Ohm's Law me te power
Ko ngā mea e rua o V, I, R me P ka whakahoki i ētahi atu e rua.
Whakamātau mahi 4
Ka whakakī ia mahi i ngā uara tauira. Whakakapia ērā ki ō uara.
- Find the resistance of a heater elementThe working resistance and current of a 2 kW heater on 230 V.
- Find the current of a 12 V lampThe current a 50 W lamp draws on 12 V d.c.
- Find the power of a three‑phase heater bankThe kW of a star heater bank of 20 Ω per phase on 400 V.
- Find the voltage across a resistorThe volts across 470 Ω carrying 20 mA d.c.
Ngā whakauru
He tauira mahi me ngā whakauru taunoa.
- Puna hiko
- Kotahi wāhanga a.c.
- Kua mōhio koe
- Whakahiato me te pūngao
- Voltage
- 230 V
- Real power
- 2,300 W
- Power factor
- 1
Iahiko
10A
- Impedance
- 23Ω
- Apparent power
- 2.3kVA
He āwhina hoahoa tēnei hua. Manatokohia ki te paerewa o nāianei.
Ngā hīkoi tātaitanga
- Tātai
I = P ÷ (V × pf)
- Voltage230V
- AuHua10A
- Ārai23Ω
- Pūngao tūturu2,300W
- Pūngao ariā2.3kVA
- Pūngao rerekē0kvar
- Wāhanga ōkeroa, Z × pf23Ω
- Wāhanga reactance, Z × sin φ0Ω
Mō tēnei tātaitai
How this is calculated
Ohm's law says the voltage across a resistance is the current through it times the resistance. The power formula says the power is the voltage times the current. Together they link four quantities, so any two of them give the other two. You pick the pair you know, and the tool solves for the rest.
On d.c. the four are voltage, current, resistance and power, and nothing else enters. On a.c. the current and voltage can be out of step, so two things change. The opposition to current is the impedance, Z, which the tool uses wherever d.c. uses R. The real power is the voltage times the current times the power factor. The tool then splits the impedance into its resistance part, Z times the power factor, and its reactance part, and gives the apparent and reactive power too.
On three phase the tool takes the line voltage, between two phases, and the line current. The real power is the square root of three times the line voltage times the line current times the power factor. The impedance it gives is one phase of a balanced star load: the voltage from line to neutral over the line current. A delta load with the same line current has three times that impedance in each branch.
No standard is read. These are the physics every electrician learns, and the tool shows the formula it used for the pair you gave.
| Step | Table or clause | What it decides |
|---|---|---|
| d.c. | V = I × R and P = V × I | The two unknowns from the pair you know |
| Single‑phase a.c. | V = I × Z and P = V × I × pf | The current or voltage, the impedance and the real power |
| Three‑phase a.c. | V = √3 × I × Z and P = √3 × V × I × pf | Line values and the impedance per phase of a star load |
| Power from resistance and current | P = I² × R | The power when the voltage is not given |
| The parts of the impedance | R = Z × pf and X = Z × sin φ | The resistance and reactance on a.c. |
Worked examples
- A 2 kW heater on 230 V draws 8.7 A. Its working resistance is 26.45 Ω.
- A 50 W lamp on 12 V d.c. draws 4.17 A through 2.88 Ω.
- A star heater bank of 20 Ω per phase on 400 V draws 11.55 A in each line and gives 8,000 W.
- A 470 Ω resistor carrying 20 mA has 9.4 V across it and gives off 0.188 W.
- A motor on 400 V drawing 32 A at 0.85 power factor looks like 7.22 Ω per phase and takes 18,844.71 W.
Limits
The tool treats every load as linear and balanced. It does not model harmonics, an unbalanced three‑phase load or a load whose resistance changes with temperature, such as a lamp filament or a cold heater element. A meter reading on a cold element is lower than its working value.
For a cable's voltage drop, use the voltage drop tool: it reads the cable's resistance and reactance at its operating temperature from AS/NZS 3008.1.
See also: Three phase power, Full load current and Voltage drop.
Pātai
He āwhina hoahoa te hua. Manatokohia ki te paerewa o nāianei.
He aha te Ohm's Law?
Ka rite te voltage puta i tētahi aukati ki te iahiko e rere ana mā taua aukati, me te aukati: V = I × R. Ko P = V × I e hono ana i ngā rahinga e whā, nō reira, ko ētahi rua ka whakatau i ngā mea e rua.
Me pēhea te power factor e whakarerekē ai te whakautu i te a.c.?
I runga i te a.c., ko te impedance Z te whakahē ki te iahiko, ehara ko te aukati anake. Ko te mana pono he V × I × power factor i tētahi wāhanga. Ka hoatu hoki e te taputapu te wāhanga aukati, Z × pf, me te wāhanga reactance.
Me pēhea te mahi a te taputapu mō te wāhanga toru?
Ka tango ia i te voltage raina me te iahiko raina. Ko te mana pono ko te √3 × V × I × power factor. Ko te impedance ka hoatu e ia he wāhanga kotahi o tētahi uta whetū taurite. He toru ngā wā o tērā i ia pou o tētahi uta delta.
He aha i iti ake ai te aukati o te huānga whakamahana ina mātao ana?
Ka piki te aukati o tētahi hāwea whakarākau i tōna pāmahana. He iti ake te pānuitanga makariri i tētahi mita i te aukati mahi. Whakamahia ngā volts me ngā watts whakatauhia mō te uara mahi.
Ka taea te whakamahi mō te heke voltage o tētahi taura?
Me te whakatata noa anake. Ka rerekē te aukati o te taura i te pāmahana, ā, ka tāpiri te a.c. i te reactance. Whakamahia te taputapu heke voltage, e pānui ana i AS/NZS 3008.1 mō te taura.
Ngā tauira mahi
Kōrero o muri
Ngā taputapu hononga
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