BBC Bitesize - GCSE Physics (Single Science) - Current, voltage and resistance - Revision 3
Compare two circuits with batteries of the same voltage, with the same length of wire, made of A higher current for a given voltage means a lower resistance. The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire has fewer electrons to carry the current. The relationship between. Resistance is dependant on the dimensions of the sample of material. Resistively to measure the Voltage across the test wire and the current through the wire.
Substances in which electricity can flow are called conductors. A piece of conducting material of a particular resistance meant for use in a circuit is called a resistor.
Electrical resistance and conductance - Wikipedia
Conductors are made of high- conductivity materials such as metals, in particular copper and aluminium. Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision, and costs. Ohm's law The current-voltage characteristics of four devices: Two resistorsa diodeand a battery.
The horizontal axis is voltage dropthe vertical axis is current. Ohm's law is satisfied when the graph is a straight line through the origin.
Therefore, the two resistors are ohmic, but the diode and battery are not. For many materials, the current I through the material is proportional to the voltage V applied across it: Therefore, the resistance and conductance of objects or electronic components made of these materials is constant. This relationship is called Ohm's lawand materials which obey it are called ohmic materials.
Examples of ohmic components are wires and resistors. The current-voltage IV graph of an ohmic device consists of a straight line through the origin with positive slope. Other components and materials used in electronics do not obey Ohm's law; the current is not proportional to the voltage, so the resistance varies with the voltage and current through them.
These are called nonlinear or nonohmic. Examples include diodes and fluorescent lamps. The IV curve of a nonohmic device is a curved line.
Relation to resistivity and conductivity[ edit ] Main article: Electrical resistivity and conductivity A piece of resistive material with electrical contacts on both ends.
The resistance of a given object depends primarily on two factors: What material it is made of, and its shape. For a given material, the resistance is inversely proportional to the cross-sectional area; for example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length; for example, a long copper wire has higher resistance than an otherwise-identical short copper wire. Batteries and power supplies supply power to a circuit, and this power is used up by motors as well as by anything that has resistance.
investigating the relationship between the diameter and the current in a wire at its melting point
The power dissipated in a resistor goes into heating the resistor; this is know as Joule heating. In many cases, Joule heating is wasted energy. In some cases, however, Joule heating is exploited as a source of heat, such as in a toaster or an electric heater. The electric company bills not for power but for energy, using units of kilowatt-hours.
It does add up, though. The following equation gives the total cost of operating something electrical: Try this at home - figure out the monthly cost of using a particular appliance you use every day. Possibilities include hair dryers, microwaves, TV's, etc. The power rating of an appliance like a TV is usually written on the back, and if it doesn't give the power it should give the current. Anything you plug into a wall socket runs at V, so if you know that and the current you can figure out how much power it uses.
- Ohm's Law Calculations With Power
- Current, voltage and resistance
- Electrical resistance and conductance
The cost for power that comes from a wall socket is relatively cheap. On the other hand, the cost of battery power is much higher. Although power is cheap, it is not limitless. Electricity use continues to increase, so it is important to use energy more efficiently to offset consumption.
Appliances that use energy most efficiently sometimes cost more but in the long run, when the energy savings are accounted for, they can end up being the cheaper alternative. Direct current DC vs.
Electric circuits || Resistance and thickness of wire
If the circuit has capacitors, which store charge, the current may not be constant, but it will still flow in one direction. The current that comes from a wall socket, on the other hand, is alternating current.
With alternating current, the current continually changes direction. This is because the voltage emf is following a sine wave oscillation. For a wall socket in North America, the voltage changes from positive to negative and back again 60 times each second. You might think this value of V should really be - volts. That's actually a kind of average of the voltage, but the peak really is about V. This oscillating voltage produces an oscillating electric field; the electrons respond to this oscillating field and oscillate back and forth, producing an oscillating current in the circuit.
The graph above shows voltage as a function of time, but it could just as well show current as a function of time: Root mean square This average value we use for the voltage from a wall socket is known as the root mean square, or rms, average.
Because the voltage varies sinusoidally, with as much positive as negative, doing a straight average would get you zero for the average voltage.
The rms value, however, is obtained in this way: To find the rms average, you square everything to get 1, 1, 9, and Finally, take the square root to get 3.
The average is 2, but the rms average is 3. Doing this for a sine wave gets you an rms average that is the peak value of the sine wave divided by the square root of two. This is the same as multiplying by 0. If you need to know about the average power used, it is the rms values that go into the calculation. Series circuits A series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take.
The current is the same through each resistor. The total resistance of the circuit is found by simply adding up the resistance values of the individual resistors: A series circuit is shown in the diagram above. The current flows through each resistor in turn. If the values of the three resistors are: The current through each resistor would be 0.
Parallel circuits A parallel circuit is a circuit in which the resistors are arranged with their heads connected together, and their tails connected together. The current in a parallel circuit breaks up, with some flowing along each parallel branch and re-combining when the branches meet again.