Category:Teaching Aids
1. The amount of matter contained in an object is called mass. Any object has mass, and the mass of an object does not change with changes in the object's shape, state, location, or temperature. The unit of mass is the kilogram (kg), as well as the ton (t), gram (g), and milligram (mg).
(1) the use of the balance balance adjustment: the balance on a horizontal platform, the vernier in the left end of the scale at the zero mark; adjust the beam balance nut, so that the pointer in the middle of the dial at the center line, this time the beam is balanced. a. The object to be measured on the left plate, with tweezers to the right plate to add or subtract weights and adjust the vernier in the scale position, until the beam is back in balance. b. This time the total mass of weights in the plate plus the vernier in the scale on the total mass. c. This is the total mass of the object in the plate plus the vernier in the scale on the vernier in the scale on the total mass of the object in the plate on the left plate. The total mass of the weights in the disk plus the vernier on the scale corresponding to the scale value, equal to the mass of the object to be measured. Note: 1, adjust the balance nut according to: the pointer left deviation to the right; right deviation to the left. 2, after the balance is adjusted and balanced, the left and right plates can not be adjusted, the balance nut can not be moved. 3, the weights must be used with tweezers. 4, to the plate to add weights should be estimated to the mass of the object to be measured, and then add weights from large to small, when added to the smallest one weight is too heavy, then it should be changed to shift the tourniquet. 5, the tourniquet readings are read on the left scale of the scale to the vernier. 6, the vernier is read in the scale of the scale to the left, the total weight in the disk is equal to the mass of the object to be measured. The left side of the scale value of the ruler.
(2) notes on the use of balances: A. Can not exceed the weighing capacity (balance of the weighing capacity = the total mass of the weights + maximum reading of the traveler's weights). B. Take the weights with tweezers, and gently holding and placing. C. Keep the balance dry and clean.
2. The mass per unit volume of a substance is called the density of the substance. Density of the international main unit is kg/m3, usually with the letter ρ for density, m for mass, V for volume, ρ = m / V. Density is a property of the substance itself, the same kind of material is generally unchanged, different kinds of material is generally different, will look up the density table.
To measure the density of an object, you should first measure the mass and volume of the object to be measured, and then use the density formula ρ=m/V to find the density value. For liquids and irregularly shaped solid volume can be measured with a measuring cylinder or measuring cup. Measure the volume with a measuring cylinder measuring cup readings, the line of sight to be level with the liquid surface. 1L = 1dm3 1ml = 1cm3 1g/cm3 = 1000kg/m3.
3. The density of water is 1.0 × 103kg/m3, which indicates the physical meaning: 1m3 of water is 1.0 × 103kg.
4. Application of the density: (1) use the density formula ρ = m/V to find the density value. Formula ρ = m / V density, the use of density to identify substances.
(2) Use the formula m = ρV to find the mass. (3) Use the formula V = m/ρ to find volume.
5. The measuring tool for length is the scale, and the main international unit is m.
6. A change in the position of an object is called mechanical motion, and the simplest form of mechanical motion is uniform linear motion.
7. Speed is a physical quantity that indicates how fast or slow an object is moving, and speed is equal to the distance traveled by a moving object in a unit of time. It is expressed by the formula: v=s/t, and the main unit of speed is m/s.
8. Force is the action of an object on an object, and the force between objects is mutual. The effect of force is (1) to change the state of motion of the object, (2) to change the shape of the object. The unit of force is the newton, abbreviated as ox. The symbol for force is N. The instrument used to measure force is the dynamometer, and the spring loaded dynamometer is commonly used in the laboratory. The spring dynamometer works on the principle that the elongation of a spring is proportional to the tension applied to it. (In the elastic range)
9. The magnitude, direction and point of action of a force are called the three elements of force. The method of representing the three elements of a force by a line segment with an arrow is called force diagramming. It is important to be able to draw a force diagram.
10. The force on an object due to the attraction of the Earth is called gravity, and the force exerted by gravity is the Earth. Direction: vertically downward, point of action: center of gravity.
11. Gravity is directly proportional to mass, and the relationship between them is G=mg, where g=9.8N/kg.
12. To find the combined force of two forces is called two-force synthesis. If there are two forces F1, F2, and the direction of the same, the combined force is F = F1 + F2 direction and the direction of the two forces. If the two forces are in opposite directions, the combined force is F=∣F1 - F2∣ in the same direction as the larger force.
