Electric Charge - Charging an Insulator
- When you rub two different insulating materials against each other they become electrically charged. This only works for insulated objects - conductors direct the charge flow to earth.
When the materials are rubbed against each other:
- Negatively charged particles called electrons move from one material to the other.
- The material that loses electrons becomes positively charged.
- The material that gains electrons becomes negatively charged.
- Both materials gain an equal amount of charge, but the charges are opposite.
Electric Charge - Attraction and Repulsion
If two charged objects are brought close together they are either attracted towards each other, or they are repelled away from each other. If they have the same type of charge they will repel and push away from each other. Two negative charges placed near each other will repel each other, and so will two positive charges.
If they have opposite charges they will attract each other. When a positive charge and a negative charge are placed near each other they will attract each other.
Electrically charged objects can attract small uncharged objects that are close to them. For example, an electrically charged plastic comb can pick up small pieces of paper.
Electric Charge - Metals and Electrical Circuits
- Metal atoms release some of their electrons, and these electrons are free to move through the structure of the metal. This is why metals are good conductors of electricity.
- Insulators have no charges free to move, which is why they do not conduct electricity.
In an electrical circuit, all the components have charges that are free to move. When a circuit is made, the battery causes these free charges to move. They move in a continuous loop around the circuit.
The strength of an electric current is measured in amperes, usually shortened to amps. Amps are a measure of how much electric charge is flowing round the circuit.
Resistance and Current - Resistance and Ohm's Law
- An electric current flows when charged particles, electrons move through a conductor. The moving electrons can collide with the atoms of the conductor. This is resistance ,making it harder for current to flow. These collisions make the conductor hot. It is this that makes a lamp filament hot enough to glow.
Resistance is measured in ohms, Ω. The greater the number of ohms, the greater the resistance.
The equation below shows the relationship between resistance, voltage, current: resistance = voltage / current, ohms (Ω) = volts (V) / amperes (A)
The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. So, if you double the voltage, the current also doubles. This is called Ohm’s Law. The graph shows what happens to the current and voltage when a resistor follows Ohm’s Law.
Resistance and Current - Changing the resistance
- You should know how to change the resistance in a circuit and how to work out the resistance in a series circuit.
- You will not need to worry about the resistance of the wires joining the circuit together. In most circuits, the resistance of the connecting wires is so small that it can be ignored.
When components are connected in series, their total resistance is the sum of their individual resistances. For example, if a 2Ω resistor, a 1Ω resistor and a 3Ω resistor are connected side by side, their total resistance is 2 + 1 + 3 = 6Ω
If you increase the number of lamps in a series circuit, the total resistance will increase and less current will flow. Check your understanding of this using this simulation. Click on the switch to close the circuits.
Resistance and Current - Changing the resistance 2
Resistance and Current - Thermistors and LDR's
The Thermistor - Thermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature increases:
- At low temperatures, the resistance of a thermistor is high and little current can flow through them.
- At high temperatures, the resistance of a thermistor is low and more current can flow through them.
The LDR - LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:
- In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it.
- In bright light, the resistance of an LDR is low and more current can flow through it.