Heating effect in resistors
Resistors get hot when a current passes through them and consequently, their resistance increases so a larger voltage is required to produce the same amount of current.
This heating effect can cause components in the circuit to melt, therefore disconnecting the circuit. Fuses melt and break the circuit if the current becomes too high, to protect the circuit.
However, this heating effect can be useful in:
- old-fashioned light-bulbs
Electrical Power Electrical Energy
Electrical power - the rate at which an appliance transfers energy
An appliance with a high power rating transfers a lot of energy in a short space of time. This means that an appliance with a high power rating will draw a large current from the supply.
The equation for electrical power is:
Electrical Power (P) = Voltage (V) x Current (I)
Electrical appliances turn electrical energy into sound, light, heat etc.
The equation for energy transferred is:
Energy Transferred (E) = Current (I) x Voltage (V) x time (t)
Current - the rate of the flow of charge around the circuit
Voltage - the driving force that pushes the current around the circuit
Resistance - anything in the circuit which slows the flow down
Increase in voltage - more current will flow.
Increase in resistance - less current will flow.
Ammeter - measures the current flowing across the component in amps. It must be placed in series.
Voltmeter - measures the voltage across the component in volts, must be placed in parallel around the component under test.
Basic circuits, allow you to take readings for a V-I graph, by altering the variable resistor which changes the amount of current flowing through the circuit, allowing you to take multiple readings from the ammeter and voltmeter.
A.C. - alternating current D.C. - direct current
Resistance, Voltage and Current
The equation linking these three is:
Voltage (V) = Resistance (R) x Current (I)
Important V-I Graphs:
- Metal Filament Lamp - as temperature of the metal filament increase, resistance increases, causing a slight curve.
- Wire - Current is proportional to voltage
- Different resistors - current through a resistor is proportional to voltage
- Diode - current will only flow through a diode in one direction
LED's, LDR's and thermistors
LED (Light-Emitting Diode) - they are used to indicate the presence of current in a circuit and have many practical uses including:
- digital clocks
- traffic lights
- remote controls
Unlike lightbulbs, they don't have a filament that could burn out.
LDR (Light-Dependent Resistor) -In bright light the resistance falls. In the darkness resistance is highest.
They are used in:
- Burglar detectors
Thermistor - temperature-dependent resistor (used in car engine temperature sensors)
In hot conditions, resistance drops, in cool conditions resistance rises
Series and Parallel Circuits
All components are connected in a straight line between the positive and negative of the power supply. Therefore, you cannot control which components current flows through and if one component is faulty or removed, the whole circuit becomes disconnected. The same amount of current flows through all parts of the circuit.
- Fairy lights
Each component is individually connected to the positive and negative of the power supply, so removing one component will hardly affect the others. This is how most practical circuits operate, eg interior domestic lighting
Charge, Voltage and Energy Change
In solid metal conductors, charge is carried by negatively charged ions.
More charge passes when a bigger current flows.
The equation for charge is:
Charge = Current x time
When electrical charge goes through a change in voltage, energy is transferred.
Voltage - the energy transferred per unit charge passed
Volt - One joule peer coulomb
A build up of static is caused by friction. Two examples include:
- Polythene rod - electrons move from duster -> rod
- Acetone rod - electrons move from rod -> duster
Positive charge NEVER moves. Objects are only left with a positive charge if they have lost electrons.
The greater the charge on an isolated object, the greater the voltage between it and Earth. If the voltage is large enough, a spark may jump across the gap.
Like charges repel.
Opposite charges attract.
Gold-Leaf Electroscope and Suspending a Charged Ro
There is a metal disc, which is connected to a metal rod, which is connected to two pieces of thin metal gold leaf.
When a rod with a known charge is brought near to the metal disc, the charge travels down the metal rod into the gold leaves.
Both leaves rise as they have the same charge and so repel each other.
Suspended Charged Rod
Suspend a rod and bring a charged rod near to it. If they are attracted they are oppositely charged. If they repel they are similarly charged. If nothing happens, the suspended rod is not charged.
Van de Graaff Generator and Inkjet Printer
Van de Graaff Generator
It is made up of a rubber belt moving around plastic rollers under a metal dome. As the belt goes around, an electrostatic charge is built up. If you stand on an insulated object with your hand placed on the dome, your hair will stand on edge as each hair has the same charge and so they are repelling each other.
1.Tiny droplets of ink are forced out of a fine nozzle, making them electrically charged.
2. The drops are deflected between two plates. One +ve one -ve. They are attracted to the plate with the opposite charge.
3.The size and direction of the voltage on each plate differents so each drop hits the paper at a different place.
Photocopier, Clothes and Lightning
1. The image that you want to copy is projected onto a positively charged image plate.
2. Whiter bits of the image cause light to fall on it and charge leaks away.
3. The charged bits attract negatively charged black powder onto the positively charged white paper
4. The paper is heated so that the powder sticks.
When synthetic clothes are dragged over one another, electrons are scraped off leaving static charges. The clothes crackle and spark as the charges rearrange themselves.
Rain drops and ice bump together inside clouds, knocking off electrons, leaving the top of the cloud +ve charged and the bottom -ve charged. This creates a huge voltage and a big spark.