13. It is Newton's first law that all objects remain at rest or in uniform linear motion when they are not acted upon by an external force.
14. The property of an object to remain at rest or in uniform linear motion is called inertia. That is why Newton's first law is also called the law of inertia. All objects have inertia.
15. The conditions for the balance of two forces are: ① two forces acting on an object, ② if the size is equal, ③ opposite direction, ④ acting on the same line, the two forces are balanced. The combined force of the two balanced forces is zero. If an object is acted upon by balanced forces, the object may be at rest or in uniform linear motion.
16. two objects in contact with each other, when they want to occur or have occurred relative motion, in the contact surface to produce a kind of obstacle to relative motion called friction. Friction is divided into static friction, sliding friction and rolling friction. The magnitude of sliding friction is related to both the magnitude of the pressure and the roughness of the contact surface.
17. The force that presses perpendicularly on the surface of an object is called pressure. The direction of pressure is perpendicular to the surface of the object. Pressure is not necessarily equal to gravity.
18. The pressure per unit area of an object is called pressure. The formula for pressure is P= F/S. The unit of pressure is "N/m2", usually called "Pa". 1Pa=1 N/m 2, and the commonly used units are hectopascal (102Pa), kilopascal (103Pa), and megapascal (106Pa).
19. The liquid has pressure on the bottom and side walls of the container, and there is pressure inside the liquid in all directions. The pressure of a liquid increases with depth. At the same depth, the pressure of the liquid in all directions is equal; the pressure of different liquids is also related to the density. The instrument used to measure the pressure of a liquid is called a piezometer.
20. The formula p = ρgh is often applied to liquids. The significance of the object of the formula is: the pressure of the liquid is only related to the density and depth of the liquid, but not with the weight, volume, shape of the liquid. The "h" in the formula is the vertical distance from a point in the liquid to the free surface of the liquid. The formula also applies to regular, solid uniform and horizontally placed solids such as squares, rectangles, cylinders.
21. A container that is open at the top and connected at the bottom is called a connector. Its nature is: the liquid in the connecting vessel does not flow, the liquid level in each container always remain level. Tea kettles and boiler level gauges are connectors. Ship locks work by utilizing the principle of a communicator.
22. The layer of air that surrounds the earth is called the atmosphere, and the pressure of the atmosphere on objects immersed in it is called atmospheric pressure. in May 1654, Otto Griecke, the mayor of Madrid, Germany, made a famous statement. Griecke made a famous experiment of the Madburg hemisphere, proved the existence of atmospheric pressure.
23. Torricelli first measured the value of atmospheric pressure. The atmospheric pressure equal to 760 millimeters of mercury column called a standard atmospheric pressure, 1 standard atmospheric pressure ≈ 1.01 × 105Pa (1 standard atmospheric pressure can support about 10.3m high column of water)
24. Atmospheric pressure decreases with height. The instrument used to measure atmospheric pressure is called a barometer. The boiling point of a liquid is related to air pressure. The boiling point of all liquids decreases when air pressure decreases and increases when air pressure increases. A pressure cooker is used to cook rice on high mountains.
25. Piston pumps and centrifugal pumps, fountain pens that suck in ink, etc. all work on the principle of atmospheric pressure.
26. An object immersed in a liquid is subjected to a difference in pressure upward and downward. This is the buoyant force of the liquid on the object (F float = F down - F up). This is the cause of buoyancy. The buoyant force is always vertically upward. There is F buoyancy = G when the object is suspended and floating, but there is a difference between the two (V rows are different).
27. Archimedes' Principle: An object immersed in a liquid is buoyed upward by a force equal to the force of gravity on the liquid it displaces. The formula is F buoyancy = G displacement = ρ liquid gV displacement. Archimedes' principle also applies to gases. Substances that are denser than water (e.g., iron, etc.) are usually made hollow to float on water. Ships, submarines, balloons and airships utilize buoyancy.
28. A stiff rod is called a lever if it can turn around a fixed point under the action of a force. Differentiate between fulcrum, power, resistance, power arm and resistance arm of a lever.
