- Certain insulating materials become charged when rubbed together
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 with the same type of charge repel each other
- Two objects with different types of charges attract each other
Electric current- the flow of charge. The size of an electric current is the rate of flow of electric charge. This is the flow of charge per second.
- Electric charge is measured in coulombs (C)
- Electric current is measured in amperes (A)
The current in amperes = charge flow (C) / time taken (seconds)
I= current (A)
Q= charge (C)
t= time taken (s)
- Ammeters measure current. They are connected in series circuits.
- Voltmetes measure the potential difference. This is the amount of work done or energy transferred to the bulb by each coulomb of charge that passes through it. They are connected in par
The potential difference across the component in volts = work done (Joules) / charge (Coulombs)
Resistance (ohms) = potential difference (volts) / current (amperes)
Ohms law: the current through the resistor at a constant temperature is directly proportional to the potential difference across the resistor.
Ohmic conductor: A conductor that has a constant resistance and therefore obeys Ohm's law.
Filament bulb: Resistance increases with increase of the filament temperature. This is because ions in the metal filament vibrate more as the temperature increases so they resist the passage of the electrons through the filament more.
Diode: It allows current to only flow in one direction. In the forward direction the current is not directly proportional to the potential difference and the resistance is low. In the reverse direction, the current is much higher.
LDR: The resistance decreases if the light intensity on it increases.
- The same current passes through components in series with each other
- The total potential difference of the voltage supply in a series circuit is shared between the components
- The total potential difference of cells in series is the sum of the potential difference of each cell
- The total resistance of components in series is equal to the sum of the resistance of each component
- The total current through the whole circuit is the sum of the currents through the seperate components
- The bigger the resistance of a compenent, the smaller the current is.
- For components in parallel, the potential difference across each component is the same
Current (amperes) = Potential difference (volts) / resistance (ohms)