection or radiation from hot areas to cold it can also be a by product of wasted energy during energy conversions http://www.oxfordreference.com/pages/VED_samples 6.
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The aim of my investigation was to explore the viscosity of golden syrup using stokes law to calculate the viscosity of the liquid.
Newtonian fluids viscosity stays the same regardless of temperature or any other force. Liquids with a high viscosity flow slowly (like golden syrup), whereas liquids which have lower viscosities flow faster (like water). There are many ways to measure viscosity. However I chose to do so by using a chrome ball bearing as this was the only sphere shaped object available to be used in the lab. Stokes Law was derived by George Gabriel Stokes in 1851.
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Viscosity Experiment. The aim of my investigation will be to analyse the relationship between several variables, which are defined by stokes law, and conclusively to apply these in order, to calculate the viscosity of the fluid from my results.
surface or "redwood viscometer", which involves the liquid to flow through a narrow tube driven by its own head of pressure, but due to the lack of apparatus, I chose to do the falling ball viscometer, as this experiment gave an absolute measurement of viscosity, and was too, the most feasible method to be performed in a lab. I will be unable to calculate the viscosity using the above equation, because "F" will not be able, to be measured directly with the method I will be using.
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The graph of period squared against mass can then be plotted. From this, the value for the spring constant, k, of each spring can be calculated by comparing the equation of the best fit line of the graph to the squared version of the equation above,. The second part of this experiment is concerned with Hooke's law, which states that the extension of a spring is directly proportional to the mass applied to it. Mathematically, this is stated as: Where x is the extension of the spring in metres, k is the spring constant of the spring measured in Nm-1 and F is the restoring force, measured in Newtons.
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The mass of the cart is recorded, and extra mass bars are added to it, in order for the cart to have differing periods depending on what mass is on it. The theoretical period for each of these masses is calculated and compared to the period calculated experimentally. Procedure Apparatus * Trolley * Two springs * Force sensor * Dynamics track * Photo gate * Mass bars * Five pattern picket fence * Computer with datastudio software * Interface Part 1 * The interface was connected to the computer, and the data studio document P15 Prelab Oscillation.DS was opened up.
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The beam was visually inspected for faults and damages. 4. Length and cross-sectional dimensions were measured accurately with a caliper. 5. Test device was wiped with a dry cloth. 6. The beam was put centrally in flexural loading device with the rough as-cast top surface vertical. 7. All rolling and supports were in evenly contact with the beam before applying the load. 8. Appropriate loading was chosen for the test. The load was increased at a rate between 0.03 to 0.06N/mm2 per second.
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Once again, numerous measurements could be taken to give a more reliable result. Breaking of loop The loop breaking has a massive detrimental effect on the experiment, completely voiding its accuracy. Thickness of wire (±1.8%) Taking more readings at further points down the wire and then finding an average will give a more certain result for this. The micrometer must also be calibrated. Values of extension between 10N increments Using 5N weights instead of the 10N ones should make this experiment more precise, as more values will be used to determine the average, making my result more accurate.
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? Centripetal force = T sin ? Centripetal force = m?2r T sin ? = m?2r T sin ? = m?2L sin ? ? T = m?2L The vertical component of the tension balances the weight of the rubber bung. T cos ? = W The nylon string is passed though a glass tube to prevent external force acting on the system by the hand. During the experiment, the paper marker was kept in a constant position below the bottom of the glass tube to help us to keep the angular velocity of the rubber bung constant.
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Objective of Experiment. To use a search coil and CRO to investigate the magnetic field due to a straight wire carrying an alternating current
the wire is long). The proportionality constant is written as ?o/2? , thus, The value of the constant ?o, which is called the permeability of free space, is 4? ? 10-7 H m-1. Procedure The circuit was set up as shown below. The signal generator was set to 5 kHz. The CRO was adjusted such that a trace was displayed. The frequency was changed to find out how the trace on the CRO was affected. The output of the signal generator was adjusted to produce a current. The time base was switched off and the length of the vertical trace on the CRO was measured.
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Using a search coil and CRO to investigate the magnetic fields generated by alternating currents (A.C.) through a straight wire and a slinky solenoid.
