Current - is the flow of electrons around a circuit (Amps)
Potential difference - is the measure of voltage between two points usually across a component (Volts)
Resistance - something that slows down the flow of current (Ohms)
Standard test circuit
The standard test circuit is used to test components. It has an ammeter in series to measure the current, a voltmeter in paralell to the component to measure the potential difference across it and a variable resistor to alter the flow of current by increasing/decreasing the resistance. This allows you to take several reading and plot a V-I graph.
Voltage, Current and Resistance
- Current is the same all around the circuit
- Voltage is shared across components
- Resistance is added uo
- Current is shared through components
- Voltage is the same all around the circuit
- Resistance is calculated by R=V/I
Series and paralell - Examples
Fairy lights are connected in series. This splits 230V between all these lights so each bulb has a very low voltage across it. If one bulb breaks however they all break.
Cars are in paralell - everything from stereo to fans is in paralell so everything can be switched on and off individually and get the maximum voltage from the car battery. However if everything is turned on at once you will notice lights getting dimmer as all the appliances are not getting their full voltage from the car battery.
Resistance slows down the flow of current in a circuit. These factors affect resistance.
Temperature - the higher the temperature the higher the resistance as the electrons are provided with more energy and so are more likely to collide with atoms of the wire.
Thickness of wire - The thiner the wire the higher the resistance as there is less space making collisions between particles more frequent. The thicker the wire the lower the resistance.
Length of wire - The longer the wire, the more chance of colliding particles, meaning higher resistance. The shorter the wire the lower the resistance.
Material - Conductors have low resistance wheras insulators have high resistance. Within metals you have different degrees of conductivity and resistance e.g copper is a better conductor than steel and so copper has less resistance.
Voltage-Current graph - Filament Lamp
A filament lamp contains a thin peice of material called filament which produces heat when current passes through it producing light as a result. As the voltage increases the light gets brighter as the filament gets hotter. However as the filament gets hotter, this heat produces resistance as a result which slows down the flow of current which is why the graph levels off at the end.
A filament lamp is more likely to blow when it is first turned on a the filament would be cold meaning that it would have very low resistance. This low resistance would cause a surge of current through the bulb causing it to blow.
Resistor at a constant temperature
As the voltage increases so does the current - the two are directly propotional and so the resistance remains constant providing there is no change in temperature.
B, the steeper line, has higher resistance than A but the resistance still remains constant.
A diode allows current to flow in one direction only as it has very high resistance in the other direction preventing current to flow that way. A diode requires a small voltage before the current is able to flow. As the voltage increases the current increases also. Usually used in alternating current. No current flows in the opposite direction.
LDR and Thermistors
An LDR or light dependant resistor is used in automatic night lights. As the lightintesity increases the resistance decreases. So in the dark it has very high resistance but in the light it has low resistance.A thermistor is used in thermostats. As the temperature increases the resistacne decreases. So in cold temperatures it has very high resistance. In hot temperature it has low resistance.
Mains electricity is a 230V alternating current supply. Its frequency is 50Hz meaning that it alternates between -325 and +325 50 times a second.
Alternating current is when electons flow in one direction then in the opposite direction. Alternating current is constantly changing direction from 325V to -325V.
Direct current, electrons flow in one direction only.
An osilloscope trace can be uses to show this.
Frequencey = 1/time period(s)
Energy and power in circuits
Electricity is also know as energy transferred as when a current flows through a circuit e.g. a battery will transform chemical energy into electrical energy which could be used to turn a bulb on, in which case electrical energy will be converted into light and heat. If there is any form of resistance in a circuit (which there always is) this will also transfer energy as heat due to the resistance. The enrgy tranferred can be calculated using this formula:
Energy transferred = Power x Time
(Joules) (W) (s)
Power is the rate at which energy is transferred. It can be calculated by:
Power = Voltage x Current
(W) = (V) x (A)
The charge transferred depends on the size of the current and how long it flows for. The Coulomb is the unit of charge.
Charge = Current x Time
(C) = (A) x (s)
The symbol of charge is sometimes shown as Q (capital Q)
When electric charge flows through a circuit and there is a change in voltage energy is transormed. If the voltage is increased then more chemical energy is transferred as electrical energy. However if voltage is decreased then the charges lose energy in the form of heat - transforming electrical energy into heat energy into the atomosphere.
Energy Transferred = Charge x Potential difference
(J) = (C) x (V)