Chapter 10 D.C. Circuits Flashcards
Switch:
Turn the circuit on (closed), or off (open)
Fixed resistor:
A resistor limits the flow of current. A fixed resistor has a resistance it cannot change
Variable resistor:
A resistor with a slider that can be used to change its resistance. Used often in dimmer switches and volume controls
Thermistor:
The resistance of a thermistor depends on its temperature. As its temperature increases, its resistance decreases and vice versa
Light-dependent resistor (LDR):
The resistance of an LDR depends on the light intensity. As the light intensity increases, its resistance decreases and vice versa
Diode:
A diode allows current to flow in one direction only. They are used to convert AC to DC current
Light-emitting diode (LED):
This is equivalent to a diode and emits light when a current passes through it. These are used for aviation lighting and displays (TVs, road signs)
Ammeter:
Used to measure the current in a circuit. Connected in series with other component
Voltmeter:
Use to measure the potential difference of an electrical component. Connected in parallel with component
Electromotive Force
- When charge passes through a power supply such as a battery, it gains electrical energy
- The electromotive force (e.m.f) is the amount of chemical energy converted to electrical energy per coulomb of charge (C) when charge passes through a power supply
- e.m.f is measured in Volts (V)
e.m.f is also
- the potential difference across the cell when no current is flowing e.m.f can be measured by connecting a high-resistance voltmeter around the terminals of the cell in an open circuit
potential difference
- Potential difference describes the loss of energy from charges; ie. when electrical energy is transferred to other forms of energy in a component
-e.m.f. describes the transfer of energy
- from the power supply to electrical charges within the circuit
Internal Resistance
- All power supplies have some resistance between their terminals
- This is called internal resistance (r)
internal resistance causes the
- charge circulating to dissipate some electrical energy from the power supply itself
- This is why the cell becomes warm after a period of time
e.m.f equation
The internal resistance therefore causes
- loss of voltage or energy loss in a power supply
- This voltage is not available to the rest of the circuit so is called the ‘lost volts’
- Vr is the lost volts
- This is the voltage lost in the cell due to internal resistance, so, from conservation of energy:
- Lost volts = e.m.f − terminal p.d
- Lost volts = I × r (Ohm’s law)
Kirchhoff’s First Law
- The sum of the currents entering a junction always equal the sum of the currents out of the junction
- This is a consequence of conservation of charge
- current shouldn’t decrease or increase in a circuit when it splits
In a circuit:
- A junction
- A branch
- A junction is a point where at least three circuit paths meet
- A branch is a path connecting two junctions
In a parallel circuit, the current …. at the junctions and each branch has a different value. Kirchhoff’s first law applies at each junction
In a series circuit, the current is the … at any point
Kirchhoff’s Second Law
- The sum of the e.m.f’s in a closed circuit equals the sum of the potential differences
- This is a consequence of conservation of energy
In a series circuit, the voltage is … across all components depending on their resistance
- split
- The sum of the voltages is equal to the total e.m.f of the power supply
In a parallel circuit, the voltage is the … across each closed loop
- Same
- The sum of the voltages in each closed circuit loop is equal to the total e.m.f of the power supply:
A closed circuit loop acts as its own independent series circuit and each one separates at a junction. A parallel circuit is made up of two or more of these loops
Resistors in Series
- When two or more components are connected in series:
- The combined resistance of the components is equal to the sum of individual resistances
The equation for combined resistors in series is derived using Kirchhoff’s laws:
Deriving the Equation for Resistors in Parallel
- I= I1 + I2
Resistors in Parallel
- When two or component are connected in parallel:
- The reciprocal of the combined resistance is the sum of the reciprocals of the individual resistances
Potential Divider Circuit
- When two resistors are connected in series, through Kirchhoff’s Second Law, the potential difference across the power source is divided between them
- Potential dividers are circuits which produce an output voltage as a fraction of its input voltage
- Potential dividers have two main purposes:
- To provide a variable potential difference
- To enable a specific potential difference to be chosen
- To split the potential difference of a power source between two or more components
Potential dividers are used widely in
volume controls and sensory circuits using LDRs and thermistors
Potential divider diagram and equation
The potential difference V across each resistor depends upon its resistance R
- The resistor with the largest resistance will have a greater potential difference than the other one from V = IR
- If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share
- In potential divider circuits, the p.d across a component is proportional to its resistance from V = IR
Variable Resistance Components
- Variable and sensory resistors are used in potential dividers to vary the output voltage
- This could cause an external component to switch on or off e.g. a heater switching off automatically when its surroundings are at room temperature
- Sensory resistors used are Light Dependent Resistors (LDRs) and thermistors
From Ohm’s law V = IR, the potential difference Vout from a resistor in a potential divider circuit is proportional to its resistance
- If an LDR or thermistor’s resistance decreases, the potential difference through it also decreases
- If an LDR or thermistor’s resistance increases, the potential difference through it also increases
-Since the total p.d of the components must be equal to Vin, if the p.d of the sensory resistor decreases then the p.d of the other resistor in the circuit must increase and vice versa
The Potentiometer
- A potentiometer is similar to a variable resistor connected as a potential divider to give a continuously variable output voltage
- It can be used as a means of comparing potential differences in different parts of the circuit
- The circuit symbol is recognised by an arrow next to the resistor
- A potentiometer is a single component that (in its simplest form) consists of a coil of wire with a sliding contact, midway along it
A potentiometer is a type of variable resistor
The Galvanometer
- A galvanometer is a type of sensitive ammeter used to detect electric current
- It is used in a potentiometer to measure e.m.f between two points in a circuit
- A galvanometer is made from a coil of wire wrapped around an iron core that rotates inside a magnetic field
- The circuit symbol is recognised by an arrow in a circle:
The galvanometer instrument
- The arrow represents a needle which deflects depending on the amount of current passing through
- When the arrow is facing directly upwards, there is no current
- This is called null deflection
Ohm’s law tells us that the current through a conductor (wire) is
- directly proportional to the potential difference through it i.e. no p.d means no current flows through the galvanometer
- A galvanometer has p.d of zero when the potential on one side equals the potential on the other side
- This is at the position at which it is connected on the wire (which varies with the sliding contact) gives a p.d equal to the EMF of the cell connected to the galvanometer
The cell should be connected such that its potential opposes the potential on the wire i.e. the positive terminal of the power supply faces the positive terminal of the cell:
- When the sliding contact moves along the potentiometer wire, you add or remove resistance from/to the external circuit. This changes the potential drop across X and Y
- Location of the sliding point is adjusted until the galvanometer reads zero. This is until the potential difference equals E2
- The direction of the two e.m.fs oppose each other and there is no current