29. The equilibrium condition of the lever is: power × power arm = resistance × resistance arm, the formula F1L1 = F2L2
30. Lever is divided into three cases: ① power arm is greater than the resistance arm, for force-saving levers; ② power arm is less than the resistance arm, for force-saving levers; such as: tweezers, barber scissors, chopsticks, fishing rods. ③ power arm is equal to the resistance arm, that is, L1 = L2, for the arm lever, specific applications for the balance, fixed pulley.
31. Many mass weighing scales, such as pole scales, case scales, are made according to the principle of leverage.
32. There are two types of pulleys: the fixed pulley and the movable pulley. Fixed pulley is essentially an arm lever, so fixed pulley does not save force, but it can change the direction of the force; pulley is essentially a power arm for the resistance arm of the two times the lever, so the pulley can save half of the force, but can not change the direction of the force.
33. The use of pulley block, pulley block with a few sections of rope hanging objects, lift the force used by the object is a fraction of the weight. And the object is raised "h", the tension force moves "nh", where "n" is the number of strands of rope.
Work and energy
34. Work in mechanics consists of two necessary elements: the force acting on the object and the distance the object passes in the direction of the force. The formula is W = FS. The unit of work is the joule, 1J = 1N?m.
35. No work is saved by using any machinery. This conclusion is called the principle of work. Applied to an inclined plane it is: FL=Gh.
36. The work done to overcome useful resistance is called useful work and the work done to overcome useless resistance is called extra work. Useful work plus extra work equals total work. The ratio of useful work to total work is called mechanical efficiency. The formula is η= W useful/W total. It is usually expressed as a percentage. η is always less than 1.
37. The work done per unit of time is called power. The formula is P=W/t . The unit is w, 1w=1J/s, P= W/t =FS/t = F?v. The formula states that when a vehicle is going uphill, the speed (v) must decrease as the force (F) increases due to a certain amount of power (P).
38. When an object can do work, we say that it has energy. The energy that an object has as a result of its motion is called kinetic energy. Kinetic energy is related to the speed and mass of the object. The greater the speed and mass of the moving object, the greater the kinetic energy. All moving objects have kinetic energy.
39. Potential energy is divided into gravitational potential energy and elastic potential energy. The energy of a lifted object is called gravitational potential energy. The greater the mass of the object and the higher it is lifted, the greater the gravitational potential energy. The energy of an object undergoing elastic deformation is called elastic potential energy. In the elastic range, the greater the elastic deformation of the object, the greater the elastic potential energy it has.
40. Kinetic energy and potential energy are collectively called mechanical energy. Energy, work, and heat are all measured in joules. Kinetic and potential energy can be converted into each other.
41. The phenomenon in which different substances enter each other when they come into contact with each other is called diffusion. The phenomenon of diffusion illustrates the never-ending irregular motion of molecules.
42. The sum of the kinetic and potential energies of all the molecules in an object that are in irregular motion is called the internal energy of the object. All objects have internal energy. The internal energy of an object is related to its temperature. The higher the temperature, the more intense the irregular motion of the molecules inside the object, and the greater the internal energy of the object. The higher the temperature, the faster the diffusion.
43. The irregular motion of a large number of molecules within an object is called thermal motion, and internal energy is also called heat. Two ways to change the internal energy of an object are: work and heat transfer. Doing work on an object increases its internal energy, and doing work on an object decreases its internal energy; when an object absorbs heat, its internal energy increases, and when an object excretes heat, its internal energy decreases.
44. unit mass of a material temperature increase (or decrease) 1 ℃ absorbed (or released) heat called the specific heat capacity of this material, referred to as specific heat. The unit of specific heat is J/(kg?℃). The specific heat of water is 4.2x103J/(kg?℃). Its physical significance is: 1kg of water temperature increase (or decrease) 1 ℃ absorbed (or released) heat is 4.2x103 J. The specific heat of water is large. So the temperature change in coastal places is not as significant as inland.
45. Q absorbs = cm(t - t0); Q releases = cm(t0 - t); or written as Q = cmΔt. At thermal equilibrium, Q absorbs = Q releases, i.e., c1m1(t - t01) = c2m2(t02 - t). Where t represents the later temperature, t0 represents the original temperature.
