Current Electricity

The Basic Symbols cell wire A Ammeter battery switch V Voltmeter Bulb / lamp Diode Power supplies Resistor Variable Resistor

The basic ideas…. Amps (A) Current I Voltage V Volts (V) R Ohms (Ω) Symbol Definition Unit Current is a measure of how many electrons are actually flowing round the circuit Amps (A) Current I Voltage is a measure of how much energy is being given to, or taken away from, the electrons as they move round the circuit. Voltage V Volts (V) Resistance indicates how difficult it is for the current to pass through a component. R Ohms (Ω) Resistance As before, power measures the rate of energy transfer from one form to another. (i.e. how fast work is being done.) Power P Watts (W)

This means the electrons are free to move between them. Conductors In an electrical conductor the atom nuclei are arranged in a regular order. e- This means the electrons are free to move between them. Insulators Insulators are quite different. There is no easy route for the electrons to flow.

( The flow of electrons around a circuit) Current ( The flow of electrons around a circuit) Conventional current runs from the positive terminal of a cell, around the circuit, and back to the negative terminal. _ I I + I I I

( The flow of electrons around a circuit) Current ( The flow of electrons around a circuit) Conventional current runs from the positive terminal of a cell, around the circuit, and back to the negative terminal. e- e- _ + e- e- Because they are negatively charged the electrons are actually moving in the opposite direction e-

( The flow of electrons around a circuit) Current ( The flow of electrons around a circuit) Conventional current runs from the positive terminal of a cell, around the circuit, and back to the negative terminal. _ + A Because they are negatively charged the electrons are actually moving in the opposite direction Current is measured by an ammeter placed into the circuit at any point.

Current _ + 6 amps 6 amps A 6 amps In a series circuit there is only one path round for the electrons – They all travel the same way round the circuit. + _ A 6 amps 6 amps 6 amps Because of this the current (which measures the number of electrons going past) is the same at all the points of the circuit.

Current + _ On the other hand……. 6 amps 6 amps A A …. In a parallel circuit there is more than one route round for the electrons – so they get split up between the different routes. A 3 amps A ( However, the actual number of amps going round each route depends on the components in the circuit. They don’t have to be evenly split like in this example). 3 amps

Voltage When they leave the cell to travel around the circuit, all the electrons have been given a certain amount of energy. This is the voltage of the circuit. This voltage can be measured by placing a voltmeter in parallel with the power supply V

Voltage Components, like resistors and bulbs convert electrical energy into another form (such as heat or light). V The voltage drop in a component is measured using a voltmeter that is added in parallel to the component So….because the electrons give up their energy as they go through the component there is a drop in voltage. V

Voltage This voltmeter is showing that the cell is adding 1.5 volts of energy to the electrons 1.5V _ + This voltmeter is showing that the bulb is taking 1.5 volts of energy from the electrons (and converting it to light) 1.5V

Voltage 3V In a series circuit the voltage given to the electrons at the battery is divided up between the components in the circuit 2V 1V

So they don’t have any voltage! The wires of a circuit simply conduct the electricity from one point to another. 3V 0V So they don’t have any voltage! 2V 1V They don’t add or remove energy from the electrons flowing through them.

Voltage 3V In a parallel circuit the components are not sharing the energy from particular electrons. They are each able to remove the full amount of energy from the electrons passing through them. 3V 3V

Circuits may contain a mixture of parallel and series components. Voltage 3V Circuits may contain a mixture of parallel and series components. 3V 1.5V 1.5V

4 amps worth of electrons are flowing round the circuit. The electrons are being given 6 volts of energy 6V 4A 3A Electrons aren’t gained or lost in a circuit so there must also be 4 amps worth here. 4 amps worth of electrons are flowing round the circuit. ? 4A This resistor is on its own branch in the circuit, so it can take all 6 volts of energy from the electrons. ? As 3 amps worth of electrons have passed along the first branch of this circuit, 1 amps worth must be left for this one 6V ? 1A 3V ? This bulb, however, is having to share the electrons’ energy with the other bulb on this branch 3V

Resistance Resistance measures how hard it is for the electrons in a circuit to travel through a component. Wires have a very low resistance, so it is easy for the current to flow through them Efficient bulbs should produce more light than heat so although they have more resistance than the wires, it is far less than the resistors. Resistors are designed to have a high resistance. Anything that gets hot usually has a high resistance.

The unit of resistance is the Ohm - Ω V R I Resistance is measured by dividing the voltage drop in a component by the current travelling through it. The unit of resistance is the Ohm - Ω voltage Resistance = current

Resistance 2V 1V 1A R = V I 2V 1A = 2Ω 1V 1A = 1Ω = 2Ω 1V 1A = 1Ω In this example it is twice as hard for the current to pass through the resistor compared to the bulb.

But this is only true if the temperature remains the same! Ohms Law 10Ω Ohms Law states that: If you change the current passing through an electrical component then its voltage drop will change proportionally 10Ω = 100 V 10A 10Ω = 200 V 20A But this is only true if the temperature remains the same!

Alternating Current and Diodes An alternating current doesn’t just flow in one direction around a circuit, it changes direction rapidly many times a second. How many times a second it changes is called the frequency and is measured in units called Hertz (Hz) This is the symbol for an alternating current. Domestic (household) circuits have a frequency of 50 HZ 50Hz This means the current changes direction 50 times a second.

Alternating current and diodes A lot of electrical apparatus needs direct current. It can’t work on alternating current. 50Hz A diode converts alternating current to direct current Current will pass through the resistor 50 times a second in this direction. Diodes only allow electricity to pass through them in one direction. (In the direction of the arrow) But it doesn’t pass through it in this direction at all because of the diode.

The diode is built with a low resistance in this direction – so current can pass through it But it has a very high resistance in this direction – preventing any current flow. Green A light emitting diode (LED) lets out a pulse of light when current flows through it.

Power As in the motion section of the syllabus, power is defined as….. The rate at which energy is being transferred. Power was calculated by dividing the work done (Energy changed) by the time taken. The power of an electrical component is calculated by multiplying its voltage drop by the current passing through it. P = E T P = V x I The formula may be different, but the units are still Watts

Power 2V 1V 1A P V I P = 2V x 1A = 2W P = 1V x 1A = 1W The resistor uses more power than the bulb – it converts the electrical energy into heat faster than the bulb converts it into light

How far would this motor lift the weight in 5 seconds? 12V 100N 5A 2. Power = work done time taken So… work done = power x time = 60W x 5s 1. The power of the motor is… = 300 J P = V x I = 12V x 5A 3. Work done = force x distance moved. = 60W So…distance = work done force = 300J 100N = 3m End