3 rd /4 th form – Electric circuits

Conductors and insulators An electric current is a flow of charge. These charges are often electrons. Electrons carry a negative charge. Electrons are usually bound to atoms, but some more strongly than others In a conductor some charges are able to flow so we can use them in an electric circuit. In an insulator the electrons are held tightly in position and so are unable to move. These materials do not conduct electricity.

Conductors and insulators Generally, most metals are good conductors and most non-metals are not. An electric cable makes use of both types of material. There is a third category, semi-conductors, which are also very useful (not GCSE) Metal wires to carry electricity Plastic insulation to isolate metal wires

Conduction electrons Metals have lots of ‘free’ electrons –weakly bound outer electrons so they are good conductors. Normally they move around randomly When an electric field is applied they move in a common direction

Counting charges The charge on a single electron is tiny We define a “useful” quantity of charge which allows sensible measurements, called the Coulomb. 1C is equal to the charge on ×10 18 electrons –That’s 6,241,510,000,000,000,000 electrons! –Ammeters need to go in the circuit so the current can flow in and out of them.

Electric current An electric current is a flow of electric charge (pushed along by a voltage). Where do these charges come from? In a circuit, all the wires and components are full of electrons –so as soon as a power supply is connected a current starts flowing –No time delay with long wires! –(Electrons move quite slowly)

Electric Current Current is a flow of charge We can define current as the number of coulombs flowing past a point in 1 second When 1C of charge flows through a wire in 1s, the current is 1A. Current (A) Charge (C) time (s)

Examples 1.What is the current when 4 C of charge flows for 2 seconds? 2.What is the charge flowing through a wire in 5 s if the current is 3 A?

Measuring current AMMETER Ammeters need to go in the circuit so the current can flow in and out of them. To just indicate the flow of current without measuring it we can use an indicator light. An ideal ammeter does not affect the circuit, no energy is transferred to it: it has zero resistance

Conventional Current Charges come in two flavours, + and – Conventional current is defined as the direction in which + charges flow so in a wire, the conventional current is in the opposite direction to the electron flow

Circuits: a reminder Circuit diagrams are simplified drawings Circuit diagrams are drawn with a ruler! –Connections must connect –There are symbols for each component

Circuit symbols You’ve just got to learn them! Full list is on p. 242

Electric current When a circuit with a battery is competed, the battery “pushes” the charges around. Electric current is never “used up” as electrons flow around a circuit. –The current is the same through all components in series circuit –All ammeters read the same: –(note connect in series) A

Electrical Energy Electricity is useful because it can be easily converted into other types of energy. A battery or power supply gives electrical energy to the electrons in a circuit. Other circuit components then convert this to different forms of energy.

Potential Difference (“Voltage”) The voltage between two points in a circuit is a measure of how much energy is transferred to or from the charges as they pass between those points. The voltage of a power supply is a measure of how hard it “pushes” charge –So a larger voltage supply means a larger current will flow in a circuit

Voltage Voltage is a measure of the energy change experienced by charges We define voltage as the number of joules transferred per coulomb of charge When charge passes through a p.d. of 1V, 1J of energy is transferred per coulomb. voltage (V) energy (J) charge (C)

Examples 1.When 2C of charge pass through a battery they acquire 24J of energy. What is the voltage of the battery? 2.The p.d. across a lamp is 3V. How much energy is transferred when 5C of charge pass through it? 3.(harder) How much energy is transferred when a current of 2A flows through a lamp with a p.d of 3V across it for 1 minute?

Combining cells For batteries in series, the supply voltage is the sum of the individual batteries –Adding more batteries in series increases voltage and therefore increases current For batteries in parallel, the supply voltage is the same as a single branch, but the battery capacity is increased (battery lasts longer) –Adding more batteries in parallel does not affect voltage or current V 7.5 V V 3 V

Measuring voltage To measure the voltage between two points we connect a voltmeter in parallel across those two points Here we are measuring the voltage across the resistor R An ideal voltmeter does not affect the circuit, no energy passes through it: it has infinite resistance

Resistance Resistance is the opposition to current flow displayed by components –for a fixed voltage, the larger the resistance, the smaller the current Resistance of connecting wires is usually so small it is ignored Resistors dissipating energy get hot! –e.g. lamp filament

Resistance Resistance is caused by collisions between the free charges and the lattice of atoms which makes up the conductor Each collision transfers energy to the atoms of material – material heats up A high current means more collisions – resistor gets hotter

Resistance Resistance is a measure of the opposition to current flow We define the resistance of a device as the voltage needed to push a given current through it When a p.d. of 1V causes a current of 1A to flow through a device, its resistance is 1  resistance (  ) (Ohms) voltage (V) current (A)

Examples If a lamp has a current of 3A when there is a p.d. of 12V across it, what is the resistance of the lamp? What is the current through a 100  resistor with a p.d. of 5V across it? A real ammeter has a resistance of 0.5 . What will the p.d drop across it be when a current of 5A is flowing?

Ohm’s Law “The current through a conductor is proportional to the current across it, provided the temperature remains constant” –Generally true for metals –Not true for all components

Voltage, Resistance & Current We have So for a given circuit: –What happens to the current if we increase the voltage of the power supply? –What happens to the current if we increase the resistance of the components? resistance (  ) voltage (V) current (A)

Resistors limit current They can be used to protect vulnerable circuit components in case of a fault Variable resistors can be used to control things

Series and Parallel Circuits A series circuit has only one path for the current to flow, all the components are joined together in a continuous line. A parallel circuit contains branches, the current splits and recombines. –Each branch is unaffected by the other branches A A

Current in series circuits Remember, current is never “used up” as electrons flow around a circuit. The current is the same through all components in series circuit –All ammeters read the same A A A A A A

Voltage in a series circuit The energy transferred to the charge by the battery = the energy dissipated by all the components in the circuit V battery V1V1 V2V2 V3V3 The largest resistance has the largest voltage across it (most energy transferred) –If all resistances are equal, the battery voltage is divided equally

Think about this circuit… What happens to V 1 as the resistance is increased? What happens to V 2 ? What happens to the current? What effect will this have on the lamp? V battery V1V1 V2V2 A

Voltage in a parallel circuit All components connected to a battery in parallel have the same voltage across them. V battery The current through each component is the same as if the other components weren’t there. –We can use V=IR on each branch in turn V battery A A

Current in a parallel circuit The total current through the battery is equal to the sum of the currents through each parallel branch The smallest current flows through the branch with the highest resistance. I battery I1I1 I2I2 I3I3

Characteristics of circuits Series: –simple –one switch affects all components –one broken component affects all components –voltage is shared between components Parallel: –components can be switched individually –one broken component only affects its own branch –all branches receive the full supply voltage