46. Energy will neither disappear nor be created, it will only be transformed from one form into another form, or transferred from one object to another, and in the process of transformation, the total amount of energy remains unchanged. This law is called the law of conservation of energy. In the use of internal energy, you can use internal energy to heat and internal energy to do work.
47. The amount of heat given off by the complete combustion of 1 kg of a certain fuel is called the calorific value of the fuel. The unit of calorific value is J/Kg. The calorific value of hydrogen (maximum) is 1.4 x108J/kg, which means the physical meaning is: 1kg of hydrogen completely burned to release heat is 1.4 x108J.
48. The content of the theory of molecular kinematics: the object is made up of a large number of molecules; all the matter consists of molecules; molecules in the never-ending irregular motion; molecular interaction exists between gravitational and repulsive forces. interacting gravitational and repulsive forces; (there are gaps between molecules.)
49. Diffusion: proves that molecules are in never-ending irregular motion. Diffusion is the phenomenon in which different substances enter into each other when they come in contact with each other.
Summary of basic concepts of junior high school physics
A. Measurement
⒈ length L: the main unit: meters; measuring tool: scale; measurements should be estimated and read to the next place of the smallest scale; the unit of light-years is the unit of length.
Peak time t: main unit: second; measuring tool: clock; stop watch in the laboratory. 1 hour = 3600 seconds, 1 second = 1000 milliseconds.
Mass m: the amount of material contained in the object called mass. Primary unit: kilogram; Measuring tool: scale; pallet balance in laboratory.
Two, mechanical motion
⒈ mechanical motion: the movement of the object changes position.
Reference: to determine the motion of an object must be selected for another object as a standard, the object is selected as a standard called reference.
peal uniform linear motion:
①two ways of comparing the speed of motion: a compare the distance traveled in equal time. b compare the time it takes to pass the same distance. c compare the time it takes to pass the same distance. d compare the time it takes to pass the same distance. e compare the time it takes to pass the same distance. f compare the time it takes to pass the same distance.
② formula: 1 meter / second = 3.6 km / hour.
Three, force
⒈ force F: force is the action of objects on objects. The action of forces between objects is always mutual.
The unit of force: newton (N). Instruments for measuring force: dynamometer; spring scales are used in the laboratory.
Effect of force: deforms an object or causes a change in its state of motion.
A change in the state of motion of an object is a change in the magnitude of its velocity or the direction of its motion.
peculiar elements of force: the magnitude, direction, and point of action of a force are called the three elements of force.
The diagram of a force is to be scaled; the schematic of a force is not scaled.
Gravity G: the force on an object due to the attraction of the earth. Direction: vertically downward.
Relation between gravity and mass: G=mg m=G/g
g=9.8 N/kg. Reading: 9.8 N/kg means that the force of gravity on an object of mass 1 kg is 9.8 N.
Center of gravity: the point at which the force of gravity acts is called the center of gravity of an object. The center of gravity of a regular object is at the geometric center of the object.
Sung the conditions of equilibrium of two forces: they act on the same object; they are equal in magnitude and opposite in direction; they act in a straight line.
Objects in two-force equilibrium can be at rest or in uniform linear motion.
An object in equilibrium is at rest or in uniform linear motion. The combined force of external forces on an object in equilibrium is zero.
Be careful about the same straight line two force synthesis: the direction of the same: the combined force F = F1 + F2 ;the direction of the combined force and F1, F2 direction;
direction of the opposite: the combined force F = F1 - F2, the direction of the combined force and the direction of the larger force is the same.
Rolling friction is much smaller than sliding friction under the same conditions of selection.
Sliding friction and positive pressure, the nature of the contact surface material and roughness related to sliding friction, rolling friction, static friction
7. Newton's first law is also known as the law of inertia, which is: all objects are not subjected to external forces, always remain at rest or in uniform linear motion. Inertia: objects have the property of maintaining the original state of rest or uniform linear motion is called inertia.
Four, density
⒈ density ρ: the mass per unit volume of a substance, density is a characteristic of matter.
Formula: m = ρV international units: kg / m 3, commonly used units: g / cm 3,
Relationship: 1 g / cm 3 = 1 × 103 kg / m 3; ρ water = 1 × 103 kg / m 3;
Reading: 103 kg per cubic meter, that the mass of 1 cubic meter of water is 103 kg.
