- Current- only flows if electrons in circuit, measured in Amps.
- Voltage- drives current around the circuit, measured in Volts.
- Resistance- Slows flow down, measured in Ohms.
- If voltage increases, so does current. If resistance increases, current decreases.
- Ammeter= measures current- must be placed in series.
- Voltmeter= measures volts across components must be placed in parallel.
- Voltage= potential difference.
Voltage- Current Graph
resistance= voltage/ current
A current coil carrying a current also carries a magnetic field. A rectangular coil reinforces the magnetic field, they strengthen each other at the centre. Solenoids make strong and uniform magnetic fields which is called an electromagnetic.
LDR's and Thermistors
LDR= Light dependent resistors.
- In bright light= low resistance.
- In darkness= high resistance.
Thermistor= Temperature dependant resistors.
- Hot conditions= low resistance.
- Cold conditions= high resistance.
The creation of voltage by a change in magnetic field. Moving a magnet in a coil of wire induces a voltage. There are four factors which effect the size of the voltage induced:
- Strength of the magnet.
- Area of coil.
- Number of turns.
- Speed of movements.
The higher the resistance, the greater the voltage drops.
Vout= Vin x (R2/ R1 x R2)
Useful if you need a certain voltage off something with a different.
- Step up transformer= makes more voltage.
- Step down transformer= makes less voltage.
- Isolating transformer= doesn't change voltage.
- The iron core of a transformer carries magnetic field, not current. Transformers are about 100% efficient (vi=vi) power in= power out.
- Primary voltage/ Secondary turns = number primary turns/ number secondary turns
- Used on national grid by stepping up when leaving grid to go along the cables, then stepping down before it enters the home. Isolating transformers are used in bathrooms so you don't get electrocuted.
When a current- carrying wire is put between two poles they put a force on a wire, cutting the magnetic field and moving them.
Flemmings left hand rule tells you which way the force acts.
Simple Electric Motor
Gets sped up by:
- More current.
- More coil turns.
- Stronger magnetic field.
- A soft iron core.
AC generator- turn the coil and there's a current!
Dynamos- Turn the magnet instead of the coil!
LED's and Relays in Logic Circuits
LED's- Light Emitting Diodes, only let current flow in one direction. If the output is one current flow through the LED to light it up. They're connected in series with a resistor to prevent being damaged by a too larger current flowing through it.
A relay switch connects two circuits- the out put of a logic gate allows only a small current to flow through it unlike what a motor requires. If you have two circuits connected by a relay it isolates the low voltage from the high needed for the output device. This is also safer for the person using it.
How Relays Work
1. When switch closes, the electromagnet turns on and attracts the iron contact.
2. The rocker pivots and closes the contacts in the high current circuit- the motor spins.
3. When the circuit reopens the electromagnet stops, the rocker returns and the high current circuit is broken.
Diodes and Rectification
- Only let Current flow in one direction as it produces a very high current in the other direction.
- They're made from semi-conductors such as silicon.Extra free electrons are free to move at a p-n junction.
- At the n-junction negative electrons move.
- At the p-junction positive electrons move.
- When there's no no potential difference they recombine and become an electric insulator. When there is a potential difference electrons are free to flow, but if they're the wrong way, no current will flow.
- One diode only lets through current in half the cycle.
- To get a full wave it takes four diodes.
Capacitors store charge. The capacitor connects to a battery. The voltage of the capacitor can never rise above the voltage of the battery. If the battery is removed, it discharges. Capacitors 'smooth' circuits as they are used to cause time delays.
- Digital systems are either off or on, yes or no, 0 or 1, high or low, etc...
- A logic gate is a digital processor.
- They are stored in truth tables.
Truth Tables- AND gates.
Truth Tables- OR Gate
Truth Tables- NOT Gate
Truth Tables- NAND Gate
Truth Table- NOR Gate