A resistor in a circuit resists the flow of current.
A variable resistor (rheostat) can have it's resistance changed. It can be used to:
- control current - increasing the resistance results in a lower current
- vary the brightness of a bulb by varying the current - higher current gives a brighter bulb
- vary the speed of a motor by varying the current - higher current gives greater speed
The resistance of a rheostat is altered by changing the length of the wire. A longer wire creates a higher resistance.
- Voltage is measured in volts (V)
- Current is measured in amps (A)
- Resistance is measured in ohms
For a given ohmic conducter, the current increases as the voltage increases but the resistance remains constant, provided that the temperature doesn't change. In non-ohmic conductors, such as bulbs the resistance changes.
Resistance = voltage/current
The voltage-current graph for an ohmic conductor is a straight line. The gradient of the graph shows the resistance of the conductor. The steeper the gradient, the higher the resistance.
The voltage-current graph for a non-ohmic conductor is a curve. The increasing gradient shows that the resistance increases as the current increases.
As the temperature of a resistor rises, its resistance increases. This is why the V-I graph for a filament bulb is curved. The curve shows an increase in gradient as the current rises, showing that the resistance is increasing.
An electric current is the flow of charge carriers through a material. In metals the charge carriers are electrons. As electrons pass along a wire, they collide with atoms (ions) in the metal. This causes the atoms to vibrate more. As a result:
- the number of collisions increases
- the resistance in the wire increases
- the temperature of the wire increases
Resistors in Series and in Parallel
Two or more resistors in series will increase the overall resistance of the circuit. The total resistance of the circuit can be found by adding the individual resistances together.
Resistors in parallel reduce the overall resistance of a circuit.
A potential divider can be made of fixed resistors that are arranged to produce a required voltage, or potential difference (pd).
There is a pd across each of the two fixed resistors. If a connection is made across one of the fixed resistors, the pd across the resistor is the output voltage.
If a variable resistor is used in the potential divider, the exact pd of the output from the circuit can be chosen. The lower the resistance, the lower the pd.
LDRs and Thermistors
A light dependant resistor (LDR) changes its resistance in response to different light levels:
- bright light causes lower resistance
- dim light causes higher resistance
Using an LDR in place of a fixed resistor in a potential divider gives an output of voltage that depends upon light conditions. For example, bright light causes low output voltage because the resistance of the LDR is lower than that fixed resistor. In dim light, the ouput voltage is high.
A thermistor changes its resistance when the temperature changes:
- high temperature causes lower resistance
- low temperature causes higher resistance
Using a thermistor as the variable resistor in a potential divider gives an output of voltage that depends upon the temperature. For example, a higher temperature causes a lower ouput voltage because the resistance of the thermistor is lower than the fixed resistor.
The transistor is the basic building block of many electrical devices. It is an electronic switch. A small base current is used to switch on a larger current, which flows through the collector and emitter. Increasing miniaturisation (making the transistors smaller):
- increases the number if transistors that can be connected in a processor
- means computer processors can be made smaller
Two factors affect how small transistors can become:
- smaller components dissipate more heat as a current passes through them
- as transistors become thinner, they offer less resistance to the electrons due to 'quantum tunnelling'.
Millions of these tiny transistors are used in computers and other electrical equipment to speed up processing. They can be connected together to work like logic gates.
Inputs and Logic Gates
The input to a logic gate can be:
- a high voltage - called high, 1
- a low voltage - called low, 0
The output (Q) of a logic gate is either high or low, depending on its input signals. Switches, LDRs and thermistors can be used in series with fixed resistors to provide input signals for logic gates. In this case, they are being made into potential dividers. A pd, which can be either high (1) or low (0), id fed to the input.
Thermistors as Inputs for Logic Gates
When a thermistor and a fixed resistor are connected in series, a variable potential divider can be produced. This can provide the input to a logic gate that depends upon temperature. If the fixed resistor is changed to a variable resistor, the temperature at which the logic gate receives the high input can be set. The voltage output across the variable resistor provides a signal with adjustable threshold voltage to the logic gate.
LDRs as inputs for Logic Gates and LEDs as outputs
When and LDR is connected in series to a fixed resistor, it produces a device that can provide the input to a logic gate that depends upon light conditions.
If the fixed resistor is changed to a variable resistor, the light level at which the logic gate receives the high input can be set.
