Download TOPIC 2- controlling and using electric current

TOPIC 2- controlling and using electric current
Series and Parallel circuits
Series circuit – components connected on the same loop of the circuit. The current that flows across each
component is the same.
Parallel circuit – components connected on separate loops. The current is shared between each
component and the total current entering a junction is equal to the total current leaving a junction. The
current is described as being conserved.
Current and voltage:
Current is the rate of flow of charge around a circuit and it is measured in amperes (A) by an ammeter which
must be connected in series.
Voltage (potential difference) is the driving force that pushes the current around the circuit and it is measured
in volts (V) by a voltmeter must be connected in parallel.
If you increase the voltage then the more current will flow.
Potential difference is the energy transferred per unit charge passed.
The bigger the change in voltage, the more energy is transferred.
Equation: Energy transferred (joule, J) = potential difference (volt, V) x charge (coulombs, C)
Resistance:
Resistance is a measure of how hard it is for a current to flow through a component. Resistors are added to
reduce the flow of current.
If you increase the resistance, then less current will flow.
Bulbs have resistance, so if you add more bulbs the resistance increases therefore the current decreases.
A variable resistor (rheostat) is a device with variable resistance and it is used to vary the amount of current in
a circuit.
Voltage, Current and Resistance
The size of current depends on the voltage driving it and the amount of resistance.
If you increase resistance, the current decreases.
If you increase potential difference, then the current also increases.
Equation: resistance (ohm, Ω) = voltage (volts, V) x current (amperes, A)
Resistance graphs:
Voltage – Current graphs show how current varies as you change the voltage.
Resistor at constant temperature: the current flowing through a resistor at constant temperature is
proportional to voltage.
The filament lamp (a light bulb that contains a thin coil of wire called filament and it heats up when an
electrical current passes through it): as the temperature of the filament increases, the resistance increases.
The diode (electrical component that regulates the voltage in circuits): current will only flow through a diode in
one direction.
The LDR (light dependent resistor): a resistor that changes its resistance depending on how much light there is.


In bright light, the resistance falls therefore more current can flow through it.
In darkness, the resistance increases therefore less current can flow through it.
LDR are used to detect light levels e.g. automatic security lights.
The thermistor: resistance depends on the temperature.


At high temperatures, the resistance drops therefore more current can flow.
At low temperatures, the resistance increases therefore less current can flow
Thermistors are used as temperature sensors e.g. in fire alarms.
Energy transfer in Resistors
When an electrical current flows through a resistor some of the energy is transferred to heat energy resulting
in the resistor heating up.
This energy transfer is due to the electrons colliding with ions in the lattice, these collisions give ions extra
energy which is emitted as heat. As a result, the metal heats up and increases resistance.
However this makes electrical circuits less efficient as some energy is wasted as heat. Also it causes
components to melt or catch fire if the current gets to high which means the circuit will stop working (fuses
work like this).
On the other hand it is useful if you want to heat something for example toasters contain a coil of wire with a
high resistance, when an electrical current passes the temperature increases and gives off heat radiation
(infrared) which cooks the food. Light bulbs work similar.
Charge is measured in coulombs: charge (coulombs, C) = current (amps, A) x time (seconds, s)
Energy transferred (joules, J) = Potential difference (volts, V) x charge (coulombs, C)
=
Energy transferred (joules, J) = current (A) x potential difference (V) x time (s)
Electrical Power (watts, W) = potential difference (volts, V) x current (amps, A)