Peculiar density determination: use a pallet balance to measure the mass and a measuring cylinder to measure the volume of a solid or liquid.
Area unit conversion: 1 cm2 = 1 × 10-4 m2, 1 mm2 = 1 × 10-6 m2.
V. Pressure
⒈ pressure P: the pressure per unit area of the object is called pressure.
Pressure F: force acting vertically on the surface of the object, unit: Nm (N)
Pressure produces the effect of pressure expressed in the size of the pressure, with the size of the pressure, the size of the area of force.
Pressure unit: Nm / m 2; special name: Pascal (Pa)
Formula: F = PS S: force area, the two objects in contact with the public **** part; unit: m 2.
Changing the size of the pressure: ① reduce the pressure or increase the force area, you can reduce the pressure; ② increase the pressure or reduce the force area, you can increase the pressure.
Peak liquid internal pressure: measure the pressure inside the liquid: the use of liquid piezometer (U-tube piezometer).
Causes: because the liquid has gravity, the bottom of the container pressure; due to liquid mobility, the wall pressure.
Law: ① at the same depth, the size of the pressure in all directions is equal ② the greater the depth, the greater the pressure ③ different liquids at the same depth, the liquid density, the pressure is also large. [Depth h, the liquid surface to the vertical height of the liquid at a certain point].
Formula: P = ρgh h: unit: meters; ρ: kg / m 3; g = 9.8 N / kg.
Atmospheric pressure: the atmosphere is subjected to gravity produces pressure, proving that atmospheric pressure exists and is very large is the Madsenburg hemisphere experiments, the determination of the value of atmospheric pressure is Torricelli (Italian scientist). When the Torricelli tube is tilted, the height of the mercury column remains the same and the length becomes longer. 1 standard atmospheric pressure = 76 cm mercury column height = 1.01 × 105 Pa = 10.336 m water column height
Instruments for determining atmospheric pressure: barometers (mercury barometer, box barometer).
Atmospheric pressure with height change rule: the higher the altitude, the smaller the air pressure, that is, with the increase in height and decrease, the boiling point also decreased.
Six, buoyancy
1. buoyancy and causes: immersed in liquid (or gas) in the object by the liquid (or gas) on it up to the force called buoyancy. Direction: vertically upward; cause: the liquid on the object of the upper and lower pressure difference.
2. Archimedes' principle: the object in the liquid by the upward buoyancy; buoyancy is equal to the size of the object discharged from the liquid by gravity.
That is, F buoyancy = G liquid displacement = ρ liquid gV displacement. (V row indicates the volume of liquid discharged by the object)
3. Buoyancy formula: F buoyancy = G-T = ρliquid gV row = F pressure difference between the upper and lower
4. When the object floats: F buoyancy = G and ρ and ρ<ρliquid When the object is suspended: F buoyancy = G and ρ = ρliquid
When the object is upwardly floating: F buoyancy & gt; G and ρ and ρ<ρliquid
When the object sinks: F buoyancy = G and ρ<ρliquid
When the object is upwardly floating: F buoyancy = G and G and ρ< ρ<ρliquid When the object sinks: F floating & lt; G thing and ρ thing & gt; ρ liquid
VII, simple machinery
⒈ lever equilibrium conditions: F1l1 = F2l2. force arm: from the pivot point to the perpendicular distance from the line of action of the force
By adjusting the lever ends of the nut so that the lever is in the position of the water: the purpose of the lever: to facilitate the direct determination of the length of the power arm and resistance arm.
Stationary pulley: equivalent to an equal-armed lever, does not save force, but can change the direction of force.
Active pulley: equivalent to the power arm is twice the resistance arm of the lever, can save half the force, but can not change the direction of force.
Peak work: two necessary factors: ① force acting on the object; ② object in the direction of the force through the distance. W = FS unit of work: joule
3. power: the work done by the object in a unit of time. It is a physical quantity that indicates how fast an object does work, i.e., an object with high power does work fast.
W=Pt P in watts; W in joules; t in seconds.