The output from a logic gate can switch on a light emitting diode (LED). When the logic gate gives a high output, the LED lights up. This could be used to show, for example, when a heater comes on. An LED can be used to indicate the output of a logic gate because it emits light when a voltage is fed to it. An LED only requires a very small current. A resistor is put in series with the LED to ensure that the current flowing through it isn't too large.
Latches and Relays
A relay can be used as a switch. A small current in the relay coil switches on a circuit in which a larger current.
A relay is needed in order for a logic gate to switch on a current in main circuit, because:
- a logic gate has low output output (whereas the mains has a higher power)
- the relay isolates the low voltage from the high voltage mains
Magnetic Field Around a Wire and Around Coils
A straight wire carrying an electric current has a circular magnetic field around it. The magnetic field is made up of concentric circles. If the wire is put near the magnet, the two magnetic fields interact and the wire can move.
The magnetic field around a rectangular coild forms straight lines through the centre of the coil.
Wires Moving in Magnetic Fields
If a current-carrying wire is placed in a magnetic field it experiences a force and moves. This is called the motor effect. For a current-carrying wire in a magnetic field to experience the maximum force, it has to be at right angles to the magnetic field.
The direction the wire moves in depends upon: the direction of the current and the direction of the magnetic field.
The direction the wire moves in can be reversed by: reversing the direction of the current and reversing the direction of the magnetic field.
Fleming's Left Hand Rule
Fleming's Left Hand Rule can be used to predict the direction of the force on a current-carrying wire. The rule states that if:
- your first finger points in the direction of the magnetic field, N to S
- your second finger points in the direction of the current, + to -
- your thumb will point in the direction of the force on the wire
Coils Rotating in Magnetic Fields
A simple direct current (dc) electric motor works by using a current-carrying coil. When a current-carrying coil is placed in a magnetic, it will rotate in the following way:
- the current flowing through the coil creates a magnetic field
- the magnetic field of the magnet and the magnetic field of the coil interact
- each side of the coil experiences a force in an opposite direction because the current is flowing in opposite directions in the two parts of the coil
- the forces combine to make the coil rotate
Electric motors transfer energy to the device. Some energy is wasted to the surroundings, often as heat. The speed of a motor can be increased by:
- increasing the size of the electric current
- increasing the number of turns on the coil
- increasing the strength of the magnetic field
The direction of the current affects the direction of the force on the motor coil. The current must always flow in the same direction relative to the magnet in order to keep the coil rotating. This is achieved by using a split-ring communicator. A split-ring communicator changes the direction of the current in the coil every half turn.
Because the maximum force is produced when the coil and the magnetic field are at right angles, curved pole pieces are used to give a radial field. The effect of the radial field is that the magnetic field lines and the coil are always at the correct angle to give maximum force.
Generating Electricity and DC Generators
Generating electricity is known as the dynamo effect. Electricity can be generated by moving a wire near a magnet, or a magnet near a wire.
A DC generator is a DC motor working in reverse. Instead of feeding a voltage to the coil and watching it move, the coil is moved to produce a voltage. A DC generator enables energy to be stored for later use.
An alternating current (AC) can be generated by rotating a magnet inside a coil of wire. In a power station, the electricity is generated by rotating electromagnets inside coils of wire.
Where a DC generator has commutators, an AC generator has slip rings and brushes.
As the wire moves up (past the north pole of the magnet) a current is induced in the wire. After the coil has turned half a turn this section of wire will be moving down past the south pole. A current is now induced in the wire in the opposite direction. This means that the induced current is an alternating current (AC)
A voltage is induced:
- across a wire, when the wire moves relative to a magnetic field
- across a coil, when the magnetic field linking the coil changes.
The induced voltage depends upon the rate at which the magnetic field changes. The rate of change of the magnetic field can be increased by increasing the speed of movement. Reversing the direction of the changing magnetic field also changes the direction of the induced voltage. The voltage induced can be increased:
- increasing the speed at which the magnet or coil rotates (this also increases the frequency of the AC)
- increasing the number of turns on the electromagnet's coils
- increasing the strength of the magnetic field
A transformer is made of two coils of wire wound onto an iron core. The two coils of wire aren't connected to each other. This enbles the transformer to change the size of an alternating current. A transformer only works with AC, it doesn't change AC to DC
Step up transformers
- increase voltage
- have more turns on the secondary coil than on the primary coil
Step down transformers
- decrease voltage
- have fewer turns on the secondary coil than on the primary coil
- are used in everyday applications such as phone cahrgers, laptops and radios
The voltage on the secondary coil can be calculated from the voltage on the primary coil (and vice verse): voltage across primary coil/voltage across secondary coil = no. primary turns / no. secondary turns.