Induced E.M.F. (?) / V 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 Current (I) / A 0.8 1.0 1.12 1.28 1.40 1.56 1.80 2.0 2.1 2.3 2.5 Relationship between the potential different across the capacitor and the time From the above V-t graph, the curve is a straight line passes through origin. Hence, the potential difference across the capacitor is directly proportional to the time for the CRO trace to rise in steps of 1V and the slope () which represented the charging rate, is constant. Relationship between the charge stored in the capacitor and the potential different across it when the charging current is constant The area of the graph (Vt)
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a = -g? = -g(x/l) (? ? radius=arc length) a=- (where g/l) Thus, the periodic time T of a simple pendulum of length l is given by T == 2? The period of oscillation of a simple pendulum depends on the length and is directly proportional to the square root of the length. Moreover, we know that the period of oscillation is independent of the amplitude when the amplitude is within small limits (length remaining constant). This means the amplitude can vary within small limits, but the period of oscillation will be the same.
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Centripetal Force Experiment. Measure the centripetal force and compare it with the theorectical value Fc=mrω2
3.1 Electronic balance was used to measure the mass of bung, metal weights and hanger. 3.2 Meter ruler was used to measure the length of string. 3.3 Stop watch was used to record the time for 20 revolutions. 3.4 It was not easy to keep L unchanged when whirling the bung, so we had to practise for a while before taking the data. 4. Data Analysis Mass of rubber bung (m) is 0.0192 ± 5×10-5 kg. L / m (±5×10-4) M / kg (±5×10-5) Mg / N Time for 20 revolutions (t)/ s (±0.005) ?
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What are the physical quantities that affect the value of centripetal force when a body in circular motion? What is the force exerted on the object when the object is doing a horizontal circular motion?
The experimental centripetal force (Fc) of the rubber stopper swinging around is calculated by using: Equation 2 where ms is the mass of the rubber stopper, and the other variables as before. Centripetal means "center seeking". There are two main forces at work in this laboratory exercise. "Opposing" the centripetal force is the theoretical, accepted force due to the weight of the screw nuts pulling the string down through the bottom of the hollow glass tube: Equation 3 where g = 9.8 m/s2, the accel.
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Afterwards , they try to explain this with the atomic model and here below is the common behavior of some pure metals : As for this experiment , we are trying to verify the above results and find the Young modulus of copper , which is its stiffness . And this is going to be done with the following set-up : For the sake of a more accurate measurement , this experiment will be separated into two parts . In the first part , we will add the slotted mass to the load gradually until the copper wire breaks ,
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We can see that the mass will accelerate upward. As the mass is moving in SHM, (by SHM equation) So by measuring the oscillation period and the mass of mass, we can calculate the spring constant (or say the force constant). Apparatus: Slotted mass (10g, 20g, and 50g), hanger, spring, retort stand and clamp, stop watch. Procedure: 1. Fix the spring to the clamp with the help of an eraser. Hook the spring to the eraser and place the eraser in the clamp and screw up the clamp.
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A careful quantitative study of the relationship between the velocity of a trolley down a slope and the angle of the slope.
Experiment Equipment: * one pair of light gates * computer to collect light gate data * ruler * piece of wood or other smooth material for slope, about 80cm long * stack of books * trolley * piece of card, 10cm long * blu tack or other adhesive to attach the card to the trolley Method 1. The piece of wood was positioned to rest at one end on the table, point C, and the other end on the stack of books, point E.
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Results: Mask Size (cm) 8 10 Drop # Acceleration (m s-2) 1 9.63 9.68 2 9.48 9.97 3 10.03 10.02 4 9.67 9.88 5 9.69 9.75 6 9.78 9.61 7 9.44 9.63 8 9.73 10.1 9 9.8 9.79 10 10.03 10.08 11 9.54 9.84 12 9.66 9.67 13 9.99 10.04 14 9.74 9.79 15 9.72 9.68 Average(m s-2) 9.728666667 9.83533 This is the table of results for the experiment. It is clear that the results are fairly accurate for the value of 9.8 m s-2 for g. The result for the greater mask size is most likely more accurate as any deviation from the expected mask size due to the angle it's dropped at will have less effect over a greater mask size.
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In the other part of this experiment the radius was varied while keeping the weight the same. Measurements of the period were calculated by measuring the time taken for ten rotations, and this proved easier to measure than one rotation, and then divided by ten. Glass was chosen as a material for the cylinder to reduce the energy lost due to friction between the top of the cylinder and the string; this will help increase the reliability of the final results.
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