VIII, light
⒈ light propagation in a straight line: light in the same uniform medium is propagated in a straight line. Small hole imaging, shadow, spot is the phenomenon of linear propagation of light. The maximum speed of light in a vacuum is 3 × 108 m / s = 3 × 105 km / s
Peake the law of reflection of light: one side of the two sides of the three equal. The angle between the incident light and the normal is the angle of incidence. The angle between the reflected light and the normal is the angle of reflection.
The imaging characteristics of a plane mirror: virtual image/equal size/equal distance, symmetrical with the mirror. The reflection of an object in water is an imaginary image is the phenomenon of reflection of light.
3 the phenomenon and laws of light refraction: see chopsticks in the water, the false image of fish is the phenomenon of light refraction. Convex lenses have light convergence, concave lenses have light diffusion. The law of refraction of light: one side two side three with the big four empty big.
Sung Convex Lens Imaging Laws: [No image when U=f, V=2f when U=2f, inverted real image]
Object Distance u Image Distance v Properties of Image Optical Circuit Diagrams Applications u>2f f<v<2f Inverted Reduction Camera
f<u<2f v>2f Inverted Enlargement Slide Show Machine u<f Enlargement positive imaginary magnifying glass
Careful with the convex lens imaging experiment: place the candle, the convex lens, and the light screen in sequence on the light stand so that the center of the candle flame, the center of the convex lens, and the center of the light screen are at the same height.
nine, thermal:
⒈ temperature t: indicates how hot or cold the object. Is a state quantity.
Commonly used thermometer principle: according to the liquid thermal expansion and contraction properties.
The difference between thermometers and thermometers: ① range, ② minimum scale, ③ glass bubble, bend the tube, ④ use.
Peak heat transfer conditions: there is a temperature difference. Heat: in the process of heat transfer, the amount of heat absorbed or released by the object. Is the amount of process
Heat transfer: conduction (heat transfer along the object), convection (by the flow of liquids or gases to achieve heat transfer) and radiation (high temperature objects directly to the outside of the heat emitted) three.
Vaporization: the phenomenon of a substance changing from a liquid to a gas. Way: evaporation and boiling, vaporization to absorb heat.
Factors affecting the speed of evaporation: ① liquid temperature, ② liquid surface area, ③ liquid surface air flow. Evaporation has a cooling effect.
Sung specific heat capacity C: the heat absorbed when the temperature of a substance rises by 1℃ is called the specific heat capacity of the substance.
Sung heat capacity is one of the properties of a substance, unit: joule / (kg ℃) Among the common substances, water has the largest specific heat capacity.
C water = 4.2 × 103 joules / (kg ° C) reading: 4.2 × 103 joules per kilogram of degrees Celsius.
Physical meaning: that the mass of 1 kg of water temperature increase of 1 ℃ absorbed heat for 4.2 × 103 J.
Be careful with heat calculation: Q put = cm ⊿t drop Q suck = cm ⊿t rise
Q is proportional to c, m, ⊿t and inversely proportional between c, m, ⊿t. ⊿t=Q/cm
6. Internal energy: the sum of kinetic and molecular potential energy of all molecules in an object. All objects have internal energy. Unit of internal energy: joule
The internal energy of an object is related to the temperature of the object. As the temperature of an object increases, the internal energy increases; as the temperature decreases, the internal energy decreases.
Methods of changing the internal energy of an object: work and heat transfer (which are equivalent for changing the internal energy of an object)
7. Law of conversion and conservation of energy: energy that is, it will not be created out of thin air, nor will it disappear out of thin air, it will only be converted from one form to another, or transferred from one object to another, while the total amount of energy remains unchanged.
Ten, the circuit
⒈ circuit consists of a power supply, keys, appliances, wires and other components. To make a continuous current in the circuit, the circuit must have a power supply, and the circuit should be closed. Circuits have through, broken (open), power supply and electrical appliances short-circuited.
Peake substances that conduct electricity easily are called conductors. Such as metals, aqueous solutions of acids, bases, and salts. Substances that do not readily conduct electricity are called insulators. Such as wood, glass, etc.
Insulators can be converted into conductors under certain conditions.
Identification of series and parallel circuits: series: current is not bifurcated, parallel: current is bifurcated.
Methods to convert a non-standard circuit diagram into a standard one: use the current flow path method.
XI, the law of electric current
⒈ electricity Q: the amount of charge is called the amount of electricity, unit: coulomb.