Transformers and AC
Transformers can only use AC because they rely on a changing magnetic field in the primary coil to induce a voltage in the secondary coil. DC isn't suitable because it only provides a steady magnetic field.
As the AC increases in the primary ciol, the magnetic field it produces grows and cuts through the wire of the secondary coi. This induces a current (to try to cancel out the magnetic field from the primary coil).
Transformers in the National Grid
When overhead power cables carry current, they get hot so energy is wasted as heat. This power loss can be reduced by reducing the current. The power loss in transmission relates to the current squared: power loss = current(squared) x resistance
In a step-up transformer, if you increase the voltage, the current automatically decreases. Therefore, step-up transformers are used to increse the voltages from power stations to supply the National Grid. Step-down transformers are used in sub-stations in order to reduce the voltages for domestic and commercial users.
The transformation equation shows that the power input to a transformer is equal to the power output of a transformer. This means that in a step-up transformer, when the voltage is increased, the current is decreased in the same proportion. As a lower current will reduce power loss during transmission, using a step-up transformer at the power station reduces the energy lost as the current flows along the overhead cables. A step-down transformer reduces the voltage to a safer level for consumers (but increases the current).
An isolating transformer is used in some mains circuits, for example, bathroom shaver sockets, to make them safer. In an isolating transformer, the two coils aren't connected to each other. This means that the user is isolated from the mains supply. It's particularly important to use isolating transformers in areas such as bathrooms, so there's less chace as electrocution where there is water present.
An isolating transformer has equal numbers of turns on the primary and secondary coils. This makes no difference to the voltage. The benefit of an isolating transformer is that is keeps the two halves of the circuit seperate. Therefore, there is less risk of contact between the live parts (connected to the mains) and the earth lead (connected to the body of, for example, a shaver)
A silicon is a device that allows current to flow through it in one direction only. A current-voltage characteristic can be drawn for a diode by plotting the current through the diode against the voltage across the diode. A silicon diode is made of two types of silicon:
- n-type, which contains extra electrons (so has extra negative charge carriers)
- p-type, which has holes where there should be electrons (so the holes are like positive charge carriers)
A diode is forward biased in a circuit when the n-type is connected to the negative terminal of the battery. Current can flow because: the electrons can flow towards the holes and the holes can flow towards the electrons
If the diode is reverse biased (backwards), the current can't flow. This is because the electrons seems to drop into the holes and are unable to get past the layer between the two types of silicon. The current-voltage characteristic curve shows that current flows easily in one direction through the diode. This is because it has a low resistance to current in this direction. Current doesn't flow easily in the opposite direction because the diode has a high resistance to current flow in the reverse direction.
If alternating current is passed through a single diode, the diode will allow the current flowing in one direction to pass through and will stop the current flowing in the opposite direction. This is half-wave rectification.
A group of four diodes can be connected together to make a bridge circuit to give full-wave rectification. A bridge circuit can supply full-wae rectification of AC. For each half of the AC cycle, there are two diodes that can pass the current, and send it to the output. During each half cycle, the current passes through the load in the same direction.
A capacitor stores charge that can be discharged later. When current flows in a circuit containing an uncharged capacitor, the charge is stored on the capacitor and its pd increases. When a charged capacitor is connected to a conductor, the capacitor behaves like a battery. This capacitor dicharges, sending its stored current through the conducter.
When a charged capacitor is connected to a conductor, the flow of current from the capacitor to the conductor isn't steady. Instead the current flow decreases as the charge on the capacitor decreases.
As the charge on the capacitor decreases, the pd across the capacitor also decreases. This means that the pd across the conductor has decreased and so the current flowing decreases. This continues until the capacitor is fully discharged.
A capacitor connected across a varying voltage supply produces a more constant (smoothed) output. This is useful for devices that need a more constant voltage supplied to them.
A capacitor smoothes the output by discharging when the pd falls to a certain level, putting more charge into the circuit. This boosts the current so that it remains constant.
When the pd in the circuit is high enough, the capacitor charges up again. It remains charged until the pd falls and the capacitor has to make up the difference once more.