Structure of the atom
- Tiny nucleus containing protons (positively charged) and neutrons (neutrally charged) giving it an overall positive charge. Makes up most of the mass of the atom.
- Rest of the atom is mostly empty space with negatively charged electrons moving around the nucleus giving the atom its overall size even though they have virtually no mass.
- Proton - mass, 1. Charge +1. Neutron - mass, 1. Charge, 0. Electron - mass, 1/2000. Charge, -1.
Friction causes a build-up of static
- When two insulating materials are rubbed together electrons are rubbed off one and transferred to the other leaving a positive static charge on one and a negative static charge on the other.
- Only electrons can move and the direction that they are transferred depends on the two materials involved: Polythene rod + cloth duster = electrons move from duster to rod, rod becomes negatively charged and duster positively charged. Acetate rod + cloth duster = electrons move from rod to duster, rod becomes positevly charged and cloth negatively charged.
Like and unlike charges
Two things with opposite electric charges are attracted to each other. Two things with the same electric charge repel each other. The further apart these two things are, the weaker the forces.
Static electricity can cause sparks
- Clothing crackles - when synthetic clothes are dragged over each other electrons get scraped off and leave static charges on both parts. This leads to attraction and causes little sparks as the charges rearrange each other.
- Car shocks - static charge can build up between clothes and a synthetic car sear due to the friction between the two. As you get out of the car and touch the metal door handle the charge flows and can cause a shock. Some cars have conducting strips which hang down behind the car giving a safe discharge to the earth.
- Shocks from door handles - A transfer of electrons can occur between a nylon carpet and shoes with insulating soles. A charge can build up on your body and when a metal door handle is touched a shock might happen as the charge flows to the conductor.
If a balloon is rubbed against hair or clothes, electrons are transferred to the balloon leaving it with a negative charge. If it is then held up against a wall it will stick. This is because the charges on the surface of the wall can move a little and the negative charges on the wall are replled by the negative charges on the balloon. This leaves a positive charge on the surface of the wall which attracts the balloon and holds it to the wall.
Paint sprayers use electrostatic charges
Bikes and cars are painted using electrostatic paint sprayers. The spray gun is charged which charges the small individual paint droplets. Each paint drop repels the others since they are all given the same charge and so you get a very fine spray. The object to be painted is given an opposite charge to the gun and this attracts the fine spray of paint. This method gives an even coat of paint and hardly any paint is wasted. Additionally, even parts of the bicycle or car that are pointing away from the gun receive paint. Many other electrostatic paint sprayers work this way, such as insecticide sprayers.
As fuel flows out of a filler static can build up. This can easily lead to a spark and can cause explosions in dusty or fumey places such as petrol stations. This problem can be solved by earthing.
Dangerous sparks can be prevented by connecting a charged object to the ground using a conductor such as copper wire. This provides an easy route for static charges to travel into the ground and means that no charge can build up causing a spark. The electrons flow down the conductor if the charge is negative and flow up the conductor from the ground if the charge is positive. Fuel tankers must be earthed to prevent any sparks that might cause the fuel to explode.
Charge = current x time
- Current is the rate of flow of charge around a circuit.
- In the metal wires of a circuit this charge is carried by electrons. Metals are good conductors because they have free electrons which are able to move and carry this charge.
- When current flows past a point in a circuit for a length of time then the charge that has passed is given by this formula: charge (coulombs, C) = current (amperes, A) x time (seconds, s).
- The bigger the current flowing the more charge passes around the circuit.
Cells and batteries
Cells and batteries can be used in an electrical circuit to supply direct current, which is current that keeps flowing in the same direction and therefore the charge moves in one direction only. On an oscilloscope a direct current source will always be a straight line as it is always at the same voltage.
Current, voltage and resistance
- Current - the rate of flow of charge around the circuit. Current will only flow through a component if there is a voltage across that component. Ampere, A.
- Voltage - the driving force that pushes the current round. Also known as potential difference. Volt, v.
- Resistance - anything in the circuit which slows the flow down. Ohm, Ω.
- The voltage is trying to push the current round the circuit and the resistance is opposing it. If you increase the voltage, the more current will flow. If you increase the resistance, the less current will flow (or more voltage will be needed to keep the same current flowing).
Current is conserved at a junction
In a parallel circuit each component is separately connected to the +ve and -ve of the supply. There are junctions where the current either splits or rejoins. Current doesn't get used up or lost in a circuit - it is conserved. The total current entering a junction is equal to the total current leaving a junction.
Standard test circuit
The ammeter: measures the current in amps flowing through the component, must be placed in series (connected in a line with the component).
The voltmeter: measures the voltage in volts across the component, must be places in parallel around the component under test.
The current through a resistor at constant temperature is proportional to voltage. Different resistors have different resistances and so the slopes may be different.
As the temperature of the filament increases the resistance increases, hence the curve.
Current will only flow through a diode in one direction, as shown.
Voltage = current x resistance
Light dependent resistors and thermistors
An LDR is a special type of resistor that changes its resistance depending on how much light there is. In bright light, the resistance falls. In darkness, the resistance is highest.
A thermistor is like an LDR but its resistance depends on temperature. In hot conditions, the resistance drops. In cool conditions, the resistance goes up.
Resistors get hot when current
- When there is electric current in a resistor there is an energy transfer which heats the resistor. This energy transfer is due to the electrons colliding with ions in the lattice that make up the resistor as they move through it. The collisions give the ions in the lattice extra energy which is emitted as heat. This heating effect increases the resistors resistance so less current will flow. If it gets too hot no current will flow. This heating effect can make electrical circuits less efficient as some energy is wasted as heat. It can also cause components in the circuit to melt which means that the circuit will stop working or not work properly. Fuses use this effect to protect circuits - they melt and break the circuit if the current gets too high.
- This heating effect of an electric circuit can have other advantages. For example it's good for heating things. Toasters contain a coil of wire with a really high resistance, When a current passes through the coil its temperature increases so much that it glows and gives off infrared radiation which cooks the break. Light bulbs work in the same way.
Electrical power and energy transferred
- An appliance with a high power rating transfers a lot of energy in a short time. This energy comes from the current flowing through it. This means that an appliance with a high power rating will draw a large current from the supply.
Electrical power (watts, W) = voltage (volts, v) x current (ampere, A)
- The energy transferred by an appliance depends on the current through it, the voltage supplied to it and how long it is on for.
Energy transferred (joules, J) = current x voltage x time
Speed and velocity
Speed and velocity are both measure in m/s (or km/h or mph). They both say how fast something is going but there is a difference:
- Speed just how fast you are going with no regard to the direction.
- Velocity must also have a direction specified. The distance in a particular direction is called displacement. Both velocity and displacement are vector quantities.