- Created by: elle cooper
- Created on: 13-05-09 14:44
What is Current?
- Electric current is a flow of charge which is measured in Amps, Amperes (A). The charge is already present in most components (e.g bulb, switch, ammeter) and wires but it needs electricity from the mains or a battery to move or 'flow'.
- Current is not used up but the chemical energy in the battery is as it is transferred to other components.
- The current flows from the negative to the positive end of the battery or cell. It was previously thought current flowed from positive to negative we call this conventional current.
Changing the Currrent
- Current can only flow through a component if there is a voltage. Voltage is like pressure, it pushes charge around a circuit. Voltage is measured in Volts (V). Voltage is also known as Potential Difference, this term is often used in exams so don't be fooled!
Resistance is the opposite of Voltage, it works against the flow of charge. Resistance is measured in Ohms.
So if you increase the voltage - then more current will flow . But if you increase the resistance - then less current will flow
To find Voltage, Resistance or Current you can use the following equations:
Voltage = Resistance x Current
Resistance = Voltage / Current
Current = Voltage / Resistance
The resistors can either be have a fixed resistance (the resistance is always the same) or the resistance can be changed.
Variable resistors - they can control resistance with a dial or a slider
Thermistors - their resistance depend on temperature.
- In hot conditions, the resistance decreases
- In cool conditions, the resistance increase
Thermistors can be used in thermostats and car engine temperature sensors.
Light - Dependent Resistors (LDRs) - their resistance changes according to light
- In bright light, the resistance falls
- In darkness, the resistance is at its highest
Series Circuits - Working Together
There are four things you need to know about series circuits.
- Voltage (Potential Difference) is shared between all the components, so the voltages running through each component add up to equal the source voltage. For example if I put two bulbs; assuming they are identical, in a series circuit, and the voltage of the cell is 3V, then the two bulbs must each have a voltage of 1.5 running through them.
- Current is the same throughout the circuit. The size of the current is determined by the total potential difference and the total resistance of the circuit. Remember that current is not used up.
- Cell voltages add up. This very simple the sum of the individual voltages in each cell add up is the total potential difference. If i have 2 12V cells what is the total voltage?
- Resistance adds up in a series circuit.The total resistance is the sum of all the indiviual resistances. For example, if i have three fixed resistors and their resistances are: 6 Ohms, 3 and 7 Ohms, then the total resistance is 16 Ohms.
Parallel circuits are very different because components work independently. Unlike series circuits if one component stops working the circuit has not yet been broken.
Here is what you need to know:
- Potential Difference is the same across all components in a parallel circuits.
- Current is shared between branches. If you add up the current in each branch of the circuit it will add up to the total current that leaves the battery. This because in a parallel circuit you get junctions where the current splits, then when the current rejoins we get the total current. Remember what I said earlier the current is not used up .
- Resistance in a tricky one. But it is always less than that of the branch with the smallest resistance. The resistance is lower because the current has more than one branch to take which weakens the resistance. On the other hand the current will be higher.
Generating Electricity Part 1
- Mains electricity supply AC (alternating current), which means the current is constantly changing direction. Batteries supply DC direct current. This means electricity flow in just one direction.
- The UK mains supply is approximately 230 volts which is very dangerous! AC is used instead of DC because its easy to generate and it can be distributed efficiently.
AC Generators generate electricity
- In a generator a magnet rotates in a coil. As the magnet turns, the magnetic field through the coil changes - this change in the magnetic field induces a voltage, which makes a current flow through the coil.
- When the magnet does a 180 degrees turn, the direction of the magnetic field field through the coil reverses. When this happens, the voltage reverses, so the current flows in the opposite direction! ¿Comprende? This is how we get an AC current.
- Three factors affect the size of the induced voltage: The strength of the magnet, the speed of the movement and the number of turns on the coil. Or simply add an iron core
Generating Electricity Part 2
- Electromagnetic induction - This is when a a magnet is moved in a and out of a coil of wire.As you move the magnet, the magnetic field through the coil changes - this change in the magnetic field induces a voltage and a current flows through the wire. The direction of the voltage depends on the way which you move the magnet.
- Transformers Change the voltage but only AC voltages. They have two coils of wire, the primary and the secondary. Transformers use electromagnetic induction. They have to coils of wire around a iron core. They have STEP UP TRANSFORMERS step up the voltage up They have more turns of wire on the secondary coil than the primary coil. STEP DOWN TRANSFORMERS step the voltage down. They have more turns on the primary coil than the secondary.
You can calculate the output voltage from a transformer if you know the input voltage and the number of turns of wire on each coil. Primary Voltage / Secondary Voltage = Number of turns on Primary / Number of turns on Secondary
Example: these are 40 turns on the primary and 800 on the secondary. The input voltage is 1000 V, find the output voltage.
Power Part 1
The power of an appliance tells you how fast it transfers energy from charge passing through it. It is measured in watts (W) or kilowatts (kW).
A light bulb converts electrical energy into light and heat energy. It has a power rating of 2.5 kW, it transfers 2500 joules every second. To find the energy transferred you use this formula: Energy transferred (in joules) = Power (in Watts) x Time ( in seconds).
Kilowatt - hours (kWh) is the amount of electrical energy coverted by a 1 kW appliance left on for 1 hour.
to find the energy transferred in one hour you use this equation:
Energy (in Kilowatt - hours) = Power (in Kilowatts) x Time (in hours).
Power Part 2
To find the power of an appliance we use this formula: Power = Voltage x Current
When appliances convert electrical energy some is often wasted as heat or sound. The efficiency is calculated like this. Efficiency = Energy usefully transferred / Total energy energy supplied x 100%
Example: An electric kettle is supplied with 180000 J of electrical energy 9000 J is wasted, what is the kettles efficiency?
Well if 9000 is watse we subtract it from 180000 to get 171 000, which is the energy that was used usefully. So 171 000 / 180 000 x 100% = 95%