How do insulating materials become charged?
When they get rubbed together (friction).
Like and unlike charges
Like charges repel; unlike charges attract
Equation for electric current
Electric current (Amperes) = Charge flow (Coulombs) / Time taken (seconds) I = Q/t
What is the size of electric current?
The rate of the flow of charge
Necessary for pushing electrons around a complete circuit. A battery consists of two or more cells.
Enables the current in a circuit to be switched on or off.
Designed to emit light as a signal when a current passes through it or as a light source such as a bulb.
Allows current through in one direction only.
Light emitting diode (LED)
Emits light when a current passes through it.
Used to measure electric current.
Limits the current in a circuit
Allows the current to be varied
Designed to melt and 'break' the circuit if the current through it is greater than a certain amount.
Designed to transfer electrical energy to heat the surroundings.
Used to measure potential difference (voltage).
What is current?
Current is a flow of charge. The size of the current is the rate of the flow of electric charge
What does the ammeter do?
The ammeter measures the current THROUGH a component. It is connected in SERIES with the component so the current through them is the same.
What does the voltmeter do?
The voltmeter measures potential difference ACROSS a component. This is the amount of work done or energy transferred to the bulb by each coulomb of charge that passes through it. The voltmeter is connected in PARALLEL with the component so the pd is measured across it.
Potential difference equation
Potential difference across the component in volts = Work done in joules / charge in coulombs V=W/Q
Resistance in ohms = Potential difference in volts / current in amperes R=V/I
What does a resistor do?
A resistor limits the current.
What does Ohm's law state?
The current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.
Current- Potential difference Graphs
Filament Bulb: resistance increases with increase of filament temperature. A flat S shape curve, not an ohmic conductor. Diode: in the forward direction, resistance is low as current is greater than pd. In the reverse direction, resistance is high. Not an ohmic conductor and a flat line with a sharp exponential increase. Thermistor: If temperature increases, the resistance decreases. Light-dependent resistor LDR: Resistance decreases if the light intensity on it increases.
For components in series: •The current is the same in each component •Adding the potential difference of each component gives the total potential difference •Adding the resistances gives the total resistance •The pd of cells in series is the sum of the pd of each cell
For components in parallel: •The total current is the sum of the currents through the separate components •The bigger the resistance of a component, the smaller the current is •In a parallel circuit, the pd is the same across each component
Why does resistance increase as temperature increases
This is because the ions in the metal filament vibrate more as the temperature increases. So they resist the passage of electrons through the filament more.