Unit D - Electricity

2c (2.7-2.19)

Series circuits

• Components connected in a line, end to end, between +ve and -ve of power supply

• If you remove/disconnect one component, circuit is broken + all stop working

• Not very useful, only a few things are practically connected in series e.g. fairy lights

Voltage in series circuit

• There’s a bigger supply p.d. when more cells are connected in series

• Total p.d. of supply is shared between components

• p.d. of each component depends on its resistance

Current in series circuit

• Current same everywhere, I₁ = I₂

• Size of current depends on total p.d. and total resistance of circuit

Resistance in series circuit

• Total resistance of circuit depends on number + type of components

• Total resistance is sum of resistance of each component in circuit

Parallel circuits

• Each component is separately connected to +ve and -ve of supply

• If you remove/disconnect one component, others are hardly affected

• This is how most things are connected
  e.g. cars and household electrics
  Each light switch in your house is part of branch of parallel circuit (turns one light on/off)

• Everyday circuits often contain mixture of series and parallel parts - rules of series circuits apply to components on same branch

Current in parallel circuit

• Current shared between branches

• Total current flowing around circuit = total of all currents through separate components

• There are junctions where current splits or rejoins

• Total current going into junction = total current leaving it

• Current through branch depends on resistance, higher resistance = lower current

Voltage in parallel circuit

• P.d. is same across all branches, V₁ = V₂

Resistance in parallel circuit

• Total resistance of circuit decreases if you add second resistor in parallel

Wire voltage-current characteristics

Current through wire is proportional to voltage

Resistor voltage-current characteristics

Current through resistor is proportional to voltage

Different resistors have different resistances

Metal filament lamp voltage-current characteristics

As temp of metal filament increases, resistance increases

Diode voltage-current characteristics

Current only flows through diode in one direction

Ammeter

• Measures current (in amps) through component

• Must be placed in series anywhere in main circuit

Voltmeter

• Measures voltage (in volts) across component

• Must be placed in parallel around component

Investigating V-I characteristics

• Component, ammeter and variable resistor are in series, so can be put in any order

• Voltmeter must be placed in parallel around component under test

• As you vary variable resistor, it alters current flowing through circuit

• Allowing you to take pairs of readings from ammeter and voltmeter

Effect of changing resistance on current

More resistance = less current

LDR

• Changes resistance depending on how much light it receives

• In bright light, resistance falls

• In darkness, resistance is highest

• Useful for various electronic circuits e.g. burglar detectors

Thermistor

• Temperature-dependent resistor

• Hot = resistance drops

• Cool = resistance increases

• Useful for temperature detectors e.g. car engine temp sensors, thermostats

LED

• Light-emitting diodes emit light when current flows through them in forward direction

• Used for numbers on digital clocks and traffic lights

• Unlike light bulb, don’t have filament that can burn out

LEDs and lamps can be used to…

indicate presence of current in circuit

Often used in appliances to show they’re switched on

Equation: Voltage, Current and Resistance

V = IR

Voltage = Current x Resistance

[V] = [A] x [Ω]

Current

Rate of flow of charge around a circuit

Equation: Charge, Current and Time

Q = It

Charge = Current x Time

[C] = [I] x [s]

In solid metal conductors, current is…

a flow of negatively charged electrons