Current I: the amount of electricity passing through the cross-section of the conductor in 1 second is called the current intensity. Q = It
Current unit: amperage (A) 1 ampere = 1000 milliamps Directional movement of positive charge direction is specified as the direction of the current.
Measurement of current is done with an ammeter, connected in series in a circuit and considered suitable for the range. It is not permissible to connect the ammeter directly to the ends of the power supply.
peak voltage U: cause the free charge in a circuit to make a directional movement to form an electric current. Voltage unit: volt (V).
Measurement of voltage with a voltmeter (voltmeter), connected in parallel at both ends of the circuit (appliance, power supply), and consider the range suitable.
Resistance R: the obstruction of current by a conducting object. Symbol: R, unit: ohm, kilo-ohm, megohm.
The size of the resistance is directly proportional to the length of the wire, the cross-sectional area is inversely proportional, but also related to the material.
Conductors with different resistances, when connected in series in the circuit, the current is the same (1:1). Conductors with different resistances are connected in parallel with the same voltage (1:1)
Singing Ohm's law: I = U/R U = IR R = U/I
The strength of the current in a conductor is directly proportional to the voltage at the ends of the conductor, and inversely proportional to the resistance of the conductor.
The resistance of a conductor R = U/I. For a definite conductor if the voltage varies, the current also varies, but the resistance remains the same.
Be careful about the characteristics of series circuits:
① I = I1 = I2 ② U = U1 + U2 ③ R = R1 + R2 ④ U1/R1 = U2/R2
The two conductors with different resistances are connected in series, and the one with higher resistance has a higher voltage, and the one with lower voltage has a lower resistance.
Example: a lamp labeled "6V, 3W", connected to a circuit labeled 8 volts, how to connect a resistor how big, in order to make the small light bulb normal light?
Solution: Since P = 3 watts, U = 6 volts ∴ I = P / U = 3 watts / 6 volts = 0.5 amperes
Because the total voltage of 8 volts is greater than the lamp's rated voltage of 6 volts, should be connected in series with a resistor R2, as shown in the figure on the right,
Therefore, U2 = U-U1 = 8 volts - 6 volts = 2 volts ∴ R2 = U2 / I = 2 volts / 0.5 amp = 4 ohms. A: (Omitted)
Chosen characteristics of parallel circuits:
①U = U1 = U2 ②I = I1 + I2 ③1/R = 1/R1 + 1/R2 or ④I1R1 = I2R2
Parallel connection of two conductors with different resistances: current through the one with higher resistance is less, and resistance of the one through which the current is higher is less.
Example: Figure R2 = 6 ohms, K is disconnected when the ammeter shows 0.4 amps, K is closed, A is 1.2 amps. Requirements: ① R1 resistance ② supply voltage ③ total resistance
Known: I = 1.2A I1 = 0.4A R2 = 6Ohm Requirements: R1; U; R
Solution: ∵ R1, R2 is connected in parallel ∴ I2 = I - I1 = 1.2A - 0.4A = 0.8A
According to Ohm's law, U2 = I2R2 = 0.8A × 6Ohm = 4.8 volts and R1, R2 parallel ∴ U = U1 = U2 = 4.8 volts
∴ R1 = U1 / I1 = 4.8 volts / 0.4 amps = 12 ohms
∴ R = U/I = 4.8 volts / 1.2 amps = 4 ohms (or use the formula for calculating the total resistance) Answer: (omitted)
XII, electrical energy
⒈ electric power W: the work of the electric current is called electric power. Current work process is the conversion of electrical energy into other forms of energy.
Equation: W = UQ W = UIt = U2t/R = I2Rt W = Pt Unit: W Jou U Volt I Ampere t seconds Q Q library P watts
⒉Electrical power P: electric current in a unit of time made by the electrical work, said the current work faster or slower. The electric power of the appliance current work fast.
1 kWh = 1 kilowatt-hour = 1000 watts x 3600 seconds = 3.6 x 106 joules
Example: How many hours can one kilowatt-hour make two "220V, 40W" lamps work?
