physics

• in distance time graphs, the gradient shows the speed, curves represent acceleration
• for velocity time graphs, the gradient shows the acceleration, flat sections represent constant speed, and the area is equal to the distance travelled in that time
• reaction force acts perpendicular to a surface
• thrust is the push or pull force due to an engine speeding something up
• drag / air resistance / friction is what is slowing the object
• newton’s first law: balanced forces mean no change in velocity. to keep going at a steady speed, the resultant force would be zero
• newton’s second law: a resultant force means acceleration in that direction
• newton’s third law: for every action there is an equal and opposite reaction
• vector quantities have both size and direction. these include force or acceleration
• scalar quantities have only size and no direction. these include mass or temperature
• terminal velocity: when an object first falls, the weight / accelerating force is more than the resistance slowing it down. as the velocity increases, the resistance builds up. the resistance force gradually reduces the acceleration, until eventually they are equal.
• hookes law says that the extension of a wire is proportional to the load or force
• stopping distance = thinking distance + braking distance
• thinking distance can be affected by how fast you’re going and your reaction time
• braking distance can be affected by how fast you’re going, the mass of the vehicle, the quality of brakes and grip
• the principle of moments says, if an object is balanced, then total anticlockwise moments = total clockwise moments
• if total anticlockwise moments doesnt equal total clockwise moments, there will be resultant moment
• a thermistor is a temperature dependent resistor. in hot conditions, the resistance increases
• voltage is the energy transferred per unit charge passed
• one volt in one joule per coulomb
• the three wires in a plug are live, neutral and earth
• the live wire alternates between a voltage of 230V
• the neutral wire is always at 0V
• the earth wire and fuse (or circuit breaker) are for safety. appliances with metal casing must be earthed to reduce electric shock. plastic is also an insulator as it stops current flowing, and anything with double insulation doesn’t need an earth wire
• the earth wire provides a low resistance path to the earth
• the high current though the fuse causes it to melt and break, which cuts off electricity supply to the appliance
• when circuit breakers detect a surge in current, they break the circuit by opening a switch
• circuit breakers are more convenient than fuses, as they can be reset, while fuses have to be replaced once melted. circuit breaker also operate faster than fuses
• when there is an electrical current in a resistor, there is an energy transfer that heats the resistor. this happens because the electrons collide with the ions that makes up the resistor; this gives the ions energy, causing them to vibrate and heat up
• the heating effect on resistors increases its resistance
• electrical power is the rate at which an appliance transfers energy
• an insulator is a material that has no free charges, so does not allow flow of charge through them. they can conduct some electricity in the form of static electricity
• when certain insulating materials are rubbed against each other, they become electrically charged. this is called charging by friction. the charges remain and cannot immediately flow away; one becomes positive and one becomes negative
• charging by friction occurs because electrons are transferred from one material to the other
• rubbing cloth on a rod, the rod becomes negatively charged due to the cloth’s lost electrons. the cloth is left with a positive charge
• as electric charge builds up on an isolated object, the voltage between the object and earth increases. if the voltage becomes large enough, electrons can jump from the object to the earth, which causes a spark
• van de graaff generators show electrostatic charges. it is made up of a rubber belt moving around plastic rollers under a metal dome. a charge is built up on the belt. if you stand on an insulting chair, electrons move between your body and the done, giving you a charge
• an inkjet uses static electricity. droplets of ink are forced out a thin nozzle, making them electrically charged. the droplets are deflected as they pass between two metal plates (one is negative, one is positive). the droplets are attracted to the plate of opposite charge and repelled from the plate with the same charge. the size and direction of the voltage across each plate changes, so each droplet deflects to a different place on the paper
• insecticide sprayers use static electricity. the insecticide is given a positive electrostatic charge as it leaves the sprayer. the droplets repel eachother as they are the same charge. they are also attracted to the negative charges on earth; will fall quickly and wont be blown away
• lightning is an example of a danger of static electricity. rain drops and ice bump together, knocking off electrons. this leaves the top of the cloud positive and the bottom negative. this creates huge voltage and a big spark (lightning)