4.1 Electrical Charges
When two electrically insulating materials are rubbed together, electrons are rubbed off one material and deposited on the other.
Electrons are transferred when objects become charged:
- insulating materials that become positively charged when rubbed lose electrons
- insulating materials that become negatively charged when rubbed gain electrons
Two objects that have opposite electric charges attract each other.
Two objects that have the same electric charges repel each other.
The bigger the distance between the objects, the weaker the forces between them.
4.2 Electric Circuits
Every component in a circuit has its own agreed symbol. A circuit diagram shows how components are connected together.
A battery consists of two or more cells connected together.
The size of an electric current is the rate of flow of charge.
Electric current = charge flow time taken
I = Q t
I = current in amperes (A) Q = charge in coulombs (C) t = time in seconds (s)
4.2 Components and Symbols
Every component has its own agreed symbol:
Current is measured with an ammeter, in amperes (A). Ammeters are always placed in series with the component.
The potential difference is measured with a voltmeter, in volts (V). Voltmeters are always placed in parallel with the component
The potential difference across a component (V) = Work done(Energy transferred) (J) Charge (C)
Resistance is the opposition to current flow in ohms. Resistance ( ) = PD (V) Current (A)
Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.
Any component that obeys Ohm's law is called an ohmic conductor.
Reversing the current through a component reverses the PD across it.
4.4 Current-Potential Difference graphs
The line on a current-potential difference graph for a filament bulb is a curve. So the current is not directly proportional to the potential difference ( and therefore does not obey Ohm's Law)
The resistance of the filament increases as the current increases as the current increases. This is because the resistance increases as the temperature increases.
Reversing the potential difference makes no difference to the shape of the curve.
The current through a diode flows flows in one direction only. In the reverse direction the diode has a very high resistance so the current is zero.
As the light falling on it gets brighter, the resistance of a light-dependent resistor (LDR) decreases.
As the temperature goes up, the resistance of a thermistor goes down.
4.5 Series Circuits
In a series circuit the components are connected one after another. Therefore if there is a break anywhere in the circuit, charge stops flowing.
There is no choice of route for the charge as it flows around the circuit so the current through the circuit so the current through each component is the same.
The current depends on the potential difference of the supply and the total resistance of the circuit. I = V/R I = current (A) V = PD (V) R = resistance (ohms)
Adding the resistances gives the total resistance of of the circuit.
For cells in series, acting in the same direction, the total potential difference is the sum of their individual potential differences.
The bigger the resistance of a component, the bigger its share of the supply PD
4.6 Parallel Circuits
In a parallel circuit each component is connected across the supply, so if there is a break in one part of the circuit, charge can still flow in the other parts.
There are junctions in the circuit so different amounts of charge can flow through different components. The current through each component depends on its resistance.
The bigger the resistance of a component, the smaller the current through it.
Current (A) = Potential Difference (V) Resistance (ohms)
The total current = the sum of the currents through the separate components
The potential difference is the same across each component.
Bigger resistance > smaller current