Solution t = W/P = 1 kWh / (2 × 40 watts) = 1000 watt-hours / 80 watts = 12.5 hours
Thirteen, magnetism
1. magnets, magnetic poles of the same name magnetic poles repel each other, the magnetic poles of the same name attract each other
Objects are capable of attracting the iron, cobalt, nickel, and other substances of the nature of the magnetic properties called magnetism. A substance with magnetic properties is called a magnet. The poles of a magnet always appear in pairs.
2. Magnetic field: there exists a region of space around a magnet that acts on other magnets.
The basic property of a magnetic field is that it exerts a magnetic force on the magnets placed in it.
Direction of magnetic field: the direction of the N pole when the small magnetic needle is at rest is the direction of the magnetic field at that point. The magnetic field around a magnet is represented by lines of magnetic inductance.
The geomagnetic north pole is near the geographic south pole and the geomagnetic south pole is near the geographic north pole.
3. Magnetic field of electric current: Auster's experiment showed the existence of a magnetic field around an electric current.
The energized solenoid is externally equivalent to a bar magnet.
The direction of the current in the energized solenoid and the relationship between the polarity of the two ends of the solenoid can be determined by the right-hand spiral rule
Secondary school review of physics knowledge summary
I. Thermometer
1, the degree of heat and cold of the object is called the temperature
2, to accurately determine and measure the temperature, we must choose the scientific tool for measuring ----- Thermometer. Thermometers are made according to the law of thermal expansion and contraction of liquids.
3, the use of thermometers: first of all, it should be clear that the range (the highest temperature can be measured and the lowest temperature temperature range); and then see its divisional value (a small grid represents the value).
4, the use of thermometers Note: ○ 1, the thermometer glass bubble into the measured liquid, do not touch the bottom of the container or container wall. ○2, the thermometer glass bubble into the measured liquid to wait a while, to be stabilized after the thermometer reading. ○3, read the thermometer's glass bubble to remain in the liquid, the line of sight should be level with the upper surface of the liquid column in the thermometer.
Two, melting and solidification
1, the process of changing a substance from a solid to a liquid is called melting.
2. The process of changing a substance from a liquid to a solid is called solidification.
3. solid state melting (heat absorption) liquid
solid state solidification (exothermic) liquid
4. crystals: some solids try to keep absorbing heat as much as possible during the process of melting, while the temperature remains the same, for example, seawaves, ice, various metals, such solids have a definite melting temperature.
5, non-crystalline: some solids in the melting process, as long as the continuous absorption of heat, the temperature continues to rise, there is no fixed melting temperature, such as wax, rosin, glass, asphalt.
6. The temperature at which crystals melt is called the melting point, and amorphous crystals have no definite melting point.
7. There is also a definite temperature at which an amazing solidification takes place and this temperature is called the freezing point. The solidification point of the same substance and its melting point are the same. Amorphous substances do not have a definite freezing point.
8, melting heat absorption, solidification exothermic.
Three, vaporization and liquefaction
1, substances from liquid to gas is called vaporization
2, substances from gas to liquid is called liquefaction
3, boiling is the liquid inside and the surface of the phenomenon of simultaneous occurrence of the intense vaporization (water boiling is a phenomenon in the intense vaporization)
4, a variety of liquids boiling have a certain temperature, this temperature is called the boiling point
5, the temperature is called the boiling point. temperature is called the boiling point
5, at any temperature can occur in the vaporization phenomenon called evaporation
6, evaporation and boiling is the vaporization of the two ways
Four, sublimation and condensation
1, the material from the solid state directly into a gaseous state is called sublimation
2, the material from the gaseous state directly into a solid state is called sublimation
3, the need to sublime Absorption of heat, sublimation requires exothermic
I. Charge
1, friction over the object has the phenomenon of attracting light objects, is the phenomenon of frictional electricity
2, there are only two kinds of electric charge in nature. The charge on a glass rod rubbed by silk is called positive charge; the charge on a rubber rod rubbed by fur is called negative charge.
3, the same kind of charge repel each other, different kinds of charge attract each other
4, the amount of charge is called the amount of charge, referred to as charge. The unit of charge is the coulomb, or Coulomb, symbol C
5, charge can move directionally in a metal rod, metal is conductive. Some objects are good at conducting electricity, called conductors. Metals, the human body, salt water solution, etc. are conductors. Some objects are not good at conducting electricity and are called insulators. Rubber, glass, plastic, etc. are insulators.